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