Mendel's first law is the law of segregation. It states that each characteristic is controlled by a pair of alleles, and that the two alleles separate (segregate) during gamete formation, so each gamete receives only one allele of each gene. At fertilisation, the pair is restored, one allele coming from each parent.Mendel proposed this from his results before anyone knew about chromosomes or meiosis. We now know the physical explanation. The two alleles of a gene are at the same locus on a pair of homologous chromosomes. In anaphase I of meiosis the homologous chromosomes are pulled to opposite poles, so each gamete ends up with one chromosome of the pair and therefore one allele.
Worked example: a heterozygous pea plant Tt makes gametes. Half the gametes carry T and half carry t, in a 1 : 1 ratio. This is why Tt × Tt gives offspring in the ratio 1 TT : 2 Tt : 1 tt, and a phenotype ratio of 3 : 1.The law also explains why a recessive characteristic can disappear in one generation and reappear in the next. The recessive allele is not lost or blended in the heterozygote. It is passed on unchanged and is expressed again when two gametes carrying it fuse.This idea of particulate inheritance replaced the earlier belief in 'blending inheritance', where parental characteristics were thought to mix permanently in the offspring.
Worked example: a heterozygous pea plant Tt makes gametes. Half the gametes carry T and half carry t, in a 1 : 1 ratio. This is why Tt × Tt gives offspring in the ratio 1 TT : 2 Tt : 1 tt, and a phenotype ratio of 3 : 1.The law also explains why a recessive characteristic can disappear in one generation and reappear in the next. The recessive allele is not lost or blended in the heterozygote. It is passed on unchanged and is expressed again when two gametes carrying it fuse.This idea of particulate inheritance replaced the earlier belief in 'blending inheritance', where parental characteristics were thought to mix permanently in the offspring.