Most anyone that has taken a genetics class has heard of Gregor Mendel. From his close observations of inheritance in Pisum stivum, the garden pea, he came to deduce the laws of inheritance that we apply today. The first of his laws is the law of equal segregation and the second is the law of independent assortment. To deduce this information Mendel spent time crossing pea plants for seven different traits. The seven phenotypic pairs studied by Mendel were round or wrinkled ripe seeds, yellow or green seeds, purple or white petals, inflated or pinched ripe pods, green or yellow unripe pods, axial or terminal flowers, and long or short stems. All of the lines used by Mendel were pure lines. To cross these pea plants he applied the methods of cross-pollination and selfing. In cross-pollination he would take the pollen from anthers of one pea plant and transfer them to the stigma of a different pea plant. In selfing Mendel would take the pollen from the anther of a pea plant and transfer it to the stigma of the same plant. Before getting into the laws a few terms should be defined. A homozygote is a plant with a pair of identical alleles. A zygote is a fertilized egg which develops into the offspring. A heterozygote is a plant with a differing pair of alleles. An individual allele that is always expressed will be referred to as dominant and an individual that is overshadowed by this dominant allele is known as the recessive allele.
Mendel's Law of Equal Segregation:
Every pea plant had a pair of hereditary factors called genes. These genes are made up of two alleles. When a characteristic such as color is being examined we can represent these by Y for yellow phenotype and y for green phenotype ( with Y being the dominant allele and y being recessive). A plant can be homozygous dominant Y/Y, homozygous recessive y/y, or heterozygous Y/y. When meiosis occurs the gene pairs will separate equally into the gametes and this separation is Mendel's first law, the law of equal segregation.
Mendel's Law of Independent Assortment:
This law simply states that the alleles of different genes are independent and do not assort with other when gametes are forming (for genes not linked, or distantly linked, to each other). For example for heterozygous gene pairs R/r and Y/y, the y allele may end up in a gamete with an r allele and is equally as likely to end up with an R allele. The same goes for the Y allele. Exceptions to this law are of course genes on the same chromosome which do not assort independently because they are linked.