Mendel's law of inheritance

Mendel's law of inheritance, test cross, punnett square.

1. Mendel’s Law of Inheritance

2. History

3. Term gene was coined by : W. Johannsen 3 February 1857 – 11 November 1927

4. Father of genetics: Gregor Johann Mendel 20 July 1822 – 6 January 1884

5. Father of modern genetics: W. Bateson 8 August 1861 – 8 February 1926

6. Father of experimental genetics: T.H. Morgan September 25, 1866 – December 4, 1945

7. Father of Human and Biochemical genetics: A. Garrod 25 November 1857 – 28 March 1936

8. Mendel’s Laws of Inheritance Genetics is the branch of biology, which deals with the inheritance, as well as the variation of characters from parents to their offsprings. Inheritance is the process by which characters or traits are passed from parent to their progeny i.e. from one generation to the next. It is the basis of heredity. Variation is the degree by which progeny differs from their parents. One of the causes of variation in living things was hidden in sexual reproduction.

9. Gregor Johann Mendel, also known as ‘Father of Genetics’, for the first time conducted experiment to understand the pattern of inheritance of variations in living things. He conducted hybridization experiments on garden pea ( Pisum sativum ) for seven years and proposed the laws of inheritance in living organisms. His observation became the foundation of modern genetics and the study of heredity. Mendel ’s Experiment (Inheritance of one gene)

10. Steps in making a cross in pea

11. He selected garden pea plant as a sample due to the following reasons: Pea plant are readily available on large scale and has bisexual flowers. Peas are self- pollinated and be cross- pollinated also. These are annual plants with short life cycle. So, several generations can be studied within a short period. Pea plants could be easily raised, maintained and handled. Pea plants differ in distinct/ contrasting characteristics, which provide many easily detectable contrasting characteristics. Mendel conducted artificial/ cross pollination experiments using several true- breeding pea lines. Mendel selected 14 true- breeding pea plant varieties, as pairs, which were similar except for one character with contrasting traits. M endel ’s Experimental Material

12. Mendel observed one trait or character at a time, e.g. he crossed tall and dwarf pea plants to study the inheritance of one gene. He hybridised plants with alternate forms of a single trait (monohybrid cross). The seeds produced by these crosses were grown to develop into plants of Filial1 progeny or F1- generation (F1 plants). He then self pollinated the tall F1- plants to produce plants of Filial2 progeny or F2- generation (F2 plants). In later experiments, Mendel also crossed pea plants with two contrasting characters known as Dihybrid Cross. Mendel’s Experimental Procedure

13. In F1- generation, Mendel found that all pea plants were tall and none was dwarf . He also observed other pair of traits and found that F1- plant always resembled either one of its parent and the trait of other parent was not found. In F2- generation, he found that some of the offsprings were ‘dwarf’, i.e. the character which were not seen in F1- generation were expressed in F2- generation. These contrasting traits (tall/ dwarf) did not show any mixing either in F1 or in F2- generation. Similar results were obtained with the other traits that he studied. Only one of the parental traits that he studied. Only one of the parental traits was expressed in F1- generation, while at F2- generation stage, both the traits were expressed in the ratio of 3:1. Mendel also found identical results in dihybrid cross as in monohybrid cross. The trait that appeared in the F1- generation is called dominant trait, while the other trait is recessive trait. In tall/ dwarf trait, tallness is dominant over dwarfness that is recessive. Mendel’s Observation on Experiment

14. Following inferences were made by Mendel based on his observations : He proposed that some ‘factors’ passed down from parent to offsprings through the gametes over successive generations. Now-a-days, these factors are known as Genes . Genes are hence, the units of inheritance. Genes which code for a pair of contrasting traits are known as alleles or allelomorph , i.e. they are slightly different forms of the same gene. Genes occur in pairs in which, one dominates the other called dominant factor expresses itself, while the other remains hidden and its factor called recessive . Allele can be similar in case of homologous (TT or tt) and dissimilar in case of heterozygous (Tt). In a true-breeding tall or dwarf pea variety, the allelic pair of genes for height are identical or homologous. TT and tt are called genotype of the plant, while the term tall and dwarf are the phenotype . When the tall and dwarf plant produce gametes, by the process of meiosis, the alleles of the parental pair segregation and only one of the allele gets transmitted to a gamete. During fertilisation, the two alleles, T from one parent and t from another parent are united to produce a zygote, that has one T and one t allele or the hybrids have Tt. Since, these hybrids contain alleles which express contrasting traits, the plants are heterozygous . Mendel’s Inferences

15. The production of gametes by the parents, the formation of zygotes, the F1 and F2- generations can be explained by a diagram called punnett square. It was developed by British geneticist Reginald C Punnett . The 1/4 : 1/2 : 1/4 ratio of TT: Tt: tt is mathematically condensable to the form of the binomial expression (ax +by)2 , that has the gametes bearing genes T or t in equal frequency of ½. The expression is expanded as given below : (1/2T + 1/2 t) 2 = (1/2T + 1/2t) X (1/2T + 1/2t) = 1/4 TT + 1/2Tt + 1/4 tt Punnett Square

16. This is a method devised by Mendel to determine the genotype of an organism. In this cross, the organism with an unknown dominant genotype is crossed with the recessive parent, instead of self- crossing. E.g. in a monohybrid cross, between violet colour flower (W), and white colour flower (w), the F1- hybrid was a violet colour flower. If all the F1- progeny are of violet colour, then the dominant flower is homozygous and if the progenies are in 1:1 ratio, then the dominant flower is heterozygous. Test Cross

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