Inheritance.

As only the father can pass on either an X or Y chromosome, they are responsible for determining the sex of the child

The inheritance of gender can be shown using a genetic diagram ( known as a Punnett square), with the X and Y chromosomes taking the place of the alleles usually written in the boxes

Most body cells have two copies of each chromosome We describe these cells as diploid

When cells divide their chromosomes double beforehand This ensures that when the cell splits in two, each new cell still has two copies of each chromosome (is still diploid)

This type of cell division is used for growth and repair in the body and is known as mitosis

Process:

Just before mitosis, each chromosome in the nucleus copies itself exactly (forms x – shaped chromosomes)

Chromosomes line up along the centre of the cell where cell fibers pull them apart

The cell divides into two; each new cell has a copy of each of the chromosomes

All cells in the body (excluding gametes) are produced by mitosis of the zygote

Mitosis is important for replacing cells e.g, skin cells, red blood cells and for allowing growth (production of new cells e.g. when a zygote divides to form an embryo)

Growth: mitosis produces new cells

Repair: to replace damaged or dead cells

Asexual reproduction: mitosis produces offspring that are genetically identical to the parent

The number of chromosomes must be halved when the gametes (sex cells) are formed

Otherwise there would be double the number of chromosomes after they join at fertilisation in the zygote (fertilized egg)

This halving occurs during a type of cell division called meiosis

It starts with chromosomes doubling themselves as in mitosis and lining up in the centre of the cell

After this has happened the cells divide twice so that only one copy of each chromosome passes to each gamete

We describe gametes as being haploid – having half the normal number of chromosomes

Because of this double division, meiosis produces four haploid cells

Each chromosome makes identical copies of itself (forming X-shaped chromosomes)

First division: chromosomes pair up along the centre of the cell, recombination occurs and then cell fibres will pull the pairs apart, each new cell will have one of each recombinant chromosome pair 

Second division: chromosomes will line up along the centre of the cell, cell fibres will pull them apart (as with mitosis)

A total of four haploid daughter cells will be produced

Production of gametes e.g. sperm cells and egg cells, pollen grains and ovum

Increases genetic variation of offspring

Meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made, meaning that when gametes fuse randomly at fertilisation, each offspring will be different from any others