Principles of Genetics : 12th Class Biology Question Answer Lesson 22 – NIOS

Principles of Genetics : 12th Class Biology Question Answer Lesson 22 – NIOS

Principles of Genetics : 12th Class Biology Question Answer Lesson 22 - NIOS


  1. Name the founder of genetics and state why he is called so.
    Ans. Gregor John Mendel, was the first to suggest principles underlying heredity
  2. State one difference between
    (i) homozygous and heterozygous individuals
    Ans. homozygous = bearing identical alleles controlling a trait;
    heterozygous = bearing dissimilar alleles controlling a trait.
    (ii) dominant and recessive traits
    Ans. Dominant allele = expressing in both heterozygous and homozygous
    Recessive = expressing only in homozygous condition.
    (iii) genotype and phenotype
    Ans. Genotype = genetic constitution of an individual, represented with
    the help of symbols.
    Phenotype = class of individuals recognised based on externally/
    internally visible characters.
    (iv) monohybrid and dihybrid crosses.
    Ans. monohybrid = cross between two parents differing in a single pair
    of contrasting character;
    dihybrid corss = cross of two parents
    differing in two pairs of contrasting characters.
  3. Define heredity and Genetics.
    Ans. Heredity : is the study of transmission of characters from one generation to next generation.
    Variation: Differences between individuals of same species.
  4. Give the monohybrid and dihybrid phenotypic ratios for Mendelian inheritancce.
    Ans. Monohybrid ratio = 3 : 1, Dihybrid ratio = 9 : 3 : 3 : 1.
  5. Mention two sources of variation.
    Ans. Mutation, Recombination.


  1. Define :
    (i) An allele – Alleles are diffrerent forms of a gene.
    (ii) Codominance – Both alleles express as dominant phenotype.
    (iii) Polygenes – Many genes controlling same trait.
    (iv) Lethal genes – Presence of which kind of genes in an individual proves to be fatal?
  2. Name the kind of inheritance in terms of expression of
    (i) blood groups of humans – Codominance and multiple alleles
    (ii) wheat kernel colour – Incomplete dominance
    (iii) human skin colour? – Polygenic inheritance
  3. State the phenotypic monohybrid ratio in case of incomplete dominance.
    Ans. 1 : 2 : 1


  1. What are genes and where are they located?
    Ans. Genes are segments of DNA. They are located in chromosomes.
  2. State the names of the scientists who proposed the chromosomal theory of inheritance.
    Ans. Sutton and Boveri
  3. Define (i) linkage and (ii) crossing over.
    Ans. Linkage is the tendency of genes residing on the same chromosome
    to be inherited together.
    (ii) Breakage and exchange of genes between two chromatids of a
    homologous pair is termed crossing over.
  4. When does gene exchange through chiasma formation occur between homologous chromosomes?
    Ans. During prophase I of meiosis
  5. Why is the human female called the homogametic sex?
    Ans. Human female produces only one kind of gametes (homo = same)
  6. A colour blind man married a normal woman whose father and mother both had normal colour vision. Will any of their sons be colour blind? If not why not.
    Ans. No. Because gene for color blindness on X chromocomes is a recessive gene so it gets marked by renual gene from mother.
  7. With the help of flow chart explain the difference in sex determination in birds and mammals.
    Ans. Female
  8. Name an insect in which all males are produced parthenogenetically.
  9. In honey bees “males have no father and cannot have sons but have a
    grandfather”. Justify the statement.
    Ans. Because males develop from unfertilised or haploid eggs and females from fertilised or diploid eggs.
  10. Which sex in birds is heterogametic?
  11. Why is sex determination in honeybees called haplodiploidy
    Ans. Because males develop from unfertilised or haploid eggs and females from fertilised or diploid eggs.


  1. Why is mitochondrial inheritance treated as a case of maternal inheritance?
    Ans. Because mitochondia are inherited from the mother through the ovum.
  2. Into how many groups have human chromosomes been grouped in the human karyotype?
    Ans. Seven
  3. State the chromosomal abnormality in Kline felters, Turners syndrome and in Mongolism.
    Ans. Kline felter : 2n = 47;
    XXY Turner : 2n = 45;
    XO Mongolism : 2n = 47;
    Trisomy of chromosome 21


  1. State the three Mendel’s laws of inheritance. Which one of these laws is Universal.
  2. Consider a hypothetical case of a cross between a tall plant (TT) and a dwarf Heredityplant (tt). Work out the phenotypic and genotypic ratios of the F2 progeny ifthe cross were to show
    (a) dominance (b) incomplete dominance
  3. What will be the blood group of the progeny of parents with AB and O groups.
  4. Write notes on :
    (a) recessive lethal genes (b) pleiotropy
    (c) linkage groups (d) mitochondrial inheritance
    (e) human karyotype (f) human genome
  5. Why do we find so many different complexions among humans?
  6. State the chromosome theory of inheritance.
  7. Work out the following crosses and mention the phenotypic ratio of their
    (a) A colour blind man marries a carrier woman
    (b) A man with normal colour vision marries a carrier woman.
  8. Why is X-linked inheritance termed cris-cross inheritance?
  9. Give an account of genetic disorders caused by abnormal chromosomal number.
  10. What is amniocentesis? How and for what is it carried out?
  11. In what way is chromosomal sex determination of humans different from thatof birds?
  12. From which kind of eggs do males and females of honeybees emerge.

NIOS 12th class biology Q Ans

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