Gr 11 Monohybrid

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Gametes have only one allele of each gene

A Punnett grid can be used to show how the alleles of parents are split between their gametes and how new combinations of alleles can show up in their offspring.

The purpose of a Punnett grid is to show all the possible combinations of genetic information for a particular trait in a monohybrid cross. A monohybrid cross is one in which the parents have different alleles and which shows the results for only one trait.

For Example

Worked Example of Punnet Squares and Punnet Grids

Let’s consider a condition called albinism. Most animals are unaffected by albinism and have pigmented skin, hair, eyes, fur, or feathers. But some animals lack pigmentation. An individual with little or no pigmentation is called an albino. For the sake of this illustration, we will assume albinism is controlled by a single gene with two alleles. In reality, the genetics of albinism is more complex, notably because there are multiple types of albinism. However, using our simplification, A will represent the allele for pigmentation and a will represent the allele for albinism. We can trace the inheritance of albinism with a Punnett grid.

This is an Penguin with albinism

Steps to Solve a Punnet Square. (2 examples)

1 Choose a letter to show the alleles.

Use the capital and lower case versions of the letter to represent the different alleles. Usually, a capital letter represents the dominant allele and the lower case letter represents the recessive allele. For example:

• A = dominant allele, allows pigments to form
• a = recessive allele, albinism, allows few or no pigments to form.

Get used to saying ‘big A’ and ‘little a’ when reading alleles and genotypes. Also, do not mix letters: for example, you cannot use P for pigmented and a for albino. Once you have chosen a letter, write down what it means so that it is clear which allele is which.

2 Determine the parents’ genotypes.

To be sure that no possibilities are forgotten, write out all three possibilities and decide by a process of elimination which genotype or genotypes fit each parent.

The three possibilities here are:

• homozygous dominant (AA) – in this case, the phenotype shows pigmentation

• heterozygous (Aa) – in this case, the phenotype shows pigmentation but the heterozygote is a carrier of the albino allele

• homozygous recessive (aa) – in this case, the phenotype shows albinism.

The easiest genotype to determine by simply looking at a person or animal is aa. The other two are more of a challenge. To determine whether an individual is AA or Aa, we have to look for evidence that the recessive gene was received from an albino parent or was passed on to the individual’s offspring. In effect, the only way to produce an albino is for each parent to donate one a.

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