Mendel's Law of Segregation
Mendel's Law of Segregation states that during the formation of gametes (sex cells), the two alleles for each gene separate (segregate) so that each gamete receives only one allele. This segregation is random, and therefore each allele has an equal probability of ending up in any given gamete. Proposed by Gregor Mendel in 1865 based on his pea plant experiments, this law forms the foundation of modern genetics and explains why offspring inherit one allele from each parent.
Key Principles of the Law of Segregation
| Principle | Description | Example |
|---|---|---|
| Allele pairs separate | Each gene has two alleles that split during meiosis | Yy → Y gametes and y gametes |
| Equal probability | Each allele has 50% chance of entering a gamete | Yy → 50% Y, 50% y |
| Random fertilisation | Gametes combine randomly at fertilisation | Any Y can meet any y |
| Offspring inherit one from each parent | Zygote gets one allele per parent | Y (from mum) + y (from dad) = Yy |
| Basis of 3:1 ratio | Explains phenotypic ratio in F2 generation | Yy × Yy → 3 yellow : 1 green |
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Related Terms
Punnett Square
A Punnett square is a diagram used to predict the probability of genotypes and phenotypes in the offspring of two parents, based on the alleles each parent can contribute. Developed by British geneticist Reginald Crundall Punnett in the early 20th century, it arranges parental alleles along the axes of a grid, and each cell represents a possible offspring genotype. It is a foundational tool in Mendelian genetics for calculating ratios such as 3:1 (phenotypic) or 1:2:1 (genotypic) for monohybrid crosses.
Law of Independent Assortment
The Law of Independent Assortment states that alleles of different genes assort independently of one another during gamete formation, meaning the inheritance of one gene does not influence the inheritance of another gene located on a different chromosome. This law, Mendel's Second Law, explains why a dihybrid cross (RrYy × RrYy) produces a 9:3:3:1 phenotypic ratio. It applies specifically to genes on non-homologous (different) chromosomes; genes on the same chromosome may be linked and do not assort independently.
Homozygous
An organism is homozygous for a particular gene when it carries two identical alleles at that genetic locus — either two dominant alleles (homozygous dominant, e.g., AA) or two recessive alleles (homozygous recessive, e.g., aa). Homozygous individuals breed true for the trait associated with those alleles, meaning all offspring from two homozygous parents with the same alleles will also be homozygous. This condition is significant in selective breeding and genetic disease risk assessment.
Named after Gregor Johann Mendel (1822–1884), an Augustinian friar and scientist who conducted breeding experiments with pea plants in Brno (now Czech Republic). "Segregation" comes from Latin "segregare" meaning "to separate from the flock" (se- = apart, grex = flock).