Genetics Problems: Mendelian Inheritance

Try your hand at solving these problems on Mendelian inheritance:

Monohybrid Cross

1. Mendel crossed pea plants heterozygous for the height gene (Tt) and obtained the monohybrid phenotypic ratio of 3:1 and the genotypic ratio 1:2:1. Calculate the genotypic and phenotypic ratios for the following crosses:
a. Homozygous dominant crossed to homozygous recessive
b. Homozygous dominant crossed to heterozygous
c. Homozygous recessive crossed to heterozygous

2. Bob and Joan know from a blood test that they are each heterozygous for the autosomal recessive gene that causes sickle cell disease. If their first three children are healthy, what is the probability that their fourth child will have the disease?

3. In garden pea plants, purple flowers are dominant over white flowers.
a. A heterozygous purple flower is allowed to self-pollinate. What are the probable genotypic and phenotypic ratios in the offspring of this plant?
b. 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 of this cross?
c. You have a pea plant with purple flowers. Design a cross to determine if this plant is homozygous or heterozygous. Use a Punnet square to show all possible crosses.

4. Use a Punnett squares to illustrate test crosses to determine whether a black male guinea is homozygous or heterozygous for black. The black coat is produced by the dominant allele (B).

5. Assume that 50 percent of 10,000 pea plant offspring are short. Use a Punnett square to
show the probable genotypes of the parents and the offspring. Let T stand for the dominant allele, and t for the recessive allele.

Dihybdrid/Trihybrid Cross

1. In the fruit fly Drosophila, wings (A) are dominant over lack of wings (a), and red eyes (E) are dominant over sepia (brownish) eyes (e). A wingless fly that is heterozygous for eye color is crossed with a fly that is heterozygous for both eye color and the presence of wings.
a. What are the genotypic and phenotypic ratios for this cross?
b. What fraction of the offspring from this cross will be wingless and have sepia eyes?
c. What fraction will have the genotype AaEe?

2. A man and a woman each have dark eyes, dark hair and freckles. The genes for these traits assort independently. The woman is heterozygous for each of these autosomal trait, but the man is homozygous. The dominance relationships of the alleles are as follows:
B = dark eyes b = blue eyes
H = dark hair h = blond hair
F = freckles f = no freckles
a. What is the probability that their child sill have the same phenotype as the parents?
b. What is the probability that their child will have the same genotype as each parent?
Use probability or a Punnett square to obtain your answers.