Law of Segregation

Before we move onto explaining the law, have you ever wondered why individuals belonging to the same family or plant progeny, have similar characteristics? Why do offspring look similar to their parents? Well, most of you might be knowing the Mendelian inheritance or Mendelian genetics theory. It answers these questions.

Simply put, this is possible because of the transfer of genes or inherited traits, in humans and plants alike. Gregor Johann Mendel, a German monk, needs to be credited with this important discovery that he made in the 19th century.

To start with, all of us living things, including plants and microorganisms, have chromosomes, which carry factors responsible for various characteristics that we possess. These factors basically control the phenotype and genotype of that individual.

For instance, a particular flower is red colored or a person is blue-eyed, because of the factor coding it, which is inherited from the parent generation.

Mendel performed all his experiments on Pisum sativum, the pea plants, because they were easier to propagate, showed a variety of distinguishing traits, and produced faster results. His experiments and theories can be proven with the help of the Punett's square.

The term factor was replaced with the word gene, after years of research, which is why the branch of study is termed as Genetics. However, considering the number of traits that define or make an individual, there should be several genes, each of which is made up of for various alleles.

For understanding the Mendelian laws, you need to understand a few related terms given here.

Phenotype: It is the physical appearance or physical trait of an individual, controlled by the genes as well as the environmental factors.

Genotype: It is the genetic makeup of an individual, controlling any particular trait.

Alleles: These are specific DNA sequences present on chromosomes at a particular position or loci. Each allele codes for a specific trait and they are generally present in pairs. For instance, GG, gg or Gg.

Dominant Allele: It is the allele or the trait that is expressed in the first generation individuals. For instance, GG, which denotes green pods in pea plants.

Recessive Allele: It is the allele or trait that is hidden in the first generation individuals, but expressed in the second generation and henceforth. For instance, gg, which denotes yellow pods in pea plants.

Homozygous Individuals: Individuals having two alleles, both are either dominant or recessive. For instance, GG and gg.

Heterozygous Individuals: Individuals having two different alleles of the gene pair. For instance, Gg.

According to the law of segregation, the genetic characteristics of a species are represented in the somatic cells by a pair of units called genes, that separate during meiosis so that each gamete receives only one gene for each trait.

Cells are generally of two types, the somatic cells (the body cells) and the sex cells (gametes). All the genetic traits are defined on somatic cells. Reproduction is a result of fusion of a male and a female gamete.

During meiosis, the alleles from a gene pair, separate, therefore, the offspring gets one allele from the male gamete, and the other one from the female gamete, which means there is segregation of the alleles.

According to the law of independent assortment, the members of a gene pair on different chromosomes, segregate independently from other pairs during meiosis, so that the gametes offer all possible combinations of factors.

As mentioned, one gene pair consists of 2 distinguishing alleles. For every meiosis, one of the alleles segregates or assorts independently, offering a variety of combinations in the successive generations.

Because of this, all the offspring show variations and breed true. Few of the alleles might be dominant while few might be recessive and that is shown in the generations that follow. We will see an example for better understanding.

Parents: GG x gg
Offspring: Gg (heterozygous plants with green pods)

Gametes	g (male gamete)
G (female gamete)	Gg (heterozygous plant with green pods)
g (female gamete)	gg (homozygous plant with yellow pods)

The aforementioned table is a sample of one of the experiments carried out by Mendel. Several alleles can be included at a time, but the complexity increases, therefore, we are going to take into consideration one gene pair with 2 alleles, one denoting green pods and one denoting yellow pods in Pisum sativum, the pea plant.

Mendel considered several male plants with green pods (GG) and female plants with yellow pods (gg), then cross fertilized them. The first generation or the F1 generation gave rise to 4 heterozygous individuals (Gg), all having green pods, wherein the allele for the yellow pods was suppressed, and thus recessive.

This proves the first law wherein the gametes are separated. Later, he self fertilized the F1 generation. He got 3 plants with green pods, while 1 with yellow pods, wherein the recessive gene was expressed. The ratio was 3:1 (1 dominant homozygote : 2 hybrid heterozygotes : 1 recessive homozygote) in the second or the F2 generation.

This means 3 different combinations are obtained, because the gametes assort independently in the successive generation, the F2 generation. If the resultant individuals are self crossed again, offspring with genotypic variations are obtained. After a series of experiments and research on various plants and lab animals, the law has been accepted universally.

Apart from this, a related term is incomplete dominance, wherein, neither of the alleles are expressed fully. Instead, the combination of the two is evident in the successive generations. For instance, a plant with white flowers, when crossed with a plant having red flowers, the successive generation shows flowers of pink color.

After studying this law, you must have realized that Mendel's contribution to the field of human genetics was indeed extremely valuable.