Genetics is the study of how genetically controlled characteristics are transferred from one generation to another i.e. from parents to offspring. There are two classes of characteristics – inheritable and non-inheritable. Inheritable characteristics are those that are controlled by genes. Non-inheritable characteristics are not controlled by genes but determined by the environment or learned.
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Chromosomes are microscopic thread-like structures that, due to special staining, show up in the nucleus of a cell during cell division. The chromosomes carry the genes.
Normal tissue cells of humans contain 46 chromosomes; one set of 23 received from the mother’s egg cell and a second set of 23 from the father’s sperm cell at fertilisation. Therefore there are 23 pairs of chromosomes in the nucleus of a normal human tissue cell. As a result there are two genes controlling each characteristic – one gene from each set of chromosomes, a maternal and a paternal gene. This is why children show some characteristics of their mother and some of their father. This double set of information has many advantages for survival and evolution.
`A chromosome is composed of two major chemicals, DNA and protein.
The genes are specific sections of the DNA. DNA = deoxyribonucleic acid – there is about 2.5 metres of DNA in a human tissue cell.
Genes A gene is a physical unit of inheritance. A gene has the information for a particular inheritable characteristic.
A normal human tissue cell has about 33,000 different genes. There are 23 different chromosomes so on average there are 1,400 genes per chromosome. There is a unique sequence of genes along each chromosome and two different chromosomes never have any genes in common. A particular gene has a specific address – it is only present on one specific chromosome at a particular location on that chromosome.
Tissue Cells and Chromosomes
All the tissue cells are genetically identical. The tissue cells are produced by a special type of cell division that copies and allocates the chromosomes perfectly producing two daughter cells genetically identical to the original zygote. Each tissue cell has all the genetic information needed to produce a new individual and this is how the cloning of plants and animals has been possible.
Gametes and Chromosome Number
Gametes are sex cells that contain only one set of chromosomes and at fertilisation produce a zygote cell with two sets of chromosomes. Gametes are produced in the ovaries and testes by a special type of cell division that reduces the number of chromosomes from two sets to one set per daughter cell.
This reduction division is essential for sexual reproduction to occur without changing the normal chromosome number of tissue cells.