Genetics Problem Sets--1

Punnett Square and Monohybrid Cross

The Punnett square is a chart used to predict the chances of an offspring receiving a particular trait. It will not tell you how many offspring will be produced, or the order in which they will be born.

A husband who is homozygous dominant for brown hair (BB) has a wife who is homozygous recessive for blonde hair (bb). The allele for brown hair (B) is dominant to the allele for blonde hair (b). Use the Punnett square below to predict the probability that they will have offspring with brown or blonde hair.

Gametes / b / b
B
B

1.  What are the chances of the offspring having blonde hair?

2.  What are the chances of the offspring having brown hair?

3.  What is the genotypic ratio?

4.  What is the phenotypic ratio?

Now, assume both husband and wife are both heterozygous for brown hair. Use the Punnett square below to predict the genotypes and phenotypes of their offspring.

Gametes / B / b
B
b

5.  What are the chances of the offspring having blonde hair?

6.  What are the chances of the offspring having brown hair?

7.  What is the genotypic ratio?

8.  What is the phenotypic ratio?

9.  What is the dominant gene?

10.  Is there a heterozygous blonde offspring? Why?

If curly hair is dominant to straight hair, what letters will you use to represent these genes?

11.  If a heterozygous curly-haired male marries a straight-haired female, how would you write their genotypes? ______x ______

12.  What would the possible gametes be for the male parent?

13.  What would the possible gametes be for the female parent?

Use the Punnett square to work out the cross and then answer the questions.

Gametes

14.  What are the chances of the offspring being homozygous curly-haired?

15.  What are the chances of the offspring having straight hair?

16.  What are the chances of the offspring being heterozygous curly-haired?

17.  What is the genotypic ratio?

18.  What is the phenotypic ratio?

Monohybrid Cross

In fruit flies, Long wing (L) is dominant to short wing (l).

Predict the outcome of the following monohybrid crosses. Give the (1) Parental genotypes; (2) Genotypic Ratio; (3) Phenotypic Ratio

1. Cross two homozygous long winged flies.

2. Cross two heterozygous long winged flies.

3. Cross a heterozygous long winged fly with a fly homozygous for long wings.

4. Cross two short winged flies.

5. Cross a short winged fly with a homozygous long winged fly.

6. Cross a short winged fly with a heterozygous fly.

Application Problems

1. Suppose that two long winged flies (from above) mate. The offspring produced is 49 short winged and 148 long winged flies. What were the most probable genotypes of the parent flies. What proportion of the offspring should be heterozygous?

3. In guinea pigs, long hair (H), is dominant to short hair (h). A long haired pig mates with a short haired pig. They have a litter of 8 baby pigs all of which have long hair. What are the probable genotypes of the parents and offspring?

5. The parents of a short haired male guinea pig were both long haired. The short haired male is mated with a long haired female whose father was long haired and mother was short haired. The male and female guinea pig produce one offspring that is short haired. What is the genotype of each of the guinea pigs?