Use alleles, genotype and phenotype, dominant and recessive, and Punnett squares to predict the genotype and phenotype ratios and probabilities of monohybrid crosses (Virginia 2018 Biology SOL BIO.5.b).

What this topic is asking

Virginia Biology SOL standard BIO.5.b states that genetic information is passed from generation to generation. The most quantitative part is using Punnett squares to predict the outcome of a cross. The Biology EOC expects you to be comfortable with alleles, genotype and phenotype, dominant and recessive, and to predict ratios and probabilities from a monohybrid cross. Many items are technology-enhanced, asking you to fill or read a Punnett square on screen.

Alleles, genotype, and phenotype

Alleles are written as letters: a capital for the dominant allele and the same letter lowercase for the recessive one. For pea-plant height, $T$ is tall (dominant) and $t$ is short (recessive). An organism with two of the same allele ($TT$ or $tt$) is homozygous; with two different alleles ($Tt$) it is heterozygous. The EOC frequently asks you to give the genotype or phenotype, so answer the one the question wants.

Dominant and recessive

This masking explains pedigrees and carriers. An organism showing a recessive trait must be homozygous recessive ($tt$); an organism showing the dominant trait could be either $TT$ or $Tt$.

Punnett squares: predicting a cross

A Punnett square sets out the alleles each parent can pass and combines them. To use one: write each parent's possible gametes (each gamete carries one allele, because alleles separate during meiosis), place one parent's gametes along the top and the other's down the side, then fill each box by combining the row and column allele. Counting the boxes gives the expected ratio and probability of each genotype and phenotype.

For a cross between two heterozygotes ($Tt \times Tt$), the four boxes are $TT$, $Tt$, $Tt$, $tt$: a genotype ratio of $1:2:1$ and a phenotype ratio of 3 tall to 1 short. Each offspring has a $\frac{3}{4}$ probability of being tall and a $\frac{1}{4}$ probability of being short. A cross of a heterozygote with a recessive ($Tt \times tt$) instead gives $Tt$, $Tt$, $tt$, $tt$: a 1:1 ratio.

Try this

Q1. A cross of $Tt \times Tt$ is carried out. State the genotype ratio and the phenotype ratio. [2]

• Cue. Genotype ratio $1\,TT : 2\,Tt : 1\,tt$; phenotype ratio 3 dominant to 1 recessive.

Q2. An organism shows a recessive trait. What must its genotype be, and why? [2]

• Cue. Homozygous recessive ($tt$), because a recessive trait appears only when no dominant allele is present.