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Name _____________________
Honors Biology
Date _________
Practice Genetic Problems
1. In humans, the allele for six fingers (F) is dominant
to the allele for five fingers (f). If both parents are
heterozygous for six fingers, what is the probability that
their first child will be “normal” (have five fingers)?
2. If the father is heterozygous for six fingers and the
mother has five fingers, what percentage of their children
should have six fingers?
3.
In horses, black color (B) dominates chestnut color
(b). The trotting gait (T) dominates the pacing gait (t).
A cross is made between a horse homozygous for black color
and the pacing gait and a horse homozygous for chestnut
color and the trotting gait. What is the probability that
an offspring will be a black trotter?
4. In humans, normal color vision dominates red-green
colorblindness. This is a sex-linked trait. Two parents
produce daughters who are all carriers and sons who are all
normal. What are the probable genotypes of the parents?
5. Assume a male with type O blood mates with a female who
has type A blood. The female’s mother had type A blood and
her father had type B. What is the probability that their
first child will have type A blood? What is the
probability that their third child will have type O blood?
6.In humans, brachydactylism is the result of a dominant
allele. Individuals that suffer from this trait have very
short fingers. Defective dentine is also the result of a
dominant allele but it is carried on the X chromosomes.
The allele causes the teeth to wear down rapidly and
usually only stubs remain by adolescence. Assume a female
who has brachydactylism and defective dentine (she is
heterozygous for both traits) mates with a male who has
normal fingers and teeth. What is the probability that a
son will have both brachydactylism and defective dentine?
What is the probability that a daughter will have both
defects?
7. You are a PTC taster, your brother is not, your father
is, and your mother is not. What is your genotype for this
trait?
8. Baldness in humans is a dominant, sex-influenced trait.
This gene is on the autosomes, not the sex chromosomes, but
how it is expressed is influenced by the person’s sex (due
to hormones present, etc.). A man who is BB or Bb will be
bald and will be non-bald only if he is bb. A woman will
only be bald if she is BB and non-bald if she is Bb or bb
(it’s almost like B is dominant in males and b is dominant
in females). Actually, because of the influence of other
sex-related factors, most women who are BB never become
totally bald like men do, but rather, their hair becomes
“thin” or sparse. If two parents are heterozygous for
baldness, what are the chances of their children being
bald? Use a Punnett square to illustrate this. Note:
because the sex of a person does make a difference in how
the gene is expressed, you need to set this up as a
dihybrid cross to account for the sex of the children.
9.
In smileys, eye shape can be starred, circular or a
circle with a star. Show the cross between a star-eyed and
a circle eyed.
Show the cross between two circle-star
eyed. What are the genotypic and phenotypic percentages?