Video Transcript
Huntington’s disease is an
inherited disease caused by a dominant allele uppercase H. The Punnett square shows the
genotypes of a male and a female and the predicted genotypes for their
offspring. What is the missing genotype?
This question discusses
Huntington’s disease, a condition that begins in midlife and causes progressive
nerve damage. This disease is caused by the
inheritance of a dominant allele, designated here by the uppercase letter H, since
dominant alleles are typically denoted by uppercase letters. An allele refers to different
variations of a particular gene. Dominant alleles are always
expressed if they are inherited. This contrasts with recessive
alleles, which require two copies, one inherited from each parent, in order to be
expressed.
We can see that this Punnett square
contains the genotypes of the mother and father of these offspring. The mother has a dominant allele,
so apparently she has or will likely develop Huntington’s disease, while the father,
with two recessive alleles, does not have Huntington’s disease. We may remember that a parent’s two
alleles for a gene will split up when gametes are formed so that only one allele for
each gene will be passed on to offspring. And when the single alleles from
each parent combine, we get the genotype of the offspring.
The Punnett square is a tool that
helps us quickly determine how the alleles from the parents may be combined. When filling out a Punnett square,
we combine the letter from the row header of each square with the letter of the
column header. Three possibilities in this Punnett
square have already been filled out. If we fill out this last square in
the grid in the same way by combining the row and column header letters, we should
get our answer, which is uppercase H lowercase h. The missing genotype is uppercase H
lowercase h.
