Inheritance and phenotype, at a glance

Alleles are alternative versions of a gene at a locus. Gametes carry one allele at each autosomal locus in the ordinary diploid model, and fertilisation restores a pair.

• Locus is position; allele is a version.
• Genotype is not the same as phenotype.
• Gametes transmit allele contributions between generations.

Keep in mind: An Aa heterozygote normally produces A and a gametes in equal expected proportions. Dominance does not override segregation.

The heterozygote reveals the dominance relationship. Codominance expresses both allele-associated effects; incomplete dominance gives an intermediate phenotype.

• Codominance: both effects detectable.
• Incomplete dominance: intermediate heterozygote.
• A 1:2:1 genotype ratio can give three phenotypes.

Keep in mind: The alleles remain distinct and segregate into gametes. The intermediate phenotype describes their combined expression.

List complete gametes before combining them. AaBb can produce AB, Ab, aB and ab equally when the loci assort independently.

• AaBb gametes: AB, Ab, aB, ab under independence.
• 9:3:3:1 is conditional on the cross and dominance model.

Keep in mind: A gamete must have one allele from each locus, such as AB or ab, not both alleles from just one locus.

Solve the segregation at each locus, then apply its own phenotype rule. An intermediate or codominant locus produces three distinguishable genotype classes.

• RWTt gives RT, Rt, WT and Wt gametes.
• An incomplete-dominant locus retains three phenotype categories.

Keep in mind: Alleles still segregate. The change is how genotypes appear as phenotypes, not whether gametes carry separate alleles.

A population can have more than two alleles at a locus, but an ordinary diploid individual has two. Solve that locus before combining it with a second independent locus.

• ABO has three alleles in the population.
• IA and IB are codominant; both dominate i.
• Dihybrid gametes contain an allele at each locus.

Keep in mind: An AB individual carries IA and IB. The third allele exists in the population but need not be present in that person.

For a usual X-linked recessive model, sons receive their X from the mother. Combine that inheritance with the second locus and state the denominator.

• Sex-linked alleles stay associated with their chromosome symbols.
• An X-linked recessive allele can be expressed with one copy in an XY male.
• Always specify all offspring versus one sex.

Keep in mind: In the usual XY model, a son receives Y from his father and X from his mother.

A test cross uses a homozygous recessive tester so offspring phenotypes reveal the alleles contributed by the unknown parent under the stated dominance model.

• A tester is homozygous recessive.
• AaBb x aabb gives 1:1:1:1 under independent assortment.
• Small samples can miss possible classes.

Keep in mind: A dominant tester can mask the allele contributed by the unknown parent. A homozygous recessive tester makes that contribution visible under the model.

Phenotype depends on genotype interacting with environmental conditions. Different larval nutrition can direct female honeybees towards queen or worker development.

• Honeybee female development is influenced by larval nutrition.
• Environmental effects can operate through gene expression.
• Phenotype reflects genotype-environment interaction.

Keep in mind: Diet can alter developmental signalling and gene expression without changing the DNA sequence.