pedigrees chapter 14. pedigree a pedigree is a chart for tracing genes in a family they can be used...
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PEDIGREESChapter 14
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Pedigree• A pedigree is a chart for tracing genes in a family• They can be used to study the transmission of a
hereditary condition• Phenotypes are used to infer genotypes on a pedigree
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Symbols Used in a Pedigree Normal male Affected male Normal female Affected female
Vertical Lines- offspring Marriage/Mating
A marriage with five children, two daughters and three sons. The middle-aged son is affected by the condition.
Oldest child Youngest child
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Organizing the Pedigree
• Generations are identified by Roman numerals
I
II
III
IV
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Organizing the Pedigree• Individuals in each generation are identified by numbers. • Therefore the affected individuals are II3, IV2 and IV3
I
II
III
IV
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Patterns of Inheritance Basic patterns of inheritance
autosomal, recessiveautosomal, dominantX-linked, recessiveX-linked, dominant (very rare)
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Autosomal Recessive Traits
• Trait is rare in pedigree
• Trait often skips generations • hidden in heterozygous carriers
(sometimes shown half shaded)
• Trait affects males and females equally
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Autosomal Recessive Traits Most common ones • Cystic fibrosis • Sickle cell anemia• Phenylketonuria (PKU)• Tay-Sachs disease
For each of these, over dominance (heterozygote superiority) has been suggested as a factor in maintaining the disease alleles at high frequency in some populations
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Autosomal Recessive Trait• If you ever see this situation
in the pedigree (two unaffected parents have an affected child) then the trait MUST be recessive!
• The parents are heterozygous.
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Autosomal Dominant Trait• Trait is common in the
pedigree
• Trait is found in every generation
• Affected individuals transmit the trait to ~1/2 of their children (regardless of sex)
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X-linked Recessive Trait
• Trait is rare in pedigree
• Trait skips generations
• Affected fathers DO NOT pass to their sons,
• Males are more often affected than females
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X-linked Recessive Trait• ex. Hemophilia in European royalty
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Steps to Determine Inheritance Pattern of a Trait
Step 1: Determine whether it’s dominant or recessive.
• Does it skip generations? YES= recessive
NO= dominant.
Step 2. Determine whether is is autosomal or x-linked.
• Does it affect one gender more than the other?
YES= X-linked
NO= autosomal
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What Inheritance Pattern Does this Trait Follow?
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• What is the inheritance pattern?• What is the genotype of III-1, III-2, and II-3?• What are the odds that IV-5 would have an affected son?
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Pedigree Analysis in Real Life
• Remember: • dominant traits may be rare in population
• recessive traits may be common in population
• alleles may come into the pedigree from 2 sources
• mutation happens
• often traits are more complex
• affected by environment & other genes