Monohybrid Inheritance

Inheritance, genotype and phenotype

Inheritance is the transmission of genetic information from one generation to the next. During sexual reproduction, offspring receive their genetic information through the gametes of each parent. Two terms describe what that information looks like at each end of the process:

The genotype is the genetic make-up of an organism, described in terms of the alleles present. The phenotype is the observable features of an organism. Suppose a gene has a dominant allele T and a recessive allele t. The possible genotypes are TT, Tt and tt — that’s allele information. A phenotype such as “tall” is what you actually observe. The two should never be used as interchangeable answers: a question asking for a genotype wants allele symbols, and a question asking for a phenotype wants an observable characteristic, not a letter combination.

This distinction also explains why two organisms can look the same while carrying different genetic information. If T is dominant, both TT and Tt show the dominant phenotype, even though only one of them is homozygous. Observing the dominant phenotype on its own does not tell you which of those two genotypes you are looking at — a problem the test cross exists specifically to solve.

Homozygous, heterozygous and pure-breeding

An organism is homozygous for a gene when its two alleles of that gene are identical — TT or tt. An organism is heterozygous when its two alleles are different — Tt. A useful way to hold the two apart: homo means same, hetero means different.

Two identical homozygous individuals that breed together are pure-breeding for that characteristic; a heterozygous individual is not. The reason comes down to what each parent can actually pass on through its gametes. A homozygous parent such as TT can only pass a T allele for this gene — there is no alternative for it to contribute. A heterozygous parent such as Tt can pass either T or t, so its offspring do not necessarily inherit the same allele from that parent every time. Pure-breeding is a statement about one particular gene or characteristic, not a claim that two organisms are genetically identical at every gene they carry.

Dominant and recessive alleles

A dominant allele is expressed whenever it is present in the genotype. A recessive allele is expressed only when no dominant allele of that gene is present.

It’s worth stating plainly what dominant and recessive do not mean, because this is where a surprising number of otherwise-correct answers go wrong: dominant does not mean stronger, more common, better, more useful, or more likely to be inherited. It means, specifically, that the allele’s effect shows up in the phenotype whenever that allele is present. A dominant allele can be rare in a population, and a recessive allele can be extremely common — dominance describes an expression rule, not a popularity contest. Upper-case and lower-case letters are just a labelling convention for dominant and recessive; the choice of letter doesn’t create the dominance, it only represents it.

One more detail worth being exact about: in a heterozygous genotype like Tt, the recessive allele has not disappeared. It remains part of the genotype and can still be passed on to offspring — it simply isn’t producing the recessive phenotype while a dominant allele is present alongside it.

What this sets up next

This vocabulary — genotype, phenotype, homozygous, heterozygous, dominant, recessive — is what every cross and every pedigree in the rest of this chapter is built from. The next step is turning it into a method for actually predicting or interpreting inheritance: Punnett squares and genetic diagrams for predicting a cross’s outcome, and pedigree diagrams for reading inheritance backwards out of a real family.