Pedigrees & Genetic Analysis

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Pedigrees & Genetic Analysis

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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 - PowerPoint PPT Presentation

Transcript of Pedigrees & Genetic Analysis

Page 1: Pedigrees & Genetic Analysis

Pedigrees & Genetic Analysis

Page 2: 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

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Patterns of Inheritance

Autosomal chromosomes Autosomal dominant Autosomal recessive

Sex-linked X-linked Y-linked Mitochondrial

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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

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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

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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!

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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

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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

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Autosomal Dominant

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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

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Autosomal Recessive

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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

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Example of Ambiguity

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Dominant or Recessive?

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Sex-linked Traits

X-linked Can be dominant or

recessive Y-linked

Passed from father to son

Mitochondrial Passed from mother to

all children

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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

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Y-Linked Inheritance

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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

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Mitochondrial Inheritance

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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!

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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

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X-Linked Dominant

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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

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X-Linked Recessive

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Patterns of Inheritance

Autosomal dominant Autosomal recessive X-linked dominant X-linked recessive Y-linked Mitochondrial

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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

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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?

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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!

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Have a good weekend!