Mendel's Laws and Segregation
Mendel's two laws describe how the paired hereditary factors of an individual separate into gametes and combine in offspring, generating the predictable phenotype ratios that founded genetics.
Definition
Segregation is the separation of the two alleles of a gene into different gametes during meiosis; together with independent assortment of unlinked genes, it constitutes Mendel's laws and yields the classical monohybrid and dihybrid ratios.
Scope
This topic covers the law of segregation (the two alleles at a locus separate equally into gametes), the law of independent assortment (alleles of genes on different chromosomes are distributed independently), the use of Punnett squares and the product and sum rules to predict offspring proportions, the testcross as a method to infer genotype, and the characteristic 3:1 and 9:3:3:1 ratios. It does not cover deviations from simple dominance, which are treated as extensions to Mendelian inheritance.
Core questions
- Why does a heterozygote produce two kinds of gametes in equal proportion?
- Under what conditions do two genes assort independently, and when do they not?
- How do the product and sum rules let one compute offspring ratios for multiple genes?
- How does a testcross reveal whether an individual showing a dominant trait is homozygous or heterozygous?
Key concepts
- Law of segregation
- Law of independent assortment
- Monohybrid and dihybrid crosses; 3:1 and 9:3:3:1 ratios
- Punnett square and the product/sum rules
- Testcross and homozygous versus heterozygous genotypes
Mechanisms
Segregation reflects the disjunction of homologous chromosomes at anaphase I of meiosis, and independent assortment reflects the random orientation of different homologous pairs at the metaphase I plate, so that alleles on separate chromosomes combine freely in gametes.
Clinical relevance
These laws supply the quantitative basis for predicting the chance that offspring inherit a single-gene trait, the everyday arithmetic of genetic counseling, and the design of breeding crosses in agriculture.
History
Mendel inferred both laws from quantitative ratios in garden peas reported in 1866; the principles were confirmed and named after their rediscovery around 1900 and were soon reconciled with the meiotic behaviour of chromosomes.
Key figures
- Gregor Mendel
- Carl Correns
- Hugo de Vries
Related topics
Seminal works
- mendel1866
Frequently asked questions
- What is a testcross used for?
- A testcross mates an individual of unknown genotype to a homozygous recessive partner; the proportions of offspring phenotypes reveal whether the tested individual was homozygous dominant or heterozygous.
- Does independent assortment always hold?
- No. It holds for genes on different chromosomes or very far apart on the same chromosome; genes that lie close together are linked and tend to be inherited together rather than assorting independently.