Genes determine most physical characteristics in a person, including gender (sex), blood type, eye colour and height. Many health conditions or diseases are also genetic. Genes are paired. In each pair, one gene is inherited from the mother and the other from the father. Some genes are dominant and others are recessive.
Traits or distinguishing characteristics are passed on from parents to their children through their genes.
The estimated 23,000 genes that make up a human being are arranged 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.
Many health conditions and diseases are genetic. Behavioural characteristics, such as intelligence and natural talents, may also be influenced in some way by genes.
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, both essential to the growth and maintenance of the human body) is contained in the letter sequence of each gene. .
The genes are copied ‘letter for letter’ to a similar substance called ribonucleic acid (RNA). The RNA is read by the working parts of the cell to create the protein or hormone according to the instructions. Each gene codes the instruction for a single protein only, but one protein may have many different roles in the human body. One characteristic, such as eye colour, may be influenced by many genes.
One set of genes is inherited from each parent
The 23,000 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 (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. As well as determining gender, the sex chromosomes carry genes that control other body functions.
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 (pronounced ‘al-eel’). Different alleles of the gene pair (one from each parent) can be inherited in different ways.
Dominant and recessive blood group inheritance
Dominant inheritance is when one 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 A allele and O allele has the blood group A O. Another way of saying this is that the O group is recessive – a person needs two O alleles 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 it to represent inheritance patterns. The Punnett Square is used in the following section to explain blood group inheritance.
Mother AO and father OO
If the mother has an A allele and an O allele (AO), 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 represents this information below. The paired alleles show the possible variations in the genes of their offspring.
Mother AO and father AA
If the mother has an A allele and an O allele (AO), her blood group will be A because the A is dominant. The father has two A alleles (AA), 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 below contains the paired alleles that show the possible variations in the genes of their offspring
The AO children in the 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.
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 terms commonly used in 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 with blood group A and AO alleles 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
· Victorian Clinical Genetics Services (VCGS), Royal Children’s Hospital Tel. (03) 8341 6201
Things to remember
· Genes are the means by which children inherit physical characteristics from their parents.
· Alleles are different forms of the same gene.
· In 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.
You might also be interested in:
- Genes and genetics.
- Genes and genetics - related parents.
- Genetic disorders.
- Genetic services in Victoria.
- Genetically modified (GM) foods.
Want to know more?
Go to More information for support groups, related links and references.
This page has been produced in consultation with and approved by:
(Logo links to further information)
Victorian Clinical Genetics Services (VCGS)
Last reviewed: October 2012
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