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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
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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 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
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
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
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
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 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
Patterns of Inheritance • Autosomal dominant • Autosomal recessive • X-linked dominant • X-linked recessive • Y-linked • Mitochondrial
Dihybrid Cross AaXX' x AaXY • 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 AX AX' aX aX' AX AY aX aY
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!
