Solving the Pedigree:
Solving a genetics
pedigree will prove that you really know your stuff!
A pedigree will show a
family history of a certain genetic trait. The only information you will be
given is who is exhibiting the trait. You must then look at the pattern of
inheritance and decide how the gene works. You must figure out if the gene for
the trait is autosomal dominant, or autosomal recessive or sex-linked recessive.
To solve a pedigree you
must first learn the symbol code.
drawn as a square
drawn as a circle
If the square or circle is
colored in that means the individual is showing the trait, if it is not colored
in the individual is not showing the trait in their phenotype. A line connecting
a male and female means they will have kids together. The kids are drawn down
from this connecting line.
Letís practice writing
out the genotypes for each type of inheritance pattern.
If a trait is governed by an autosomal dominant gene you must have at least one
dominant allele to show the trait. If you do not exhibit the trait in your
phenotype you must be recessive in your genotype.
If a trait is governed by an autosomal recessive gene you must have a homozygous
recessive genotype to show the trait. If you do not exhibit the trait in your
phenotype you must have a least one dominant allele in your genotype.
If a trait is governed by a sex linked recessive gene males must have the
recessive X to show the trait and females must have two recessive Xs to show the
trait. A dominant X male will not show the trait in his phenotype and a
homozygous dominant or heterozygous female will also lack the trait in her
The best way to learn
about solving pedigrees is to just try and solve one. Let's work on solving the
We will refer to each
individual by number. We will assign each person a number working from top to
bottom and left to right. So starting with the top level the man will be
referred to as number 1 and the woman number 2.
Let's see if the pedigree
works by being autosomal recessive in nature.
First assign all of the
genotypes possible if the trait is caused by an autosomal recessive gene. This
means if the individual is showing the trait they must be autosomal recessive in
their geneotype (aa).
This would make numbers 2,
4, 6, 7, 11, and 12 aa. Now look at the rest of the pedigree and see if it is
possible to have the pattern inherited in this manner.
As you can see, the gene
cannot be autosomal recessive and produce the pattern. Why you say? Well look at
number 11 and 12. They both show the trait and thus would be aa and aa. Now look
at their offspring. Some are not showing the trait, which would mean they must
have at least one A. This is not possible if both parents are autosomal
OK, we can cross off
autosomal recessive from the list. Now let's try sex-linked recessive.
Start out the same way. Assign the genotypes you know for sure and then take a
look at the overall picture and see if it is possible.
All the males not showing
the trait (1, 9, 10, 14, and 15) must have the genotype XRY. All of
the males showing the trait (4, 7, and 12) must have the genotype XrY.
All of the females showing the trait (2, 6, and 11) must have the genotype XrXr.
Now take a close look at
the pedigree and you will see that this pattern of inheritance is not possible
with a sex-linked recessive gene. The female number 6 (XrXr)
is not possible with the parents shown in the pedigree. The father would need to
show the tait to produce a daughter showing a sex-linked recessive trait.
This narrows the field
down to just one possibility. The gene must be autosomal dominant to
produce this pedigree pattern. Fill in what you can and see if the pedigree
All individuals not
showing the trait must be aa (numbers 1, 3, 5, 8, 9, 10, 13, 14, and 15). All
individuals showing the trait must have at least one dominant gene, AA or Aa.
(Numbers 2, 4, 6, 7, 11, and 12)
Thus number 1 (aa) crossed
with number 2 (Aa) could give you number 4 (Aa), number 5 (aa), and number 6
(Aa). Number 3 (aa) crossed with number 4 (Aa) could give you numbers 8, 9, and
10 (all aa). Number 6 (Aa) crossed with number 7 (Aa) could give you numbers 11
and 12 (AA or Aa) and numbers 12, 14, and 15 (all aa). Thus the pedigree pattern
is possible at ALL levels if the gene causing the trait is autosomal dominant.
A suggestion is to work on
these in PENCIL!!!