Video Transcript
Assume the gene for brown eyes is
dominant over the gene for blue eyes. If a mother with blue eyes and a
father with brown eyes have a child, what eye color is the child most likely to
have?
This question asks about
inheritance of eye color. In order to work out the correct
answer, let’s discuss inheritance in terms of dominant and recessive alleles.
Recall that a trait refers to a
specific characteristic, for example, eye color, hair color, or height, which is
inherited from an organism’s parents. It is encoded by genes. And most body cells contain two
genes coding for a trait, one from each parent.
In this question, we are asked
about eye color as a trait. And the two different alleles of
the gene coding for eye color are brown and blue. Note that brown eyes are
represented by the color orange in our diagram. The question states that the allele
coding for brown eyes is dominant over the allele coding for blue eyes. This means that the allele for
brown eyes will be expressed if it is inherited from either parent, while to inherit
blue eyes, the person must have two alleles coding for blue eyes. This means they must inherit the
allele for blue eyes from both parents.
Let’s annotate the alleles coding
for brown eyes with a capital B and the allele coding for blue eyes with a lowercase
b. The father, who has brown eyes,
will have one of two possible genotypes: capital B capital B or capital B lowercase
b. Because the mother has blue eyes
and the allele which codes for blue eyes is recessive, the mother must have the
genotype lowercase b lowercase b. We can now use this information to
create two Punnett squares and work out which eye colors are possible in a child of
these two parents.
If the father has two dominant
alleles, the child will have brown eyes because they will definitely inherit the
dominant allele coding for brown eyes from the father. If the father has one dominant and
one recessive allele, the child has a 50 percent chance of having brown eyes and a
50 percent chance of having blue eyes. We can observe that of the eight
possible combinations of genes the child could inherit, only two will code for blue
eyes.
Therefore, the answer to our
question “What eye color is the child most likely to have?” is brown.
