Genetics Chapter 4

allelic

an individual who has two variants of a gene Gg

non-allelic

an individual who has two copies of the same form of a gene, GG or gg

Wild type

the phenotype of the most common form of a species as it occurs in nature

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

Loss-of function mutation

a genetic alteration that disrupts the normal function of a gene

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 - snapdragon

Incomplete penetrance

Identical known genotypes yield <100% expected phenotype

Variable expressivity

Identical known genotypes with an expressivity effect yield a range of phenotypes

<100% penetrance

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

100% penetrance

all have color but are showing it in different ways

Polydactylyl

caused by a dominant mutant allele that exhibits ~90% penetrance

Variable expressivity of the Drosophila eyeless mutation

Flies that are homozygous for the recessive eyeless mutant allele show different degrees of eye size reduction (< 800 ommatidia); wild-type phenotype, partial eye reduction, and complete eyeless

Complete penetrance

Identical known genotypes yield 100%% expected phenotype

Constant Expressivity

Identical known genotypes with no expressivity effect yield 100% expected phenotype

Penetrance

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

Expressivity

the degree or range in which a phenotype for a given trait is expressed

Pleiotropy

a condition in which a single mutation causes multiple phenotypic effects

Marfan syndrome

autosomal dominant mutation in fibrillin

Fibrillin

a gene

that codes for a connective

tissue protein present in

many tissues in the body

Essential gene

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

Recessive lethal

cause death in an organism who carries two copies of the recessive allele

Result of essential genes/recessive lethal alleles

modified Mendelian monohybrid ratios

Agouti locus in mice

A (brown) is an essential gene

A^Y (yellow) is dominant and pleiotropic mutant allele

A^Y A^Y is recessive lethal

Multiple alleles in mallards

M^R is restricted

M is mallard (wild type)

m^d is dusky

restricted is dominant over mallard and dusky

mallard is dominant over dusky

Codominance

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

The human blood groups

ABO

A and B blood antigens

carbohydrate groups that are bound to lipids on the surface of erythrocytes; I^A and I^B

What chromosome are the blood type alleles on

ABO locus of chromosome 9

What if both an I^A and I^B allele is present in an individual

the two antigens will be present simultaneously

What if an individual is I^O I^O

neither antigen is present

I^A and I^B are the _______ alleles

dominant

Type A blood

Erythrocytes with type A surface antigens and plasma with anti-B antibodies

Type B blood

Erythrocytes with type B surface antigens and plasma with anti-A antibodies

Type AB blood

Erythrocytes with type A and B surface antigens and plasma with neither anti-A nor anti-B antibodies; codominant

Type O blood

Erythrocytes with neither type A and B surface antigens and plasma with both anti-A and anti-B antibodies; recessive

I^O is a ______ allele

null

Extensions of Mendelian genetics

• multiple alleles of the same locus

• codominance of blood types

• I^O (i) is recessive to both I^A and I^B

Inheritance of human skin pigmentation

A: normal pigmentation (dominant)

a: albino (recessive)

Follows strict mendelian rules

the H Gene

codes for an enzyme that adds a fucose to the compound H precursor to yield compound H on chromosome 19

The product of the I^A gene

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

The product of the I^B gene

an enzyme that adds a terminal

galactose to compound H to yield the B antigen

The inheritance of the H locus

• H (a.k.a. the FUT1 allele) is dominant over the null allele h

• homozygous hh individuals do not produce compound H

• follows strict Mendelian rules

Bombay phenotype

a rare blood type that occurs when red blood cells (RBCs) lack the H antigen, will caue type O blood type

Besides the bombay phenotype, type O blood can still occur due to

the absence of a terminal sugar on the compound H

Proband

a person serving as the starting point for the genetic study of a family; the entire pedigree was built around that person

Epistasis

occurs when the effect of one gene masks or modifies the effect of another gene; modified Mendelian dihybrid ratios; the h allele over I^A and I^B

Recessive epistasis

The effect of h on the expression of the alleles of the ABO locus because the

h allele is recessive in causing the epistatic effect

Recessive epistasis in Labradors

Locus B and Locus E

Locus B in Labradors

black: determines the color of the pigment produced by the hair follicle cells:

• B (dominant): black

• b (recessive): brown

Locus E in Labradors

extension: affects the deposition of the pigment in the shafts of the hairs:

• E (dominant): allows the deposition of black or brown

• e (recessive): prevents dark pigment deposition =>yellow

Dominant epistasis in summer squash

• w (recessive to W) is necessary for converting the colorless

compound A into the green pigment (compound B)

• Y (dominant over y) is necessary for converting the green

pigment into the yellow pigment (compound C)

• W is dominant negative over w and dominant epistatic

over Y

Duplicate recessive epistasis in snails

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

Complementation

occurs when two parents that exhibit a recessive phenotype can yield offspring that exhibit the dominant phenotype for the character- can occur if the recessive phenotypes of the two parents are caused by recessive alleles of two different genes that affect the same character

Complementation in pigment synthesis snails

two enzymes (encoded by separate genes) are needed to produce the final product

• if either enzyme is absent: same albino phenotype (no

pathway product, only colorless intermediates)