59 Alleles

Key Idea: Eukaryotes generally have paired chromosomes. Each chromosome contains many genes, and each gene may have a number of versions called alleles.

Homologous chromosomes

` In most sexually reproducing organisms, the cells of the body (not gametic cells) contain two complete sets of chromosomes (called diploid) and the chromosomes are found in pairs. Each parent contributes one chromosome to the pair. ` The pairs are called homologues or homologous pairs. Each homologue carries an identical collection of genes, but the version of the gene (the allele) from each parent may differ. This diagram shows the position on the same chromosome of three different genes (A, B and C) controlling three different traits (features).

Having two different versions of gene A is a heterozygous condition. Only the dominant allele (A) will be expressed.

When both chromosomes have identical copies of the dominant allele for gene B the organism is homozygous dominant for that gene.

When both chromosomes have identical copies of the recessive allele for gene C the organism is said to be homozygous recessive for that gene.

Maternal chromosome originating from the egg of the female parent.

a a A

BB

c c

5 4 3

1 4

3

1 5 2

2

The diagram above shows the complete chromosome complement for a hypothetical organism. It has a total diploid (2N) number of ten chromosomes, as five, nearly identical pairs (each pair is numbered).

Genes occupying the same position or locus on a chromosome code for the same characteristic (e.g. hair length).

Paternal chromosome originating from the sperm of the male parent.

1. Define the following terms describing the allele combinations of a gene in a diploid sexually reproducing organism:

(a) Heterozygous:

(b) Homozygous dominant:

(c) Homozygous recessive:

2. For a gene given the symbol ‘A’, identify the alleles present in an organism that is:

(a) Heterozygous: (b) Homozygous dominant:

3. What is a homologous pair of chromosomes? (c) Homozygous recessive: