Use the rules of inheritance (dominant and recessive alleles, genotype and phenotype) and Punnett squares to predict the outcomes of genetic crosses, and interpret pedigrees (NYSSLS LS3, patterns; using mathematics).

What this topic is asking

NYSSLS LS3 wants you to predict how traits pass from parents to offspring using the rules of inheritance and a Punnett square, and to read a pedigree. On the Life Science: Biology Regents this is one of the most quantitative topics, so the science practice of using mathematics (ratios and probabilities) is central, alongside the crosscutting concept of patterns.

Alleles, genotype and phenotype

Alleles are often written as letters: a capital for the dominant allele and the same letter in lower case for the recessive allele. So for height in pea plants, $T$ 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.

Dominant and recessive

This masking is why a recessive trait can skip a generation: carriers ($Tt$) show the dominant trait but can still pass the recessive allele to offspring. Recognizing carriers is a frequent pedigree task.

Punnett squares: predicting a cross

A Punnett square sets out the alleles each parent can pass and combines them to show the possible offspring. To use one: write each parent's alleles, place one parent's 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 the probability of each genotype and phenotype.

For example, a cross between two heterozygous tall plants ($Tt \times Tt$) gives genotypes $TT$, $Tt$, $Tt$, $tt$, a genotype ratio of $1:2:1$ and a phenotype ratio of 3 tall to 1 short. Each offspring therefore has a $\frac{3}{4}$ probability of being tall and a $\frac{1}{4}$ probability of being short.

Reading a pedigree

A pedigree is a chart showing how a trait passes through a family, with squares for males, circles for females, and shading for individuals showing the trait. By applying the rules of dominance you can work out genotypes and predict the chance that future children inherit a trait. A useful clue: if two unaffected parents have an affected child, the trait must be recessive (both parents are carriers).

Try this

Q1. Define genotype and phenotype. [2]

• Cue. Genotype is the alleles an organism has (for example $Tt$); phenotype is the observable trait that results (for example tall).

Q2. Two heterozygous black guinea pigs ($Bb$) are crossed. State the expected ratio of black to white offspring. [2]

• Cue. $Bb \times Bb$ gives 3 black to 1 white (genotypes $1\ BB : 2\ Bb : 1\ bb$).