This illustrates Mendel's "Principle of Independent Assortment." See Figure 13.9b for another illustration from your reading.
BIOL 114.02 Spring 2006
Quiz 3 Answer Key
1. [3 points] Suppose that you are heterozygous for two (2) genes located on two (2) different chromosomes. You carry alleles A and a for one gene and alleles B and b for the other. Draw a diagram that illustrates what happens to these genes and alleles when meiosis occurs in your reproductive tissues. Clearly label:
(a) the meiotic divisions (i.e. meiosis I and meiosis II),
(b) the homologous chromosomes, sister chromatids, genes and alleles.
(c) all the genetically different gametes that could form and how frequently each type should be observed.
This illustrates Mendel's "Principle of Independent Assortment." See Figure 13.9b for another illustration from your reading.
2. [3 points total] In the television program Star Trek, a group of humanoid organisms called Klingons are the arch-enemies of the Enterprise crew. In Klingons, one gene determines the length of the bicuspid teeth while another determines whether the individual will have a saggital crest (a protrusion on the forehead). The two genes are on different chromosomes.
T = long sharp bicuspids (dominant)
t = shorter earthling-like bicuspids (recessive)
F = large saggital crest (dominant)
f = smooth, flat earthling-like forehead (recessive)
The character Kayless is a half-human, half-Klingon with the genotype TtFf. He mates with Lieutenant Worf's sister, who is also heteorzygous at the tooth locus but has a strikingly smooth forehead.
[2 points] What are the possible phenotypes that may result from this mating? Describe both teeth and forehead for each. What is the probability of their first born child having each phenotype?
Kayless' genotype: TtFf
Worf's sister's genotype: Ttff
Offspring probabilities:
3/4 sharp teeth; 1/4 earthling teeth
1/2 large saggital crest; 1/2 smooth forehead
Combine these probabilities by multiplying:
3/8 sharp teeth and large crest
3/8 sharp teeth and smooth forehead
1/8 earthling teeth and large crest
1/8 earthling teeth and smooth forehead
[1 point] What proportion of their children are expected to be heterozygous for both genes?
1/4 TT; 1/2 Tt; 1/4 tt
1/2 Ff; 1/2 ff
combine probabilities of heterozygous genotype at each locus by multiplying:
1/4 TtFf
3. Answers are:
1. BOTH A and E.
2. 0 (zero)
3. A
4. [2 points] A BIOL 110 student is conducting her independent project on fruit fly genetics. She suspects that a particular recessive trait in fruit flies (dumpy wings, which are somewhat smaller and more bell-shaped than the wild-type) is sex-linked. A single mating between a fly having dumpy wings (dp; female) and a fly with wild-type wings (Dp; male) produces 3 dumpy-wing females and 2 wild-type males.
On the basis of these data, is the trait sex-linked or autosomal? What were the genotypes of the parents? Explain how these conclusions can be reach with such a small data set.
If the Dp gene were X-linked, then parental genotypes would be:
female: dp dp
male: Dp YA cross of these parents would always produce dumpy-wing males (dp Y) and normal females (Dp dp). The data show that both normal males and dumpy-wing females resulted, inconsistent with the prediction. Thus, the gene must be autosomal. The sample size is entirely adequate to make this determination, since even one exception to the prediction can disprove the hypothesis.