Mendel’s Law of Segregation states that, in diploid organisms, each parent randomly passes down just one of two alleles (different versions of a gene) for a particular trait to its offspring. A diploid organism, such as humans, has paired chromosomes, one from each parent. In total, humans have 23 pairs of chromosomes. 

Mendel’s Law of Segregation is a fundamental principle of genetics that determines the chances of a particular genotype arising from a genetic cross.

The laws of inheritance

There’s a lot of chatter nowadays about gene-editing tools like CRISPR or genetically modified organisms in general. Regardless of your stance on GMOs, everyone can agree that genetics is now a fundamental pillar of biology — and it all started more than a century before the famed discovery of the double-helix DNA molecule, in a Bohemian Abbey.

Although farmers were aware for thousands of years that the crossbreeding of animals and plants could favor certain desirable traits, it wasn’t until Gregor Mendel, a 19th-century Augustinian friar, came along that the modern science of genetics was founded. Mendel was the first to demonstrate using his now-famous pea plant experiments that “invisible factors” — now known as genes — predictably determined the traits of an organism. He also coined many of the concepts and terms used in the field to this day, such as “recessive” and “dominant”.

Mendel’s work led him to discover several principles of inheritance, which we now refer to as Mendelian laws.

• Law of dominance and uniformity.  Individuals receive two versions of each gene, known as alleles, from each parent. Some alleles are dominant, meaning they are expressed in the organism, while others are recessive, meaning they are masked. An organism with at least one dominant allele will display the effect of the dominant allele. So rather than both alleles contributing to a phenotype, the dominant allele will be expressed exclusively. The recessive trait will only be expressed by offspring that have two copies of this allele, but these offspring will breed true (two parents with that phenotype produce offspring of that same phenotype) when self-crossed.
• Law of independent assortment. Genes do not impact one another with regard to sorting into gametes. The allele received for one gene does not influence the allele received for another gene.
• Law of segregation. When two heterozygous individuals are crossed, a parent gives just one allele for a gene to each gamete (ovum and sperm cells) they produce.

Mendel’s law of segregation made easy

Mendel defined this important principle of inheritance while observing pea plants. The 19th-century scientist crossed a true-breeding tall plant with a true-breeding short plant (the parental generation), and every single time he found that the “F1” generation (first filial generation) of offspring was always tall.

In fact, when Mendel repeated the experiment for other traits, such as the pea’s size or color, he found that the first generation offspring always resembled one of the parents and never the other parent.

This begs the question: what happened to the trait for shortness? Did it simply vanish? Does this F1 generation completely lose the trait that is responsible for short height in the plant? These were the kind of questions that Mendel must have thought of, and to answer them he crossed F1 generation offspring among themselves to produce the F2 generation offspring.

What he saw was that, although about 75% of the F2 offspring (second filial generation) were in fact tall, the remaining 25% were short. Mendel found that this 3:1 ratio remained consistent even after he cross-bred thousands of pea plants. This means that the F1 offspring had that short hereditary factor (i.e. gene) all this time, it’s just that it wasn’t actually expressed.

In light of these findings, Mendel proposed that each of these plants, tall and short, contained two hereditary factors, later known as alleles, that code for that given trait. This also means that the tall trait (T) is dominant over the short trait (t), which is recessive. So seeds with the genotype of (TT) or (Tt) are always tall, while seeds that are (tt) are always short.

Likewise, the allele for the yellow seed color in pea plants is dominant (Y) while the allele for green seed color is recessive (y). Seeds with the genotype (YY) and (Yy) will be yellow and seeds with the genotype (yy) will be green.

When an organism has the same alleles for a trait, it is called homozygous, and when the organism has two different alleles — one dominant, one recessive — for the same trait, it is called heterozygous. Individuals that are homozygous dominant have two dominant alleles, and those that are homozygous recessive have two recessive alleles.

The way a trait is expressed is called a phenotype — yellow or green is the phenotype of the trait for pea color. The genotype, on the other hand, is the genetic composition of an organism. For instance, the genotype of a yellow pea plant is either (YY) or (Yy), whereas the genotype of a green plant is always (yy).

These ideas proved groundbreaking. Before Mendel, scientists used to think that heredity involved mixing different traits from the parents, such that offspring traits were a blend of the parents’ traits. However, this thinking is flawed because, over the course of countless generations, all this mixing would cause every trait to become a constant average of all previous traits. The ideas of dominant and recessive traits were a much better model reality.

Meiosis and Mendel’s law of segregation

Nowadays, we know that what Mendel referred to as hereditary factors are actually genes found on homologous chromosomes.

Gametes are eggs and sperm created through the process of meiosis, which is a process of cell division that reduces the amount of genetic information. During meiosis the parent splits the genome in half, giving each gamete just one copy of each gene. Therefore, a heterozygote (which has different alleles) parent can only pass down one of two alleles. A parent can’t give both alleles to a single gamete.

When the sperm and egg meet and fertilization occurs, the gametes fuse together giving the organism two copies of each gene to complete their genome.

So the law of segregation simply means that a parent gives just one of its alleles for a gene to its offspring. Which of the parents’ two alleles is given to an offspring is random.

What’s amazing about this discovery is that it was made at a time when we knew nothing about meiosis or mitosis. Even though at the time Mendel made his groundbreaking findings we knew what gametes were, and we knew that gametes must fertilize to form the zygote, we knew nothing about how meiosis takes place, and hence how the gametes are actually formed.

Mendel’s achievements, first published in 1866, were ignored at the time. The importance of his discoveries on the heredity of traits in plants was only recognized after three researchers independently rediscovered them in 1900. And although Mendel’s work never mentions DNA or genes, his work is so well-documented and verified that his principles are still taught in schools and even in undergrad courses on genetics.