Chapter 4: Modification of Mendelian RatiosAllele

*Wild-type allele*Mutant allele

Conventional symbols for alleles:recessive allele- initial letter of the name of

the recessive trait, lowercased and italicizeddominant allele- same letter in uppercase

Genetic nomenclature is extremely diverse!

Incomplete or Partial DominanceCross between parents

with contrasting traits: Red flowers or white

flowers

Offspring with an intermediate

phenotype: pink flowers

Codominance:

Example:MN Blood group- red blood cells contain a

transmembrane glycoprotein (glycophorin); two different forms of this protein exist, M and N

Examples:*Table 4.1: over 100 alleles at a given locus in

Drosophila*ABO Blood group in humans

Multiple Alleles:

*Characterized by the presence of glycoprotein antigens on the surface of red blood cells*Distinct from the M and N antigens*Also exhibits codomiance

Lethal Alleles:Example: Coat color in mice*A = agouti = wild-type allele*AY = yellow = mutant allele

Combining modified modes of inheritance:

Gene interaction:*Epistasis

Example: *In Drosophila, the recessive gene eyeless (when homozygous) prevents the expression of eye color genes present in genome*Coat color in mice

*Black (B) is dominant to brown (b)*Second gene responsible for allowing pigment to be deposited in hairC = presence, c = absence (colorless)

*Novel phenotypes due to gene interactionExample:disc-shaped fruit (AABB) X long fruit (aabb)pepper color

Genes on the X Chromosome:

*Sex chromosomes*Autosomes

Example: In Drosophila and all mammalssex chromosomes designated as X and YXX=femaleXY=male

*1909 Thomas Hunt Morgan

II III IV

XX

XY

or

*X-linkage

X-linkage in Drosophila: white mutation (eyes)

Genes on the X Chromosome con’t:

Sex-limited Inheritance:*Sex-limited trait

*holandric genes: genes on the Y chromosome

*autosomal genesExample: milk production in mammals; L=lots, l=little

*Sex-influenced trait

Sex-influenced Inheritance:

Examples:*cleft palate in humans*horns in sheep *pattern baldness in

humans

Summary:Sex-linked on X or Y sex-chromosomeSex-limited all or none expression by sexSex-influenced genotype + sex determines phenotype

Phenotypic Expression:

*Penetrance

*Expressivity

Gene expression often governed by genotype and environment

If 9/10 of individuals carrying an allele express the trait, the trait is said to be 90% penetrant

*Onset of genetic expression

*Temperature