Chapter 4: Gene interaction

How do the genes and alleles of genes interact to produce the phenotypes we see?

• more than two alleles often exist

• Dominance of one allele over another may not be complete

• A trait may be impacted by more than one or two genes

• Expression of a trait may depend on non-genie factors

Dominance determined by

• the products of alleles

Haplosufficient

heterozygous traits

Haplo

single

Haploinsufficient

homozygous dominant

• amorphsc allele is dominant

Loss of function

changes that result in a decrease or loss of activity by the gene product

• most are recessive but some can be dominant

• if one subunit is faulty → nonfunctional

Gain of Function

an allele expresses a protein with a new function or increased activity

• excessive expression of the gene product

• hypermorphic mutations

• usually dominant

Null/amorphic mutations

• mutations disables the gene product completely

• dominant gene

Leaky/hypomorphic mutations

• reduces but doesn’t eliminate the gene product

• recessive gene

Incomplete dominance

when heterozygotes display intermediate phenotypes

• both show and alleles blend their effects

Codominance

when heterozygotes display a difference phenotype than either homozygote

• both alleles show but do not blend effects

Allelic series

Any diploid individual can have only two alleles but more than two alleles are usually present in a population

lethal alleles

single gene mutations that are so detrimental as to prevent development

• low frequencies

• mostly are rapidly eliminated through natural selection

  • unless the phenotype doesn’t arise until after reproduction

  • its takes longer to get eliminated

• homozygous

Sex-limited gene expression and traits

some genes found on all autosomes and are present in both sexes but phenotypes associated with them may only be visible in one sex

• determined by hormones acting on the genes differentially

• phenotype is not limited to one sex but sex influences whether it appears or not

Penetrance

correlation between genotype and phenotype

• usually determined by gene-gene interactions or gene-environment interactions

• when phenotype matches genotype

Complete penetrance

1:1 correspondence between genotype and phenotype

Incomplete penetrance

non 1:1 correspondence

Non-penetrant

different phenotypes

Variable expressivity

the phenomenon of differing phenotypes among individuals carrying the small allele

Gene-environment interactions

environmental factors impacting the expression of genes and altering phenotype

• mender’s tall vs short plants

  • the tall allele predisposes to tall stems

• flowering time

  • there are genes that say a plant should flower when average temp reaches a certain point

Pleiotropy

multiple apparently unrelated phenotypic affects by one gene/allele

One gene-one enzyme

• each gene produces an enzyme, and each enzyme has a specific role in a biosynthetic pathway that produces the phenotype

• each mutant phenotype was attributable to the loss or malfunction of a specific enzyme

• Each enzyme defect was inherited as a single gene defect

• Prototroph: wildtype

• auxotroph: mutant

Epistasis

gene contributing to different steps of a multistep pathway work together for an end product

• Null hypothesis = no epistasis

Complementary gene interaction

gene work together to yield a phenotype

• gene isn’t functional

• phenotype is altered

Duplicate gene action

two genes do the exact same job and produce redundancy

• at least one functional allele

Dominant gene interaction

two dominant alleles have similar effect when separate but a different effect when expressed together

• if neither is present, a third phenotype results

Recessive epistasis

Recessive alleles from one gene prevent phenotype of dominant alleles at another

Dominant epistasis

dominant alleles from one gene to prevent phenotype of any alleles at another

Dominant suppression

dominant allele from one gene prevents phenotype of dominant allele at another