Principles of Inheritance, Class 12 CBSE
- 1. Principles of Inheritance and Variation
- 3. Genetics • Organisms reproduce- formation of offspring of the same kind. • The resulting offspring most often do not totally resemble the parent. • Branch of biology that deals with the inheritance and variation- Genetics. • Inheritance- the process by which characters are passed on from parent to progeny. • Variation-it is the degree by which progeny differ from their parents.
- 5. history • Human knew before 8000- 1000 B. C variation is due to sexual reproduction • Exploited variations present in wild plants & animals to selectively breed & select organism with desirable characters • Artificial selection & domestication of wild cow- Sahiwal cows in Punjab
- 6. • Genetics is the branch of life science that deals with the study of heredity and variation. • Heredity is the transmission of characters from parents to their offsprings. • Variation is the difference among the offsprings and with their parents. • Hereditary variations: These are genetical and inheritable. • Environmental variation: These are acquired and non inheritable. Terminology
- 7. Gregor Johann Mendel: Father of Genetics • Known as the father of modern genetics • Gregor Mendel developed the principles of heredity while studying seven pairs of inherited characteristics in pea plants. • Although the significance of his work was not recognized during his lifetime, it has become the basis for the present-day field of genetics.
- 8. • Conducted hybridization (artificial pollination/ cross pollination) experiment for 7 years 1856-1863 & proposed law of inheritance • Applied statistical analysis & mathematical logic for biology problems • Large sampling size- greater credibility to data • Experiments- true breeding pea lines (continuous self pollination) • Confirmation of inference from experiments on successive generations of test plants, proved general rules of inheritance • Mendel investigated two opposing traits- tall & dwarf, yellow & green seed Mendel’s ApproAch
- 9. Seven pair of contrasting characters selected by Mendel for his experiment.
- 10. • Phenotype: The external appearance of an organism due to the influence of genes and environmental factors. • Genotype: The genetic constitution of an individual responsible for the phenotype . • Phenotypic ratio: The correct proportion of phenotype in population. • Genotypic ratio: The correct proportion of genotype in population. • Homozygous: The individual heaving identical genes in an allelic pair for a character. Ex: TT, tt. • Heterozygous: The individual heaving un-identical genes in an allelic pair for a character. Ex: Tt. Terminologies
- 11. • Dominant gene: The gene that expresses its character in heterozygous condition. • Recessive: The gene that fails to express its character in heterozygous condition. • Hybrid: The progeny obtained by crossing two parents that differ in characters. • Back cross: The cross between F1 hybrid and one of its parents. • Test cross: The cross between hybrid and its homozygous recessive parent. It is used to identify the genotype of the hybrid.
- 12. Why Mendel selected pea plant? • Pure variety are available. • Pea plants are easy to cultivate. • Life cycle of plants are only few months. So that result can be got early. • Contrasting trait are observed. • Flowers are bisexual and normally self pollinated. • Flowers can be cross pollinated only manually. • Hybrids are fertile.
- 13. Inheritance of one gene. • Inheritance of one gene can be explained by monohybrid cross. • The cross between two parents differing in one pair of contrasting character is called monohybrid cross. • Crossed tall & dwarf pea plants- Collected seeds & grew to generate first hybrid generation/ Filial generation/F1 • F1 plants- Tall & none were dwarf • For other traits also- F1 generation resembled only one parent & trait of other parent were not shown • Self pollinated F1 – Filial 2 generation/ F2 • F2 generation- 1/4th were dwarf & 3/4th tall- identical to parents • F1 generation one parent trait shown & F2 both parent trait shown in the ratio- 3:1 & no blending were seen
- 15. • Mendel proposed- Something is stably being passed to the next generation through gametes ‘factors’ – genes • Genes/factors- unit of inheritance, contain the information required to express particular trait • Genes which code for pair of contrasting trait- alleles • Alphabetical symbols were used; T-Tall, t- dwarf • Plants pair of alleles for height- TT, Tt & tt • Mendel proposed- true breeding tall or dwarf plant- identical or homozygous allele pair of TT or tt (genotype) • Descriptive term tall or dwarf- phenotype • Mendel found phenotype of heterozygote Tt of F1 was same as parent with TT & proposed, in a pair of dissimilar factors one dominates the other & hence called dominant (T) & recessive (t)
- 16. TallP Dwarfx F1 All Tall Phenotype Monohybrid cross F2 Tall is dominant to Dwarf TT tt Tt Genotype Homozygous Dominant Homozygous Recessive HeterozygousSelf pollinated Gamets T t T TT tall Tt tall t Tt Tall tt dwarf Phenotypic ratio 3:1 Genotypic ratio: 1:2:1
- 17. • Production of gametes & formation of zygotes- Punnett Square • Developed by- British scientist Reginald C. Punnett • Graphical representation- calculate probability of possible genotypes in genetic cross • Gametes- on two sides, top row & left columns • Self- pollination- 50% • F2- 3/4th tall & 1/4th Dwarf- phenotypically • 1/4 : ½ : ¼ ratio of TT: Tt: tt- genotype
- 18. Test cross: The cross between hybrid and its homozygous recessive parent I called test cross. It is used to identify the genotype of the hybrid.
- 19. Mendelian laws of heredity. • Rules were proposed- Principles or Laws of Inheritance: First Law or Law of Dominance & Second law or Law of Segregation • Law of dominance 1. Characters are controlled by discrete units called Factors 2. Factors occurs in pair 3. In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive) Used to explain the expression of only one of the parental characters in monohybrid cross (F1) & expression of both in F2. Also explains proportion 3:1 in F2
- 20. Law of segregation • It states that, ‘when a pair of factors for a character brought together in a hybrid, they segregate (separate) during the formation of gametes. • Alleles do not blend & both characters recovered in F2 & one in F1 • Factors which is present in parent segregate & gametes receives only one of two factors • Homozygous parent- one kind gamete • Heterozygous parent- two kind gamete each one have one allele with equal proportion
- 21. Incomplete dominance: • Correns discovered Incomplete dominance in Merabilis jalapa. • It is also called partial dominance, semi dominance. • The inheritance in which allele for a specific character is not completely dominant over other allele is called Incomplete dominance. • Snapdragon or Antirrhinum sp.- Cross between true breed red flower (RR) & white flower (rr), F1 generation- Pink (Rr) & after self pollination in F2 generation- 1 (RR) Red: 2 (Rr) Pink: 1 (rr) white • Genotype ratio same as Mendelian cross & Phenotype ratio different than Mendelian cross
- 22. Incomplete dominance: Ex snapdragon. ( Dog flower plant)
- 24. Parent: Red X White Genotype. RR WW Gametes R W F1 generation Pink (Hybrid) RW Self pollination F2 generation Gametes R W R RR Red RW Pink W RW Pink WW white The phenotypic ratio is 1:2:1. The genotypic ratio is 1:2:1
- 25. CO-DOMINANCE • Both the alleles for a character are dominant and express its full character is called co-dominance. • Ex AB blood group of human being. • Blood group in humans are controlled by 3 alleles of a gene I. • They are IA IB and i. • The ABO locus is located on chromosome 9. • IA is responsible for production of antigen –A. • IB is responsible for production of antigen –B. • i does not produces any antigen.
- 26. • I A and I B are co-dominant and dominant over i. Blood Group Genotype A- Group IAIA or IA i B-Group IBIB or IBi AB-Group IAIB O-Group ii
- 28. • ABO blood grouping- multiple allele • Three alleles govern same character • Multiple allele is found when population studies are made • Single gene product may produce more than one effect • Eg.- Starch Synthesis in Pea seeds- controlled by a gene having two allele B & b • Starch synthesis effective if homozygote BB & produce large starch grains • Homozygote bb – lesser efficiency in starch synthesis & seeds are wrinkled • Heterozygote Bb – round seeds, intermediate size
- 29. Inheritance of two gene: Mendel’s 2nd law or Law of independent assortment: • It states that, ‘factors for different pairs of contrasting characters in a hybrid assorted (distributed) independently during gamete formation. Mendel’s 2nd law can be explained by dihybrid cross. • Dihybrid cross: The cross between two parents, which differs in two pairs of contrasting characters.
- 30. Dihybrid cross: Parents Round Yellow Wrinkled Green Genotype Phenotype RRYY rryy Gametes RY ry F1 generation Round Yellow RrYy
- 31. Phenotypic ratio : 9 : 3 : 3 : 1
- 32. Dihybrid test cross. • F1 hybrid is crossed with recessive green wrinkled pea plant. • Recessive green wrinkled – rryy, Gamete ry • F1 hybrid : round yellow- RrYy, Gametes: RY, Ry, rY, ry. Gametes RY Ry rY ry ry RrYy Rryy rryY rryy Phenotypic ratio – 1 : 1 : 1 :1
- 33. • Mendel work published 0n 1865 but remain unrecognized till 1900 • Reasons for that: 1. Lack of communication 2. Concept of genes / factors- clear 3. Mathematical approach for biology was not acceted 4. No proof for existence of factors
- 34. Chromosomal theory of inheritance: • It was proposed by Walter Sutton and Theodore Boveri . • They work out the chromosome movement during meiosis. • The movement behavior of chromosomes was parallel to the behavior of genes. The chromosome movement is used to explain Mendel’s laws. • The knowledge of chromosomal segregation with Mendelian principles is called chromosomal theory of inheritance. • According to this, Chromosome and genes are present in pairs in diploid cells. • Homologous chromosomes separate during gamete formation (meiosis) • Fertilization restores the chromosome number to diploid condition.
- 35. Chromosomal Theory of inheritance
- 36. • Thomas Hunt Morgan and his colleagues conducted experimental verification of chromosomal theory of inheritance • Morgan worked with tiny fruit flies, Drosophila melanogaster.
- 37. • He selected Drosophila because, • It is suitable for genetic studies. • Grown on simple synthetic medium in the laboratory. • They complete their life cycle in about two weeks. • A single mating could produce a large number of progeny flies. • Clear differentiation of male and female flies • Many types of hereditary variations can be seen with low power microscopes.
- 38. SEX DETERMINATION • Henking (1891) traced specific nuclear structure during spermatogenesis of some insects. • 50 % of the sperm received these specific structures, whereas 50% sperm did not receive it. • He gave a name to this structure as the X-body. • This was later on named as X-chromosome.
- 39. XX-XO type • Sex-determination of grass hopper: • The grasshopper contains 12 pairs or 24 chromosomes. The male has only 23 chromosome. • All egg bears one ‘X’ chromosome along with autosomes. • Some sperms (50%) bear’s one ‘X’ chromosome and 50% do not. • Egg fertilized with sperm having ‘X’ chromosome became female (22+XX). • Egg fertilized with sperm without ‘X’ chromosome became male (22 + X0)
- 40. XX-XY type Sex determination in insects and mammals • In this type both male and female has same number of chromosomes. • Female has autosomes and a pair of X chromosomes. (AA+ XX) • Male has autosomes and one large ‘X’ chromosome and one very small ‘Y-chromosomes. (AA+XY) • In this type male is heterogamety and female homogamety.
- 41. ZZ – ZW type Sex determination in birds: • In this type female birds has two different sex chromosomes named as Z and W. • Male birds have two similar sex chromosomes and called ZZ. • In this type of sex determination female is heterogamety and male is homogamety.
- 42. Linkage & recombination • Morgan carried dihybrid cross in Drosophila to study genes that are sex linked • Crossed- yellow bodied, white eyed females with brown bodied, red eyed males & intercourse F1 progeny • Two genes did not segregate independently of each other & F2 ratio deviated from 9:3:3:1 • The genes present on X –chromosome & two genes in a dihybrid cross- situated on same chromosome- parental gene combination is much higher than non parental- this is due to physical association/ linkage of two genes on chromosome- Linkage • Generation of non parental combination- Recombination
- 43. • He found genes are grouped in same chromosome, some genes are tightly linked- less recombination • When genes are present in different chromosome- higher recombination • Eg.- Genes for white & yellow- tightly linked- 1.3% recombination while genes for white & miniature wings- 37.2% recombination • Student Alferd Sturtevant used frequency of recombination between gene pairs on chromosome as a measure of the distance between genes & mapped genes position on chromosome
- 45. • Linkage: physical association of genes on a chromosome is called linage. • Recombination: The generation of non-parental gene combinations is called recombination. • It occurs in crossing over of chromosomes during meiosis.
- 46. MUTATION • Phenotypic variation occurs due to change in gene or DNA sequence is called mutation. The organism that undergoes mutation is mutant. • Phenomenon which result in alternation of DNA sequence & result in change in genotype & phenotype 1. Loss (deletion) or gain (insertion/duplication) of a segment of DNA results in alteration in chromosomes- abnormalities/ aberrations- Chromosomal aberrations 2. Gene Mutations: The mutation takes place due to change in a single base pair of DNA is called gene mutation or point mutation. E.g. sickle cell anemia. 3. Frame shift mutations: Deletion or insertions of base pairs of DNA is called frame shift mutations.
- 47. Pedigree Analysis: • The study of inheritance of genetic traits in several generations of a family is called the pedigree analysis. • Pedigree study- strong tool of human genetics to trace inheritance of specific trait/ abnormality/ diseases • Pedigree analysis of inheritance of a traits is represented in family tree • It helps in genetic counseling to avoid genetic disorders.
- 48. Genetic disorders • Genetic disorders grouped into two categories – 1. Mendelian disorder 2. Chromosomal disorder Mendelian Disorders • Mendelian disorders are mainly determined by alteration or mutation in the single gene. • It obey the principle of Mendelian inheritance (principles of inheritance) during transmission from one generation to other. • Mendelian disorder- traced in family by pedigree analysis • E.g. Haemophilia, Colorblindness, Cystic fibrosis, Sickle cell anemia, Phenylketonuria, Thalesemia etc. • Dominant or recessive- pedigree analysis • Trait may linked to sex chromosome, Eg. Haemophilia • X- linked recessive trait- transmitted from carrier female to male progeny
- 49. Hemophilia: • It is a sex linked recessive disease. • The defective individual continuously bleed to a simple cut. • The gene for hemophilia is located on X chromosome. • In this disease a single protein that is a part of cascade of proteins that involved in the clotting of blood is affected. • The diseases transmitted from unaffected carrier female to some of the male progeny. • Heterozygous female (carrier)- transmit to sons • Female being hemophilic is rare- Mother should be carrier & father Haemophilic
- 52. H
- 54. • Autosome linked recessive trait • Transmitted from parents- both partners are carrier/ heterozygous • Controlled by single pair of allele HbA & Hbs • Homozygous individuals of Hbs (HbSHbS)- diseased • Heterozygous individuals HbAHbS- unaffected but carrier • Defect is due to substitution of Glutamic acid(Glu) by Valine (Val)- at the 6th position beta globin chain of Hb • Due to substitution of single base at 6th codon of beta globin gene from GAG to GUG • Mutant haemoglobin- polymerization under low oxygen tension causing change in shape of RBC from biconcave to sickle like structure
- 56. phenylketonuria • Inborn error of metabolism- inherited as autosomal recessive trait • Affected individual lack enzyme that convert amino acid phenylalanine to tyrosine • Phenylalanine accumulates & convert to phenylpyruvic acid & other derivatives • Accumulation in brain result- mental retardation • Excreted in urine- poor absorption by kidney
- 58. Chromosomal disorder • Caused due to absence or excess or abnormal arrangement of one or more chromosome. Causes: 1. Failure of segregation of chromatids- cell division cycle- gain or loss chromosome- Aneuploidy, Eg.- Down’s syndrome (Extra copy of 21 chromosome)- Trisomy, Turner’s syndrome (loss of an X chromosome in female)- Monosomy 2. Failure of cytokinesis after telophase- increase in whole set chromosomes- Polyploidy, seen in plants
- 59. down's syndromes • Presence of an additional copy of chromosome no. 21- Trisomy of 21 • Described- Langdon Down (1866) • Short statured, small round head, furrowed tongue, partially open mouth, broad palm with palm crease; physical, psychomotor & mental- retardation
- 60. Klinefelter’s syndrome • Presence of an additional copy of X- chromosome • Karyotype- 47, XXY • Overall masculine development along with feminine development- development of breast (Gyanaecomastia), Sterile
- 61. turner’s syndroMe • Absence of one of X- chromosome, Monosomy • Karyotype- 45, X0 • Females-sterile, ovaries are rudimentary, lack of secondary sexual character