Principles of Inheritance - NCERT Questions & Answers PDF

PRINCIPLES OF INHERITANCE AND VARIATION 5 NCERT QUESTIONS WITH ANSWERS 1. Mention the advantages of selecting pea plant for experiment by Mendel. (i) The plant shows clear-cut contrasting characters. (ii) It has perfect self-pollinat...

PRINCIPLES OF INHERITANCE AND VARIATION 5 NCERT QUESTIONS WITH ANSWERS 1. Mention the advantages of selecting pea plant for experiment by Mendel. (i) The plant shows clear-cut contrasting characters. (ii) It has perfect self-pollinating bisexual flowers. (iii) Easy to get pure lines for several generations. (iv) Its short life cycle helps to study several generations within a short period. (v) It is easy to cultivate. (vi) It is easy to cross because pollen from stigma of another plant. 2. Differentiate between the following – (a) Dominance and Recessive (b) Homozygous and Heterozygous (c) Monohybrid and Dihybrid. Differences between: (a) Dominant character: It is the character which is expressed in heterozygous condition. Recessive character: It is the character which is suppressed in heterozygous condition. (b) Homozygous is the condition in which chromosome pair carries similar alleles of a gene. Also known as pure line (True breeding). E.g. TT, tt, YY, yy etc. Heterozygous is the condition in which chromosome pair carries dissimilar alleles of a gene. E.g. Tt, Yy etc. (c) Monohybrid: It is a cross involving 2 plants differing in one character pair. Dihybrid: It is a cross between two parents differing in 2 pairs of contrasting characters. 3. A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced? If an organism is heterozygous at 4 loci (e.g. Aa, Bb, Cc, Dd), 24 = 16 types of gametes can be produced. But, if all the loci are linked, then 2 gametes are produced. 4. Explain the Law of Dominance using a monohybrid cross. Monohybrid cross is a cross involving 2 plants differing in one character pair. E.g. Mendel crossed tall and dwarf pea plants to study the inheritance of one gene. Monohybrid phenotypic ratio: 3 Tall: 1 Dwarf = 3:1 Monohybrid genotypic ratio: 1 TT: 2 Tt: 1 tt: = 1:2:1 Mendel’s Law of Dominance: Characters are controlled by discrete units called factors. Factors occur in pairs. In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive). 5. Define and design a test-cross. 1  Testcross: Crossing of F1 hybrid with its recessive parent. Hence monohybrid test cross ratio= 1:1 6. Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus. Cross between a homozygous female and a heterozygous male: Phenotypic features of F1 generation = Tall: dwarf 7. When a cross in made between tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy), what proportions of phenotype in the offspring could be expected to be (a) Tall and green (b) Dwarf and green. Parents: TtYy x Ttyy Gametes: TY Ty tY ty Ty ty Ty ty TTYy TtYy TY Tall yellow Tall yellow TTyy Ttyy Ty Tall green Tall green TtYy ttYy tY Tall yellow Dwarf yellow ty Ttyy Ttyy Tall green Dwarf green Phenotypes of the offspring: Tall yellow: 3 Tall green: 3 Dwarf yellow: 1 Dwarf green: 1 (a) Proportion of tall & green: 3/8 (b) Proportion of dwarf & green: 1/8 8. Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dibybrid cross? Consider the genotypes of two heterozygous parents are AaBb. If the 2 loci are linked, then it will produce only 2 types of gametes, i.e., AB & ab. Parents: AaBb X AaBb Gametes: AB ab AB ab F1: AABB AaBb AaBb aabb 9. Briefly mention the contribution of T.H. Morgan in genetics. Thomas Hunt Morgan proved chromosomal theory of inheritance using fruit flies (Drosophila melanogaster). Morgan carried out several dihybrid crosses in Drosophila to study sex-linked genes. E.g. Cross 1: Yellow-bodied, white-eyed females X Brown-bodied, red-eyed males (wild type) Cross 2: White-eyed, miniature winged X Red eyed, large winged (wild type) 2  Morgan intercrossed their F1 progeny. He found that  The two genes did not segregate independently of each other and the F2 ratio deviated from the 9:3:3:1 ratio.  Genes were located on the X chromosome.  When two genes were situated on the same chromosome, the proportion of parental gene combinations was much higher than the non-parental type. This is due to linkage.  Genes of white eye & yellow body were very tightly linked and showed only 1.3% recombination.  Genes of white eye & miniature wing were loosely linked and showed 37.2% recombination.  Tightly linked genes show low recombination. Loosely linked genes show high recombination. 10. What is pedigree analysis? Suggest how such an analysis, can be useful. In human, control crosses are not possible. So the study of family history about inheritance is used. Such an analysis of genetic traits in several generations of a family is called pedigree analysis. The representation or chart showing family history is called family tree (pedigree). In human genetics, pedigree study is utilized to trace the inheritance of a specific trait, abnormality or disease. It helps to identify whether a particular disease is due to recessive or dominant allele. 11. How is sex determined in human beings? Human has 22 pairs of autosomes and 1 pair of sex chromosomes (X & Y) - XX in the female, XY in male. During spermatogenesis, males produce 2 types of gametes: 50 % with X-chromosome and 50 % with Y- chromosome. Females produce only ovum with an X-chromosome. There is an equal probability of fertilization of the ovum with the sperm carrying either X or Y chromosome. Thus the sperm determines whether the offspring male or female. 12. A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offspring. Genotype of child is ii. Each allele in this genotype was inherited from parents. So parents must have an i allele in their genotypes. Thus genotype of father: IAi (Heterozygous A group) Genotype of mother: IBi (Heterozygous B group) IAi x IBi Gametes: IA i x IB i IAIB IAi IBi ii Thus the possible genotypes of other offspring are I I (AB group), IAi (A group) and IBi (B group). A B 13. Explain the following terms with example (a) Co-dominance (b) Incomplete dominance (a) Co-dominance is the inheritance in which both alleles of a gene are expressed in a hybrid. E.g. ABO blood grouping in human. ABO blood groups are controlled by the gene I. It has three alleles IA, IB & i. IA and IB produce a slightly different form of the sugar antigens (A & B respectively). So both are dominant alleles. The allele i doesn’t produce any sugar. When IA and IB are present together they both express their own types of sugars. This is due to co-dominance. (b) Incomplete dominance is an inheritance in which heterozygous offspring shows intermediate character b/w two parental characteristics. E.g. Flower colour in snapdragon (dog flower or Antirrhinum sp.) and Mirabilis jalapa (4’O clock plant). 3  Here, phenotypic and genotypic ratios are same. Phenotypic ratio= 1 Red: 2 Pink: 1 White Genotypic ratio= 1 (RR): 2 (Rr): 1(rr) This means that R was not completely dominant over r. 14. What is point mutation? Give one example. It is the mutation due to change in a single base pair of DNA. E.g. sickle cell anaemia. 15. Who had proposed the chromosomal theory of the inheritance? Walter Sutton & Theodore Boveri. 16. Mention any two autosomal genetic disorders with their symptoms. Sickle cell anaemia. Symptoms: Aggregation of RBC and anaemia. Phenyl Ketonuria (PKU). Symptoms: Poorly developed brain and mental retardation. 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