Pedigrees & Genetic Analysis
Learning Objectives
By the end of this class you should understand:
The purpose of a pedigree
How to read and interpret a pedigree chart
How to construct a pedigree chart given sufficient information
How to use a pedigree chart to identify the behavior of a particular allele
How to construct a Punnett square for a particular pedigree chart crossing
The nature of autosomal vs. sex-linked genes
Patterns of Inheritance
Autosomal chromosomes Autosomal dominant Autosomal recessive
Sex-linked X-linked Y-linked Mitochondrial
Pedigree Chart
A pedigree chart, or just pedigree, shows family history for a particular condition
Can be for hair color, eye color, etc
Most commonly for a genetic disorder
Can be used to determine the nature of the inheritance
Pedigree Symbols
A key is typically provided
If it is not, these are the standards:
Male is square, female is circle
Age left-right Marriage is
horizontal line Offspring is vertical
branched line
Single-gene Tracking
Tracking a genetic disorder typically involves monohybrid crosses only
More dihybrid cross practice later Typically disorders are at two-allele loci
Multi-allele locus is like blood type & hair color Each cross can also be represented by a
Punnett Square We will practice this!
Proband
The first person to be identified as having a genetic disorder is called the proband
The proband may be the first person to receive treatment or be diagnosed
Alternatively, sometimes the progenitor ancestor is identified as the proband
Autosomal Dominant Disorder
Does not “skip” any generations
Approximately 50% of the offspring of every affected individual is also affected
Punnett Squares typically heterozygote crossed with recessive homozygote
Autosomal Dominant
Autosomal Recessive Disorder
Often “skips” generations When both parents are
carriers, about 1 in 4 offspring are affected
When one parent has the condition: 1 in 2 offspring are
affected and other half are carriers
OR all are carriers
Autosomal Recessive
SPECIAL NOTE: An autosomal recessive may
resemble an autosomal dominant if the allele is very common Essentially most crosses become aa
x Aa which is hard to distinguish from Aa x aa
Look for unaffected offspring of two affected parents Indicates dominant
Look for affected offspring of two unaffected parents Indicates recessive
Example of Ambiguity
Dominant or Recessive?
Sex-linked Traits
X-linked Can be dominant or
recessive Y-linked
Passed from father to son
Mitochondrial Passed from mother to
all children
Y-Linked Inheritance
The Y chromosome causes a fetus to become male
Any gene on the Y chromosome will only be passed on to male children Exception: CAIS, an
XY individual who is female
Y-Linked Inheritance
Mitochondrial Inheritance
All of a human's mitochondria are passed down from the mother
Sperm mitochondria are not absorbed into the fertilized egg
All offspring of an affected female have the disorder, but not an affected male
Mitochondrial Inheritance
X-Linked Traits
X-linked traits are coded for by genes on the X chromosome
Women have two X chromosomes and men have one This means
expression patterns are different in men and women!
X-Linked Dominant
X-Linked Dominant will affect men and women differently
All of an affected man's daughters will express the disorder
None of an affected man's sons will express the disorder
Half an affected woman's offspring will express the disorder
X-Linked Dominant
X-Linked Recessive
Several disorders are X-linked recessive
Colorblindness, hemophilia
For a woman, both X chromosomes must be defective
Men only have one X so if it is defective they express the disorder
X-Linked Recessive
Patterns of Inheritance
Autosomal dominant Autosomal recessive X-linked dominant X-linked recessive Y-linked Mitochondrial
Dihybrid Cross
One practical use for a dihybrid cross is to consider gender as a factor in an autosomal cross Particularly if there is
a sex-linked trait as well
Strategy: determine what the gametes are first
AaXX' x AaXY
AX AX'aX aX'
AX AYaX aY
AX AX' aX aX'
AX AAXX AAXX' AaXX AaXX'
AY AAXY AAX'Y AaXY AaX'Y
aX AaXX AaXX' aaXX aaX'Y
aY AaXY AaX'Y aaXY aaX'Y
Dihybrid Practice:
Heterozygous Aa x Aa cross between a colorblind man and a noncarrier woman
What are the possible outcomes? If the autosomal gene is a recessive disorder
what is the probability of a child having the disorder?
What is the probability of a boy having each disorder? A girl?
What is the probability of a child having both?
Partner Practice
(time permitting) Everyone choose one of the five patterns and
draw your own pedigree chart! Be sure it has at least 3 generations and thhere
should be at least five crosses of interest Trade with a partner and analyze which
pattern(s) it matches!
Have a good weekend!