The number of chromosomes is characteristic of each species and can vary enormously between species.
Sexual reproduction always includes two distinctive processes: the production of gametes, which involves meiosis, and fertilisation. The two processes are accompanied by changes in the chromosome number, from diploid to haploid and from haploid to diploid, respectively.
Genetics is based on the concept of the gene as the unit of inheritance. A particular phenotypic character is determined by the two copies of a gene that an organism possesses and these two copies are identical in a pure-breeding variety.
When organisms with contrasting characters for which they are pure-breeding are crossed, the dominant character appears in the F1 generation and the recessive character is masked. Crossing the F1 offspring gives rise to the F2 offspring with a phenotypic ratio of 3 :1 (three with the dominant phenotype : one with the recessive phenotype) and a corresponding genotypic ratio of 1: 2 :1 (one homozygous dominant: two heterozygous : one homozygous recessive). A cross of a heterozygote with a homozygous recessive individual produces offspring with a 1 :1 phenotypic and genotypic ratio (one heterozygous : one homozygous recessive).
The genotypic ratios of a cross result from the separation of the two copies of a gene to different gametes in equal numbers, and because gametes combine at random at fertilisation. The expected ratios in genetics do not tell us the actual ratios observed, but rather the most probable ratios.
The behaviour of chromosomes at meiosis explains the segregation of the two copies of a gene and the independent assortment of genes. The linkage of genes on a chromosome can be broken by means of crossing over.
Recombination – the production of new combinations of alleles – arises during meiosis from independent assortment of chromosomes and crossing over between homologous chromosomes.
The amount of crossing over between linked genes differs according to their distance apart. This observation is the basis for mapping the order of genes along a chromosome, and hence drawing up genetic (or linkage) maps.