Genetics Problem Set I : One trait crosses
Directions: Use the five steps for each genetic problem. Show your work
1. A purebred tall plant (homozygous dominant) is crossed with a purebred short plant (homozygous recessive).
What is the probability of getting a tall plant?
2. Cross a heterozygous tall (hybrid tall) with a homozygous short (purebred short). What are the phenotypic ratios
of the offspring?
3. In peas, the trait for tallness is dominant over the dwarf (small) trait.
a. If a plant homozygous for tall is crossed with one homozygous for dwarf, what is the probability of producing
dwarf offspring? What would be the genotype of the offspring produced?
b. If a plant heterozygous for tall is crossed with a dwarf plant, what is the probability of producing tall
offspring? Express your answer as a fraction, ratio, and percent.
4. Big ears are dominant to tiny ears. A couple decides to have kids. They have four children. Three of them have
big ears only one has small ears. What are the genotypes of the parents? What are the genotypes of the kids?
What is the probability or percent that they will have another small-eared kid?
5. Tay-sachs disease occurs when a child receives two recessive alleles. At first the baby appears healthy, but it
produces a defective enzyme hexosaminidase A that does not clear away lipids accumulated in body cells and
nerve cells. The child dies by the age of four. If you are a carrier (heterozygous) you do not suffer from the
disease. What is the probability that two heterozygous carrier parents will have a child with Tay-sachs disease?
6. A couple (both do not have cystic fibrosis) decides to have children. They are both carriers for a genetic disease
called Cystic fibrosis. What are the chances that their children will be normal? What are the chances that their
kids will get the disease?
7. A man and his wife are both heterozygous for dark colored eyes. Dark eyes are dominant over blue eyes. They
have three blue eyed children. What is the probability of this happening? What is the probability that their next
child will have blue eyes? What are the genotypic and phenotypic ratios of the offspring?
8. In sheep white hair color is dominant over black hair color. A white female is mated to a white male; their lamb
is black. What are the genotypes of all the animals mentioned in this problem? What is the genotypic ratio of the
offspring? If they produce another offspring, could it be white? If so, what is the probability of its being white?
Express your answer as a fraction, ratio, and a percent.
9. Two individual that both have unattached earlobes have children. They have four boys. Three have unattached
earlobes and one has attached earlobes. What are the genotypes of the parents and offspring? What are the
phenotypic and genotypic ratios of the offspring?