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For each problem I need to know the following plus whatever specifics the problem requests:
Key: (Which symbol stands for which allele) – Use the same letter for a gene using a capital to designate dominant and lower case to show recessive.
Parental genotypes
Possible gametes for each parent
The Punnett Square
1. In guinea pigs black coat color is dominant to brown coat color. A black guinea pig (from a long line of black guinea pigs) is bred to a brown guinea. If one of these offspring is then bred to a different brown guinea pig, what are the expected genotypes and phenotypes of their offspring?
2. In pea plants tall stems are dominant to short stems and purple flowers are dominant to white flowers. A true breeding plant with tall stems and white flowers is crossed with a true breeding plant having short stems and purple flowers. The offspring from this cross are grown up and allowed to interbreed. What are the expected genotypes and phenotypes of their offspring?
3. In dogs, black is dominant to golden. You have a black Labrador retriever you would like to stand at stud, but first you want to know if it is homozygous black or heterozygous. You make a testcross to determine this. What are the expected genotypes and phenotypes if your dog is heterozygous? If it is homozygous?
4. Color-blindness is X-linked in humans. A color-blind man marries a woman whose father was color-blind. What are the expected genotypes and phenotypes of their children?
Explanation / Answer
1). The genotype of a black guinea is either BB or Bb. The genotype of brown guinea pig is bb. Cross between these two will have the children with the following genotypes.
BB* bb = 100% Bb (All are black)
The cross between F1 offspring and brown guinea pig will have the offspring with the following genotypes.
Bb* Bb = BB (25%), Bb (50%) and bb (25%).
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