ACT® · Science: Background Knowledge - Life Science
Genetics
Genetics questions test whether you can turn a cross between two parents into predicted ratios of offspring, and whether you know that "dominant" describes how an allele behaves, not how common it is.
Every offspring gets one allele per gene from each parent. If a brown-eyed parent carries one brown allele (B) and one blue allele (b), they're Bb: brown shows up because B is dominant, but the b is still there, ready to pair up with another b from the other parent. Cross two Bb parents and a quarter of their children end up bb, showing the recessive blue trait, even though neither parent looked blue-eyed.
A Punnett square just organizes this. List one parent's possible alleles across the top, the other's down the side, fill in the boxes with the combinations, and count. For Aa x Aa, the boxes read AA, Aa, Aa, aa: three quarters show the dominant trait, one quarter shows the recessive one. That one-in-four ratio is the single most useful number to recognize on sight.
Two ideas sit right next to this mechanics question and the test likes to check both. First, dominant does not mean common: a dominant allele just wins the matchup whenever it's paired with a recessive one, which is a statement about one individual's two copies, not about how often that allele appears across a population. Second, the clean ratios above assume the two genes assort independently. When two genes sit close together on the same chromosome, they get inherited as a package more often than chance predicts, so a test cross shows an excess of the original parental combinations over the 50-50 split independent genes would give, with the leftover offspring being recombinants produced by crossing over.
Key terms
- Allele
- One version of a gene, inherited one copy from each parent.
- Genotype
- The actual pair of alleles an individual carries for a gene, such as Aa or aa.
- Phenotype
- The observable trait produced by a genotype, such as purple or white flowers.
- Test cross
- A cross between an individual of unknown genotype and one that is homozygous recessive, used to reveal hidden alleles or linkage.
- Linked genes
- Genes located close together on the same chromosome that tend to be inherited together unless separated by crossing over.
Watch it explained
- How it worksHow does a Punnett square turn two parents' alleles into odds for their offspring?
- Worked exampleFind the fraction of offspring showing the recessive trait when two Aa parents are crossed.
- Common mistakeWhy isn't a dominant allele the one that shows up most often in a population?
- Going furtherWhat changes about a cross when two genes sit close together on the very same chromosome?
Check yourself
End of topic.