Gregor Mendel started experiments on inheritance in the 19. century. The basis of inheritance patterns is the fact that diploid cells contain two copies of the genome and that a gene has different variants, called alleles.Alleles can be dominant (for example, marked A) and recessive (for example, marked a). In a gene locus (position of a gene) there are only two alleles, one from the mother and the other from the father.If individuals have two copies of the same allele they are homozygous (AA (dominant homozygous) or aa (recessive homozygous)). If individuals have two different alleles they are heterozygous (Aa).In autosomal dominant traits, the phenotype is present if both copies of the dominant allele (A) are present (homozygous individuals AA) or only one copy of the dominant allele is present (heterozygous individuals Aa).In autosomal recessive traits, the phenotype is present if both copies of the recessive allele (a) are present (homozygous individuals aa).This is complicated in sex-linked trait, because there are two X cromosomes in females and one X chromosome and one Y chromosomes in males, so terms dominant and recessive are not reliable.It should also be taken into consideration that many traits are complex and caused by many genes interaction and/or environment conditions.
