Genetics Problem Set #2
Monohybrid and Dihybrid Crosses
Monohybrid Crosses:
1. Pollen from a pea plant with white flowers is used to fertilize the ovules (female gametes) of a heterozygous plant. What are the possible phenotypes in the offspring from this cross? Assume purple flowers (P) is dominant to white flowers (p).
2. In corn, yellow seed color is dominant and white is recessive. A certain ear of corn had a mixture of yellow seeds and white seeds. What color seeds could the parents have grown from? (Hint #1: Each corn kernel is a potential seed or “baby corn plant” and therefore, it is an individual offspring. Hint #2: There is more than one parental genotype combination for the answer.)
3. In a certain animal, a breed is known that always has a hairy tail; another breed is known that always has a naked tail. How would you determine which trait is dominant?
4. The Jones family has eight children, all of whom are girls. What is the chance that the next child will be a boy? Explain.
5. In humans, normal pigmentation is due to a dominant factor "C," albinism to its recessive allele "c." A normal man marries an albino woman. Their first child is an albino.
(a) What are the genotypes of these three people?
(b) If they have more children, what would they probably be like?
(c) In a different scenario, an albino man marries a normally pigmented woman. They have 2 children, both normally pigmented. What are the genotypes of the parents and children?
6. In sheep, white coat is dominant. Black is recessive. Occasionally, a black sheep appears in a flock. Black wool is worthless. How could a farmer eliminate the genes black coat from the flock without killing the sheep?
7. This is a PEDIGREE problem. PEDIGREES, or family histories, are charts which show the inheritance of a particular trait. MALES are squares; FEMALES are circles. A horizontal line between a male and female stands for parentage. When a square is black, a person SHOWS the trait in question. If the circle/square is half filled in, they are a carrier but do not exhibit the trait.
The pedigree at the right is for albinism. Albinism is the inability
to make skin pigment. It is a recessive trait. Persons 1 & 2
have children 3, 4, 5, and 6. Your job is to figure out the
GENOTYPES of each person in the pedigree & list them.
(Let A = make pigment, and a = albinism allele)
Two-Gene Crosses:
8. Hearing (D) is known to be dominant to a type of deafness (d), and having healthy metabolism (P) is known to be dominant to having the disease pku (p). A married couple -- let’s call them Barbie and Ken – have these phenotypes and genotypes.
Barbie is ddPP (deaf but normal metabolism)
Ken is Ddpp (can hear but he has pku)
They ask their genetic counselor, “What is the probability of our having a normal child (one that can hear and does not have pku)?” What does their counselor tell them?
9. In humans, nearsightedness (N) is dominant to normal vision (n), and polydactyly (P) (having more than 5 fingers) is dominant to 5-fingered hands (p). A man has normal vision and is heterozygous for polydactyly; a woman is heterozygous for nearsightedness and has normal hands. If they marry, what is the probability of this couple having a child with normal vision and normal hands?
10. Jenny has red hair and freckles. We know that non-red hair (R) is dominant to red hair (r), and that freckles (F) are dominant to plain skin (f). She tells you that her mom has brown hair and freckles, and that her dad has red hair and plain skin. Furthermore, she says that her maternal grandmother (her mom’s mom) had red hair and plain skin.
a) What is Jenny’s genotype?
b) Jenny’s mom is pregnant with a second child. What is the probability that Jenny will have a brother or sister with brown hair and plain skin?