MENDEL’S LAWS OF INHERITANCE

MENDEL’S LAWS OF INHERITANCE.

·      Introduction:

                                             Gregor Mendel was a monk (priest) in Austria. He developed the fundamental principles of genetics. Mendel proposed that there are “special factors” in organisms, which control the expression of traits and their transmission to next generations. These factors were eventually termed genes.

Mendel selected pea plant (Pisum sativum) to carry out a large number of experiments. In his writings, he gave reasons for this selection. He argued that an organism for genetic experiments should have the following features:

• There should be a number of different traits that can be studied.

• The organism should have contrasting traits e.g. for the trait of height there should be only two very different phenotypes i.e. tallness and dwarfness.

 • The organism (if it is a plant) should be self-fertilizing but cross fertilization should also be possible.

 • The organism should have a short but fast life cycle.

All these features are present in pea plant. Normally, the flowers of pea plant allow self-pollination. Cross pollination can also be done by transferring the pollen grains from the flower on one plant to the flower on another plant. Each trait studied in pea plant had two distinct forms. Mendel succeeded in his work not only because he selected the right organisms for his experiments but also because he analyzed the results by using the principles of statistics (ratios).

 

INHERITANCE:

                                Inheritance refers to the assets that an individual bequeaths to their loved ones after they pass away. An inheritance may contain cash, investments such as stocks or bonds, and other assets such as jewelry, automobiles, art, antiques, and real estate.

 

·     Mendel’s Law of Segregation:

                                                                                                                             Mendel studied the inheritance of seed shape first. For this purpose, he crossed (reproduced) two plants having one contrasting trait i.e. seed shape. A cross in which only one trait is studied at a time, is called as a monohybrid cross.

 

 

Monohybrid Cross:

                                                “A monohybrid cross is the hybrid of two individuals with homozygous genotypes which result in the opposite phenotype for a certain genetic trait.”

“The cross between two monohybrid traits (TT and tt) is called a Monohybrid Cross.”

Monohybrid cross is responsible for the inheritance of one gene. It can be easily shown through a Punnett Square.

Monohybrid cross is used by geneticists to observe how homozygous offspring express heterozygous genotypes inherited from their parents.

 

 

 

            Mendel crossed a true-breeding round-seeded plant with a true-breeding wrinkled-seeded plant.

              

                                                                       

 

 

All resulting seeds of the next generation were round. Mendel declared the trait “round seeds” as dominant, while “wrinkled seeds” as recessive. The following year, Mendel planted these seeds and allowed the new plants to self-fertilize. As a result, he got 7324 seeds: 5474 round and 1850 wrinkled (3 round : 1wrinkled). The parental generation is denoted as P1 generation. The offspring of P1 generation are F1 generation (first filial). The cross in F1 generation produces F2 generation (second filial).

 

 

 

 

 

 

Similarly, when “true-breeding” tall plants were crossed with “true-breeding” short plants, all offspring of F1 were tall plants i.e. tallness was a dominant trait. When members of F1 generation were self-fertilized, Mendel got the ratio of tall to short plants in F2 as 3:1.

 

True-Breeding:

                    A kind of breeding in which the parents with a particular phenotype produce offspring only with the same phenotype .

 

Mendel concluded that the traits under study were controlled by discrete (separable) factors or genes. In each organism, the genes are present in pairs. During gamete formation, the genes (alleles) of each pair segregate from each other and each gamete receives one gene from the pair. When the gametes of male and female parents unite, the resulting offspring agains gets the genes in pairs. These conclusions were called the Law of Segregation.

 

·      Mendel’s Law of Independent Assortment:

 

Dihybrid Crosses:

 

                             “Dihybrid cross is the cross between two different genes that differ in two observed traits.”

 

A dihybrid cross is a breeding experiment between two organisms which are identical hybrids for two traits. In other words, a dihybrid cross is a cross between two organisms, with both being heterozygous for two different traits. The individuals in this type of trait are homozygous for a specific trait. These traits are determined by DNA segments called genes.

 

 

 

 

In the next crosses, Mendel studied two contrasting traits at a time. Such crosses are called dihybrid crosses. He performed experiments on two seed traits i.e. shape and colour. The trait of round seeds (controlled by allele R) was dominant over wrinkled (controlled by allele r) seeds. Similarly, yellow seed colour (controlled by Y) was dominant over green (controlled by y). Mendel crossed a truebreeding plant that had round yellow seeds (RRYY) with a truebreeding plant having wrinkled green seeds (rryy). All seeds in F1 generation were round yellow.

When F1 seeds grew into plants, they were self-fertilized. This cross produced seeds with four phenotypes. There were 315 round yellow seeds, 108 round green seeds, 101 wrinkled yellow seeds and 32 wrinkled green seeds. The ratio of these phenotypes was 9:3:3:1.

 

 

The Punnett square is a diagram that is used to predict an outcome of a particular cross or breeding experiment. It is named after R. C. Punnett (an English mathematician). The gametes of both parents having all possible genetic set-ups are determined. A checker board is used to cross all the possible gametes of one parent with all the gametes of other parent. In this way, a biologist can find all the possible genotypes of offsprings.

 

 

Mendel explained that the two traits i.e. seed shape and seed colour are not tied with each other. The segregation of ‘R’ and ‘r’ alleles happens independently of the segregation of ‘Y’ and ‘y’ alleles. From his second experiment, Mendel concluded that different traits are inherited independently of one another. This principle is known as the Law of independent assortment.

 

 It states as: “the alleles of a gene pair segregate (get separated and distributed to gametes) independently from the alleles of other gene pairs”.