Principle of inheritance Chapter 4 NCERT

Principal of Inheritance

1. Chapter 5 PRINCIPLES OF INHERITANCE AND VARIATION Pooja Singh PGT Biology

2. The Story of Mendel and His Pea Plants A long time ago, in a quiet monastery in Austria, lived a curious monk named Gregor Mendel. He wasn’t just a man of prayer; he had a deep love for nature and science. Mendel had a special garden where he spent hours every day—not to grow food, but to understand how traits are passed from one generation to another.

3. Mendel chose to work with pea plants , not because they were delicious, but because they had clear, visible traits.

4. Some plants had tall stems , others were short .

5. Some had round seeds , others had wrinkled ones .

6. Some had purple flowers , and others had white . These differences fascinated Mendel.

7. One fine spring morning, Mendel had an idea: “What if I cross a tall pea plant with a short one and see what happens to their children?” So he carefully took pollen from one plant and transferred it to another. This was the beginning of what would become the world’s first controlled genetic experiment .

8. The 7 characters of Pea plant

9. INHERITANCE OF ONE CHARACTER

10. Gene and Allele Genes are the units of inheritance. They contain the information that is required to express a particular trait in an organism. Example- Height, colour , shape etc Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly different forms of the same gene. (Tall/Dwarf), (Green/Yellow), (Round/Constricted)

11. Gene and Allele Gene Allele 1 Allele 2 Plant Height Tall (TT) Dwarf ( tt ) Pod Colour Green (GG) Yellow (gg) Seed shape Round (RR) Wrinkled ( rr )

12. Dominant and Recessive

13. Monohybrid cross

14. Monohybrid cross

15. Monohybrid cross

16. Phenotype and Genotype Phenotype- Genotype- TT/Tt tt

17. Phenotype- Genotype RR or Rr rr

18. Snapdragon plant

19. Co-dominance

20. Multiple alleles Height- TT tt Colour YY yy Flower VV vv Blood group AA BB AB ii

21. INHERITANCE OF TWO GENES

22. Selfing

23. Genotypic ratio :

24. Law of Independent Assortment The law states that ‘when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters

25. Home-Assignment Dihybrid cross between a pea plant having tall height with violet flower and a dwarf plant with white flower

26. Chromosomal Theory of Inheritance Walter Sutton and Theodore Boveri noted that the behaviour of chromosomes was parallel to the behaviour of genes and used chromosome movement to explain Mendel’s laws 46 23 23

27. 23 23

28. t t T T T T t t Tt

29. Can you tell which of these columns A or B represent the chromosome and which represents the gene?

30. POLYGENIC INHERITANCE The genotype with all the dominant alleles (AABBCC) will have the darkest skin colour and that with all the recessive alleles ( aabbcc ) will have the lightest skin colour . As expected the genotype with three dominant alleles and three recessive alleles will have an intermediate skin colour AABBCC x aabbcc F1 generation- AaBbCc

31. PLEIOTROPY There are instances where a single gene can exhibit multiple phenotypic expression. Such a gene is called a pleiotropic gene. An example of this is the disease phenylketonuria phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation.

32. SEX DETERMINATION

33. Drosophila

34. Birds

35. Grasshopper Male XO Female XX

36. Heterogamety Male Human- XY Male Drosophila- XY Male Grasshopper XO Female Bird- ZW Homogamety Female Human- XX Female drosophila- XX Female Grasshopper- XX Male Bird- ZZ

37. G enetic Disorder

38. Colour Blindness

39. Colour blindness is a sex-linked recessive disorder due to defect in either red or green cone of eye resulting in failure to discriminate between red and green colour . This defect is due to mutation in certain genes present in the X chromosome. It occurs in about 8 per cent of males and only about 0.4 per cent of females. This is because the genes that lead to red-green colour blindness are on the X chromosome. Males have only one X chromosome and females have two.

40. If the mother is a carrier of colour blindness and the father is normal, show the possible genotype and phenotype of the offspring of the next generation, with the help of a punnet square. If the mother is colour blind and the father is normal, show show the possible genotype and phenotype of the offspring of the next generation, with the help of a punnet square.

41. Haemophilia sex linked recessive disease in an affected individual a simple cut will result in non-stop bleeding.

42. A carrier woman for haemophilia marries a haemophilic man.Show the type of progeny formed with the help of punnet square?

43. Sickle-cell anaemia autosome linked recessive trait

44. Sickle cell anaemia Normal

45. In the Punnett Square below, both parents are carriers of the sickle cell allele, AA Aa Aa aa A a A a

46. Phenylketonuria autosomal recessive trait The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine As a result of this phenylalanine is accumulated and converted into phenylpyruvic acid Accumulation of these in brain results in mental retardation. These are also excreted through urine because of its poor absorption by kidney.

47. Thalassemia autosome-linked recessive blood disease The defect could be due to either mutation or deletion which ultimately results in reduced rate of synthesis of one of the globin chains (α and β chains) that make up haemoglobin . Thalassemia can be classified according to which chain of the haemoglobin molecule is affected. In α Thalassemia , production of α globin chain is affected while in β Thalassemia , production of β globin chain is affected. chromosome 16 is affected in α Thalassemia Chromosome 11 is affected in β Thalassemia