Biology

Notes on Reproduction

Some key points about mitosis and meiosis.

Mitosis

· Mitosis is where diploid cells produce more diploid cells. This is how all the cells are reproduced, except, the gametes ( sex cells)

· The diploid cells produced by mitosis are the same

· Mitosis is how cells are copied for growth and repair

Meiosis

· Meiosis is how gametes are produced

· It is a reduction division

· The gametes are haploid, they have half the number of chromosomes.

· Meiosis occurs in two stages, meiosis 1 and meiosis 2, in meiosis 1, the homologous chromosomes are separated. This is the reduction division. The second stage is where the chromatids are separated. The result is that the gametes have 23 chromosomes.

A specific area of DNA on a chromosome that codes for a particular characteristic is called a gene. The sex cells will have one set of genes for each characteristic; some will be from the father and some from the father. However, it is chance that determines which particular set of genes are in the sperm or egg and each one will be different. When fertilization occurs, the fertilized egg will have two sets of genes for each characteristic, one from the mother and one from the father. Again it is chance which sperm reaches the egg and manages to fertilise it.

The fertilised egg will have two sets of genes for each characteristic. Which characteristic is actually shown, depends on which gene is dominant over the other. For example, the gene for brown eye colour is dominant over the gene for blue eye colour. Therefore, if the fertilised egg has one gene for blue eye colour and one for brown, the child will have brown eyes. In order for the child to have blue eyes, it must have both the recessive genes for blue eye colour. The dominant gene is represented by a capital letter and the recessive one by a lower case letter.

Genetic Crosses

This shows us the percentage chance of particular characteristics being shown. However, over a small sample these %?s may not be shown at all. An example is shown below:

Mother Father

Phenotype Brown eyes Blue eyes

(characteristics shown)

Genotype B B b b

(the genes present)

production of gametes B b

and fertilisation

B b

You can use the punnet square system:

	b	b
B	Bb	Bb
B	Bb	Bb

Genotype of offspring Bb, Bb, Bb, Bb

Phenotype 100% Heterozygous brown eyes

If the male and female are heterozygous for blue eyes, what eye colour might the offspring ?

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Biology