Describe patterns of inheritance beyond simple dominance (incomplete dominance, codominance, multiple alleles, and sex-linked traits) and interpret pedigrees (Virginia 2018 Biology SOL BIO.5.b).

What this topic is asking

Virginia Biology SOL standard BIO.5.b covers how genetic information is passed on, including patterns beyond simple dominance. The Biology EOC expects you to recognize incomplete dominance, codominance, multiple alleles (such as the ABO blood groups), and sex-linked traits, and to interpret pedigrees. These patterns extend the basic Punnett-square reasoning to the more realistic ways traits are inherited.

Incomplete dominance

The giveaway for incomplete dominance is a new, in-between phenotype in the heterozygote that is a blend of the two parents.

Codominance

In codominance, both alleles are fully and separately expressed in the heterozygote, rather than blended. An example is a chicken with both black and white feathers (speckled), or the AB blood type, where a person with one A allele and one B allele expresses both A and B markers. The contrast with incomplete dominance matters: incomplete dominance is a blend (pink), while codominance shows both distinct traits at once (black and white, or A and B).

Multiple alleles and blood type

In the ABO system, $I^A$ and $I^B$ are codominant to each other and both dominant over $i$. So genotype $I^A I^A$ or $I^A i$ is type A, $I^B I^B$ or $I^B i$ is type B, $I^A I^B$ is type AB (codominance), and $ii$ is type O. This example combines codominance and multiple alleles, and the EOC may ask you to work out blood-type crosses.

Sex-linked traits

Sex-linked traits are carried on the sex chromosomes, usually the X chromosome. Females are typically XX and males XY. Because males have only one X, a single recessive allele on the X is expressed in males with no second X to mask it, so X-linked recessive conditions (such as red-green color blindness and hemophilia) appear more often in males. Females are more often unaffected carriers. The EOC tests this by asking why a trait appears more in one sex.

Reading pedigrees

A pedigree is a diagram that tracks a trait through a family, using squares for males and circles for females, with shaded shapes for affected individuals. The key reasoning: if two unaffected parents have an affected child, the trait is recessive and both parents are heterozygous carriers. If an affected child has a parent who is also affected in every generation, the trait is more likely dominant. Pedigrees turn the inheritance rules into a puzzle to solve from the diagram.

Try this

Q1. Explain the difference between incomplete dominance and codominance. [2]

• Cue. Incomplete dominance gives a blended, intermediate heterozygote (pink); codominance shows both alleles fully and separately (black and white, or A and B).

Q2. Explain why X-linked recessive conditions are more common in males. [2]

• Cue. Males have only one X chromosome, so a single recessive allele on the X is expressed with no second X to mask it; females need two copies to be affected.