2. In the Eastern Rocky Mountain Ground Squirrel, skinny tails (t) are recessive
ID: 211770 • Letter: 2
Question
2. In the Eastern Rocky Mountain Ground Squirrel, skinny tails (t) are recessive to fluffy tails (T) buck teeth (b) are recessive to small teeth (B). Two fluffy tailed squirrels that both had a skinny-tailed parent mate. One of the pair has buck teeth, and the other small teeth. If at least one of their offspring has buck teeth, what are the expected genotype and phenotype ratios of their offspring The two mates must each have one t allele from their skinny-tailed parent, and must have one T allele because they have fluffy tails. The buck-toothed parent must be bb because it is a recessive trait, and the other parent must have at least one B allele. Because we know one offspring is also buck-toothed, the small-toothed parent must have a b allele to donate. Thus, the cross is (Tt Bb) x (Tt bb) TT-0.5 0.5 0.25 Tt-Tt+tT-2 0.5 *0.5- 0.5 t-0.50.5- 0.25 BB = 0.5 * 0-0 Bb- bB +Bb 0.500.51-0.5 bb = 0.5 * 1 = 0.5 TTBB = TT * BB = 0.25 * 0-0 TTBb = TT * Bb-0.25 * 0.5 = 0.125 TTbb-TT * bb-0.25 * 0.5 = 0.125 TtBB- Tt BB-0.5 * 0-0 TtBb = 0.5 * 0.5 = 0.25 Ttbb-0.5 0.5- 0.25 ttBB = 0.25 * 0-0 tBb-0.25 0.5 0.125 tbb-0.250.5 0.125 Fluffy tailed & small-toothed (TT Tt) (BB Bb) (0.25 0.5)* (0 +0.5) 0. 375 Fluffy tailed & buck-toothed (TTTt(bb) (0.250.5) (0.5) 0.375 Skinny tailed & small-toothed-(t) * (BB + Bb)-(0.25) * (0 + 0.5)-0.125 Skinny tailed & buck-toothed- (tt) * (bb)-(0.25)(0.5)-0.125Explanation / Answer
When we cross Bb Tt to Ttbb then in the f1 generation we do not get BB in any progeny because there is only one B allele in both parents. This suggests BB = 0.5 x 0 = 0. One B due to the independent assortment rule it must has 0.5 allelic frequencies, however due to absence of another B allele, we would take its frequency as 0.
Therefore, if we go through all F1 generation of the above cross we would the given genotypic ratio as in the right hand side table.
If there are two alleles B and b then both will have 0.5 frequencies because of the law of independent assortment and the Hardy-Weinberg rule.
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