#5 Complex Phenotypes and gene interactions

Pseudoautosomal Inheritance

Refers to the very few genes found on both X and Y chromosomes

• Small regions at the tips of X and Y chromosomes that are homologous

• These genes behave like autosomes (non sex chromosomes)

Hemizygous

X-only genes are hemizygous

• In males: only has 1 X gene

• Results in unique inheritance patterns

Y Chromosome

Very difficult to sequence

• Little sequence variation between father and son

  • Can trace Y chromosome directly to father, paternal grandfather

X-Linked Chromosome

• Hemizygous in males

• Only one copy

• Males are more likely to be affected (as they only inherit 1 copy of X chromosome)

Y-Link

• Relatively few in human genes

• Holandric genes

• Transmitted only from father to son

Lethal Alleles

Gene that can cause death

• Alters phenotype ratios

• Result: mutations in essential genes and recessive (required for survival)

• About 1/3 of all genes are essential for survival

• lead to deviation from expected Mendelian ratios, because some genotypes die before birth.

EX: Manx

• M = allele for taillessness

• Mm = tailless (viable)

• MM = lethal (dies before birth), this allele is lethal as a homozygote

• mm = normal tail

Pleiotropy

Most genes affect more than 1 trait

• Single gene cause multiple phenotypes

EX: Cystic fibrosis

• Mutation reduces Cl- conductance

  • Lungs → mucus

  • Skin → salty sweat

  • GI → Enzyme block → weight gain

Gene Interactions

When two or more different genes influence the outcome of a single trait

Complementation

Two affected parents have unaffected offspring due to having mutations in separate genes

• 2 affected, recessive parents = unaffected offspring

Redundancy

One gene compensates for another

• Need to have mutant gene in more than 1 gene to see phenotype

• Only when both genes are homozygous recessive (aabb) do you see the recessive phenotype.

• 15:1 ratio

Epistasis

One gene masks the phenotype of another gene

• Usually genes in the same pathway

• Homozygous allelic versions of either white gene (cc or pp) = both lead to white flower even if there is Pp 

  • Ex: ccPp = white (cc masks the purple)

• 9:7 ratio

Modifiers

Modifiers alter the phenotype

• Ex: Mouse coat color

  • A primary gene determines coat pigment (black vs brown).

  • A modifier gene can change pigment distribution → giving different shades or patterns.