Download single cross problems

SINGLE CROSS PROBLEMS
1. In guinea pigs, rough fur (F) is dominant over smooth fur (f). What percent of the F2
generation would be homozygous dominant when a homozygous rough fur guinea pig is
crossed with a homozygous smooth fur guinea pig?
2. A person’s second toe is sometimes longer than their “big” toe. This condition is due to a
dominant gene (L). What percent of the kids would you expect to have long second toes
when a heterozygous man (Ll) marries a homozygous recessive woman (ll)?
3. Facial dimples in man are dominant over no dimples. Show the expected children of a
marriage between a woman with no dimples and a man with dimples whose mother did not
have any facial dimples. What percent of the children would have dimples?
4. In hamsters, short legs are dominant over long legs. Use the five point plan to show how
two short-limbed hamsters could have offspring with long limbs. If this one pair of hamsters
has a total of 40 young, how many of the 40 would you expect to have short limbs?
5. In a study of a Danish town, the following data was obtained concerning eye color:
Parents
A. Blue x Blue
B. Blue x Brown
C. Brown x Brown
Number of
Couples
158
150
29
Number of Children
with Blue Eyes
625
317
25
Number of Children
with Brown Eyes
0
301
82
Which trait is dominant – blue eyes or brown eyes? Which line(s) of data proves which trait
is dominant? Use the five-point plan to verify your conclusion.
6. Free earlobes (E) in humans is dominant over attached earlobes (e). What would be the
percent of free earlobes in the offspring when a man homozygous for free earlobes marries a
woman who is heterozygous for free earlobes?
7. The Basenji is a breed of dog noted for its inability to bark. When a Basenji is mated to a
Beagle, all of the eight puppies can bark. How would you describe the genotype of the
Basenji in genetic terms?
8. Webbed finders and toes in humans is due to a rare dominant gene (F). What percent of the
offspring would you expect to have normal fingers and toes when a man heterozygous for
webbed fingers and toes marries a woman with normal hands and feet?
Example Problem
Sample Problem:
When a purebred black guinea pig is crossed with a purebred white
guinea pig, all of the offspring are black. What are the genotypes of
the F2 generation and ratios? What are the phenotypes of the F2
generation and their ratios?
Solution:
1. Legend:
b = white
B = black
2. Parents:
BB x bb
3. Gametes:
Bxb
4. Square:
B
b
Bb
2. Parents:
Bb x Bb
3. Gametes:
B, b x B, b
4. Square:
B
b
B
b
BB Bb
Bb bb
5. Answer:
Genotypes –
25% homozygous dominant
50% heterozygous
25% homozygous recessive
Ratio 1:2:1
Phenotypes –
75% black
25% white
Ratio 3:1