3.8 Inheritance patterns beyond dominant/recessive traits

How does gene expression differ from phenotypic expression?

Gene expression is the molecular process of making a protein from a gene (transcription and translation). Phenotypic expression is the resulting visible trait. Often, both dominant and recessive alleles are expressed at the molecular level, but the dominant protein's function masks the recessive one's effect in the phenotype.

Define incomplete dominance.

Heterozygotes display an intermediate phenotype that is a blend or mix of the two homozygous traits. Example: Red X White snapdragons produce Pink.

What are the genotypic and phenotypic ratios for a cross between two heterozygotes exhibiting incomplete dominance?

The ratios are the same: 1:2:1 (e.g., 1 Red : 2 Pink : 1 White). This is a key way to identify incomplete dominance.

Define co-dominance.

Heterozygotes display both phenotypes associated with each allele equally and simultaneously. Example: AB blood type, where both A and B sugars are present.

What are the genotypic and phenotypic ratios for a cross between two heterozygotes exhibiting co-dominance?

The ratios are the same: 1:2:1 (e.g., 1 A : 2 AB : 1 B). This is a key way to identify co-dominance.

How can you differentiate between incomplete and co-dominance based on the heterozygote?

Incomplete: Heterozygote phenotype is a blend or new intermediate. Co-dominance: Heterozygote phenotype shows both original traits clearly (not blended).

What is multiple allelism?

When a single gene has more than two alleles circulating within a population. Example: Human ABO blood type (with A, B, and O alleles).

What are quantitative traits?

Traits that show a continuous range of variation in the population (e.g., height, weight) rather than falling into discrete classes. They are typically controlled by many genes.

What is polygenic inheritance?

An inheritance pattern where a single trait is controlled by the additive effects of multiple genes (more than two genes). Quantitative traits are usually polygenic.

Explain gene-by-gene interactions (Epistasis).

The phenotype produced by an allele at one gene is modified or masked by the allele(s) present at a different gene. This changes the expected Mendelian phenotypic ratios (e.g., from 9:3:3:1 to something different).

Explain gene-by-environment interactions.

The final phenotype is determined by the interaction between an organism's alleles (genotype) and the non-genetic environmental conditions. Example: Genetic potential for height is realized only with good childhood nutrition.

Why are simple Mendelian patterns rare?

Most traits are complex, meaning they are influenced by multiple genes (polygenic), multiple alleles, gene-by-gene interactions, and gene-by-environment interactions, making the simple 3:1 and 9:3:3:1 ratios uncommon.

How do patterns of inheritance differ based on the number of genes involved?

One gene: Simple dominant/recessive, incomplete, co-dominance, or multiple allelism. Two or more genes: Gene-by-gene interactions (epistasis). Many genes: Polygenic inheritance (resulting in quantitative traits).