8.6- Phenotype Expression

Phenotypic expression is the result of

a complex, dynamic relationship between an individual’s genes and its surroundings during development.

Sex-linked genes:

• genes located on the sex chromosomes; either X-linked or Y-linked. • Color blindness is X-linked recessive. Men only have one X chromosome, so they express colorblindness with just one affected allele.

• Only men can have Y-linked genes (e.g. webbed toes). Autosomal genes are located on autosomal (non-sex) chromosomes. Individuals with a dominant condition carry an affected allele and express the affected phenotype. For recessive conditions, a heterozygous individual (a carrier) carries the affected allele but still displays a normal phenotype.

• All males with X-linked conditions must have inherited it from their mother. Fathers pass only the Y chromosome to their sons (not the X chromosome).

Sex-influenced genes:

• expression is influenced by the sex of the individual. Gene is not necessarily on a sex chromosome.

• E.g. a sex-influenced gene for baldness may result in males being bald, but females unaffected

Genomic imprinting:

the deactivation of one copy of a gene depending on which parent it came from. 

Penetrance:

the probability that an organism with a specific genotype will express the corresponding phenotype (e.g. likelihood that a gene for baldness will cause a bald phenotype).

Expressivity:

describes the level of expression of a phenotype for a specific genotype (e.g. gene for body hair → people with the same genes may express different amounts of body hair due to different levels of expressivity). 

X-inactivation:

a female phenomenon in which one of the two X chromosomes (randomly chosen) condenses down into an inactivated Barr body. Expressed genes come from the remaining X chromosome.

Heterozygote Advantage:

when the heterozygous genotype has an advantage over homozygous dominant or recessive (e.g. heterozygotes for sickle-cell anemia have malaria resistance without suffering from disease).

 Linked genes:

genes that are physically close together on the same chromosome. Linked genes are less likely to be separated by crossing over during meiosis I and more likely to be inherited together.

Recombination frequency:

the percentage likelihood that two genes will be separated by crossing over. a. Linked genes have a low recombination frequency