Mendel’s Laws of Inheritance
Gregor Johann Mendel (1822 – 1884) laid the foundation of classical genetics by formulating two laws of genetics
- Law of segregation
- Law of independent assortment
He was a priest. He performed a series of breeding experiments on the garden pea, Pisum sativum in his abbey garden for eleven years (1854– 1865).
Selection of Pea Plant
Mendel selected the pea plant due to the following factors.
- Pisum sativum was simple to cultivate.
- It grew well in his garden.
- Its flowers were hermaphrodite.
It was generally self-fertilizing, however could also be cross-fertilized.
- As the time gap between generations was short, Mendel could raise many generations of pea within a short time.
- Pea had many sharply distinct qualities.
Each trait had two clear cut alternative forms or varieties; e.g., seed shape had a round or wrinkled phenotype, plant height was either tall or short, seed color could be yellow or green. Mendel called them contrasting set of a characteristic.
He focused on 7 such sets.
Mendel’s Law of Segregation
True breeding varieties
He first established true-breeding lines or varieties for each trait. A real – breeding variety upon self – fertilization constantly produced offspring similar to the parents, e.g., a true-breeding “round” seed plant produced just “round” seeds. Similarly, a true-breeding “wrinkled” seed plant produced only “wrinkled” seeds.
After establishing 14 pure – breeding lines of 7 characters, the cross-fertilized plants that differed in one character just. The offspring of such a cross was called monohybrids. He cross-fertilized a true-breeding round-seeded male plant with a true-breeding wrinkled-seeded female plant.
First Parental and First Filial Generation
He called it the first parental generation (P1). Their offspring were called F1 or first filial generation. All F1offspring were round like one of the parent plants. wrinkled phenotype did not appear at all.
Dominant and Recessive
Round seed shape-controlled wrinkled seed plant. Its dominance was total due to the fact that no offspring intermediate between parents was discovered. He called the trait that appeared in F, as dominant; while the quality, which was masked, as recessive.
Self-Fertilization of F1 Offspring
Then Mendel allowed self-fertilization amongst F1 monohybrids to raise F2 offspring. As a result of monohybrid cross 3/4 of F2 were round and 1/4 wrinkled.
Mendel got comparable outcomes and the exact same 3:1 ratio in offspring of monohybrid crosses for all the 7 contrasting pairs of traits.
Phenotypic ratio= 3: 1
Genotypic ratio= pure round: hybrid: Pure wrinkled
1: 2: 1
Self-fertilization of F2 plants
He self- fertilized F2 plants to raise F3. He kept in mind that 1/3 of F2 round produced just round, while 2/3 of F2 round produced both round and wrinkled in. 3:1 ratio; but F1 wrinkled produced only wrinkled. He concluded that 1/3 of F2 rounds were true-breeding like P1 round, and 2/3 of F2 rounds were monohybrids like F1 round.
Mendel proposed that each contrasting kind of characteristic, e.g., roundness or wrinkledness of seed was identified by particulate genetic factors, which he called ‘elementen’. These factors carrying hereditary information were transmitted from parents to offspring through gametes. Each pea plant had a pair of these elements, one originated from the male parent and the other from the female parent.
Both of these elements together managed the expression of a characteristic. He designated the dominant factor with a capital letter and recessive factor with a small letter; e.g., R for roundness factor and r for wrinkledness element. Johannsen renamed them ‘genes’.
When both the alleles of a gene set in an organism are very same, the organism is homozygous for that gene pair. An individual with a homozygous genotype is a homozygote.
The true-breeding round seed plant of P1 generation carried ‘RR’ alleles while the true-breeding wrinkled seed plant of P1 brought ‘it’ alleles.
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 e.g. Rr.
Interpretation of F1 and F2 Crosses
(a) F1 Cross
Mendel inferred that the factors of a pair (alleles) separated from each other during gamete development so that each gamete got only one element (allele) for each characteristic. So, half the gametes got one allele, and the other half got the other allele. Fertilization was random. When male gamete carrying element (R) fertilized female gamete with the element (r), the total set of the two factors (Rr) for the trait was brought back in the zygote.
The zygote turned into F1 offspring that was heterozygous ‘Rr’, since the two alleles of its gene pair were different from each other. A specific with a heterozygous genotype is a heterozygote. F1offspring (Rr) was a monohybrid for seed shape; it was round in phenotype however heterozygous in genotype. Its alleles likewise segregated during gamete formation.
Punnett square shows that 1/4 of F2 progeny would have been ‘RR’ (homozygous round), 1/4 + 1/4 = 1/2Rr (heterozygous round), and 1/4 rr (wrinkled). Mendel actually observed 3: 1 phenotypic ratio in F2. His phenotypic information of F3 can likewise be described on the basis of 1: 2: 1 genotypic ratio of F2. Mendel compared the results of all the 7 independently studied characters and found them noticeably similar to create a law of segregation.
Law of Segregation: According to the law of segregation, the two-existing side-by-side alleles for each characteristic in an individual segregate (separate) from each other at meiosis, so that each gamete gets only one of the two alleles. Alleles unite once again at random fertilization of gametes when the zygote is formed.
Mendel designed a cross called test cross, which is used to test the genotype of an individual showing a dominant phenotype. It is breeding in which an individual revealing a dominant phenotype is crossed with a specific showing its recessive phenotype. This cross finds out the homozygous or heterozygous nature of the genotype.
If the seed is homozygous round (RR) it will turn into a pea plant that forms all gametes with only the ‘R’ allele. Wrinkled seed plant is always homozygous recessive.it will form all gametes with the ‘r’ allele. Fertilization will result in 100% round seed progeny.
If the seed is heterozygous round (Rr), it will become a plant that forms half the gametes, with ‘R’ and half with ‘r ‘allele. Wrinkled seed plant will form just ‘r ‘kind of gametes. Fertilization will result in 50% round and 50% wrinkled seed progeny. Even a single wrinkled seed in the progeny is convincing evidence for the heterozygous nature of the round parent.
The law of Mendel states that the allele of gene present on the homologous chromosomes segregate during meiosis in such a way that each gamete gets one allele, not both.
The genes in each parent are incorporated into separate gametes during gamete formation. The homologous chromosome moves towards opposite poles of the cell during anaphase I of meiosis.
Therefore, the gametes have only one member of each chromosome pair. The allele of genes present on one member of a pair of homologous chromosomes enters into one gamete.
The other allele of that gene is present on the other member. These alleles are segregated into a different gamete. There is random combination of gametes during fertilization. It brings homologous chromosomes together again.