What is Mendel’s Law of Segregation?
Mendel's law of segregation states: "The two members of a gene pair segregate from each other into the gametes, so that one-half of the gametes carry one member of the pair and the other one-half of the gametes carry the other member of the pair."
This means that each individual has two copies of every gene, that one copy came from the father and the other from the mother, and when the individual reproduces, they will contribute only one of the two copies to their offspring (while the other parent contributes the other). Which copy ends up in any given offspring will be random.
The law of segregation, also called Mendel's first law of inheritance, applies to all diploid organisms with meiosis and sexual reproduction.
To Be More Precise …
Diploid organisms have two complete copies of their genome. Every gene in the genome has two copies, called a gene pair.
When a diploid organism reproduces, it begins by making haploid gametes through meiosis. Each gamete carries one complete copy of the genome – and each gene of a pair separates (or segregates) from the other into separate gamete cells. If the copies are identical, all gametes produced will have the same version of the gene, or allele. If they aren't, half the gametes will have one allele and half will have the other.
All of this may seem basic and obvious to us now, but at the time Gregor Mendel deduced the law of segregation from his pea plant experiments, no one knew about mitosis, meiosis, genes, chromosomes, DNA, haploid vs. diploid or genotypes. Scientists only knew about differences in observable physical traits, or phenotypes, of individual organisms. The prevailing thought was of blending inheritance where offspring would have a blend of the parents' traits and would have an appearance intermediate between the two.
By breeding together lines of peas with a difference of only one phenotype, Mendel worked out several things:
The carriers of inheritance are particulate (we now call them genes).
- Each individual has two copies of the gene, or a gene pair of alleles.
- Some alleles override others. When an offspring receives copies of alleles for both phenotypes, one from the father and one from the mother, it will display only one of them, not a blend of both phenotypes. Mendel described this phenomenon with the words "dominant" and "recessive" – which are terms we still use today.
- Lines that breed true have two copies of the same allele, or are homozygous (as we now call them).
- Individuals with the dominant phenotype that can produce offspring with the recessive phenotype have one copy each of both alleles, and are heterozygotes. When two heterozygotes are crossed, the ratios of the offspring population always work out as 1:2:1 – 1 part dominant homozygotes, 2 parts heterozygotes, and 1 part recessive homozygotes.
- Given all of the above: gene pairs segregate equally into the gametes, and pair up randomly with other gametes.
That last, of course, is what became known as Mendel's law of segregation.
Griffiths, Anthony J.F., Jeffrey H. Miller, David T. Suzuki, Richard C. Lewontin, and William M. Gelbart. 1993. An Introduction to Genetic Analysis 5th ed. W.H. Freeman and Company.
Mendelian Inheritance diagram by Magnus Manske, public domain.
This post is part of the series: Mendelian Inheritance in Classical Genetics
- Classical Genetics: Mendel’s Law of Segregation
- Classical Genetics: Mendel’s Law of Independent Assortment
- Autosomal Dominant Inheritance
- Autosomal Recessive Inheritance