Download GENETICS PROBLEMS

Name:__Answers_____
GENETICS PROBLEMS
1. What is the probability of rolling a five with a pair of dice? The probability of
rolling a five is 4/36 or 1/9.
There are four ways to roll a five: 1-4, 4-1, 2-3, 3-2.
The chance of rolling each of those combinations is 1/36. For example, the chance of
rolling a one on the first die and a four on the second die is 1/6 (the probability of
rolling the one on the first die) times 1/6 (the probability of roling a six on the
second) = 1/36 [remember - to find the probability of two independent events
occurring together, multiply the probabilities of each occurring alone]
Adding the separate probabilities for each way you can roll a five:
1/36 + 1/36 +1/36 +1/36 = 4/36 or 1/9.
[remember: to find the probability of an outcome that can be obtained in more than
one way, add the separate probabilities]
2. In peas, the allele for purple flower color (W) is dominant to the allele for white
flower color (w), and the allele for yellow seeds (Y) is dominant to the allele for green
seeds (y). (use this information for this and the next question.) A plant with purple
flowers and yellow seeds is crossed with a plant with white flowers and green seeds. The
cross produces 800 offspring with the following phenotypes:
Type
number
purple/yellow purple/green
206
199
white/yellow
193
white/green
202
What are the genotypes of the two parental plants? WwYy x wwyy. The only cross
that produces young in a 1:1:1:1 phenotypic ratio is a cross between a double
heterozygote and a double recessive. The possible gametes from each parent are in
italics below:
Parental genotype
gametes
wwyy wy
WwYy
WY (1/4)
Wy (1/4)
wY (1/4)
WwYy
P/Y
Wwyy
P/G
wwYy
W/Y
wy (1/4)
wwyy <= offspring genotypes
W/G
<= offspring phen.
3. A pea plant with white flowers and green seeds is crossed with a plant with purple
flowers and yellow seeds. Both plants are from pure, true-breeding lines. What is (are)
the expected phenotype (s) of the F1 generation of plants? If two F1 plants are crossed,
what phenotypes result, and in what ratio?
All F1 plants have purple flowers and yellow seeds.
Phenotypes of F2 plants: 9 Purple/yellow: 3 Purple/green: 3 white/yellow:
1white/green
Pea plant with white flowers and green seeds = wwgg
Pea plant with purple flowers and yellow seeds = WWYY
(both are from pure breeding lines, so you know they're homozygous)
First cross: one parent can only produce wy gametes, the other produces only WY
gametes, so all offspring are WwYy.
Cross of F1s. Both parents are WwYy, so each parent can produce four types of
gametes: WY, Wy, wY, wy. This is a typical dihybrid cross - see figure 10-8 in the
text or your notes for the outcome.
4. Red-green colorblindness is an X-linked recessive trait in humans. If a normal man
marries a woman who is red-green colorblind, what are the expected phenotypes of their
children?
All boys will be colorblind. All girls will be carriers.
If C is the allele for normal vision, and c is the allele for red-green colorblindness:
Normal man's genotype: C0 (where 0 means no allele for color vision, since the male
only has one X chromosome)
Colorblind woman's genotype: cc
If the male passes on his Y chromosome, the offspring will be a colorblind male; if
the male passes on his X chromosome (with the C allele for normal vision), the
offspring will be a female with normal vision, but carrying one recessive allele for
colorblindness.