GENETIC PROBLEMS (dihybrid crosses)
About 70% of Americans perceive a bitter taste from the chemical phenylthiocarbamide (PTC). The ability to taste this chemical results from a dominant allele (T) and not being able to taste PTC is the result of having two recessive alleles (t). Albinism is also a single locus trait with normal pigment being dominant (A) and the lack of pigment being recessive (a). A normally pigmented woman who cannot taste PTC has a father who is an albino taster. She marries a homozygous, normally pigmented man who is a taster but who has a mother that does not taste PTC. What are the genotypes of the possible children (choose all that apply)?
What percentage of the children will be albinos?
What percentage of the children will be non-tasters of PTC?
Ans:
First determine the genotypes of the parents. We know that the woman has normal pigment which means she must have at least one A. Her father is albino and because the albino allele is recessive, his genotype is aa. What does this make her skin genotype?
We also know that the woman cannot taste PTC. Because the ability to taste PTC is dominant, what does this make her genotype for the tasting trait?
Putting both traits together, we see that the woman's overall genotype is Aatt.
Now, what about her husband? You have been told that he is homozygous for normal pigment. What is his genotype for skin color?
He is a taster and so must have at least one T. However, we also know that his mother can not taste PTC so she must be homozygous recessive. With this information, what is his genotype for taste?
Putting both traits together, you see that his overall genotype is AATt.
for both traits. How many of the offspring (assume 16) living in the pack will have each of the
following genotypes?
NNBB: NNBb: NNbb: NnBB: NnBb: Nnbb: nnBB: nnBb: nnbb
What percent of the offspring will be normal colored with blue eyes?
Ans:
First determine the genotypes of the parents. We know that the female is heterozygous for both traits so her genotype must be which of the following?
The male is black with blue eyes. These are both recessive traits and must be homozygous to be expressed. What, then, is the male's genotype?
3. Suppose in a strain of soybeans, high oil (H) content in the seeds is dominant to low oil content and four seeds (E) in a pod is dominant to two seeds in a pod. A farmer crosses two soybean plants, both with high oil content and four seeds per pod. The resulting F1 offspring have a phenotypic ratio of 9:3:3:1 (High oil / four seeds : High oil / two seeds : Low oil / four seeds : Low oil / two seeds). What genotype were the parent plants?
Ans:
The easiest way to answer this question, is to have remembered what type of cross will produce a 9:3:3:1 phenotypic ratio. However, if you don't remember that, then the only thing to do is to make repeated punnett square crosses until you find the one that results in the phenotypic ratio you are looking for. You can narrow down the possibilities by realizing that all of the phenotypes are present in the offspring so you can rule out right away crosses that only involve homozygous traits. For example crossing two individuals with a HHEE genotype will not result in any offspring that have the low oil / two seed phenotype (hhee). Below is a blank punnett square for you to perform crosses in.
GENETIC PROBLEMS (incomplete dominance)
1.
In northeast Kansas there is a creature know as a wildcat. It comes in three colors, blue, red, and purple. This trait is controlled by a single locus gene with incomplete dominance. A homozygous (BB) individual is blue, a homozygous (bb) individual is red, and a heterozygous (Bb) individual is purple. What would be the genotypes and phenotypes of the offspring if a blue wildcat were crossed with a red one?What are the genotypic and phenotypic ratios of the F2generation? Ans:
One parent is blue and you were told in the question that blue color in wildcats comes from the homozygous BB state. Likewise, for the red parent which must be homozygous bb.
Count up how many of the four possible offspring in the square have the purple color and the heterozygous genotype.
2.
The lubber grasshopper is a very large grasshopper, and is black with red and yellow stripes. Assume that red stripes are expressed from the homozygous RR genotype, yellow stripes from the homozygous rr genotype, and both from the heterozygous genotype. What will be the phenotypic ratio of the F1 generation resulting from a cross of two grasshoppers, both with red and yellow stripes (red : both : yellow)?What would be the genotypic ratio of the F1 generation (RR : Rr : rr)?
What genotypes would be produced by crossing a grasshopper with both color stripes and one with yellow stripes (choose all that apply)?
What phenotypes would be produced by crossing a grasshopper with both color stripes and one with yellow stripes (choose all that apply)?
Ans:
First determine the genotypes of the parents, both of which have red and yellow stripes. In a case of incomplete dominance with these grasshoppers, what genotype would this make the parents?
Once again, determine the genotypes of the parents first. A grasshopper with stripes of both colors would have what genotype?
..and the grasshopper with yellow stripes has what genotype?
BBWW:
BBWw: BBww: BbWW: BbWw: Bbww: bbWW: bbWw: bbww:
How many of the F1 generation will have wide tail bands and won't wash any of their food?
Ans:
Again, determine the genotypes of the parents first. The racoon parents have the same phenotype and therefore the same genotype. Medium sized tail bands and washing food some of the times are both heterozygous traits. What would that make the genotypes of the parent racoons?
Reference:
