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Complete Dominance
1. A woman heterozygous for widow’s peak has a child with a man who has straight hair
(homozygous recessive). What are the possible genotypes for their child? What are the
possible phenotypes? What are the chances that the child will have a widow’s peak?
(H=widow’s peak, h=straight)
2. A woman who can roll her tongue is homozygous dominant for that trait. She has a
child with a man who is homozygous recessive and cannot roll his tongue. What are the
possible genotypes and phenotypes for their child? What are the chances that their child
can roll his/her tongue? (R=roller, r=non-roller)
3. Both parents have a hitchhiker’s thumb. However, one parent is homozygous for this
trait and the other is heterozygous. Give the possible genotypes and phenotypes of their
child. (H=hitchhiker’s, h=straight)
4. A student can clearly taste PTC paper. He brings this paper home and his father cannot
taste it and his mother can. What must be the genotype of the father? What must be the
child’s genotype? (T=taster, t=non-taster)
5. A couple is having their second child. Both parents are hybrid for eyelash length. Their
first child has short eyelashes. What is the probability that the second child will have
short eyelashes? (L=long, l=short)
6. What genotypic and phenotypic ratios can be expected in the offspring of a cross
between hybrid tall pea plants and short pea plants? (T=tall, t=short)
7. If two pea plants hybrid for a single trait produce 60 pea plants, about how many of the
60 plants would you expect to show the recessive trait?
8. A mother and father are each heterozygous for curly hair which is dominant over
straight hair. What are the chances that their child will also have curly hair? (C=curly,
c=straight)
9. In a certain meadow mouse, a dark coat is dominant over a cream coat. If a
heterozygous dark coated male was mated with a homozygous dark coated female what
would be the phenotype of their offspring? What would be the genotypes? (D=dark,
d=cream)
10. An albino male rat is crossed with a normally pigmented female who had an albino
mother. The gene for pigmentation is dominant over the gene for albinism. What would
be the phenotypic and genotypic ratios of their offspring? (A=pigmented, a=albino)