Dominant and Recessive:
You inherit characteristics from your parents through their chromosomes. A
chromosome has small parts called genes all the way along it. The genes are made of a
chemical called DNA. Genes control the development of inherited characteristics. For
example, there are genes which control eye colour, hair colour and skin colour.
What happens to genes during fertilization:
A sperm has 23 chromosomes with genes from a man. An ovum (egg) has
23 chromosomes with genes from a woman. During fertilization the sperm
and ovum join together.
Each chromosome from the sperm then pairs up with a matching
chromosome from the ovum. This brings the two sets of genes together.
The genes for hair colour pair up, the genes for skin colour pair up, and so
on. Genes always work in pairs.
The genes in a pair may be identical. For example, they may both produce
black hair. But if one is a gene for black hair and the other a gene for
blond hair they are in competition.
Dominant and recessive genes:
The child has black hair because the gene for black hair is more powerful than the gene
for blond hair. It dominates the gene for blond hair and produces the final hair colour.
Genes which dominate other genes are called dominant genes.
Genes which are dominated are called recessive genes. When a dominant gene pairs up
with a recessive gene, the dominant one produces the final effect. However, it is still part
of the genotype and can be passed on to the next generation.
Homozygous and heterozygous:
A person with two identical genes for a characteristic is pure bred or homozygous for that
characteristic. (HH is a pure bred gene.) A person with two different genes (one dominant
and one recessive) for a characteristic is hybrid or heterozygous for that characteristic.
(Homo means the same and hetero means different.)
Questions:
1. Which parts of chromosomes control inherited characteristics?
2. What are genes made of?
3. In the gene pair Hh (for black and blond hair) which gene is dominant?
4. If E is the gene for brown eyes, and e is the gene for blue eyes, what eye colour will
the following gene pairs give?
a) EE
b) Ee c) ee
5. Look at question 4 and pick out a gene pair which is heterozygous, and a gene pair
which is homozygous.