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: X H X H X H X h X h X h – Male: X H y, X h y

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