Lecture 2 - Pedigrees and Conditional Probabilities (Discussion)

Pedigree

family tree that shows information regarding relevant phenotype

• can determine genotype/probable genotypes of individuals

3 types of pedigrees

Recessive, Dominant, Sex-linked

Recessive Trait

trait resulting from a recessive mutant allele

• Homozygous dominant = normal phenotype

• Heterozygous = mutant carrier

• Homozygous recessive = mutant phenotype

Dominant Trait 

trait resulting from a dominant mutant allele

• Homozygous dominant = Mutant phenotype

• Heterozygous = mutant phenotype

• Homozygous recessive = normal phenotype

Recessive Pedigree

Produces:

• 1 unaffected (AA)

• 2 unaffected carrier (Aa) children

• 1 affected (aa) child

Dominant Pedigree

Produces:

• 2 unaffected (aa) children

• 2 affected (Aa) children

Determining Recessive Traits

• Mutant trait shown in homozygous recessive organisms

• Heterozygous organisms will not show the mutant trait but will be a carrier

• Hallmark of recessive traits: normal parents produce mutant offspring

• Loss-of-function mutation

• Mutant “a” allele creates mutant phenotypic expression

• “A” allele produces normal wild-type phenotypic expression

• AA = unaffected (non-carrier), Aa = unaffected (carrier), aa = affected

Determining Dominant Traits

• Mutant trait shown in heterozygous and homozygous dominant organisms

• Parents who are homozygous recessive and do not show the mutant trait cannot have offspring that possess the mutant trait

• Hallmark of dominant traits: mutant parents produce normal offspring

• Gain-of-function mutations

• Mutant “A” allele creates mutant phenotypic expression

• “a” allele produces normal wild-type phenotypic expression

• aa = unaffected, Aa = affected, AA = affected

Recessive trait

Recessive and dominant trait