Genetics - Lecture 5: Mediation Genetics 2

Mendel Dominance

• Although an individual organism possesses two different alleles for a characteristic, the trait encoded by only one of the alleles is observed in the phenotype

• With dominance, the heterozygote possesses the same phenotype as one of the homozygotes.

• Not all characteristics exhibit dominance.

• Ex: Time peas take to flower

Types of dominance

• complete

• incomplete

• Codominance

Complete dominance

Type of dominance in which the same phenotype is expressed in homozygotes (AA) and in heterozygotes (Aa); only the dominant allele is expressed in a heterozygote

Incomplete dominance

•The relationship between two alleles of a gene where the heterozygote has a phenotype intermediate between that of the two homozygotes

Codominance

Type of allelic interaction in which the heterozygote simultaneously expresses the phenotypes of both homozygotes.

• ImcomDiffers from incomplete dominance in that phenotype of the heterozygote is not an intermediate somewhere between phenotypes of the homozygotes; instead, it expresses the phenotype of both homozygotes at the same time.

Incomplete vs. Codominace

• In incomplete dominance, the offspring exhibit a blend of parental traits, neither allele fully asserting dominance over the other

• Codominance allows both alleles to manifest simultaneously without blending, leading to offspring that express both parental traits distinctly

• Determinations of dominance relationships depend on the appearance of the F1 generation.

• Dominance can be understood in regard to how the phenotype of the heterozygote relates to the phenotypes of the two homozygotes.

Penetrance

in a population, the  fraction of individuals with a particular genotype that show the associated phenotype

Incomplete penetrance

The genotype does not always produce the expected phenotype

Expressivity

the degree or intensity with which a particular genotype is expressed as a phenotype

• individuals with the genotype may show the characteristic, but to different degrees

Lethal Alleles

Allele that causes death at an early stage in development—often before birth

so that some genotypes do not appear among the progeny.

• A recessive lethal allele kills individuals that are homozygous for the allele

  • E.g., yellow coat color in mice (pleiotropy)

• A dominant lethal allele kills both heterozygotes and homozygotes.

Multiple Alleles

Presence of more than two alleles within a group

Compound heterozygous

An individual who carries two different alleles at a locus that result in a recessive phenotype

• Different alleles can give rise to the same phenotype

Ex: Cystic Fibrosis

Gene Interaction

Interaction between genes at different loci that affect the same characteristic

• Non-allelic genes

• Allelic genes

Non-allelic genes

Genes that are at different locations on a chromosome but still affect the same phenotyping

Allelic Genes

Genes that share the same locus or location on the chromosome to affect the phenotype of the individual

Gene Interaction

the effects of genes at one locus depend on the presence of genes at other loci (genes that are not allelic).

• It’s the products of genes at different loci that interact to produce new phenotypes that are not predictable from the single-locus effects alone

Epistasis

A gene interaction in which the effects of alleles at one gene hide the effects of alleles at another gene

• Epistatic Gene

• Hypostatic Gene

Epistatic Gene

The gene doing the masking

• Can be Recessive or Dominantin effect

Hypostatic Gene

The gene being masked

Recessive Epistasis

A gene interaction in which the effects of homozygous recessive alleles at one gene hide the effects of alleles at another gene

Ex: coat colour in Labrador

Dominant Epistasis

A gene interaction in which the effects of a dominant allele at one gene hide the effects of alleles at another gene.

• only a single copy of an allele is required to inhibit the expression of an allele at a different locus

Ex: fruit colour in summer squash

Duplicate Recessive

A gene interaction in which two recessive alleles at either of two different loci are capable of suppressing a phenotype

Complementation

when two mutations together result in a wild-type phenotype

• indicating that the mutations are at nonallelic genes

Complementation will occur only if the mutations are in different genes.

• Complementation will not occur if the mutations are in the same gene

Non-Complementation

when two mutations together result in a mutant phenotype

Complementation test

• To carry out a complementation test on recessive mutations (a and b), parents that are homozygous for different mutations are crossed, producing offspring that are heterozygous

• If the mutations are allelic (occur at the same locus), then the heterozygous offspring have only mutant alleles (a b) and exhibit a mutant phenotype

• If the mutations occur at different loci, each of the homozygous parents possesses wild-type genes at the other locus (aa 𝑏+𝑏+ and 𝑎+𝑎+ bb); so the heterozygous offspring inherit a mutant allele and a wild-type allele at each locus. In this case, the presence of a wild-type allele complements the mutation at each locus, and the heterozygous offspring have the wild-type phenotype