Gregor Mendel, a 19th-century Austrian monk, is widely recognized as the founder of modern genetics based on his experiments with pea plants. One of his discoveries is the Law of Segregation, which explains how traits are passed from one generation to the next. This law posits that the heritable units for a trait, which occur in pairs within an organism, must physically separate during the creation of reproductive cells.
The Statement of Segregation
Mendel’s observations, primarily from crosses involving a single trait, led him to propose that traits are determined by paired “factors.” Today, these factors are known as genes, and the specific variations of a gene are called alleles. The Law of Segregation states that when an organism produces gametes, the two alleles for any given trait separate from each other so that each gamete receives only one allele.
This means a parent contributes only one of its two copies of a gene to its offspring, and the selection of which copy is entirely random. An individual may possess two identical alleles, a condition called homozygous, or two different alleles, which is known as heterozygous. In a heterozygous individual, one allele may be dominant, masking the effect of the recessive allele, but both copies remain distinct and do not blend. This principle of separation is the reason why a recessive trait, hidden in one generation, can reappear unchanged in a later generation.
The Biological Basis in Gamete Formation
The physical mechanism underpinning the Law of Segregation is the process of meiosis, which is the specialized cell division that produces gametes, or sex cells. Organisms that reproduce sexually are diploid, meaning their cells contain two sets of chromosomes, with one set inherited from each parent. These paired chromosomes are called homologous chromosomes, and they carry the two alleles for every gene.
During the first stage of meiosis, known as Meiosis I, the homologous chromosome pairs line up and then physically pull apart. Because the two alleles for a single gene are located on these separate homologous chromosomes, their physical separation is guaranteed. As the cell divides, each resulting daughter cell, which will mature into a gamete, receives only one chromosome from the pair. Consequently, each gamete contains just one allele for that trait. This process ensures an equal, 50% probability of receiving either allele.
Predicting Inheritance Ratios
The predictable separation of alleles allows geneticists to use probability to forecast the ratios of traits in the next generation. This prediction is commonly visualized using a Punnett square, a tool that maps all possible combinations of gametes from two parents. For example, in a monohybrid cross involving two heterozygous parents (carrying one dominant and one recessive allele), each parent produces gametes with a 50% chance of carrying either allele.
When these gametes randomly combine during fertilization, the resulting offspring can have three possible combinations of alleles, known as genotypes, in a 1:2:1 ratio. Specifically, one quarter of the offspring will be homozygous dominant, half will be heterozygous, and one quarter will be homozygous recessive. Because the dominant allele masks the recessive allele, the physical appearance, or phenotype, will follow a different ratio. This monohybrid cross results in a predictable 3:1 phenotypic ratio, where three quarters of the offspring display the dominant trait and one quarter displays the recessive trait.
Segregation Versus Independent Assortment
Mendel’s work established two primary laws, and the Law of Segregation is often confused with his Law of Independent Assortment. The crucial distinction lies in the number of genes being considered during gamete formation. Segregation focuses exclusively on the separation of the two alleles for a single gene.
The Law of Independent Assortment, conversely, addresses the inheritance of alleles for two or more different genes. It states that the alleles for one gene are sorted into gametes independently of the alleles for another gene. For instance, the separation of the alleles for flower color occurs independently of the separation of the alleles for plant height, provided those genes are located on different chromosomes. Segregation is therefore a rule for a single gene, while independent assortment describes the relationship between multiple genes.