Mendel’s Law of Segregation asserts that allele pairs separate during gamete formation and randomly unite during fertilization.

This law, also referred to as the first law of inheritance, is a foundational principle in genetics, articulated by Gregor Mendel, a 19th-century Austrian monk and scientist. Mendel conducted his groundbreaking experiments using pea plants, focusing on seven distinct traits, including seed shape, flower color, and stem length. His meticulous observations led him to formulate this law, which elucidates the mechanism by which traits are transmitted from parents to their offspring.

According to the Law of Segregation, each individual organism carries two alleles for any given trait. During the formation of gametes (eggs and sperm), these alleles segregate, meaning they separate so that each gamete contains only one allele for each trait. Upon fertilization, the offspring inherits one allele from each parent, thereby reestablishing the pair of alleles within the offspring. This explains why offspring inherit traits from both parents but do not appear as exact replicas of either parent.

For instance, let’s examine the trait of seed color in pea plants, which can be either yellow or green. The yellow color is governed by the dominant allele $Y$, while green is determined by the recessive allele $y$. A plant with either the $YY$ or $Yy$ genotype will produce yellow seeds, whereas only a plant with the $yy$ genotype will yield green seeds. When a $Yy$ plant self-fertilizes, the alleles segregate during gamete formation, resulting in half of the gametes carrying $Y$ and the other half carrying $y$. When these gametes combine during fertilization, they produce offspring with the genotypes $YY$, $Yy$, and $yy$ in a predictable ratio.

The Law of Segregation serves as a cornerstone of genetics, providing insight not only into how traits are inherited from one generation to the next, but also into the variations that exist within species. It is crucial to note, however, that this law applies primarily to genes located on separate chromosomes or those that are sufficiently distant from each other on the same chromosome. Genes that are closely linked on the same chromosome tend to be inherited together, a phenomenon known as genetic linkage.