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Copying genes - meiosis

 
Genetics - Introduction
Cellular processes
Reading the instructions
Copying genes - Mitosis
Copying genes - Meiosis
Meiosis and Fertilisation
The sex chromosomes
Is there a gene for it?
Inheritance
Genes and personality
Non-Mendelian Inheritance
Epigenesis
The Maternal Effect

 
The beginning of sex in the world came with two organisms, usually referred to as male and female, merging their genetic material in a process called meiosis. Why this should have happened, and in the way that it did, is a whole debate on its own.

In all the complex organisms, such as humans, the genes are contained on numbers of separate strands of DNA, the chromosomes. These are arranged in pairs, one half of each from the father, one half from the mother, in humans, 46 pairs.

While most chromosomes pairs are similar, and known as the autosomes, the sex chromosomes differ. In humans, there are 44 autosomes and two sex chromosomes known as X and Y. Females have a pair of X chromosomes, males have one X and one Y.

We are trying to avoid long words but they are referred to as homozygous and heterozygous respectively. It isn't a universal rule. With birds, for instance, it is the female that is heterozygous, ZW, and the male homozygous, ZZ. In other words, the biological definition of a female is that she produces eggs, the male produces sperm.

In meiosis, in each parent, DNA is split to form the sperm or the egg. In each of the sex cells, the sperm or the egg, known as gametes, therefore, there is only one set of chromosomes instead of a set of pairs. In humans, the egg carries twenty three chromosomes. One will always be an X chromosome, for the female has no Y chromosomes. The sperm also contains twenty three chromosomes, but one of them will be either X or Y. Individuals inherit half of their chromosomes from their mothers, half from their fathers.

At fertilisation, the sperm and the egg combine to form a complete new double spiral of DNA. The two sets of chromosomes join and, once again, there are forty six chromosomes, producing a child that is different from either the man or the woman, yet carrying on something from each of them. But one pair of its chromosomes will either be XX, if the child is female, or XY, if the child is male.

One might expect the paternal half of the DNA to be either from his father or mother, and the maternal to be either from her mother or her father. half of the DNA. However a process called recombination occurs.

In both the father and mother, during the process whereby the DNA is split to form the sperm or egg, material is exchanged between the two parts. In other words, there will be genes from the maternal grandmother, the maternal grandfather, the paternal grandmother and the maternal grandfather.

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Citation:
Bland, J., (2003) Copying Genes - meiosis, http://www.gender.org.uk/about/03gene/33_copyb.htm
 
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Derby TV/TS Group. Text copyright Jed Bland.
01.01.99 Last amended 04.06.03, 24.11.03