PPT On Pedigree Analysis
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1. Pedigree Analysis2. Why do Pedigrees?
Punnett squares and chi-square tests work well for organisms that have large numbers of offspring and controlled matings, but humans are quite different: 1. small families. Even large human families have 20 or fewer children. 2. Uncontrolled matings, often with heterozygotes. 3. Failure to truthfully identify parentage.
3. Goals of Pedigree Analysis
1. Determine the mode of inheritance: dominant, recessive, partial dominance, sex-linked, autosomal, mitochondrial, maternal effect. 2. Determine the probability of an affected offspring for a given cross.
4. Y-Linked Inheritance
We will now look at how various kinds of traits are inherited from a pedigree point of view. Traits on the Y chromosome are only found in males, never in females. The father’s traits are passed to all sons. Dominance is irrelevant: there is only 1 copy of each Y-linked gene (hemizygous).
5. Mitochondrial Genes
Mitochondria are only inherited from the mother. If a female has a mitochondrial trait, all of her offspring inherit it. If a male has a mitochondrial trait, none of his offspring inherit it. Note that only 1 allele is present in each individual, so dominance is not an issue.
6. Outsider Rules
In any pedigree there are people whose parents are unknown. These people are called “outsiders”, and we need to make some assumptions about their genotypes. Sometimes the assumptions are proved wrong when the outsiders have children. Also, a given problem might specify the genotype of an outsider. Outsider rule for dominant pedigrees: affected outsiders are assumed to be heterozygotes. Outsider rule for recessive pedigrees: unaffected (normal) outsiders are assumed to be homozygotes. Both of these rules are derived from the observation that mutant alleles are rare.
7. Maternal Effect Genes
The maternal effect rule: “Mother’s genotype determines offspring’s phenotype.” Assume that the trait is recessive, in a complete dominance situation. Also assume all “outsiders” (people with unknown parents) are homozygous for the allele they are expressing : the dominant allele if they are unaffected, and the recessive allele if they are affected.
8. Sex-Influenced Trait
Assume that the trait is dominant in males but recessive in females. Assume all outsiders are homozygotes. Thus: DD is always affected dd is always normal Dd is affected in males, but normal in females
9. Sex-Limited Trait
There are several possibilities for dominance, but for this problem assume the trait is dominant but only expressed in males. Affected outsider males are heterozygous; unaffected males are homozygous normal Assume that outsider females are homozygous normal.
10. Thanks.
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