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


Typically disorders are at two-allele loci


More dihybrid cross practice later
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


Look for unaffected offspring of two
affected parents


Essentially most crosses become aa
x Aa which is hard to distinguish from
Aa x aa
Indicates dominant
Look for affected offspring of two
unaffected parents

Indicates recessive
Example of Ambiguity
Dominant or Recessive?
Sex-linked Traits

X-linked


Y-linked


Can be dominant or
recessive
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 Xlinked 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 AY
aX 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

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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!