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Name: __________________________________________ Date: ______________________ Period: ______
Practice with Dihybrid Crosses
For the first two questions, determine the following:
Determine the parent gene types.
Determine the possible gene pairs donated by each parent. Fill in the Punnett square.
Determine the possible offspring phenotypes.
Example:
A tall green pea plant that is homozygous dominant for both traits is crossed with a short yellow pea plant that is homozygous recessive for both traits. Show the test cross.
- Parent gene types
Parent 1 = (T T G G)
Parent 2 = (t t g g)
- Possible gene types
Parent 1: TG, TG, TG, TG
Parent 2: tg, tg, tg, tg
- Fill in the Punnett Square
T G T G T G T G
t g
t g
t g
t g
Determine offspring phenotypes
Tall/Green______ : Tall/Yellow______ : Short/Green_______ : Short/Yellow______
TtGg TtGg TtGg TtGg
TtGg
TtGg
TtGg
- A long neck yellow giraffe (T t Y y), mates with a short neck orange giraffe (t t y y).
___________ X ___________
Possible phenotypes
Long/Yellow____ : Long Orange ____ : Short/Yellow ____ : Short/Orange____
- A fast red fox (F f R R), mates with a slow grey fox (f f r r).
___________ X ___________
- In a dihybrid cross, AaBb x AaBb, what fraction of the offspring will be homozygous for both recessive traits?
- Following a SSYy x SsYy cross, what fraction of the offspring are predicted to have a genotype that is heterozygous for both characteristics?
- In a dihybrid cross, SsYy x SsYy, what fraction of the offspring will be homozygous for both traits?
- About 70% of Americans perceive a bitter taste from the chemical phenylthiocarbamide (PTC). The ability to taste this chemical results from a dominant allele (T) and not being able to taste PTC is the result of having two recessive alleles (t). Albinism is a trait with normal pigment being dominant (A) and the lack of pigment being recessive (a). A normally pigmented woman who cannot taste PTC has a father who is an albino taster. She marries a homozygous, normally pigmented man who is a taster but who has a mother that does not taste PTC. What are the genotypes of the possible children?
- In the breeding season, male Anole lizards court females by bobbing their heads up and down while displaying a colorful throat patch. Assume for this question that both males and females bob their heads and have throat patches. Now, suppose that anoles prefer to mate with lizards who bob their heads fast (F) and have red throat patches (R) and that these two alleles are dominant to their counterparts, slow bobbing and yellow throats. A male lizard heterozygous for head bobbing and homozygous dominant for the red throat patch mates with a female that is also heterozygous for head bobbing but is homozygous recessive for yellow throat patches. How many of the F 1 offspring have the preferred fast bobbing / red throat phenotype (assume 16 young)?
What percentage of the offspring will lack mates because they have both slow head bobbing and yellow throats?
What percentage of the offspring will have trouble finding mates because because they lack one of the dominant traits?
- Carrion beetles lay their eggs in dead animals and then bury them in the ground until they hatch. Assume that the preference for fresh meat (F) is dominant to the preference for rotted meat and that the tendency to bury the meat shallow (S) is dominant to the tendency to bury the meat deep. Suppose a female carrion beetle homozygous dominant for both traits mates with a male homozygous recessive for both traits. What will be the genotype of the F 1 generation?
What will be the phenotype of the F 1 generation?
What will be the genotypic ratio of the F 2 generation?
