BIO 325 Genetics Problem Set 2 Questions and Answers 2026/27 Latest - UT Austin
1. This is a pedigree for a recessive trait (fully penetrant).
A.Is the trait dominant or recessive?
B. What is the probability that IV-4 and IV-5 will have a girl child with the disease.
Aa Aa
Aa aa
Why? Aa Aa AA
2/3 1/1
Aa Aa
¼ x ½ for a girl
= 2/3 x ½ x ¼ = 1/12
2. Red green color blindness is controlled by a X chromosomal gene in humans. Given the two
pedigrees below. What is the probability that their first child will be color blind.
Let X = normal and Xc = colorblind
XXc
Xcc XcY
XY XX XX
½ chance that mom donated an Xc
XXc for colorblindness and 1/2 that dad
XY
donated Y so ½ x 1/2 = ¼. If dad
1/2 did not donate Y then no chance
for child to be colorblind.
? 1/4 XcY
BIO 325 Genetics worksheet 2
, 3. Awn are the hair like appendages at the end of some plants and aids in seed dispersal. Awn growth in
wild rice is controlled by two genes (A and B for this example). The presence of a dominant allele for
either A or B will produce awn, awnless rice is thus only seen in a double homozygous recessive (aabb).
You perform a dihybrid cross with true breeding awned and awnless rice. What is the ratio of awned to
awnless rice you would expect in the F2 generation? How could you describe the relationship between A
and B?
AABB x aabb => AaBb 9A_B_ 3 A_bb 3 aaB_ 1 aabb
9 Awn 3 Awn 3 Awn 1 awnless = 15 : 1
4 All the Parental (P1) flowers are homozygotes, and blue is the wildtype phenotype. What are the
genotypes of the P1 flowers, the F1 flowers, the F2 flowers?
P1 F1 F2
1) Blue x Blue Blue all Blue
2) White x Green Blue 90 Blue, 31 Green, 39 White
3) Blue x White Blue 90 Blue, 29 Green, 41 White
Answer
We are told that the Parentals are homozygotes. Total = 160 so 1/16th = 10. At least one of these
matings shows a 9:3:4 ratio. White must be aabb and could be either aaB- or A-bb. Assume for mating 2
white is aaBB so Green must be AAbb
1) Blue (AABB)x Blue(AABB) Blue all Blue
2) White (aaBB)x Green(AAbb) Blue 90 Blue, 31 Green, 39 White
3) Blue(AABB) x White(aabb) Blue 90 Blue, 29 Green, 41 White
BIO 325 Genetics worksheet 2
1. This is a pedigree for a recessive trait (fully penetrant).
A.Is the trait dominant or recessive?
B. What is the probability that IV-4 and IV-5 will have a girl child with the disease.
Aa Aa
Aa aa
Why? Aa Aa AA
2/3 1/1
Aa Aa
¼ x ½ for a girl
= 2/3 x ½ x ¼ = 1/12
2. Red green color blindness is controlled by a X chromosomal gene in humans. Given the two
pedigrees below. What is the probability that their first child will be color blind.
Let X = normal and Xc = colorblind
XXc
Xcc XcY
XY XX XX
½ chance that mom donated an Xc
XXc for colorblindness and 1/2 that dad
XY
donated Y so ½ x 1/2 = ¼. If dad
1/2 did not donate Y then no chance
for child to be colorblind.
? 1/4 XcY
BIO 325 Genetics worksheet 2
, 3. Awn are the hair like appendages at the end of some plants and aids in seed dispersal. Awn growth in
wild rice is controlled by two genes (A and B for this example). The presence of a dominant allele for
either A or B will produce awn, awnless rice is thus only seen in a double homozygous recessive (aabb).
You perform a dihybrid cross with true breeding awned and awnless rice. What is the ratio of awned to
awnless rice you would expect in the F2 generation? How could you describe the relationship between A
and B?
AABB x aabb => AaBb 9A_B_ 3 A_bb 3 aaB_ 1 aabb
9 Awn 3 Awn 3 Awn 1 awnless = 15 : 1
4 All the Parental (P1) flowers are homozygotes, and blue is the wildtype phenotype. What are the
genotypes of the P1 flowers, the F1 flowers, the F2 flowers?
P1 F1 F2
1) Blue x Blue Blue all Blue
2) White x Green Blue 90 Blue, 31 Green, 39 White
3) Blue x White Blue 90 Blue, 29 Green, 41 White
Answer
We are told that the Parentals are homozygotes. Total = 160 so 1/16th = 10. At least one of these
matings shows a 9:3:4 ratio. White must be aabb and could be either aaB- or A-bb. Assume for mating 2
white is aaBB so Green must be AAbb
1) Blue (AABB)x Blue(AABB) Blue all Blue
2) White (aaBB)x Green(AAbb) Blue 90 Blue, 31 Green, 39 White
3) Blue(AABB) x White(aabb) Blue 90 Blue, 29 Green, 41 White
BIO 325 Genetics worksheet 2
