NIOS Class 12 Biology Chapter 22 Principles of Genetics

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NIOS Class 12 Biology Chapter 22 Principles of Genetics

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Principles of Genetics

Chapter: 22




1.  Name the founder of genetics and state why he is called so. 

Ans: Mendel is called the “Father or Founder of Genetics” because he suggested the “principles of inheritance for the first time. 

2. State one difference between: 

(i) Homozygous and heterozygous.

Ans: Difference between homozygous and heterozygous:

1. Individual having identi- cal genes for a character is called as homozygous e.g., AA, TT etc.Individual having dissi-milar genes for a charac-ter is called heterozygous e.g., Tt, Aa etc.
2. It bears identical genes/alleles a trait.It bears dissimilar genes alleles for a trait.

(ii) Dominant and recessive traits.

Ans: Difference between Dominant and Recessive:

It is trait that expresses in both homozygous and heterozygous conditions as ness trait. Tt, TT for tallness genes.It expresses only in homo-zygous state as tt for dwarf-ness trait.

(iii) Genotype and phenotype.

Ans: Difference between Genotype and Phenotype:

It is genetic constitution of an organism.It refers to the outward expression of gene/organism.

(iv) Monohybrid and dihybrid crosses.

Ans: Difference between Monohybrid and Dihybrid crosses:

Monohybrid CrossDihybrid Cross
It is a cross between two parents differing in a single pair of contrasting trait/character.It is a cross of 2 parents differing in 2 pairs of con-trasting traits/characters.

3. Define heredity and Genetics.

Ans: (i) Heredity: It is defined as “the transmission of characters from parents to their offsprings”.

(ii) Genetics: Genetics are defined “as the differences between siblings or members of same species”.

4. Give the monohybrid and dihybrid phenotypic ratios for Mendelian inheritance.

Ans: (i) Monohybrid ratio = 3: 1.

(ii) Dihybrid phenotypic ratio = 9:3:3:1. 

5. Mention two sources of variation.

Ans: (i) Gene mutation.

(ii) Recombination.


1. Define:

(i) An allele.

Ans: Allele: It refers “to the different forms of a gene”.

(ii) Codominance.

Ans: In codominance, both alleles express as dominant phenotype.

(iii) Polygenes.

Ans: Several genes controlling same trait are called polygenes.

(iv) Lethal genes.

Ans: Lethal genes presence in an individual may be fatal.

2. Name the kind of inheritance in terms of expression of:

(i) Blood groups of humans.

Ans: Codominance and Multiple Alleles.

(ii) Wheat kernel colour.

Ans: Polygenic Inheritance.

(iii) Human skin colour?

Ans: Polygenic Inheritance.

3. State the phenotypic monohybrid ratio in case of incomplete dominance.

 Ans: 1:2:1 Ratio.


1. What are genes and where are they located?

Ans: The genes are “segments of DNA”. The genes are located on the chromosomes in cells.

2. State the names of the scientists who proposed the chromosomal theory of inheritance.

Ans: Sutton and Boveri in 1902.

3. Define:

(i) Linkage. 

Ans: Linkage: It is defined “as the tendency of genes found on the same chromosome, to be inherited together and are tied together”.

(ii) Crossing over.

Ans: Crossing over: It is defined “as the breakage and exchange of genes between two chromatids of a homologous pair”.

4. When does gene exchange through chiasma formation occur between homologous chromosomes?

Ans: At Prophase Ist of Meiosis Cell Division. 

5. Why is the human female called the homogametic sex?

Ans: Because human female produces gametes of one kind with X sex chromosome plus 22 autosomes.

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. Since the gene for colour blindness is located on X chromosome, is a recessive gene; therefore gets masked by normal gene from the mother.

7. With the help of flow chart explain the difference in sex determination in birds and mammals.



8. Name an insect in which all males are produced parthenogenetically.

Ans: An since males develop from hapliad/ & females from fertilised eggs/unfertilized eggs diploid/eggs.

9. In honey bees “males have no father and cannot have sons but have a grandfather”. Justify the statement.

Ans: Males develop parthenogenetically. 

10. Which sex in birds is heterogametic?

Ans: Female is heterogametic – XW. 

11. Why is sex determination in honeybees called haplodiploidy.

Ans: Since there are haploid/deploid eggs that develop into males/females respectively.


1. Why is mitochondrial inheritance treated as a case of maternal inheritance?

Ans: Since the cell organelles mitochondria are inherited from mother to her progeny by ovum. (It is a case of maternal inheritance-cytoplasmic inheritance).

2. Into how many groups have human chromosomes been grouped in the human karyotype?

Ans: In seven groups.

3. State the chromosomal abnormality in Klinefelters, Turners and in Mongolism. 

Ans: (i) Klinefelter: 2N = 47; (44 + XXY). 

(ii) Turner: 2N = 45 (44+XO)

(iii) Mongolism:  2N = 47; Trisomy of Chromosome 21.


1. State the three Mendel’s laws of inheritance. Which one of these laws is universal?

Ans: Mendel proposed the following laws:

(i) Law of Dominance: It states that “if a cross is made between the two parents the expressed character resembles one of the parent called dominant and other suppressed is recessive.” 

(ii) Law of segregation: It states that “allelic pairs separate or segregate during gamete formation, and the paired condition is resorted by the random fusion of gametes during fertilisation”. This law states that if a cross is made between two parents, which are different in one unit character (allele pair), the suppressed character in F₁ generation, is segregated in F₂ generation in the ratio of 3: 1. It is called first law.

Dominant and Recessive characters (See below)

Fig. 22.7. Seven traits studies by Mendel.

Phenotypic Ratio 3 Tall: 1 Wrinkled 

Genotypic Ratio 1 TT: 2 Tt: 1tt

(iii) Law of Independent Assortment: This law states that if we consider the inheritance of two or more factors of genes at a time their distribution in the gametes and in the progeny of subsequent generations is independent of each other”.

Law of segregation is universal because incomplete dominance fails dominance and linkage fails law of independent assortment.

2. Consider a hypothetical case of a cross between a tall plant (TT) and a dwarf plant (tt). Work out the phenotypic and genotypic ratios of the F₂  progeny if the cross were to show:

(a) Dominance.

Ans: Dominance:

On selfing F₁


F₂ phenotypic ratio = 3 tall: 1 dwarf

F₂ genotypic ratio = 1 TT: 2Tt: 1tt

(b) Incomplete dominance.

Ans: Incomplete dominance: It is “the intermediate inheritance of dominant and recessive genes”.

The inheritance of flower colour in Snapdragon shows incomplete dominance. In incomplete dominance two alleles of a pair interact and produce an intermediate type. In Snapdragon a cross is made between red flowered (RR) and white flowered varieties. F₁ (Rr) progeny produced is all pink flowered. This type of inheritance is called incomplete dominance. It is due to incomplete genes which are also called intermediate genes.

When F₁ hybrids (Rr) are interbreed or self pollinated to raise F₂ generation, they produce phenotypic ratio – 1 Red, 2 Pink, 1 White. 2

i.e., 1: 2: 1.

F₂ genotype 1 RR: 2 Rr: 1 rr

Fig. 22.8. Incomplete Dominance in Snapdragon.

3. What will be the blood group of the progeny of parents with AB and O groups? 


Progeny blood groups are A, B. 

4. Write notes on:

(a) Recessive lethal genes.

Ans: Recessive lethal genes: These genes kill the organism in homozygous condition. 

(b) Pleiotropy.

Ans: Pleiotropy: In the ability of a gene to regulate many phenotypic characters. Example of pleiotropy is a hereditary disease called sickle cell anemia. It is caused by a recessive allele in humans.

(c) Linkage groups.

Ans: Linkage groups: All the genes found on a chromosome constitute a linkage group. The number of linkage group is proportional to the chromosomes found in an organism. There are 4 pairs of linkage groups in fruit.

(d) Mitochondrial inheritance.

Ans: Mitochondrial inheritance: It is a case of cytoplasmic inheritance. The traits are inherited by genes through maternal inheritance through the egg/ovum.

(e) Human karyotype.

Ans: Human Karyotype: Chromosomes (23 pairs) are arranged according to size in 7 groups in karyotype.

(f) Human genome.

Ans: Human genome: It is a mega project to sequence human genes (30,000 or 40,000).

5. Why do we find so many different complexions among humans?

Ans: This is due to genes.

6. State the chromosomal theory of inheritance.

Ans. Sutton and Boveri (1902) proposed the “chromosomal theory of inheritance”. The main points are:

(i) Somatic cells of an organism have two identical sets of chromosomes. Out of them one set of chromosomes is derived from mother (maternal chromosomes) and other one set from father (paternal chromosomes). The two chromosomes of one type (carrying same genes) consti-tute a homologous pair. The humans possess 23 pairs (46) of chromosomes.

(ii) Chromosomes of homologous pair sepa-rate out during meiosis cell division at time of gamete formation (gametogenesis).

(ii) Behaviour of chromosomes during meiosis cell division indicates that “Mendelian factors or genes are loca-ted linearly on the chromosomes”.

7. Work out the following crosses and mention the phenotype ratio of their progeny.

(a) A colour blind man marries a carrier woman.


Fig. 22.9. Progeny of colourblind man and carrier woman.

(b) A man with normal colour vision marries a carrier woman.


(C=gene for normal colour vision and c=gene for colour blindness).

Fig. 22.10. Progeny of carrier female for colour blindness and a man with normal colour vision.

8. Why is X-linked inheritance termed cris-cross inheritance?

Ans: Cris-cross Inheritance: The inheri-tance of a character from one sex to other sex in successive generation is called cris-cross inheritance. See example:

(Gene for Haemophilia-h):

Fig. 22.11. Cris-cross Inheritance or X-linked inheritance.

9. Give an account of genetic disorders caused by abnormal chromosomal number.

Ans: Klinefelter syndrome, Turner’s syndrome and Down’s syndrome are caused by abnormal chromosome number due to trisomy or aneuploidy etc.

(i) Down syndrome. It was described by Langdon Down in 1866. Its cause is trisomy of 21st chromosome. The afflicted persons are short statured having broad palm and crease etc.

(ii) Klinefelter syndrome. It has karyotype of 47 (44 + XXY). The individual has tall masculine development by gynecomastia. They are sterile mentally retarded males. See Fig.

(iii) Turner syndrome. It is caused by absence of one X chromosome (44+XO =45). They are sterile having rudimentary ovaries that never produce ova. They have web like skin on neck.

Fig. 22.12. Man showing Klinefelter’s Syndrome.

10. What is amniocentesis? How and for what is it carried out?

Ans: It is a technique “for detecting genetic disorder in the foetus”.

Amniocentesis is the prenatal diagnostic technique by which foetal cells can be made available to examine outside mother’s body to know genetic and metabolic disorders of foetus.

Use of aminocentesis: For detection of congential diseases and sex determination.

Fig. 22.13. Techniques of Arminocentesis.

11. In what way is chromosomal sex determination of humans different from that of birds?

Ans: Chromosomal sex determination is humans is XX♀-XY ♂ Type. In Birds it is ZZ ♂ -XW ♀. So the method of sex determination in the birds is a little different from humans. In birds both sexes (male & female) have 2 sex chromosomes but unlike human beings female has heteromorphic morphologically different sex chromosomes (ZW). But males bear homomorphic sex chromosomes (ZZ). It means the females are heterogametic & produce 2 kinds of eggs: A+Z and A+W (‘A’ stands for autosomes). Male gamete is only of one type: A+Z. This type of sex determination is termed as ZW-ZZ type or WZ-ZZ type of sex determination. Letters Z & W are used to distinguish types of sex chromosomes from X and Y chromosomes occur in X-Y type of sex determination in humans.

12. From which kind of eggs do males and females of honeybees emerge.

Ans: Sex Determination in Honey Bees: In honey bees, the fertilised eggs produce as females but unfertilised eggs develop into males. As fertilised eggs & females are diploid; the unfertilised eggs & males are haploid, the sex determination in honey bees is called haplodiploidy (arrhenotoky). Sex is determined by number of sets of chromosomes an individual gets Male (drone) is produced from unfertilised haploid eggs. Male honeybees have a single set of chromosomes. Female honeybees (worker & queen) are produced from fertilised eggs and so these are diploid. They have 2 sets of chromosomes (females produced by sexual reproduction); males parthenogenetically.

Moreover, in honey bees males have no father and so cannot have sons but have a grandfather as well as can have grandsons also.

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