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"