Download Patterns of Inheritance Worksheet #5

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Patterns of Inheritance Worksheet #5
Codominance, Incomplete Dominance, and Sex Linkage Practice Problems
Dominance is the simplest example of how genes interact with each other. Earlier, you learned that
the effects of the dominant allele are seen even when the recessive allele is present. But what causes
dominance?
Remember that a gene is a section of DNA, and DNA codes for a polypeptide, or string of amino
acids. In many cases, the dominant allele codes for a polypeptide that works, whereas the recessive allele
codes for a polypeptide that does not work. For example, suppose that the allele B codes for an enzyme
that makes a black pigment in a mouse’s fur and allele b codes for a defective enzyme that cannot make the
pigment. A mouse that has the genotype bb will have white fur because it lacks the enzyme that makes the
black pigment. But a mouse that has the genotype BB or Bb will have black fur because it possesses the
enzyme that makes the black pigment. Although each cell in the Bb animal has just one copy of the
functioning allele, that single copy can code for thousands of mRNA molecules. And each mRNA molecule
can code for thousands of enzymes. This is the reason the B allele is dominant over the b allele.
In some cases, this simple interaction between the dominant and recessive alleles is not seen.
Instead, the resulting phenotype of the heterozygous individual is somewhere in between the two
homozygous phenotypes. Incomplete dominance (aka “intermediate inheritance”) and codominance are
examples of such situations.
Solve the following problems in the space provided. Put a box around your answer to each question.
1. Petal color in carnations is an example of incomplete dominance. In these flowers, the FR allele, which
codes for an enzyme that makes red pigment, is incompletely dominant over the FW allele, which codes
for a defective enzyme that cannot make pigment. The heterozygous phenotype is pink flowers. A pink
flower is allowed to self-pollinate. What percentage of the offspring are predicted to be pink? Show all
of your work. (2 pts)
2. In certain varieties of chickens, black feathers (CB) are codominant with white feathers (CW). Thus, the
heterozygous phenotype, speckled black and white, is the expression of both alleles. A speckled male
chicken mates with a white female chicken. What percentage of the offspring are predicted to be
speckled? Show all of your work. (2 pts)
Genes that are carried on either the X or the Y-chromosome are said to be sex-linked. In humans the small
Y-chromosome carries very few genes. The much larger X-chromosome contains a number of genes that are
vital to proper growth and development. In fact, it seems to be impossible for humans to develop without
the genes of the X-chromosome.
It is particularly easy to spot recessive defects in genes located on the X-chromosome because the genes are
expressed more commonly in males than in females. What is the reason for this? Recall that males have one
X-chromosome. Thus all X-linked genes are expressed in males, even if they are recessive. In order for a
recessive gene to be expressed in females, there must be two copies of it, one on each of the two Xchromosomes. If one of the X-chromosomes contains a dominant gene, it will mask the expression of the
recessive gene. Women who have one dominant allele and one recessive allele for X-linked genes are called
carriers.
3. In humans, hemophilia is an X-linked recessive trait. A woman who is a carrier for hemophilia marries a
man who is normal. What are the chances that their children will have hemophilia? Express your answer
as a percentage. Show all of your work. (2 pts)
4. Red/Green color blindness is also an X-linked recessive trait. A color-blind woman marries a man with
normal color vision. What are the chances that their children will be color-blind? Express your answer as
a percentage. Show all of your work. (2 pts)
5. One kind of muscular dystrophy is a genetic disorder. It is due to a recessive X-linked allele. Usually, its
symptoms start in childhood. Over time, a slow loss of muscle function leads to death, usually by age 20
or so. Unlike color blindness, this disorder is nearly always restricted to males. In at least 2 complete
sentences, suggest why. (2 pts)