Monohybrid inheritance

To describe the genotype $\text{aa}$ in words, both words are required. "Homozygous" alone says there are two identical alleles but not which one; "recessive" alone says which allele but not that there are two copies. Mark schemes award the description only for the full term homozygous recessive — and this is the one genotype that shows the recessive phenotype.

A genotype is the pair of alleles in a body cell, so it is always written with two letters: $\text{AA}$, $\text{Aa}$ or $\text{aa}$. A single letter such as T or t is a gamete or an allele, not a genotype. Candidates lose the mark by answering with one letter, so read the command: "give the genotype" wants two letters, "give the gamete" wants one.

Label the gene with a single letter: upper case for the dominant allele and the same letter in lower case for the recessive one ($\text{A}$/$\text{a}$, $\text{T}$/$\text{t}$). Invented symbols such as P and Q, or X and Y — which mean the sex chromosomes — are not credited. Choose a letter whose upper- and lower-case shapes differ, or the examiner cannot tell them apart.

Gametes are haploid, so each gamete circle holds a single allele. A heterozygous $\text{Aa}$ parent makes two kinds of gamete, A and a, in separate circles — writing Aa or aA inside one gamete is wrong, and so is copying the whole genotype AA. Wrong gametes make the Punnett grid impossible to complete, so the offspring and ratio marks are forfeited too.

If one parent is homozygous dominant ($\text{AA}$), every gamete it makes carries A, so every offspring shows the dominant phenotype: 0% recessive, 100% dominant. Candidates report these the wrong way round, or default to 25%/50% out of habit. Count the boxes matching the target phenotype and convert — 0 of 4 = 0%, 1 = 25%, 2 = 50%, 3 = 75%, 4 = 100%.

A recessive phenotype needs one recessive allele from each parent, so a homozygous dominant parent can never produce homozygous recessive offspring, whatever the other parent is. Candidates asked whether a parent could be heterozygous often miss this. For recessive offspring to appear at all, both parents must carry a recessive allele — each is $\text{Aa}$ or $\text{aa}$.

A gene is a length of DNA that codes for a protein; an allele is one alternative form (version) of that gene. The gene for flower colour has a purple allele and a white allele. Examiners report both terms being defined poorly or used interchangeably, so always define the gene first, then the allele as a version of it — never the other way round.