ANSC 221 (Exam 3): G3- Extensions of Mendelian Genetics

Complete Dominance

One allele completely masks the other in heterozygotes; phenotype matches the dominant homozygote.

Incomplete Dominance

Neither allele is dominant; heterozygotes show an intermediate phenotype.

Codominance

Both alleles are fully expressed in heterozygotes, producing a phenotype that shows both traits.

Multiple Alleles

More than two allele forms exist in a population, though individuals carry only two.

Pleiotropy

A single gene affects multiple traits or phenotypes.

Albinism

A recessive trait caused by the absence or reduction of melanin due to the "a" allele.

Achondroplasia

A dominant trait causing dwarfism; individuals with one "D" allele show the phenotype.

Okazaki Fragments

Short DNA segments synthesized on the lagging strand during replication.

Agouti Gene in Mice

A gene with multiple alleles affecting coat color; dominance series: A > at > a.

Overo Lethal White Syndrome (OLWS)

A pleiotropic condition in horses where homozygous individuals are nonviable due to GI tract defects.

Sickle-Cell Disease

Caused by a mutation in the Hbβ gene; affects red blood cell shape and function.

HbβA and HbβS Alleles

HbβA is normal; HbβS causes sickling. Their interaction shows complete, incomplete, and codominance depending on context.

Carrier (Heterozygote)

An individual with two different alleles who may express a trait partially or not at all.

Phenotype Ratios in F2 Generation

For complete, incomplete, and codominance: 1:2:1 genotype and phenotype ratios.

Environmental Influence on Dominance

Traits like sickle-cell disease vary in expression depending on oxygen levels or altitude.

Degrees of Dominance

Three degrees of dominance: complete, incomplete, and codominance.

Lethal Alleles

Alleles that affect viability and phenotype.

Applied Learning: Sickle-Cell Disease

Caused by mutation in Hbβ gene; multiple dominance relationships depending on trait level.

Examples of Incomplete Dominance

Pink flowers (CRCW), Andalusian chickens.

Examples of Codominance

ABO blood types, roan coat in cattle.

Examples of Pleiotropy

Australian Shepherd coat color, OLWS in horses.