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Pedigrees are like family trees that track the inheritance of traits throughout generations. Scientists and genetic counselors use these to study how traits are passed down and to predict the probability of certain traits (particularly disorders) being inherited in future generations.
They use a series of symbols to represent individuals and relationships. Circles are females and squares are males. The trees track one particular phenotype, and a shaded in (often, but not always, black) circle/square means that that individual is "affected" - that they have the trait being followed. Genotypes are typically not written out, and need to be inferred by the data.
Roman numerals represent generations, while Arabic numerals represent the individual. So, individual III-2 is the affected female (black circle) from the third generation. And she had four kids with individual III-3.
The biggest thing, when it comes to learning pedigrees, is learning how to identify the inheritance pattern of a trait. Here's a brief overview for common patterns seen in each major type of inheritance.
Autosomal Dominant
Autosomal Dominant
Two affected parents can have unaffected children (the affected parents being heterozygous)
Two unaffected parents can never have affected children
Males and females are affected equally
Autosomal Recessive
Autosomal Recessive
Two unaffected parents can have affected children (it skips a generation!)
Two affected parents will always have affected children
Males and females are affected equally
Mitochondrial
Mitochondrial
Mom will pass the trait to all of her kids
Dad will not pass the trait on
X-Linked Recessive
X-Linked Recessive
Males are affected more often that females
Affected males cannot pass it to their sons
Can skip generations with female carriers
X-Linked Dominant
X-Linked Dominant
Does not skip generations
Affected males will pass it to all daughters, but no sons
Affected females can pass it to sons or daughters
Y-Linked
Y-Linked
Only in males
Dad will pass to all of his sons
Click here for a summary of the main differences to look for in inheritance patterns.
Click here for a summary of the main differences to look for in inheritance patterns.
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