Transcript Pedigree Analysis

PEDIGREE ANALYSIS
In humans, pedigree analysis is an important tool for
studying inherited diseases
Pedigree analysis uses family trees and information
about affected individuals to:
 figure out the genetic basis of a disease or trait from its
inheritance pattern
 predict the risk of disease in future offspring in a family
(genetic counseling)
PEDIGREE ANALYSIS
Mating
I
Normal
Female
Normal
Male
1st born
II
Affected
Siblings
How to read pedigrees
Basic patterns of inheritance
autosomal, recessive
autosomal, dominant
X-linked, recessive
X-linked, dominant (very rare)
AUTOSOMAL RECESSIVE
Trait is rare in pedigree
Trait often skips generations (hidden in
heterozygous carriers)
Trait affects males and females equally
 For each of these, overdominance (heterozygote
superiority) has been suggested as a factor in
maintaining the disease alleles at high frequency
in some populations
Most common Autosomal Recessive
disorders
Cystic fibrosis
Sickle cell anemia
Phenylketonuria (PKU)
Tay-Sachs disease
AUTOSOMAL RECESSIVE
AUTOSOMAL DOMINANT
Trait is common in the pedigree
Trait is found in every generation
Affected individuals transmit the trait to
approximately 1/2 of their children
(regardless of sex)
AUTOSOMAL DOMINANT
ex. achondroplasia
(a sketelal disorder
causing dwarfism)
X-LINKED RECESSIVE
Trait is rare in pedigree
Trait skips generations
Affected fathers DO NOT pass to their sons,
Males are more often affected than females
X-LINKED RECESSIVE
X-LINKED DOMINANT
Trait is common in pedigree
Affected fathers pass to ALL of their
daughters
Males and females are equally likely to be
affected
X-LINKED DOMINANT
X-linked dominant diseases are extremely
unusual
Often, they are lethal (before birth) in males
and only seen in females
ex. incontinentia pigmenti (skin lesions)
ex. X-linked rickets (bone lesions)
Remember:
 dominant traits may be rare in population
 recessive traits may be common in population
 alleles may come into the pedigree from 2 sources

mutation happens
 often traits are more complex
 affected by environment & other genes
 Scientists study twins to view/separate environmental
contributions vs. environmental factors.
 There is a large difference between fraternal twins and identical
twins owing to genetic influence
 EPIGENETICS OF TWINS
BELOW ARE SOME PRACTICE
EXAMPLES! 
IF TWO AFFECTED PARENTS HAVE AN
UNAFFECTED KID, RECESSIVENESS* CAN
BE RULED OUT
*both autosomal and
x-linked
GENETIC COUNSELING
A woman’s brother died from Tay Sach’s
Disease (autosomal recessive, lethal), but she
is unaffected. What are the chances that she
is a carrier of the disease?
A. 2/3 (67%) B. 1/2 (50%) C. 1/4 (25%)
D. 1/6 (17%) E. 1/8 (12.5%)
HINT
A woman’s brother died from Tay Sach’s
Disease (autosomal recessive, lethal), but she
is unaffected. What are the chances that she
is a carrier of the disease?
A. 2/3 (67%) B. 1/2 (50%) C. 1/4 (25%)
D. 1/6 (17%) E. 1/8 (12.5%)
The answer is…
A
B
D
d
D DD Dd
C
D
d Dd dd
A. 2/3 (67%) B. 1/2 (50%) C. 1/4 (25%)
D. 1/6 (17%) E. 1/8 (12.5%)
PROBLEM 2
ASSUMING YOUR GRANDFATHER
IS A CARRIER (HETEROZYGOTE)
FOR A RARE RECESSIVE,
DISEASE CAUSING ALLELE OF A
GIVEN GENE,
WHAT’S THE CHANCE THAT YOU
ARE ALSO A CARRIER OF THIS
ALLELE?
©2001 Lee Bardwell
IF
D
F
THEN
A.2/3
E
G
H
you
?
B. 1/2 C. 1/4
D. 1/6 E.1/8
D
E
1/2 (50% chance) mom
got the allele
F
G
H
1/2 (50% chance) she gave
it to you if she had it
1/2 x 1/2 = 1/4
= Chance that you got a given allele from
Grandpa D
PROBLEM 3
YOUR GRANDFATHER’S SISTER
HAD CYSTIC FIBROSIS (RARE,
AUTOSOMAL RECESSIVE).
THAT’S THE ONLY CASE IN
YOUR FAMILY.
(A) WHAT’S THE CHANCE THAT
YOU ARE A CARRIER OF CF?
A.2/3
B. 1/2 C. 1/4
D. 1/6 E.1/8
Your grandfather’s sister had CF but he, his
parents, and his descendants are unaffected, as
are those who married into the family
A
B
D
C
F
E
G
H
you
A.
B.
C.
D.
E.
2/3
1/2
1/4
1/6
1/8
A
D
B
d
D DD Dd
D
C
F
2/3
E
G
H
you
d Dd dd
IF
D
F
THEN
E
G
H
you
?
ANSWERS
= Chance that you got any particular allele from from
grandpa
A. 1/4
= correct answer = 1/4 (see A) x 2/3 (chances that
grandpa was a carrier)
B. 1/6