Mendelian principles on inheritance

Mendelian principles on inheritance

1. 1 Amity Institute of Biotechnology ADVANCED CELL BIOLOGY AND GENETICS MSB: 104 Credit Units: 03 Dr. Pallavi Singh Chauhan Assistant Professor

2. Mendelian principles on inheritance Our understanding of how inherited traits are passed between generations comes from principles first proposed by Gregor Mendel in 1866. Mendel worked on pea plants, but his principles apply to traits in plants and animals – they can explain how we inherit our eye colour , hair colour and even tongue-rolling ability. 2

3. Gregor Mendel Gregor Mendel (1822–1884) is known as the father of genetics. He proposed the key laws of genetics from this work on inheritance of traits in peas in 1866. 3

4. Inheritance in pea plants Mendel followed the inheritance of 7 traits in pea plants ( Pisum sativum ). He chose traits that had 2 forms: Pea shape (round or wrinkled) Pea colour (yellow or green) Flower colour (purple or white) Flower position (terminal or axial) Plant height (tall or short) Pod shape (inflated or constricted) Pod colour (yellow or green). 4

5. Mendel began with pure-breeding pea plants because they always produced progeny with the same characteristics as the parent plant. Mendel cross-bred these pea plants and recorded the traits of their progeny over several generations. 5

6. Mendel’s principles of inheritance Key principles of genetics were developed from Mendel’s studies on peas. 6

7. Why was Pea Plant Selected for Mendel’s Experiments? He selected a pea plant for his experiments: The pea plant can be easily grown and maintained. They are naturally self-pollinating but can also be cross-pollinated. It is an annual plant, therefore, many generations can be studied within a short period of time. It has several contrasting characters. 7

8. Mendel conducted 2 main experiments to determine the laws of inheritance. These experiments were: Monohybrid Cross Experiment Dihybrid Cross Experiment While experimenting, Mendel found that certain factors were always being transferred down to the offspring in a stable way. Those factors are now called genes i.e. genes can be called as the units of inheritance. 8

9. Mendel experimented on a pea plant and considered 7 main contrasting traits in the plants. Then, he conducted both the experiments to determine the a forementioned inheritance laws. A brief explanation of the two experiments is given below: 9

10. Monohybrid Cross In this experiment, Mendel took two pea plants of opposite traits (one short and one tall) and crossed them. He found the first generation offsprings were tall and called it F1 progeny. Then he crossed F1 progeny and obtained both tall and short plants in the ratio 3:1. Mendel even conducted this experiment with other contrasting traits like green peas vs yellow peas, round vs wrinkled, etc. In all the cases, he found that results were similar. From this, he formulated the  laws of Segregation And Dominance . 10

11. Dihybrid Cross In a dihybrid cross experiment, Mendel considered two traits, each having two alleles. He crossed wrinkled-green seed and round-yellow seeds and observed that all the first generation progeny (F1 progeny) were round-yellow. This meant that dominant traits were the round shape and yellow colour . He then self-pollinated the F1 progeny and obtained 4 different traits wrinkled-yellow, round-yellow, wrinkled-green seeds and round-green in the ratio 9:3:3:1. 11

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13. Conclusions from Mendel’s Experiments The genetic makeup of the plant is known as the genotype. On the contrary, the physical appearance of the plant is known as phenotype The genes are transferred from parents to the offsprings in pairs known as allele. During gametogenesis when the chromosomes are halved, there is a 50% chance of one of the two alleles to fuse with the other parent. When the alleles are same they are known as homozygous alleles when the alleles are different they are known as heterozygous alleles. 13

14. Mendel’s laws The two experiments lead to the formulation of Mendel’s laws known as laws of inheritance which are: Law of Dominance Law of Segregation Law of Independent Assortment 14

15. Law of Dominance Mendel found that paired pea traits were either dominant or recessive. When pure-bred parent plants were cross-bred, dominant traits were always seen in the progeny, whereas recessive traits were hidden until the first-generation (F1) hybrid plants were left to self-pollinate. Mendel counted the number of second-generation (F2) progeny with dominant or recessive traits and found a 3:1 ratio of dominant to recessive traits. He concluded that traits were not blended but remained distinct in subsequent generations, which was contrary to scientific opinion at the time. 15

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17. Law of Segregation Mendel proposed that, during reproduction, the inherited factors must separate into reproductive cells. He had observed that allowing hybrid pea plants to self-pollinate resulted in progeny that looked different from their parents. Separation occurs during meiosis when the alleles of each gene segregate into individual reproductive cells (eggs and sperm in animals, or pollen and ova in plants ). 17

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19. Principle of independent assortment Mendel observed that, when peas with more than one trait were crossed, the progeny did not always match the parents. This is because different traits are inherited independently – this is the principle of independent assortment. For example, he cross-bred pea plants with round, yellow seeds and plants with wrinkled, green seeds. Only the dominant traits (yellow and round) appeared in the F1 progeny, but all combinations of trait were seen in the self-pollinated F2 progeny. The traits were present in a 9:3:3:1 ratio (round, yellow: round, green: wrinkled, yellow: wrinkled, green). 19

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21. Exceptions to Mendel’s rules There are some exceptions to Mendel’s principles, which have been discovered as our knowledge of genes and inheritance has increased. The principle of independent assortment doesn’t apply if the genes are close together (or linked) on a chromosome. Also , alleles do not always interact in a standard dominant/recessive way, particularly if they are codominant or have differences in  expressivity or penetrance . 21

22. Penetrance is used to describe whether or not there is a clinical expression of the genotype in the individual . Expressivity is the term that describes the differences observed in the clinical phenotype between two individuals with the same genotype. 22

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25. Key Points on Mendel’s Laws The law of inheritance was proposed by Gregor Mendel after conducting experiments on pea plants for seven years. The Mendel’s laws of inheritance include law of dominance, law of segregation and law of independent assortment. The law of segregation states that every individual possesses two alleles and only one allele is passed on to the offspring. The law of independent assortment states that the inheritance of one pair of genes is independent of inheritance of another pair. 25