Q1. Where are chromosomes found?

Q2. Which are larger: chromosomes or genes?

Q3. What is the function of genes?

The nucleus

genes

The chromosomes that occur in our nucleus consist of 23 pairs. Along each chromosome are sections called genes.

These genes give a chemical potential for a characteristic to be expressed. For example eye colour.

Since one chromosome for each pair comes from each parent, we get one half of our genes for a characteristic from each parent.

Each gene can come in a variety of forms eg a gene for eye colour could be for a blue eye or for a brown eye. These different versions of a gene are called alleles.

Sometimes a pair of genes will be a matching pair, and we call this a homologous or pure bred pair.

Sometimes there will be two different alleles in a pair, and this is called a heterozygous or hybrid pair.

The chemical strength of these alleles will vary. If an allele is stronger it is called dominant, and if it is weaker it is called recessive.

We represent the dominant allele with a capital letter, eg R for the allele that gives the ability to roll your tongue, and the recessive gene is represented by a lower case letter, eg r for the allele that disables tongue rolling.

The way a gene pair is expressed eg the actual colour of the eyes, is called the phenotype, and the actual alleles in the pair is called the genotype.

Example 1:

We know that the gene for brown eyes is dominant to the gene for blue eyes. We represent these alleles as:

B for the brown eyed allele, and

b for the blue eyed allele.

Q4. What would be the phenotypes for the following gene pairs (genotypes)?BB, bb, and Bd.

A punnet square allows a geneticist to predict the possible genotypes and phenotypes of offspring.

For example if we wanted to know what types of offspring could result from a cross between two heterozygous yellow (Yy) pea plants. (ie Y represents the allele for a yellow seed and y represents the allele for a green seed.

1 Make the grid Place the alleles of one parent along the top of the grid, and those of the other parent down the left hand side.

2 Fill the grid Combine the parent alleles inside the boxes. The letter pairs show the genotypes of the offspring.

In this example the genotype ratio is 1:2:1, meaning 1 YY, for every 2 Yy for every 1 yy.

3 Fill in the offspring Use the law of dominance to work out the phenotypes and the phenotype ratio of the offspring.

In this example the phenotype ratio is 3:1, meaning 3 yellow peas for every 1green pea.