The first true geneticist What is a gene? Alleles Phenotype and genotype Dominant and recessive phenotypes Genes in gametes Widow's peak or no widow's peak? Inheritance

"She has her mother's eyes". Some characteristics obviously "run in families" with similarities between the children and parents being quite noticeable.

It was initially thought these inherited features were passed through the blood (hence the term "blood brothers"). Later the sperm was credited with carrying all the necessary information, with the ovum simply being a food supply! Now we know that both the egg and sperm carry information about inherited features.

The first true geneticist
In the 1860s, however, the Austrian monk Gregor Mendel carried out an extensive cross-breeding program with garden peas and he concluded that the inheritance of each characteristic of an organism is determined by hereditary units (genes), which can be passed from one generation to another.

What is a gene?
A gene is actually a set of instructions in DNA (deoxyribonucleic acid) for assembling a specific protein which affects one aspect of the organism's structure, function or behaviour. Each characteristic of an organism is coded by at least one gene.

Genes are made of DNA with a specific code. Mutations in gametes slightly alter this code which changes the individual's ability to make that specific protein.

Genes are located on the chromosomes found in the nucleus of each cell.
The position of a gene along a chromosome is called the gene locus (pl. loci).

Alleles
When a mutation occurs in the DNA of a gene it may generate a new alternative form of the gene called an "allele". Many gene loci have at least two alternative forms or alleles.

Phenotype and genotype
The genotype of an individual is made up of the alleles at each locus that the individual has inherited from its parents. One copy of the gene comes from the mother's (maternal) gamete, and the other copy comes from the father's (paternal) gamete.

The phenotype is the outward expression of the gene: the appearance, behaviour, biochemical and physiological properties of an individual which is influenced by the genotype, as well as the environment. For example, sunlight (an environmental factor), can change light brown hair (genetically determined) to several shades lighter during summer.

Dominant and recessive phenotypes
Suppose a gene for coat colour in dogs has two alleles B for black and b for brown. When two different alleles at a particular gene locus occur in the one organism, in this case Bb, the genotype of this organism is described as "heterozygous" at this locus. When you look at the phenotype (appearance) of this individual it will tell you which phenotype is dominant. In this case, the heterozygous dog will always be black, so black is the dominant phenotype. The brown phenotype is called the "recessive" phenotype, as it is not seen in heterozygous individuals. An individual with two b alleles ( called "homozygous") will be brown as there is no B allele to "dominate". We talk of dominant and recessive phenotypes rather than alleles because it is the expression of the allele in the phenotype which we see.

Genes in gametes
During the formation of parents' gametes (sperm and eggs), the pairs of genes are separated so that each gamete contains only one of each gene pair. At fertilisation, when the sperm and egg nuclei meet, the first cell (zygote) has a full complement of two genes for each characteristic. These genes are found in the cell nucleus with all the others needed to determine all the characteristics of an individual. They are arranged on the chromosomes which can be seen as paired threads during cell division.

Widow's peak or no widow's peak? (An example of an inherited characteristic)
If you observe the hairline of a group of individuals, some will have a straight hairline and others will have a widow's peak or 'V' shape in the middle of their hairline. This is an inherited trait. The alleles are designated W for widow's peak and w for no widow's peak. The capital letter represents the allele for the dominant phenotype.

There are three possible genotypes for this gene locus WW, Ww and ww. WW and ww are described as homozygous. The genotypes WW and Ww will produce a widow's peak (the dominant phenotype) and the ww genotype will produce no widow's peak (the recessive phenotype).

Inheritance
If we know the genotype of the parents, we can predict the possible genotypes and phenotypes in their offspring (first generation).

Genotypes of parents		WW				ww
Phenotypes of parents		widow's peak				no widow's peak
Gametes produced	W	and	W		w	and	w
Fertilisation involves one
gamete from each parent	Ww		Ww		Ww		Ww

If two individuals both heterozygous for this locus (Ww) had children, we can predict the possible genotypes and phenotypes in the offspring.

Parental genotype	Ww				Ww
Parental phenotype	widow's peak				widow's peak
Gametes formed	W	and	w		W	and	w
At fertilisation	WW		Ww		Ww		ww
Offspring phenotype	widow's peak		widow's peak		widow's peak		no widow's peak

At each pregnancy, this couple has a 1/4 chance of having a child without a widow's peak. The chance is exactly the same for each pregnancy.

If parents are known to come from families with a history of recessive genetic disorders (e.g. Cystic Fibrosis), they may attend genetic counselling sessions. If parents are known to both be heterozygous for a recessive disorder, they will be told that at each pregnancy there is a 1/4 chance that their child will have the disorder, and a 3/4 chance it will not (but it may carry the allele and perhaps pass it on to its children).

Copyright owned by the State of Victoria (Department of Education and Early Childhood Development). Used with Permission.