Apply concepts of statistics and probability, using Punnett squares, to explain the variation and distribution of expressed traits from a genetic cross (Louisiana Student Standards for Science, High School Biology, HS-LS3-3).

What this topic is asking

Louisiana's LS3 standards (HS-LS3-3) ask you to apply statistics and probability to predict the outcome of a genetic cross. For LEAP 2025 Biology that means being comfortable with alleles, genotype and phenotype, dominant and recessive, and using Punnett squares to predict ratios and probabilities from a monohybrid cross. Because the standard is explicitly about probability, the test rewards stating outcomes as a probability (a fraction or percent), not just as a count, and some items are technology-enhanced drag-and-drop Punnett squares.

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 test frequently asks you to give the genotype or the phenotype, so read which the question wants.

Dominant and recessive

This masking explains carriers and pedigrees. 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 and probability

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 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, a $\frac{1}{2}$ probability of each phenotype.

Why this is a statistics standard

The Louisiana standard frames this as probability on purpose: a Punnett square does not promise that exactly three of every four offspring will be tall. It gives the probability for each offspring ($\frac{3}{4}$ tall), the same way a coin gives a $\frac{1}{2}$ chance of heads on each flip. Over many offspring the actual numbers approach the predicted ratio, but each individual outcome is a matter of chance.

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.