Ch 4: extensions

alleles

different variants of gene that map to same locus on homologous chromosomes

allelic

has two variants of a gene

non-allelic

has two copies of same form of gene

wild type

phenotype of most common form in nature for a species

null allele

- a mutant allele with the effect of either the absence of gene product or the absence of gene product function due to a loss-of function mutation.

- Most null alleles behave as recessive

extension of mendelian genetics

- some characters that follow the basic Mendelian principles of inheritance but yield different phenotypic forms and/or proportions.

- gene expression does not always adhere to the Mendelian dominant/recessive rule

- more than one locus may influence the expression of a trait

incomplete dominance

- partial dominance

- neither one of two contrasting traits is dominant

- the phenotype of heterozygous individuals is an intermediate of the two contrasting traits.

incomplete penetrance

- <100% penetrance

- all should have color but some are showing the recessive phenotype (no color).

variable expressivity

- 100% penetrance but variable expressivity

- all have color but are showing it in different ways

incomplete penetrance example

- polydactyly

- dominant allele that exhibits 90% penetrance

variable expression example

Flies that are homozygous for the recessive eyeless mutant allele show different degrees of eye size reduction (< 800 ommatidia)

penetrance

frequency with which individuals of a given genotype manifest at least some degree of the trait

expressivity

degree or range in which a phenotype for agiven trait is expressed

pleiotropy

- condition in which a single mutation causes multiple phenotypic effects.

- Marfan syndrome results from an autosomal dominant mutation in fibrillin, a gene that codes for a connective tissue protein present in many tissues in the body

essential genes

gene that is necessary for growth and survival, and its absence results in lethality.

recessive lethal

- null allele behaves as a recessive lethal allele

- the homozygotes for the null allele die but the heterozygotes survive

- modified mendelian monohybrid ratio

Agouti

- A (agouti) is an essential gene

- one copy of the A allele is sufficient for survival

- AY (yellow) is a dominant and pleiotropic mutant allele

- AY is also a recessive lethal

multiple alleles

- inheritance of plumage color patterns in mallard ducks depends on the three alleles of the M locus

- MR: restricted M: mallard (wild type) md: dusky

- restricted is dominant over mallard and dusky

- mallard is dominant over dusky

codominance

mode of inheritance in which the phenotypic effects of two allelic genes are fully and simultaneously expressed in heterozygous individuals.

blood types (ABO) have what extension

- multiple alleles of the same locus

- codominance of blood types IA and IB

H gene

- chromosome 19

- codes for enzyme that adds a fucose to the compound H precursor to yield compound H

- hh makes O blood type

IA

an enzyme that adds a terminal N-acetylgalactosamine to compound H to yield theA antigen

IB

an enzyme that adds a terminal galactose to compound H to yield the B antigen

proband

first case in pedigree

epistasis

- occurs when the effect of one gene masks or modifies the effect of another gene.

- Result: modified Mendelian dihybrid ratios

- H gene in blood type, E gene in lab coat

duplicate recessive epistasis

- The effects of the a and b alleles on the production of pigment is an example of duplicate recessive epistasis

- a is recessive epistatic over B, and b is recessive epistatic over A

- Need at least one dominant A and B