Law of Segregation | Law of Inheritance

The law of segregation is a fundamental principle of genetics that explains how traits are passed down from one generation to the next, and it has been confirmed by numerous experiments and observations.

The cellular basis of Mendel’s law of segregation is the separation of homologous chromosomes during meiosis.

The Law of Segregation was the second law of Inheritance. It states that

”Two coexisting alleles for each trait in an individual segregate from each other during meiosis and each gamete receives only one of both alleles. Alleles unite again at random fertilization of gametes during zygote Formation”.

The law of segregation tells us that the rearrangement of chromosomes into gametes is a random process. Each gamete receives only one allele from each parent for a particular trait. It ensures that the offspring inherit two alleles for each trait, one from each parent. So the resulting genotypic ratio follows the 1:2:1 pattern.

For Mendel’s law of segregation to occur, the alleles must be located on homologous chromosomes, which are paired chromosomes that are similar in size and shape and carry genes for the same traits.

Law of Segregation and Its Relationship to Meiosis

The law of segregation and meiosis are closely related. Meiosis is the process by which gametes are formed, and the law of segregation states that alleles for a trait separate during gamete formation. It ensures that each gamete receives only one allele from each parent, allowing for the predictable inheritance of traits from parents to offspring.

Proof of Law of segregation by Monohybrid cross

Mendel carried out the following crosses to prove this law:

F₁ Cross: Mendel crossed the two P₁ generations. One was dominant with round seeds and the other was recessive with wrinkle seeds. All the offspring in F₁ were round seeds.

F₂ Cross: Then Mendel allowed self-fertilization among F₁ monohybrids. It produces F₂ progeny. 3/2 % of F₂ were round and 1/4 were wrinkled. Mendel got a 3:1 ratio in offspring of monohybrid crosses for all the seven contrasting pairs of traits.

F₃ cross: Then Mendel self- fertilized F₂ plants and got F₃ generation. He noted that. 1/3 % of the F₂ round produced only round. But 2/3 of F round produced both round and wrinkled and gave a 3:1 ratio. But F₂ wrinkled produced only wrinkled. He concluded that 1/3 of F₃ rounds were true-breeding like the P₁ round, and 2/3 of R rounds were monohybrids like the F₂ round.

Mendel’s Interpretation

Mendel gave concept of hereditary factor that was responsible of transfer of traits from parents to offspring.

Concept Of Hereditary Factor

Mendel proposed that each contrasting firm of the trait was determined by particulate hereditary factors. He called these hereditary factors elementen (allele). For example, roundness or wrinkledness of seed. These factors carry hereditary information.

Hereditary information is transmitted from parents to offspring through gametes. Each pea plant had a pair of these factors (allele). One allele is derived from a male parent and the other from a female parent. Both of these factors together control the expression of a trait.

He designated the dominant factor with a capital letter and the recessive factor with a small letter. e.g. R for roundness factor and r for wrinkledness factor. Johansen named these factors genes.

Types Of Organisms Based On The Nature Of Alleles

There are two types of organisms based on the nature of alleles.

Homozygous

When both the alleles of a gene pair in an organism are the same, it is called homozygous. An individual with a homozygous genotype is homozygote. The true-breeding round seed plant of the P₁ generation carried R alleles. But the true-breeding wrinkled seed plant of P₁ carried ‘rr’ alleles. So the organism is homozygous for that gene pair.

Heterozygous

When both the alleles of a gene pair in an organism are different, it is called heterozygous. An individual with a heterozygous genotype is a heterozygote. For example Rr.

F1 And F2 Crosses in Law of Segregation

F₁ Cross: Mendel concluded that the factors of a pair (alleles) separated from each other during gamete formation. Thus each gamete got only one factor (allele) for each trait. So half the gametes got one allele, and the other half carried the other allele.

Fertilization is random. The male gamete with factor (R) fertilized the female gamete with factor (r) and the zygote is formed. The zygote has a complete set of the two factors (Rr) for the trait. The zygote developed into F₁ offspring. The offspring was heterozygous Rr.

Thus the F₁ offspring was a rounded seed phenotype. But it is heterozygous in the genotype. Its alleles are also segregated during gamete formation.

F₂ cross: Punnett square indicates that ¼ % of F₂ progeny have ‘RR’ (homozygous round), ¼ + ¼ = ½ Rr (heterozygous round), and ¼ rr (wrinkled). Mendel observed a 3:1 phenotypic ratio in F₂. But its genotypic ratio was 1:2:1 genotypic in F₂.

Mendel separately studied all seven characters and compared their result He found similar results. It proved the law of segregation.

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