Patterns of Inheritance
Patterns of Inheritance
Autosomal Recessive Traits
• Must inherit both alleles to have disease– Widow’s peak– Hitchiker’s thumb– Tongue rolling– Tay Sach’s disease– Sickle cell anemia– Cystic fibrosis
• Genotype: bb
Autosomal Dominant Traits
• Only need to inherit one affected allele to show the trait– Polydactyly– Marfan syndrome– Huntington’s disease– Achondroplasia (form of
dwarfism)
• Genotype: BB or Bb
X-linked Inheritance
• Trait passed on X chromosome.– Mothers always pass to sons
• Hemophilia• Colorblindness
– Male’s “y” chromosome is not able to carry a trait to mask a recessive defect so if son inherits disease from mother’s X it will be expressed.
• Genotypes (ex: hemophilia)– Female: XHXH XHXh XhXh
– Male: XHy, Xhy
Consanguinity
• Genetic disorders are more likely to show up in a population if the population is closely related
• Ex: cousins marry & have children with other cousins
Lethal Alleles• A genotype that causes death• Usually causes death before the
individual can reproduce• May result in miscarriages• Can be autosomal dominant (AD) or
recessive (AR)• Ex: Mexican hairless dogs.
(autosomal dominant disorder)– If it inherits Hh- hairless– If it inherits HH- lethal– It if inherits hh- hairy– What two dogs should breeders mate
to ensure that they get some hairless pups but no stillborn pups?
• Ex: Huntington’s disease (AD), Tay Sach’s disease (AR)
Epistasis• Epistasis- when two or more
genes affect a single phenotype• The interaction is between
different genes not the different alleles
• EX: Hair color and Hair presence in Mexican Hairless dogs– Hair color genes are present in
Mexican hairless dogs but they can’t act if there’s no hair to color.
– The hair presence gene has epigenetic control over hair color
Another Epistasis Example• Ex: Comb shape in chickens
– Two genes with two alleles:• Rose gene: R or r• Pea gene: P or p
• Rose gene, if present in RR or Rr will produce a "rose type" comb-- but ONLY if Pea gene is present in pp condition.
• Pea gene, if present in PP or Pp will produce a "pea type" comb-- but ONLY if Rose gene is present in rr condition.
• If one dominant allele is present for BOTH pea and rose, a "walnut type" comb results. R_P_ will give "walnut" comb.
• If both alleles are present in double recessive condition, (rrpp), the wild type, single comb results.
pprr P?R? P?rr ppR?
Pleiotropy
• Pleiotropy- a single gene has control over more than one phenotype
• Ex: albinism– Lack the enzyme to turn
tyrosine into melanin (pigment)
– Affects not just skin color but also eye color and hair color
Penetrance• Penetrance- all or none
expression of a genotype– Complete penetrance- produces
phenotype in all who inherit it • Ex: Huntington’s disease
– Incomplete penetrance- some do not show symptoms• Ex: Polydactyly- some people
have allele for extra fingers/toes but have regular number.
– Calculated as percentage of a population• Ex: if 80 out of 100 people
express the affected phenotype then the phenotype is 80% penetrant
Expressivity
• Expressivity- severity or extent an allele is expressed
• Variable expressivity:– Ex: Parent might have
extra finger on each hand whereas child may have extra finger just on one hand
Phenocopies
• Phenocopy- environmentally caused trait that appears to be inherited but is not genetic
• Ex: underweight child that gets frequent colds may appear to have cystic fibrosis but instead has malnutrition– Genetic testing can rule out
cystic fibrosis
• Ex: Phocomelia can be caused by thalidomide drug or can be genetic disorder