Download Genetics III

Genetics III
Answered Review Questions
1. Describe a pedigree.
A pedigree is a graphical way to depict the inheritance of a trait through
several generations of a family. We use the word “affected” when
describing an individual with the trait. The symbols in a pedigree
represent the following:
■ = affected male
□ = unaffected male
● = affected female
○ = unaffected female
□○ = mated pair
Below are examples of pedigrees tracing the inheritance of widow’s peak
and attached earlobes.
2. Determine the inheritance pattern for a trait and each family
member’s genotype using a pedigree.
Imagine you are a geneticist and you have stumbled upon a previously
unknown genetic trait and you want to figure out its inheritance pattern.
You can do this by trial and error: testing it as autosomal dominant,
autosomal recessive, and then sex-linked recessive and seeing which
one works.
Autosomal Dominant
If we assume that the trait is autosomal dominant, all the affected
people have at least one dominant allele and all the unaffected are
homozygous recessive. In this situation, I like to knock out all the
unaffected first and then determine the genotypes of the affected
individuals. Here is a legend of the genotypes that may prove useful.
■ = affected male (AA or Aa or A_)
□ = unaffected male (aa)
● = affected female (AA or Aa or A_)
○ = unaffected female (aa)
Sometimes we might have a person with the dominant phenotype
where, based on the information we have, we cannot determine if
they are homozygous dominant of heterozygous. In that case the
best we can do is indicate they have at least one dominant allele
“A_”.
Aa
aa
aa
Aa
aa
aa
Aa
aa
aa
aa
Everything works for this pedigree assuming the trait is autosomal
dominant. We know that Great-Grandmother was heterozygous because
her son his homozygous recessive (he got one a from mom, he had to
have received the other from dad). Same for Grandmother and for
Father #1. Let’s test it next for autosomal recessive.
Autosomal Recessive
If we assume that the trait is autosomal recessive, all the affected
people are homozygous recessive. All the unaffected have at least one
dominant allele. In this situation, I like to knock out all the affected first
and then determine the genotypes of the unaffected individuals. Here
is a legend of the genotypes that may prove useful.
■ = affected male (aa)
□ = unaffected male (AA or Aa or A_)
● = affected female (aa)
○ = unaffected female (AA or Aa or A_)
aa
A_
Aa
aa
Aa
Aa
aa
Aa
A_
A_
Everything works for this pedigree if we assume the trait is autosomal
recessive. It is okay to have more than one inheritance pattern fit. It
usually means that we need a bigger one with more generations and
family (especially the in-laws).
We know Grandfather is heterozygous because he has the dominant
phenotype and so he must have inherited a dominant allele from GreatGrandfather and since Great-Grandmother is affected all she can donate
is the recessive allele.
We don’t have enough information to determine if Great-Grandfather is
homozygous dominant or heterozygous so the best we can do is label
him as “A_”.
Mom #2 and Mom #3 are both heterozygous because they have the
dominant phenotype but also must have received a recessive allele from
their mom.
Dad#4 and his son are both A_ because we don’t have enough
information.
Son#1 is heterozygous because Dad#1 is homozygous recessive and so
only has the recessive allele to give and he has the dominant phenotype.
Sex-linked Recessive
If we assume that the trait is sex-linked recessive, all the affected
males have the recessive allele, all the affected females are
homozygous recessive. All the unaffected males have the dominant
allele and all the unaffected females have at least one dominant allele.
In this situation, I like to knock out all the affected first, then knockout
all the unaffected males, and then all there is left to do is determine the
genotypes of the unaffected females. Here is a legend of the
genotypes that may prove useful.
■ = affected male (XaY)
□ = unaffected male (XAY)
● = affected female (XaXa)
○ = unaffected female (XAXA or XAXa or XAX_)
XaXa
XAY
XAY
XaY
XAXa
XAY
XaXa
XAXa
XAY
XAX_
A sex-linked recessive inheritance pattern does not work for this
pedigree. The problem is Grandfather. The only genotype he could be
is XaY because he got his Y chromosome from his Dad and the only
allele his Mom could donate is a recessive.
A great way to practice deciphering pedigrees is to draw a pedigree
and then at random fill in some of the individuals and then try it with
different types of inheritance patterns to see which ones fit.