Use Punnett squares and the laws of segregation and dominance to predict the genotypes and phenotypes of offspring from a monohybrid cross (Ohio's Learning Standards for Science, Biology, B.H.2).

What this topic is asking

Ohio standard B.H.2 states that "in sexual reproduction, offspring receive some genetic information from each parent." Ohio's Biology EOC turns this into items where you predict offspring using a Punnett square and Mendel's rules. The crosscutting idea is patterns: inheritance follows predictable probability patterns. Because the standards ask you to use mathematics, expect to calculate genotype and phenotype ratios and probabilities. Make sure you are fluent with the vocabulary from chromosomes, genes, and alleles first.

Mendel's key ideas

The Austrian monk Gregor Mendel worked out the basic rules of inheritance by breeding pea plants. Two of his ideas are essential for the EOC.

• Law of segregation. Each organism carries two alleles for a gene (one on each homologous chromosome), and these separate during meiosis so each gamete gets only one. At fertilization, the offspring gets one allele from each parent.
• Dominance. When the two alleles differ, a dominant allele masks a recessive one. The dominant allele's phenotype shows whenever at least one copy is present (homozygous dominant or heterozygous); the recessive phenotype shows only when both alleles are recessive (homozygous recessive).

By convention, the dominant allele is a capital letter (T) and the recessive allele the same letter in lower case (t).

Setting up a Punnett square

A Punnett square is a grid that shows every way the parents' alleles can combine in the offspring. The steps are always the same.

1. Write each parent's genotype (for example $Tt$ and $Tt$).
2. Find the gametes each parent can make. A $Tt$ parent makes two kinds of gamete: $T$ and $t$.
3. Put one parent's gametes across the top and the other's down the side.
4. Fill each box by combining the allele from its row and its column.
5. Count the genotypes and phenotypes to get the ratios.

A worked monohybrid cross

Take $Tt \times Tt$ (two heterozygous tall pea plants), with $T$ (tall) dominant over $t$ (short).

Reading the four boxes:

• Genotypes: one $TT$, two $Tt$, one $tt$, a ratio of $1 : 2 : 1$.
• Phenotypes: three plants have at least one $T$ (tall), one plant is $tt$ (short), a ratio of $3 : 1$.

So this classic cross predicts 3 tall : 1 short.

Ratios are probabilities

A Punnett square gives the probability of each outcome, not a guarantee. A $3 : 1$ ratio means each offspring has a $3/4$ chance of being tall and a $1/4$ chance of being short. With only four offspring you might not get exactly three tall and one short, just as four coin flips need not give exactly two heads. The ratio becomes accurate over large numbers of offspring. The EOC may ask for a probability ("what is the chance the next offspring is short?") rather than a whole ratio.

Try this

Q1. State Mendel's law of segregation. [1]

• Cue. Each organism's two alleles for a gene separate during meiosis so that each gamete receives only one allele.

Q2. Two heterozygous plants ($Rr$) for a dominant red flower color are crossed. State the expected phenotype ratio of the offspring. [1]

• Cue. 3 red : 1 white.