Mendelian genetics and Punnett squares — Transcript

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1. 0:03Genetics is the science that studies
2. 0:05inheritance or the way parents transmit
3. 0:07certain traits to their descendants. And
4. 0:10Mandelian genetics refers to Gregor
5. 0:12Mandel, an Austrian monk who studied
6. 0:15inheritance by experimenting on pea
7. 0:17plants.
8. 0:21He cross-pollinated the flowers of
9. 0:22different plants together, took the
10. 0:24seeds that developed from the pairing,
11. 0:26planted those seeds, and took careful
12. 0:28notes on the types of peas that resulted
13. 0:30in the subsequent generations.
14. 0:33You might say as a monk, he was trying
15. 0:35to find his inner peas. Now, in addition
16. 0:38to having lots and lots of peas in his
17. 0:40garden, he helped to formulate two
18. 0:42important laws. The law of segregation
19. 0:44and the law of independent assortment.
20. 0:48So to start out, Mandel took plants with
21. 0:50violet flowers and plants with white
22. 0:52flowers and crossbred them. This
23. 0:55original group of flowers are called the
24. 0:57P generation as in parent. And then when
25. 1:00he obtained some peas, he planted them
26. 1:02and got more plants. And the flowers in
27. 1:05this offspring generation were called F1
28. 1:07or filial 1.
29. 1:10It turns out that the F1 generation
30. 1:12consisted of all violet flowers. So we
31. 1:14called the violet trait dominant. while
32. 1:17the white trait which appeared to be
33. 1:19lost in the F1 generation was called
34. 1:21recessive.
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40. 1:41Next, Mandel let the violet flowers in
41. 1:43the F1 generation cross-pollinate
42. 1:45amongst themselves. And when they formed
43. 1:47peas, he planted them again. From this
44. 1:50he got more plants. And the flowers from
45. 1:52the second generation of plants he
46. 1:54called filial 2 or F2.
47. 1:57It turned out that some of the plants in
48. 1:58this F2 generation had white flowers,
49. 2:01whereas other plants had purple flowers.
50. 2:04In fact, the ratio was about three
51. 2:06violet flowering plants for every one
52. 2:08white flowering plant. Based on this
53. 2:11experiment, Mendel drew a few
54. 2:13conclusions.
55. 2:14First, since the F1 violet flowers had
56. 2:16some offspring plants that produce
57. 2:18violet flowers and other offspring
58. 2:20plants that produced white flowers, it
59. 2:22meant that the F1 plants must have
60. 2:24contained both of these elements. The
61. 2:26inheritable elements of Pants are its
62. 2:28gameamtes.
63. 2:31So that meant that the gametes of the F1
64. 2:33plant contained either the dominant
65. 2:34violet trait or the recessive white
66. 2:36trait.
67. 2:39The F2 plants are created with one game
68. 2:41from each parent. And Mandel worked out
69. 2:44that the white flowering plants resulted
70. 2:46when they received both white flower
71. 2:48elements and that plants that had at
72. 2:50least one violet flower element from
73. 2:52either parent would produce violet
74. 2:53flowers.
75. 2:55Mandel didn't know this at the time, but
76. 2:57the element he was referring to were
77. 2:59segments of DNA called genes that
78. 3:02encoded each flower color. These genes
79. 3:04were located on specific parts of
80. 3:06chromosomes called loi. Different
81. 3:09versions of a gene are called alals. And
82. 3:11in the case of the flowers, there were
83. 3:13two alals, a white and violet al for
84. 3:15flower color.
85. 3:17A helpful way to visualize Mandel's
86. 3:19experiment is to use a punit square.
87. 3:22Imagine a box with four squares in it
88. 3:24where we put the genetic information of
89. 3:26one parent or genotype on the horizontal
90. 3:29row and the other parent on the vertical
91. 3:31column. The dominant alil represented
92. 3:34with a capital letter codes for a violet
93. 3:36flower and the recessive alle
94. 3:39represented with a lowercase letter
95. 3:41codes for a white flower. The letter we
96. 3:43choose doesn't matter. So let's use
97. 3:45capital P for the violet flower al and a
98. 3:47lowercase P for the white flower al
99. 3:51generation. Mandel used pure breeding
100. 3:53plants. So their genotype was two of the
101. 3:56same alals. In other words, both of the
102. 3:58parent plants in this generation were
103. 4:00homozygous for flower color trait. Homo
104. 4:03meaning same and zygus referring to the
105. 4:05male and female alals.
106. 4:08The violet pea plant had two of the same
107. 4:10dominant alals capital P capital P and
108. 4:14therefore had all violet flowers.
109. 4:16Whereas the white pea plant had two of
110. 4:18the same recessive alals lowercase P
111. 4:21lowercase P and therefore had all white
112. 4:23flowers. Now the observable trait that
113. 4:26results from the genotype is called the
114. 4:27phenotype. In this case the phenotype is
115. 4:30the flower color.
116. 4:33So when the violet and white flowering
117. 4:34plants were crossbreed, each offspring
118. 4:37got a dominant alil from the violet
119. 4:39flower parent and a recessive al from
120. 4:41the white flower parent. Since the two
121. 4:44alals are different, these plants are
122. 4:46all hetererozygous, meaning that they
123. 4:48have heterero or different alals for the
124. 4:50flower color trait.
125. 4:52The phenotype of these hetererozygous
126. 4:54plants was that they all had violet
127. 4:56flowers because the dominant capital P
128. 4:58al masks the recessive lowercase P al.
129. 5:03Now when we breed any two of these
130. 5:05hetererozygous plants in the F1
131. 5:06generation, we can make a new Punet
132. 5:09square with the capital P lowercase P
133. 5:12genotype of one parent on the horizontal
134. 5:14row and the same capital P lowercase P
135. 5:16genotype of the other parent in the
136. 5:18vertical column. When we use the pet
137. 5:21square, we get one offspring with a
138. 5:22capital P capital P genotype, two with a
139. 5:25capital P lowercase P genotype, and one
140. 5:28with a lowercase P lowercase P genotype.
141. 5:32The three plants with at least one
142. 5:34capital P alil will have a violet flower
143. 5:36phenotype. And the one plant with a
144. 5:38homozygous lowercase P lowercase P
145. 5:40genotype will have a white flower
146. 5:42phenotype.
147. 5:44This was the ratio of plants that Mandel
148. 5:46observed in the F2 generation.
149. 5:49and it helped establish the law of
150. 5:51segregation which states that alals
151. 5:53segregate and that offspring acquire one
152. 5:56alil from each parent.
153. 5:59Now it turns out that in addition to
154. 6:01flower color, Mandel also observed the
155. 6:03seeds of his pea plant, specifically
156. 6:05their color and texture. He noted
157. 6:08whether the seeds were yellow, which
158. 6:10we'll call the dominant big Y alil, or
159. 6:13green, the recessive little Y alil, and
160. 6:16whether the seeds were round, which
161. 6:18we'll call the dominant big R alle, or
162. 6:21wrinkly, the recessive little R al. As
163. 6:25before, Mandel started with pure
164. 6:26breeding plants. One of them was
165. 6:28homozygous dominant for both traits,
166. 6:31which means that it was capital Y,
167. 6:32capital Y genotype for the color trait
168. 6:35and capital R, capital R for the seed
169. 6:37texture trait.
170. 6:39So this plant's phenotype was that it
171. 6:41had yellow round seeds.
172. 6:44The other plant was homozygous recessive
173. 6:46for both traits, which means that it had
174. 6:48little Y little Y genotype for color
175. 6:50trait and little R genotype for texture
176. 6:54trait. So its phenotype was that it had
177. 6:57green wrinkled seeds.
178. 6:59So Mandel cross-pollinated these two
179. 7:02plants and the result was that all of
180. 7:04the plants in the F1 generation got
181. 7:06capital Y capital R from one parent and
182. 7:09lowercase Y lowercase R from the other
183. 7:11parent and therefore were capital Y
184. 7:14lowercase Y capital R lowercase R.
185. 7:17So far so good. But then Mendel bred two
186. 7:20of these F1 plants with one another and
187. 7:22things got interesting.
188. 7:25Let's put this in a pet square. For
189. 7:27these two traits, there are four
190. 7:28different combinations for each parent.
191. 7:30Capital Y, capital R, capital Y, little
192. 7:33R, capital R, little Y, and little R,
193. 7:37little Y. When we crossbreed the plants,
194. 7:41we can expect the F2 generation will
195. 7:43have seeds that have four different
196. 7:44types of phenotypes.
197. 7:47Nine are yellow and round. These have at
198. 7:49least one dominant capital Y and one
199. 7:52dominant capital R. Three are yellow and
200. 7:55wrinkled. Those that have at least one
201. 7:57dominant capital Y and two little Rs.
202. 8:01Three are green and round. Those that
203. 8:03have two little Y's and at least one
204. 8:05dominant big R. And one that's green and
205. 8:08wrinkled. The one that has two little
206. 8:10Y's and two little Rs.
207. 8:13And that's what Mendel got, a 93 to 3:1
208. 8:17ratio. And this helped establish the law
209. 8:19of independent assortment that the genes
210. 8:22for seed color and seed texture were
211. 8:24assorting independently of each other
212. 8:26and they weren't somehow influencing one
213. 8:28another. In other words, having one
214. 8:31trait did not make having another trait
215. 8:33any more or less likely.
216. 8:37This law is generally true except in
217. 8:39certain situations like when two genes
218. 8:42are located really close to each other
219. 8:43on a chromosome.
220. 8:46When that happens, it's called genetic
221. 8:47linkage and the two genes start to move
222. 8:50together more often than not and
223. 8:52therefore don't assort independently.
224. 8:56All right, as a quick recap, the law of
225. 8:58segregation states that inherited alals
226. 9:00are separated when producing gameamtes
227. 9:03and the law of independent assortment
228. 9:05states that the alals get distributed to
229. 9:06offspring randomly and without regard to
230. 9:09what other alil the offspring might have
231. 9:11received.
232. 9:13Helping
233. 9:16current and future clinicians focus,
234. 9:18learn, retain, and thrive. Learn more.