Genes and genetics - inheritance

Traits or distinguishing characteristics are passed on from parents to their children through their genes.

The estimated 30,000 genes that make up a human being are beaded along tightly bundled strands of a chemical substance called deoxyribonucleic acid (DNA). These DNA strands are tightly packed into structures called chromosomes.

There are 23 pairs of chromosomes in almost every human cell, stored within a sac called the nucleus. Chromosomes carry all the genes that determine physical characteristics such as height and eye colour. Behavioural characteristics, such as intelligence and natural talents, may also be influenced in some way by genes.

The genes are like an instruction book
A DNA strand looks like a twisted ladder. The genes are a series of ‘letters’ strung along each rung. These letters are used like an instruction book. Information on building specific molecules (such as proteins or hormones) is contained in the letter sequence of each gene.

The genes are copied ‘letter for letter’ to another similar substance called RNA (ribonucleic acid). The RNA is read by the working parts of the cell to create the protein or hormone according to the instructions. Each gene usually codes the instruction for a single characteristic only, such as eye colour, but many genes may influence the same characteristic.

One set of genes is inherited from each parent
The 30,000 or so genes that make up each person come in pairs. One gene in each pair is inherited from the mother and the other from the father. A sperm and an egg each contain one copy of every gene needed to make up an individual (or one set of 23 chromosomes each). When the sperm fertilises the egg, two copies of each gene are present (46 chromosomes) and new life can begin.

The chromosomes that decide gender are called sex chromosomes. The mother’s egg always contributes an X, while the father’s sperm provides either an X or a Y. An XX pairing means the child is female, while an XY pairing means the child is male.

How characteristics are inherited
Characteristics can be inherited in many different ways. One characteristic can have many different forms – for example, blood type can be A, B, AB or O. Each of these different forms of a characteristic is caused by variations in the gene for that characteristic. Each variation of a gene is called an allele. Different forms (alleles) of the one gene (one from each parent) can be inherited in different ways.

Dominant and recessive inheritance
Dominant inheritance is when one form (allele) of a gene is dominant within the pair. For blood groups, the A allele is dominant over the O allele. So a person with one copy of the A allele and one copy of the O allele has the blood group A, because A is dominant over O. Another way of saying this is that the O group is recessive – a person needs two copies of O to have the blood group O.

So a child may have blood group A because the ‘blood group A’ gene inherited from its mother is dominant over the ‘blood group O’ gene inherited from its father.

This concept can be explained with the Punnett Square, named after the mathematician and biologist R C Punnett, who created a graph to represent inheritance patterns. Below it is used to look at blood group inheritance.

Mother AO and father OO
If the mother has an A allele and an O allele, her blood group will be A because the A is dominant. The father has two O alleles (OO), so he has the blood group O. Each one of their children has a 50 per cent chance of having blood group A (AO) and a 50 per cent chance of having blood group O (OO), depending on which alleles they inherit.

The Punnett Square would represent this information in the following way. The paired alleles show the possible variations in the genes of their offspring.

		Father
Mother		O	O
	A	AO	AO
	O	OO	OO

Mother AO and father AA
If the mother has an A allele and an O allele, her blood group will be A because the A is dominant. The father has two A alleles, so his blood group is also A. Their children will be either AO or AA but, either way, each one will have blood group A because the A allele is dominant. The Punnett Square would represent this information in the following way – the paired alleles show the possible variations in the genes of their offspring.

		Father
Mother		A	A
	A	AA	AA
	O	AO	AO

The AO children in the second case above will have a 50 per cent chance of passing on either the A or the O allele to their own offspring. This explains, for example, why parents with blood group A (if they are both AO) can produce a child with blood group O.

Co-dominant genes
Not all genes are either dominant or recessive. Sometimes, each allele in the gene pair carries equal weight and will show up as a combined physical characteristic. For example, with blood groups, the A allele is as ‘strong’ as the B allele. So someone with one copy of A and one copy of B has the blood group AB.

Genotype and phenotype
Genotype and phenotype are popular terms in the science of genetics. Continuing the example of blood groups, a person with the alleles AO will have the blood group A. The observable trait – blood group – is known as the phenotype.

The term ‘genotype’ refers to the genes that produce the observable trait. So the person has the blood group A phenotype, but the AO genotype.

Where to get help

• Your doctor
• The Murdoch Children’s Research Institute Tel. (03) 8341 6200
• Genetic Health Services Victoria – Genetic Clinical Coordinator, Royal Children’s Hospital Tel. (03) 8341 6201
• Genetic Health Services Victoria – Genetic Clinical Coordinator, Royal Women’s Hospital Tel. (03) 9344 2121
• Genetic Health Services Victoria – Monash Medical Centre Tel. (03) 9594 2026

• When a sperm fertilises an egg, their two sets of genes combine into a paired set of 46 chromosomes.
• Alleles are different forms of the same gene.
• Out of a gene pair, one gene is often dominant over the other – for example, the allele for blood group A is dominant over the allele for blood group O.

This page has been produced in consultation with and approved by:


(Logo links to further information)