The chromosomal theory of inheritance — Transcript

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1. 0:00hi everybody today we are going to speak about  the chromosomal basis of inheritance and the
2. 0:10chromosomal basis of inheritance is based on  meiosis which is a certain type of cell division
3. 0:22in general why cells divide first for growth  from the embryo to an adult we start as
4. 0:36one cell the zygote and we turn into an organism  that consists of many trillions of cells here we
5. 0:48have a lot of cell division during our lifetime  we have like 10 in the power of 16 cell divisions
6. 1:02replacement of dead and damaged cells
7. 1:06for example just for the immune system we are  replacing billions of cells every single day
8. 1:14cells are dying and new cells are being made  cell division and differentiation of stem cells
9. 1:24cell size is limited cells can grow up to certain  point we are making more membranes we are making
10. 1:34more proteins but at certain point if the cell  is too large the size of the cell is not going
11. 1:44to support all the activities that require simple  diffusion of material from one place to another
12. 1:54so the cell needs to stay at a certain  size and when cells are growing they can
13. 2:02be divided in order to reduce their size by  half cells cannot keep growing indefinitely
14. 2:14a sexual reproduction organisms that one parent  is enough in order to make an offspring one parent
15. 2:25produces genetically identical offspring
16. 2:30plants unit cellular cells and also multicellular  cells if you put one bob in your backyard you're
17. 2:41going to find in few hills that there are many  bulbs there that are making flowers each of them
18. 2:48actually is a result of a sexual reproduction  from the fers bulb that you put in the garden
19. 2:58hydra this is the parental organism and here  you can see a progeny which can barred from
20. 3:07the parental hydra now the bad the progeny and  the parent are identical you don't need another
21. 3:17element in order to form this bud bacterial  cells and many other unicellular organisms
22. 3:26are going mainly through a sexual reproduction one  parent produces genetically identical offspring
23. 3:38many single cell life forms plants and  other organisms can reproduce asexually
24. 3:54in all those cases we need to  use a process called mitosis
25. 4:02here one cell divides to two and both  cells are identical to the parental cell
26. 4:15in general mitosis is a sexual cell  division duplication of one cell into
27. 4:26two cells let's say that this is the cell with  one chromosome this cell need to replicate the dna
28. 4:37into two sister chromatids that are attached  to each other it is still one chromosome
29. 4:44and later on during the process of mitosis the  two sister chromatids are going to be separated
30. 4:53the cytoplasm is going to be separated as  well and two cells are going to be formed
31. 5:02from the parental cell so the cells can grow  to certain level and then by the cell division
32. 5:13you're going to make cells that each is identical  to the original cell so mitosis in general
33. 5:24here we have a pair of chromosomes we start  with an unduplicated pair of chromosome
34. 5:32in the g1 and then by g2 each chromosome  has been duplicated during the s-phase and
35. 5:45then the mitosis and the cytoplasmic division  package one copy of each chromosome into each of
36. 5:55the two new cells and the process is  going to be repeated over and over again
37. 6:03mitosis maintain the chromosome number after  mitosis is completed the parental cell cytoplasm
38. 6:13divides as well and we are going to have two cells  which are both identical to the parental cell
39. 6:24so that's about mitosis mythology  is not relevant directly
40. 6:30to inheritance when it comes to inheritance  cells need to divide for sexual reproduction
41. 6:42two parents give rise to offspring that  have a unique combination of genes inherited
42. 6:50from the two parents this is a process called  meiosis which is responsible for the gamete
43. 7:00formation the meats are sperm and egg cells and  the gametes are formed by a process called meiosis
44. 7:13so let's start with some terms the karyotype  is the number of type of chromosomes
45. 7:20present in the organism the letter  n stands for the number of distinct
46. 7:28type of chromosomes chromosome 1 chromosome 2  each of them is distinct type of a chromosome
47. 7:36the letter n is the haploid number and in  human it is 23. human have 46 chromosomes 2n
48. 7:51so we have 22 pairs of matching chromosomes they  are autosomes and in male we have two different
49. 8:02sex chromosomes x and y and the female f2xs which  act in many ways like the autosomal chromosomes
50. 8:16another important term is a  somatic cell versus germ cells
51. 8:23a somatic cell is a typical body cell it's a  diploid cell as two sets of each of its chromosome
52. 8:35human has 46 chromosomes 2n equal  46 and all the cells in the body are
53. 8:44somatic cells besides the gametes which are  the germ cells sperm egg each gamete has
54. 8:56one n chromosomes which is 23 chromosomes in human  and the gametes are involved with reproduction
55. 9:10so what are the type of chromosomes present in an  organism first homologues chromosomes homologues
56. 9:23they are two chromosomes composing  a pair chromosomes of the same type
57. 9:31with the same genes in the  same order in the same location
58. 9:36on the chromosome we call them the  autosomal chromosomes autosomes
59. 9:46and sex chromosomes represented as x and y  determine the sex of the individual general
60. 9:57xx is a female x y is a male and this is in  mammal it's more complicated in birds for example
61. 10:08and other organisms that their sex  determination happen to be different than
62. 10:14mammals let's go to some other terms when we  are talking on chromosomes here you can see
63. 10:26one chromosome in the duplicated  state you have two sister chromatids
64. 10:37which are attached to each other by its centromere
65. 10:42so pale definition as long the sister chromatids  are attached to each other we are calling this
66. 10:55one chromosome and the second that they are  separated they are becoming two chromosomes
67. 11:04so chromosomes can be with one chromatid can we  with two chromatid it's still one chromosome but
68. 11:11upon separation in between the sister chromatids  then we are talking about two chromosomes
69. 11:22so one chromosome unduplicated can turn  to a chromosome which is fully duplicated
70. 11:30chromatid is attached to each other when  chromated to its sister chromatid it's still
71. 11:37one chromosome and when those chromatids are  separated we are talking about two chromosomes
72. 11:47let's talk about the cell which the n haploid  number is three so we have the paternal
73. 11:58set of chromosomes and the maternal set of  chromosome chromosomes that are coming from
74. 12:09and chromosomes that are coming from dead
75. 12:16so again two sister chromatids in  one fully duplicated chromosome
76. 12:27here you see a pair of homologous chromosomes it's  chromosomes that one of them is from the mother
77. 12:38the other one from the father they're identical  in many ways that probably they're going to be
78. 12:46different in some ways people are different from  each other so the sequence of this chromosome is
79. 12:55going to have some differences as compared to the  other so this is pair of homologues chromosomes
80. 13:07here you have two non-sister chromatids in a  homologous spell again this sister chromatid
81. 13:20is not attached to this one and they have the  same gene at the same order although they can
82. 13:28be different to some level from each other  now when you're talking about the two sister
83. 13:36commented they are pretty much identical to each  other there is no differences almost not at all
84. 13:46so a maternal chromosome paternal chromosome  maternal paternal chromosome consists of the
85. 13:55same genes arranged at the same order but still  the dna sequence is likely to be different
86. 14:03by one to three percent or so  we are different from each other
87. 14:11what is the gene genes are the unit of hereditary
88. 14:17alleles of different versions of a specific gene  [Music] a gene is a section of dna that influence
89. 14:27one or more traits chromosomes contain their cell  hereditary material and are composed of genes
90. 14:39in most cases there are thousands  of genes on each chromosome
91. 14:49so what is the chromosomal theory  of inheritance during the end of the
92. 15:0018 onwards suton and movery carried out a careful  observation of a process called meiosis they
93. 15:14formulated the chromosomal theory of inheritance  they proposed that myosis causes the pattern
94. 15:26of inheritance that mendel observed they  brought back mendelian genetics to life
95. 15:37by figuring out that mendel's rules  can be explained by the independent
96. 15:44alignment and separation of homologous chromosomes  at meiosis one so sutton and bovary explained that
97. 15:58the mathematical model of mendel is confirmed also  when you're looking on chromosomes during meiosis
98. 16:10in this lecture i will discuss the  following questions why does meiosis exist
99. 16:18when do music and fertilization occur during  the life of the organism how does meiosis occur
100. 16:28how does the separation and distribution of  homologous chromosomes produce genetic variation
101. 16:37and what is the role of crossing over
102. 16:42let's start with a short introduction
103. 16:49meiosis is the process that results in  the formation of sperm cells and egg cells
104. 16:55the cells that will undergo meiosis are  typically found in the testes and ovaries
105. 17:00of males and females respectively these germline  cells are diploid having two sets of chromosomes
106. 17:07but will undergo meiosis to produce haploid  cells having one set of chromosomes during
107. 17:14fertilization these haploid cells  fuse to form a diploid offspring
108. 17:21during sexual reproduction a sperm  which is a male reproductive cell
109. 17:28and an egg female reproductive cell unite to form  an individual in the process called fertilization
110. 17:43the result of fertilization is a zygote it's  going to divide by the process called mitosis
111. 17:54and to differentiate to different type  of cells that are making an individual
112. 18:02among the cells of the individual there is  a development of the germ cells as well the
113. 18:13sperm the eggs are going to be formed by process  of meiosis and other level of differentiations
114. 18:23and those are going to be the germ cells  that are responsible for the fertilization
115. 18:31of the following generation because  this is necessary in sexual reproduction
116. 18:39why me or this is necessary why we cannot make  the sperm and eggs by using the regular mitosis
117. 18:49the reason why would regular mitosis be a  problem in making myths is related to the fact
118. 18:56that we are a deployed organism  and if the egg is deployed to end
119. 19:05and the sperm is deployed as well to end the  fertilization is going to result with an embryo
120. 19:17which is made of 4n chromosomes foreign embryo  is not possible it's not compatible with life
121. 19:29especially when it comes to mammals in most  cases in order to stay as a diploid organism
122. 19:39you must to make the egg and the sperm only with  the half amount of chromosomes that you have
123. 19:49in the body somatic cells and this is a process  that called meiosis meow this is a process
124. 20:02that is allowed the egg and the sperm to have  half of the genetic material that you have
125. 20:13in the somatic cells the egg and sperm are  formed from cells that are deployed to start with
126. 20:21and they need to turn into haploid  cells in the gem cells egg and sperm
127. 20:30it is essential to have only one copy of each  chromosome meiosis cell division generate
128. 20:40cells with half of the genetic material reducing  the 2n into one n in the egg and in the sperm
129. 20:53so when you are looking here on the life cycle  of people that's exactly what we have we are
130. 21:00organism with when all the cells of the body  beside the egg and sperm are 2n are deployed
131. 21:09and during the process of meiosis we are  making eggs and sperms that are haploid 1n
132. 21:20and then by process of fertilization the one set  of chromosome is contributed by the egg the other
133. 21:30set of chromosomes by the sperm in order to  make an embryo which is again deployed to end
134. 21:39so sexually reproducing organisms are deployed
135. 21:43to n-46 new men gametes are formed from germ cells  in the adult throughout the process called meiosis
136. 21:54the gametes are formed by meiosis before they can  be used for fertilization those gametes combine
137. 22:03with fertilization to form the zygote which  is again deployed to n 46 chromosomes in human
138. 22:12one set of chromosome came from the haploid  egg the other one came from the applied sperm
139. 22:22and this way we are making an organism with  the normal diploid 46 chromosomes in human
140. 22:34the cells of the zygote divide by  mitosis to produce an adult organism
141. 22:42the myosis is a nuclear division that precedes  the formation of gamete the gametes are formed by
142. 22:52a lot of other cell divisions which are going  to make them into the specific type of cell so
143. 23:02it required a lot of differentiation as well  so making the gametes this egg and spam it's
144. 23:08not just about the meals but there are a lot  of other activities that need to take place
145. 23:17the result when it comes to chromosomal number
146. 23:22is that the gamete consists of half of the  chromosome number and that's what we have after
147. 23:30meiosis one meiosis consists of two  cell division meiosis one and meiosis
148. 23:43before meiosis one we have the process called the  interface where the dna is replicated in order
149. 23:56to make a fully duplicated chromosome so before  meiosis one the dna must be replicated and
150. 24:08chromosomes are duplicated each chromosome in  the diploid two ends parent cell is duplicated
151. 24:22this is a representation of a cell before it  begins meiosis a process in the nucleus that
152. 24:27divides the chromosome number in half for  clarity the nuclear membrane is not shown
153. 24:34also the chromosomes are depicted as condensed  although during interphase of the normal cell
154. 24:39cycle they are actually thin and dispersed and  not visible under a light microscope before a cell
155. 24:46enters meiosis it first replicates its dna after  dna replication the chromosomes are duplicated
156. 24:54so that each has two identical sister chromatids  connected at a structure called the centromere
157. 25:05so from here meiosis let's look at the big picture
158. 25:14meow this reduces the number of chromosomes  set from diploid to haploid in meiosis one
159. 25:27the homologous chromosomes of each chromosome  pair separate and go to different doctor cells
160. 25:38sister chromatids stay together  although the daughter says are haploid
161. 25:471n each chromosome has two identical sister  chromatids still attached at the centromere
162. 25:59but during meiosis 2 the sister chromatids of
163. 26:02each chromosome separates and go to each  of two daughter cells resulting in four
164. 26:13non-identical haploid cells why not identical  later on you're going to see that there are
165. 26:20several changes that can take place in  between the different sister chromatids
166. 26:31let's look at this once again here you  have a cell with a haploid number of two
167. 26:40so you have maternal paternal chromosomes  and the first step during the interface
168. 26:50is that those chromosomes  are going to be duplicated
169. 26:57and now the homologous chromosomes here one  from the father the other one from the mother
170. 27:06at this stage the homologues are going to pair  and to exchange pieces in a process called
171. 27:17crossing over now this is a stage that make the  chromosome to be hybrid part of the chromosome is
172. 27:28maternal chromosome the other one is paternal  chromosome those homologous chromosomes are
173. 27:36going to be separated the spindle fibers are  going to attach to each of the chromosome
174. 27:47and to separate in between them so the chromosomes  are pulled towards the pole homologous chromosomes
175. 27:56are separated but each chromosome still have a  pair of sister chromatids which stay together
176. 28:07and now we are going to have a division of this  cell into two and this is the end of meiosis one
177. 28:23next mios is two
178. 28:27mississippi is quite similar to mitosis here the  separation is in between the cystel chromatids
179. 28:40so again spindle fibers are going to bind to  one sister chromatid from one side and to the
180. 28:51other sister chromatid from the other side and  to pull them apart towards the pull of the cell
181. 29:04so now sister chromatids are separated and the  cells are going to be divided from two to four
182. 29:22so what you have now is four sets that  each of them is different from each other
183. 29:35and each of them is different from the parental  cells as well because of the recombination that
184. 29:43you have during the crossing over so this is part  of the variability which isn't used by meiosis
185. 29:55each gamete is different from another and this  is only one level of variation later on we will
186. 30:04talk about other level of variations that are in  making each commit in a person to be different
187. 30:12from any other gamete on this planet so the cells  are making back the nucleus and then they are
188. 30:23moving towards more steps of differentiation  in order to make they appropriate the meat
189. 30:32the egg the sperm the development of each of  them require many other cell divisions mitosis
190. 30:43and differentiation sperm cell for example is a  result of like four android cell divisions and
191. 30:53differentiation until we have the fully mature  sperm cell in the female this process starts
192. 31:02when the female is still an embryo in a mother  room so before the female is born she already
193. 31:10have the eggs at some level of differentiation  in male the sperm is developed later in life
194. 31:22again me of this one reduce the number  of chromosomes from diploid to haploid
195. 31:28and myosis two produce four haploid daughter cells
196. 31:33that each of them is different from each  other and from the parental cell as well
197. 31:44let's talk with a bit more details on the  different phases of meals this one and meiosis too
198. 31:56me or this one is continuous  process with four distinct phases
199. 32:05prophase one metaphase one anaphase one telophase
200. 32:14one and at the end of meals is one the cell  divide to form two haploid doctor cells
201. 32:29during meiosis one the chromosomes condense  you can see them under the microscope
202. 32:40the homologous chromosomes  pair a process called synapsis
203. 32:49crossing over is taking place between homologous  non-system chromatids and here this process
204. 33:04make the chromosomes to be in hybrid between  the parental and the maternal chromosomes
205. 33:15during metaphase one the pair of homologous  chromosomes line up at the metaphase plan
206. 33:27and during anaphase one
207. 33:30the pair of homologous chromosomes separate  and migrate to the opposite ends of the cell
208. 33:44and during the telophase and cytokinesis
209. 33:51the homologues finish migrating to  the poles of the cell and the cell
210. 33:58is going to divide with the process of  cytokinesis to form two haploid daughter cells
211. 34:10meiosis consists of two successive cell divisions  called meiosis one and meiosis ii each of which
212. 34:18is subdivided into four phases in the first phase  of meiosis one called prophase one the harmonicus
213. 34:25chromosomes or homologues in a diploid cell come  together in a process called synapsis the synapse
214. 34:32pairs are known as tetrads each tetrad consists  of one chromosome inherited from the mother
215. 34:39colored red in the diagram and one inherited from  the father colored blue where they're synapsed
216. 34:46the chromosomes can cross over each other  forming an x-shaped structure called a chiasma
217. 34:52at the crossover sites the harmonicus chromosomes  exchange segments this exchange results in genetic
218. 34:59variability in the daughter cells the second  major phase of meiosis one is metaphase one
219. 35:06during this phase the pairs of chromosomes line up  in the middle of the cell during the third phase
220. 35:13anaphase one the chromosomes of each pair move  to opposite poles of the cell in telophase 1 the
221. 35:20fourth and final phase the chromosomes reach the  poles of the cell when meiosis 1 is complete the
222. 35:28cytoplasm divides to produce two haploid daughter  cells each having just a single set of chromosomes
223. 35:35even though a duplicated chromosome has  two chromatids the chromatids are bound by
224. 35:40a centromere and are considered one chromosome  the number of chromosomes in a cell equals the
225. 35:46number of centromeres present note that the  two daughter cells are genetically different
226. 35:54the phase of meals is slow prophase two metaphase  two anaphase two telophase two and cytokinesis
227. 36:14at the end of meals is still each cell divide  and four haploid daughter cells are produced
228. 36:23so at the start of meiosis each doctor cell  is haploid but each replicated chromosome is
229. 36:34composed of two sister chromatids with the  variation which is related to the crossing over
230. 36:46meals is still consists of five phases  prophase two the spindle apparatus form
231. 36:54the nucleus envelope formed at the end  of meals one it breaks apart metaphase
232. 37:02two replicated chromosomes consisting of two  sister chromatids line up at the metaphase plan
233. 37:14and during anaphase two sister  chromatids separate into two chromosomes
234. 37:23each chromosome begin moving to opposite sides  of the cell in telophase 2 and cytokinesis
235. 37:34chromosomes finish moving to the opposite side  of the cell a nuclear envelope forms around each
236. 37:42haploid set of chromosomes and itself  divide and foraploid dr cells are produced
237. 37:53the second event of meiosis meiosis ii  strongly resembles mitosis during prophase
238. 38:00ii duplicated chromosomes consisting of two  sister chromatids begin to move to the middle
239. 38:05of the cell in metaphase two the chromosomes  are arranged along the cell's midplane
240. 38:12the sister chromatids begin to separate in  anaphase ii becoming independent chromosomes
241. 38:17that move to opposite poles of the cell during  telophase 2 the chromosomes reach the poles
242. 38:25when meiosis 2 is complete the cytoplasm  of each cell divides to form two daughter
243. 38:31cells the four cells are haploid each  containing a single set of chromosomes
244. 38:37note that the four daughter cells are all  genetically different from one another
245. 38:50pseudonym bovary carried out a careful  observation of meiosis find the pattern
246. 38:59pair singles pair like mendel did in meiosis  the number of chromosomes is reduced by half
247. 39:10and the original number of the chromosome is  restored in the fertilized egg and in the zygote
248. 39:18they propose that meoses causes the  patterns of inheritance that mendel observed
249. 39:27and they formulated the chromosomal theory of  inheritance in 1903 susan suggested that the
250. 39:39association of parental and maternal chromosomes  in pairs and their subsequent separation during
251. 39:48the reduction division may constitute the  physical basis of mendelian law of hereditary
252. 39:59he published the formal publication in  1903 this article discussed the connection
253. 40:10in between mendelian genetics and chromosomal  theory so in 1903 mendel was back his was repeated
254. 40:24by others and they figure out that mendel was  right mendel actually is the reference of genetics
255. 40:32even today when we are looking and we can see  something that violate mendelian rules there
256. 40:39is a very good reason for this every time that we  see violation of mendelian rules we need to make
257. 40:45some explanation and usually those explanation  are making things to be even more interesting
258. 40:55what about the sex chromosomes well medallion  genetics was not dealing with sex chromosomes
259. 41:03mendelian was about autosomal chromosomes
260. 41:10sex chromosomes are one of the way that mendelian  rules are violated why because it's not pair
261. 41:20single pair when it comes to the sex  chromosomes at least in most of the organisms
262. 41:28actually violating mendelian rules by  the discovery of the sex chromosome
263. 41:36gave a strong conformation  to the chromosomal theory
264. 41:42because we were able to see that mendelian rules  are still relevant even when they are violated
265. 41:53here we can see that specific combination
266. 41:57of chromosomes dictate the  phenotype of male and female
267. 42:05this is natalie stevens this is edmund welson  they did the karyotype analysis of meatwarm
268. 42:15and they were able to show some interesting  differences in the karyotypes of different worms
269. 42:26there is a male and female worm  what they were able to say in 1905
270. 42:38is that the body cells from a female worm 20  chromosomes and all of them are similar in size
271. 42:52with females things are quite simple
272. 42:56at this level because the sex chromosomes act like  an autosomal chromosome but when male is different
273. 43:08in male body cells there are 19 similar  chromosomes and one which is much smaller
274. 43:19now the chromosomes are not  the same i mean they were able
275. 43:23to see the size of chromosome at the time and  that's the way that they were able to distinguish
276. 43:29between the large chromosomes and something  else so they found two type of male gametes
277. 43:39and that's quite similar to what  we have in our system as well
278. 43:47half of the gametes the sperm add the nine  large chromosomes and one small chromosome
279. 43:58and half of the gametes had 10 large chromosomes  so 50 of the sperm of this small unique chromosome
280. 44:13that's the first time that we were  talking about the x y chromosomes
281. 44:23the small chromosome is the y chromosome  the x chromosome is a large chromosome
282. 44:31and here the x and the y chromosomes are  the sex chromosomes they determine the sex
283. 44:40of the offspring when xx is  a female and xy is a male
284. 44:50that was the first time that we were talking  about x and y chromosomes the sex chromosome
285. 44:59they determine the sex of the offspring  the y chromosome is the small chromosome
286. 45:08the x chromosome is one of the large chromosomes
287. 45:12and genes on the sex chromosomes the x  and the y are sex-linked inheritance genes
288. 45:22so that was the first time  that they developed an idea
289. 45:30that the phenotype of gender is also have some  sort of mendelian pattern the gender is inherited
290. 45:44like mendelian trait although the sex chromosomes  violate mendelian rules human gender is indeed
291. 45:54determined in the same way x and y chromosomes  female f to x chromosomes male x and y
292. 46:10female can contribute only an x  chromosome to a egg to the next generation
293. 46:21and the male can contribute the  x and then it's a girl or the y
294. 46:34and it's a boy
295. 46:44so the chromosomal basis of inheritance  in organism sex is an inherited
296. 46:50phenotypic character determined by the  presence or absence of certain chromosomes
297. 46:59in human and other mammals there are two variety  of sex chromosomes x and y females xx male is xy
298. 47:13and the karyotype of a female is where  the x chromosome act pretty much like
299. 47:22any other autosomal chromosome it's a large  chromosome consists of a lot of different genes
300. 47:29now the karyotype of a male is similar  when it comes to autosomal chromosomes
301. 47:38but the sex chromosomes are combination of x and y  next chromosome is very important to the survival
302. 47:50of the cell because it consists a lot of genes  that have nothing to do with sex determination
303. 47:57but the y chromosome consists mainly of  genes that are involved with male fertility
304. 48:05so what's happened during me or this when we  have x and y how they can be separated from
305. 48:13one another the x and y every certain  region where they can align with each other
306. 48:26they have some homogeneous at the very end now  this is an autosomal chromosomes that can pair
307. 48:37that can exchange pieces by crossing over  as you can see here and that's what you're
308. 48:46going to see also when it comes to the sex  chromosomes x and y they compare with each other
309. 48:57and they can even exchange pieces
310. 49:02but normally it's going to take place  only in the region that they have homology
311. 49:10and now they're going to be separated from  each other in a very similar way that autosomal
312. 49:18chromosomes are to be separated they are being  separated during meiosis one and half of the
313. 49:27cells are going to have the x chromosome the  other half are going to have the y chromosome
314. 49:34and that's the way that the gametes in  this case the sperm are going to have x o
315. 49:44y so we'll determine the sex of the next  generation is the sperm because the egg
316. 49:54always have the x chromosome if it is the x  chromosome it's going to be a girl if it's
317. 50:03going to be the y chromosome it's going  to be a boy again sex chromosome meiosis
318. 50:13as the result of this the sex chromosomes are  going to be separated according to the mendelian
319. 50:21and the chromosomal theory of inheritance  and this way 50 of the sperm containing the x
320. 50:31chromosome 50 of the sperm contain the y  chromosome and the result is x y male x x female
321. 50:48life can be quite complicated when it comes to
322. 50:53sex determination because there are different  system of sex determination in other organisms
323. 51:04the xo the termination system is a system that you  can find in grass super crickets coca chairs in
324. 51:18this system there is only one sex chromosome refer  to us as x male have only one x chromosome which
325. 51:27is x zero while female have two x chromosomes  x x in roosters in chickens other birds
326. 51:40actually here the large chromosome is zed that's  actually the x chromosome of chickens and the w
327. 51:52is like the y chromosome it's  the small chromosome here
328. 51:58things are quite different male a homozygous  for the z the x chromosome of chickens two z
329. 52:09in order to be a male and females are heterozygote  so the females have like the x y in our system
330. 52:24in other animals like bees they have the full  deployed organism versus two haploid organisms
331. 52:32now the haploid is a male the diploid  with the old 16 chromosome is a female
332. 52:43if you will go back to bovery and certain  chromosomal theory well even the fact that
333. 52:53it was consistent with mendelian theory many  people thought that that's not correct direction
334. 53:01of hereditary so it was discussed and debated  for like 10 years or so before it was accepted
335. 53:12it was supported by some but strongly  rejected by others by 1915 thomas hunt morgan
336. 53:23initially strong skeptics against the chromosomal  theory but he led the controversy to west
337. 53:35with studies on the fruit fly drosophila early  found genes that are located on the x chromosome
338. 53:47when we are talking about the fruit fly you're  going to find a lot of different phenotypes
339. 53:55one of them is the color of the eye the color  of the eye is normally red but in some cases
340. 54:08it is white thomas hunt morgan was working with  the fruit flies and there are few reasons for this
341. 54:18first they breed at high rate a new  generation can be bred every two weeks or so
342. 54:26that's really important netherlands wait  for quite a while the almost entire season
343. 54:33in order to see a result from his experiment but  when you can get the results of a breeding between
344. 54:42fruit flies within a two weeks that's really a  very big boost to your research there are other
345. 54:49reasons that morgan made a choice for the fruit  fly it's very easy to find them in the kitchen
346. 54:58all you need is to keep a potato for long periods  of time and you are going to find your entire
347. 55:06home full with fruit flies so it's easy organism  to find and there are many traits that you can
348. 55:15find in fruit flies and they have quite a lot  of mutations but are expressed as a phenotype
349. 55:24wild type or where normal is a phenotype  that were common in the fly population
350. 55:34and the mutant phenotype is a  trait alternative to the wild type
351. 55:43thomas hunt morgan is doubted his  fly lab in colombia university
352. 55:51another reason why i was using a fly lab in  columbia university is because well colombia
353. 56:01university is located in manhattan and if you  wanted to grow plants there tell you i'm not going
354. 56:11to find a place it's manhattan so using flies in  a small lab it was quite a good idea at the time
355. 56:25morgan was interested in traits in the fruit fly
356. 56:34and in 1910 he found a mutant  a male with a white eye
357. 56:43he made the male with the white eye  with the female which looked normal
358. 56:50this mating between those two phenotypes  gave the first generation that all of them
359. 56:58look normal then it cost male and female from  the first generation male which have the normal
360. 57:11phenotype female with the normal phenotype so  let's see what is the genotype of the parents
361. 57:20so when you cross in between the male with the  normal phenotype and the female with the normal
362. 57:27phenotype which of the f1 of this experiment you  have the following ratio half are going to be
363. 57:37females that have a normal phenotype and from  the other half you have males that half of them
364. 57:49are normal in the thread of the color of the eye  and half of the male have white eyes here you have
365. 58:00two phenotypes one is the sex being a male being a  female and the color of i in the second generation
366. 58:11the females were always normal while half of the  male are not right now you can start to say well
367. 58:21white eye thread is connected to the sex white  eye is linked to the phenotype of being a male
368. 58:34in order to understand the genotype of all the  above let's take the f1 put it in punnett square
369. 58:47and in all order to have the  phenotype which is linked to a male
370. 58:58you must have the following gene distribution  you must have the gene of the color of the eye
371. 59:09to be located on the x chromosome female can  have the mutated x chromosomes but they can
372. 59:16still have the normal phenotype that come from  the normal allele on the other x chromosome
373. 59:27males are normal if the gene of the color of  the eyes is normal on the only x chromosome
374. 59:39what you have from this mating is  the following you're going to have
375. 59:44a female with a normal eye color you have  a male with the normal color of the eye
376. 59:55you have a female which is  curious she is still normal
377. 1:00:04and a male which add the x chromosome with  the mutation from the mother and this male
378. 1:00:14is going to have white eyes all the others are  going to have red eyes they smell where the only
379. 1:00:27eggs contain a gene which is mutated with the  mutant phenotype white eyes in order to prove
380. 1:00:37this point even further you must do a test course  and in this case to make between the f2 generation
381. 1:00:52now here you have the mutated allele  in the female and the mutated allele
382. 1:01:03in the male the male have the phenotype the  female is normal and when you mate in between them
383. 1:01:16you're going to have the following distribution
384. 1:01:20well you're going to have a female which  is homozygote for the mutated allele
385. 1:01:29this female going to have the phenotype of white  eye here you have a female carrier mutated male 1
386. 1:01:43and here is normal male which get the  x chromosome with the normal allele
387. 1:01:55and when you see female with white eye  she must be homozygote on a x chromosome
388. 1:02:05for the mutated allele that's actually the  way to prove that the color of the eye is
389. 1:02:17sex linked is linked to the x chromosome  in this type of pattern is strongly suggest
390. 1:02:26that a gene is located on the x chromosome  if the gene is located on the x chromosome
391. 1:02:37females that are homozygote for this  mutation are going to have the trait
392. 1:02:45morgan put to rest the debate about the  importance of chromosomes as carrier of genes
393. 1:02:54here you can see that there  is a chromosome x chromosome
394. 1:02:59which must have a gene which gave you a  phenotype so thomas and morgan provided the
395. 1:03:09convincing evidence that chromosomes in this case  next chromosomes are the location of mendel's
396. 1:03:18heretable facto morgan proved the chromosomal  theory of inheritance morgan showed an association
397. 1:03:31between the sex phenotype and the eye color  the sex phenotype was already known to be
398. 1:03:39linked to the sex chromosome this is the  work that was done by stevenson and wilson
399. 1:03:48hence by the indirect association the eye color  was linked to a chromosome to the x chromosome
400. 1:04:01and by generalization chromosomes skeletons and  that was the first solid evidence indicating
401. 1:04:12that a specific gene is associated  with a specific chromosome
402. 1:04:21and that was the time that sultan and mother  studies become totally acceptable 10 years later
403. 1:04:35in general genetics can explain why kids resemble  their parents but also why we are different
404. 1:04:48genetics can explain the similarities between  people but also the variation among people yes
405. 1:05:04we are different from each other  in some ways the way that we look
406. 1:05:10and certain threats some of them are visual some  of them are not and meiosis can explain genetic
407. 1:05:19variation sexual reproduction result in variation  meow this led to offsprings that are genetically
408. 1:05:32distinct from each other and from their parents  and there are several mechanisms for this
409. 1:05:40we are going to speak about two of them  independent assortment and crossing over
410. 1:05:52natural selection is about the total fitness of  a population which is a result of the balance
411. 1:06:00between positive effect and negative  effect of the genetic diversity some
412. 1:06:07genetic diversity can lead to a disease others are  going to lead towards a better natural selection
413. 1:06:20a natural selection act on variation
414. 1:06:29the independent assortment  of homologous chromosomes
415. 1:06:33in meiosis is one of the  reasons for genetic variation
416. 1:06:42the independent assortment allow the maternal  versus paternal chromosomes to align at random
417. 1:06:55here for example in this possibility we have two  chromosomes maternal and paternal and in this
418. 1:07:06possibility two of the chromosomes the maternal  chromosomes are aligned on one side of the cell
419. 1:07:16and the paternal on the other side so they  are going to be separated in a way that the
420. 1:07:24maternal is going to be in one cell and the  paternal in another cell after me or this one
421. 1:07:33and this is another option which is equally  possible in this possibility the maternal
422. 1:07:42of chromosome here and the paternal chromosomes  here that are aligned towards the same pole
423. 1:07:50after the separation during meiosis one you're  going to have a cell which have the combination
424. 1:07:59of maternal and paternal chromosomes here again  paternal and maternal chromosomes and of course
425. 1:08:08it's going to be expressed in a similar  way when it comes to the end of meiosis too
426. 1:08:17so one of the ways by which myosis promotes  variation is randomly choosing one chromosome
427. 1:08:25out of the two for the new sperm or egg cells  so you can have all the different possibilities
428. 1:08:35in each of the sperm or eggs that are going  to be produced at the end of this process
429. 1:08:45so how does the separation and distribution of  homolog chromosomes produce genetic variation
430. 1:08:53the random separation or independent assortment  of homologous chromosomes in anaphase 1
431. 1:09:00generate a great deal of genetic diversity in  the subsequent gametes so the number of different
432. 1:09:11combination equal to 2 in the power of n where  n is the upload number so in human we deployed
433. 1:09:23chromosome number 23 we can produce 2 in the  power of 23 8.4 million different combination
434. 1:09:38of chromosomes that's in one gamete right so now  there is another gamete which have the same level
435. 1:09:47of variability there is also 8.4 million  different combination in the other gamete as
436. 1:09:56well so 8.4 million times 8.4 million which is the  probability of the overall union in between male
437. 1:10:05and female here we are getting over 64 trillions  genetic combinations so yes we are quite different
438. 1:10:18from one another just because of independent  assortment during metaphase one of meiosis
439. 1:10:27each joined pair of homologs lines up on the  metaphase plate the orientation of each pair on
440. 1:10:34the spindle axis is random either the maternal or  paternal homologue may orient toward a given pole
441. 1:10:43the number of possible chromosome  orientations equals two raised to the
442. 1:10:47power of the number of chromosome pairs in this  hypothetical cell with three chromosome pairs
443. 1:10:54eight two to the third possible orientations  exist each orientation produces gametes with
444. 1:11:00different combinations of parental chromosomes  so mendelian law of independent assortment
445. 1:11:09fit well with the independent  assortment of the chromosomes
446. 1:11:16mendel said that genes of different traits are  sorted separately from one another so that the
447. 1:11:24inheritance of one trait is not  dependent on the inheritance of the other
448. 1:11:32well mendel was working with genes that each  of them was located on different chromosomes he
449. 1:11:40actually chose those traits that are showing  independent assortment and probably had other
450. 1:11:47traits that are not working in the same way and  left them aside for others to figure them out
451. 1:11:56another mechanism for variation is crossing  over let's start with a short introduction
452. 1:12:05about crossing over and genetic  variation cross-strand exchange is
453. 1:12:11an important intermediate structure in general  dna recombination the characteristic junction
454. 1:12:18is formed when two homologous double-stranded  dna molecules reciprocally exchange dna strands
455. 1:12:32this junction can be visualized  directly in the electron microscope
456. 1:12:40an important aspect of recombination is that the  junction point is not fixed after it has formed
457. 1:12:45but can migrate up and down the dna as shown  in this animation in the cell this junction is
458. 1:12:52formed and stabilized by a specific protein  which can be seen here in the background
459. 1:13:15crossing over [Applause] prophase of meiosis one  crossing over me of this one result in gametes
460. 1:13:26whose chromosome composition is different  from that of their parent after crossing
461. 1:13:35over the chromosomes are hybrid between the  parental and the maternal original chromosomes
462. 1:13:46let's look into chromosomes  that have two genes h and f
463. 1:13:55the maternal and the paternal chromosomes the  maternal have a two genes one capital f the
464. 1:14:04other capital h the paternal f small f and small h  now we are going to see what's going to happen to
465. 1:14:13the combination of those genes after crossing  over the chromosome is going to be duplicated
466. 1:14:21during the interface and then  the chromosomes are going to pair
467. 1:14:32and then the non-sister chromatids
468. 1:14:41are going to exchange pieces
469. 1:14:46so uh here in a certain region and it can be done  totally at random the chromosomes are going to
470. 1:15:01exchange pieces to make the non-sister chromatids  different than the original parental chromosome
471. 1:15:16and now what you're going to have is  a combination of the parental genes
472. 1:15:30so let's see what's happened after me or this one
473. 1:15:35separation and now you can see what's  going to happen we are going to have here
474. 1:15:43the combination of the maternal and the paternal  genes in this sister commented here we are
475. 1:15:57going to stay with the original paternal genes  here we are going to have again a combination
476. 1:16:07of capital f small h means the maternal and  the paternal on the same sister commented
477. 1:16:18while this one the original maternal chromosome  is going to keep the capital f and capital h
478. 1:16:31so let's go towards the end of this process  meiosis one in the end of me of this one we have
479. 1:16:42different combinations as compared to what we  had in the paternal or the maternal chromosomes
480. 1:16:50we have a hybrid in between the  two so we have capital f small h
481. 1:16:58means maternal and paternal on the same  chromatid while we are going to find here
482. 1:17:08the opposite contribution here the capital h  is the maternal and here we have the paternal
483. 1:17:20gene again here we are going to  have overall a new combination
484. 1:17:29if we do not have crossing over the new  generation are going to have the same
485. 1:17:36combination like their parents so here  we are going again to separate in between
486. 1:17:51the chromosomes in one cell we have the maternal
487. 1:17:57chromosome fully maternal chromosome and the  other one we have the entire paternal chromosome
488. 1:18:06but when we have the crossing over we have a  combination of both maternal and paternal paternal
489. 1:18:20and maternal while this chromatid is going to  keep the original here paternal here maternal
490. 1:18:35after meals is still we are  going to have the following
491. 1:18:44the well you don't have crossing over you're  going to keep the entire original chromosome
492. 1:18:55the maternal and the paternal but here after  crossing over you're going to find that half
493. 1:19:05of the gametes a result of a combination of  the maternal and the paternal chromosomes
494. 1:19:14this is the way that recombinant chromosomes  can be developed during the process of meiosis
495. 1:19:23during synapses in prophase one the two pairs of  sister chromatids are held tightly together and
496. 1:19:34both sets of non-sister chromatids may  undergo crossing oval and exchange segments
497. 1:19:44crossing oval during prophase one  is form of genetic recombination
498. 1:19:52produce new combination of  alleles on the same chromosome
499. 1:19:57and greatly increase the genetic  variability of gametes beyond that due
500. 1:20:05to random assortment of chromosomes during meiosis  although mitosis and meiosis have much in common
501. 1:20:14meiosis has three unique features synapsis  homologous recombination and reduction division
502. 1:20:23following chromosome replication homologous  chromosomes or homologs pair all along their
503. 1:20:29length the process of forming these complexes  of homologs is called synapsis the second unique
504. 1:20:36feature of meiosis is that genetic exchange occurs  between the homologous chromosomes while they are
505. 1:20:42physically joined the exchange process that  occurs between paired chromosomes is called
506. 1:20:48crossing over the third unique feature of meiosis  is a reduction division at the end of meiosis each
507. 1:20:56cell contains only half the original complement  of chromosomes because of the crossing over
508. 1:21:03that occurs during the first division the sister  chromatids in meiosis ii are not always identical
509. 1:21:09omitting a chromosome duplication before  meiosis ii produces haploid gametes
510. 1:21:16how does fertilization affect genetic variation
511. 1:21:21first we have many sperms billions and here  we have random genetic options for one egg
512. 1:21:30and out coursing our mating partners are not a  family members are not related to us genetically
513. 1:21:41so in many sexually reproducing species  gametes from non-related individuals
514. 1:21:48combine to form a offspring that's out causing  and in outcrossing the genetic diversity
515. 1:21:57of the offspring increases  because chromosomes form
516. 1:22:01two parents which have many different cells  in between the gene versions are combined
517. 1:22:12so a summary meiosis is called the reduction  division because the total number of chromosomes
518. 1:22:19present is cut in half during me of this a single  diploid parent cell with replicated chromosomes
519. 1:22:31gave rise to four haploid daughter cells the cells  that result from eoses are genetically unique each
520. 1:22:42receive a random assortment of maternal  and paternal chromosomes and crossing over
521. 1:22:50lead to chromosomes with random  combination of maternal and paternal
522. 1:22:58allele myosis is about the development  of sperm and eggs so meiosis is a process
523. 1:23:09which takes place before the fertilization  the union between sperm and an egg
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