Use Mendel's laws of segregation and independent assortment, with Punnett squares, to analyze patterns of inheritance and predict the genotype and phenotype ratios of monohybrid crosses (NGSSS SC.912.L.16.1; Reporting Category 2, Classification, Heredity, and Evolution).

What this topic is asking

The NGSSS benchmark SC.912.L.16.1 asks you to use Mendel's laws and Punnett squares to analyze patterns of inheritance. For the Florida Biology 1 EOC this is the most quantitative genetics topic, so you must be comfortable with alleles, genotype and phenotype, the meaning of dominant and recessive, and predicting ratios and probabilities from a cross. Items almost always involve setting up or reading a Punnett square.

Alleles, genotype, and phenotype

Alleles are written as letters: a capital for the dominant allele and the same letter in lowercase for the recessive allele. 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

Mendel's two laws

Gregor Mendel's pea-plant experiments gave two laws the EOC expects you to know:

• Law of segregation. The two alleles for a gene separate during gamete formation (meiosis), so each gamete carries only one allele for each gene.
• Law of independent assortment. Alleles for different genes are sorted into gametes independently of one another, so the inheritance of one trait does not affect another (for genes on different chromosomes).

These laws are why a Punnett square works: segregation tells you each parent passes one allele, and you combine them to predict the offspring.

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 alleles, place one parent's possible 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 heterozygous tall plants ($Tt \times Tt$), 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. 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. 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.