Predicting traits

Punnett squares are vital tools for understanding how genetic traits are inherited. They provide a simple way to predict the probabilities of offspring inheriting certain traits based on the genetic makeup of the parents. This knowledge is crucial in fields like medicine, where it helps assess the risk of genetic disorders, and in agriculture, where it aids in breeding for desirable traits. Use this resource to learn how Punnett squares can simplify complex genetic predictions.

When two organisms are crossed, the produce offspring that inherit genetic information from both parents. Punnett squares are diagrams used to predict the genetic outcomes of a cross.

They illustrate how alleles from each parent combine, showing the possible genotypes of their offspring. Punnett squares are helpful for visualising how traits are inherited, and for understanding the probability of offspring expressing certain traits.

Using Punnett squares

To set up and use a Punnett square:

1. Identify the parent genotypes.
2. Set up the grid by drawing a square and dividing it into four smaller squares.
3. Assign letters for the dominant and recessive alleles, if they are not already given. Write one parent's alleles across the top and the other's on the side. Each allele gets its own row or column.
4. Fill in the square. For each box, combine the allele from the top with the allele on the side. This will show the possible genotypes of the offspring.
5. Look at the completed Punnett square. Determine the possible genotypes and phenotypes of the offspring, including their ratios or probabilities.

Example

In pea plants, purple flowers \((P)\) are dominant over white flowers \((p)\). If a plant with heterozygous purple flowers \((Pp)\) is crossed with a plant with white flowers \((pp)\), what are the possible genotypes and phenotypes of the offspring?

1. Identify the parent genotypes.

One parent is heterozygous \((Pp)\) and the other is homozygous recessive \((pp)\).

2. Set up the grid by drawing a square and dividing it into four smaller squares.
3. Assign letters for the dominant and recessive alleles, if they are not already given. Write one parent's alleles across the top and the other's on the side. Each allele gets its own row or column.

The dominant allele for purple is \(P\) and the recessive allele for white is \(p\). Let's write the alleles for the heterozygous parent (\(P\) and \(p\)) across the top. We can then write the alleles for the homozygous recessive parent (\(p\) and \(p\)) down the side.

	P	p
p
p

4. Fill in the square. For each box, combine the allele from the top with the allele on the side. This will show the possible genotypes of the offspring.

	P	p
p	Pp	pp
p	Pp	pp

5. Look at the completed Punnett square. Determine the possible genotypes and phenotypes of the offspring, including their ratios or probabilities.

The genotypes are \(Pp\) and \(pp\). The phenotypes are purple \((Pp)\) and white \((pp)\). The ratios for the genotypes are \(2\,Pp:2\,pp\) or \(1:1\). The ratios for the phenotypes are \(2\) purple : \(2\) white or \(1:1\).

This means that \(50\%\) of the offspring will have purple flowers and \(50\%\) will have white flowers.

Exercise

1. Use a Punnett square to determine the genotypes and phenotypes, along with their ratios, for the following situations.
1. In mice, black fur \((B)\) is dominant over white fur \((b)\). If a heterozygous black-furred mouse \((Bb)\) is crossed with a white-furred mouse \((bb)\), what are the possible fur colours of the offspring?

   

2. In flowers, tall stems \((T)\) are dominant over short stems \((t)\). Cross a homozygous tall plant \((TT)\) with a heterozygous tall plant \((Tt)\). What are the possible stem heights of the offspring?
3. In guinea pigs, rough coat \((R)\) is dominant over smooth coat \((r)\). If two heterozygous rough-coated guinea pigs \((Rr)\) mate, what are the possible coat textures of their offspring?

   

4. In humans, having a cleft chin \((C)\) is dominant over having a smooth chin \((c)\). If one parent is homozygous for a cleft chin \((CC)\) and the other parent has a smooth chin \((cc)\), what are the possible chin types of their children?

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