ASSIGNMENT SHEET # 6—GENETICS

DIRECTIONS: Use the key and assume the trait is homozygous unless told otherwise.

1. If a pure variety of peas showing the character tall and terminal flowers is crossed with a short variety having axial flowers, and the hybrids are allowed to self-fertilize, what

will be the visible ratio among their offspring?

2. What is the chance that any tall axial plant(s) produced by the hybrids in the preceding

problem is homozygous for both genes? What is the chance that any tall terminal plant is

homozygous for both genes?

3. Show the result of crossing a dihybrid from problem one with a recessive

( short, terminal) plant.

4. Write out the genotypes for the gametes produced by an organism hybrid with respect to

three genes, as XxYyZz. What chance has the offspring of two such hybrids to be xxyyzz?

Assuming that dominance is complete in all three genes, what proportion of the offspring

of two such hybrids will show all three dominant effects?

5. In peas, tall is dominant over short. Four experiments in cross-pollination gave the

results shown below. In each case give the most probable genotype of the parents, as

indicated by the ratio among their offspring.

a. Tall + tall produced 95 tall, 29 short

b. Tall + short produced 50 tall, 0 short

c. Tall + tall produced 75 tall, 0 short

d. Tall + short produced 8 tall, 4 short

6. In summer squashes, white fruit is due to a dominant allele, and colored fruit is due to its

recessive allele. Disk -shaped fruit is due to a dominant allele, and sphere-shaped fruit is due to its recessive allele. In a cross between a squash plant with yellow fruit and disc shape and a white one with a sphere shape, what will be the appearance as to color and shape of fruit in the first generation?

In the second generation what will be the appearance for fruit shape and color?

7. The gene which determines whether the wings will be “miniature” or “long” in Drosophila

is located on the X-chromosome. Describe and explain genotypically and phenotypically

the results of crossing a ‘miniature” female with a “long” male. In breed the F1 and

describe the F2.

8. Red-green color blindness is a sex-linked recessive trait in humans. What are the chances

that the son of a color-blind woman and normal man will be color-blind? That the son of a

color-blind man and a normal woman will be color-blind? Consider the different

possibilities involved.