Transcript Pedigree Analysis

Pedigree Analysis
How do doctors know if a trait is
inherited?
They take a “family history”, and show it in
a diagram form known as a pedigree
Pedigree Symbols
Example 1:
 Grandparents had two
children: a son and a
daughter.
 Their son had the trait
in question. He
marries a woman
without the trait.
 One of the son’s four
children (a boy) had
the trait.
Example 2:
 Grandpa has the trait,
grandma doesn’t.
 Of their five children,
one son and two
daughters have trait.
One son and one
daughter don’t have
trait.
 One daughter with
trait marries man
without. Of their five
kids, one son and one
daughter have trait.
Can you tell from a pedigree if a
trait is dominant or recessive?
Could this trait be dominant?
 If it’s dominant, the affected
daughter would have to be either
DD or Dd.
 If she’s DD or Dd, she would have
had to get a dominant D allele
from a parent.
 But, if were dominant and the
parent had a D allele, the parent
would have the trait, too.
 Therefore, trait is recessive, NOT
dominant; daughter is dd
What are the genotypes?
Dd
dd
Dd
DD
or
Dd
 Since the trait is
recessive, daughter
must be dd.
 Parents don’t have the
trait, so they can’t be
dd.
 But, since they each
passed a recessive d
allele to daughter, they
must each be Dd.
Could this trait be dominant?
 If it’s dominant, the
affected daughter
would have to be
either DD or Dd.
 If she’s DD or Dd, she
would have had to get
a dominant D allele
from a parent.
 Dad has the trait, so
he could have given
her a D allele.
 So, trait is dominant.
What are the genotypes?
dd
Dd
Dd
dd
 Since the trait is
dominant, mom can’t have
a D, or she would show the
trait.
 So, mom is dd.
 The son doesn’t have trait,
so he must be dd, too.
 If son is dd, he got one d
from mom; one from dad.
 Dad must be Dd; daughter
is Dd.