Mendel's Laws of Segregation & Independent Assortment

Systematic studies of the inheritance of genes in controlled crosses
    were first undertaken by Gregor Mendel (1822 - 1884)
    The rediscovery of Mendel's Laws in 1900 signalled the start of modern genetics

Mendel showed that
   
Alleles separate (segregate) during the formation of gametes (eggs & sperm)

        half of the germs cells carry one allele & half carry the other [Mendel's Law of Segregation].
         [This is a result of the behaviour of chromosomes during meiosis]


   
The random union of gametes produces zygotes that develop into new individuals.

       The zygotic genotypes will occur in characteristic ratios, according to the genotypes of the parents.
       For example, a monohybrid cross between two heterozygotes ( Aa x Aa )
           produces an expected genotypic ratio of 1:2:1 among AA, Aa, & aa genotypes.

   
The genotypic ratios produce characteristic phenotypic ratios,

        according to the dominance relationships of the alleles involved.
        For example, if A is dominant to a, the cross between heterozygotes produces
            an expected phenotypic ratio of 3:1 between "A" and "a" phenotypes.

     Alleles at separate loci are inherited independently [Mendel's Law of Independent Assortment]
       This produces charactertistic genotypic and phenotypic ratios.
           For example, in a dihybrid cross between two "double heterozygotes" (
AaBb x AaBb )
                The genotypic ratios are 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
                and the phenotypic ratios are 9 "AB" : 3 "Ab" :
3 "aB" : 1 "ab"
              

All text material ©2006 by Steven M. Carr