The Principles of Genetics NIOS Biology Chapter 22, explains inheritance and variation in living organisms. It covers Mendel’s laws, genetic crosses, and the molecular basis of heredity. Students also learn about chromosomal theory and genetic disorders. Solved terminal exercises provide easy-to-understand answers for exam preparation. This chapter builds a strong foundation in genetics and is essential for students aiming to perform well in NIOS Class 12 Biology and higher-level biological studies.
Principles Of Genetics NIOS Biology Chapter 22
1. State the three Mendel’s laws of inheritance. Which one of these laws is universal?
1. Law of segregation or purity of gametes. At the formation of gametes, the two chromosomes of each pair separate (segregate) into two different cells, which form the gametes.
2. Law of dominance. During inheritance of many traits (e.g., eye colour, flower colour, seed shape) is controlled by one pair of genes.
3. Law of independent assortment meaning whereby that in the inheritance of two features (each feature controlled by a pair of genes), genes for the two different features are passed down into the offspring independently i.e. the segregation of one pair of factors is independent of the segregation of the factors belonging to any other pair of factors or allelic pair.
The Law of Segregation, or purity of gametes, is the universal law.
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, (b) incomplete dominance.
(a) TT x tt (Dominance)
| Tall Plant | Dwarf Plant | |
| Parents | TT | tt |
| Gametes | T and T | t and t |
| F1 genotype | Tt | |
| Phenotype | Tall plants | |
(b) Incomplete dominance:
In the four O’clock plant, Mirabilis jalapa, and Snapdragon or Antirrhinum law of dominance does not hold good. Thus, when a homozygous red flowered plant (RR) is crossed to a homozygous white flowered plant (rr), all flowers in the F1 are pink, while when F1 plants are self-pollinated, the phenotypic ratio in the next generation is found to be 1: 2: 1.
| Parents | RR × rr |
| Gametes | R, R × r, r |
| F1 | Rr (Pink) |
| F2 | Rr x Rr |
| Phenotypic ratio | 1 Red: 2 Pink: 1 White |
| Genotypic ratio | 1 RR: 2 Rr: 1 rr |
Here, the heterozygous (Rr) plants have an intermediate colour pink.
Their genotypic ratio is 1 RR: 2 Rr: 1 rr
Phenotypic ratio 1 Red: 2 Pink: 1 white is the same, that is, 1: 2: 1.
3. What will be the blood group of the progeny of parents with AB and O groups?
Parent 1 AB produces gametes = IA and IB
Parent 2 produces gametes = Io and Io
| Parents | Io | Io |
| IA | IA Io (A group) | IAIo (A group) |
| IB | IB Io (B group) | IB Io (B group) |
Thus, parents having blood groups AB and O can have children having either the A group or the B group.
4. Write notes on:
(a) recessive lethal genes (b) pleiotropy (c) linkage groups
(d) mitochondrial inheritance (e) human karyotype (f) human genome
- (a) Recessive Lethal Genes: Genes that have a homozygous recessive combination (yy) that is lethal to the organism are called recessive lethal genes
- (b) Pleiotropy is the ability of a gene to control several phenotypes. For example, the recessive gene for white eye in Drosophila in its homozygous condition can control the shape of the abdomen, shape of wings, etc, as well.
- (c) Linkage Groups: All the genes present on the same pair of chromosomes and with a All the genes present on the same pair of chromosomes and with tendency to be inherited together form a linkage group.
- (d) Mitochondrial Inheritance: Mitochondrial inheritance refers to the traits inherited through the genes in the mitochondria. It is also called maternal inheritance, as the mitochondria are inherited through the ovum.
- (e) Human Karyotype: Human karyotype is the arrangement of human chromosomes in seven groups according to the types of chromosomes and their size. It is prepared by arranging chromosomes seen at mitotic metaphase in descending order, with the longest pair of chromosomes drawn first, and the sex chromosomes are drawn last.
- (f) Human Genome: Genome means genes of a particular organism on its haploid set of chromosomes. The human genome may be defined as all the genes present in the haploid set of chromosomes in humans.
5. Why do we find so many different complexions among humans?
Genes are responsible for the different complexions in humans. Genes undergo recombination that gives rise to a new combination of genes that is reflected in the phenotype, as seen in complexion and other traits in humans.
6. State the chromosome theory of inheritance.
Sutton and Boveri proposed the ‘chromosomal theory of inheritance’ in 1902, and its salient features are as follows.
- 1. The somatic or body cells of an organism, which are derived from the repeated division of the zygote, have two identical sets of chromosomes, i.e., they are diploid. Out of these, one set of chromosomes is received from the mother (maternal chromosomes) and one set from the father (paternal chromosomes). Two chromosomes of one type (carrying genes controlling the same set of characters) constitute a homologous pair. Humans have 23 pairs of chromosomes.
- 2. The chromosomes of a homologous pair separate during meiosis at the time of gamete formation.
- 3. The behaviour of chromosomes during meiosis indicates that Mendelian factors or genes are located linearly on the chromosomes. With progress in molecular biology, it is now known that a chromosome is made of a molecule of DNA, and specific sets of segments of DNA are the genes.
7. Work out the following crosses and mention the phenotypic ratio of their progeny.
(a) A colour blind man marries a carrier woman
(b) A man with normal colour vision marries a carrier woman.
Let Xᶜ = allele for color blindness, Xᴺ (or Xᶜ⁺/X^C) = normal allele, and Y = Y chromosome.
(a) Colour-blind man × carrier woman
Father: XᶜY (affected) and Mother: XᴺXᶜ (carrier)
| Xᴺ (mother) | Xᶜ (mother) | |
| Xᶜ (father) | XᴺXᶜ | XᶜXᶜ |
| (daughter — carrier, normal vision) | (daughter — colour-blind) | |
| Y (father) | XᴺY | XᶜY |
| (son — normal vision) | (son — colour-blind) |
Phenotypic ratio (overall): 3 normal : 1 colour-blind (i.e., 25% affected).
By sex: Daughters all normal (½ carriers)
Sons 1 normal : 1 colour-blind.
Phenotypic ratio (overall): 1 normal : 1 colour-blind (i.e., 50% affected).
By sex: Daughters 1 carrier : 1 colour-blind; Sons 1 normal : 1 colour-blind.
(b) Man with normal colour vision × carrier woman
Father: XᴺY (normal)
Mother: XᴺXᶜ (carrier)
| Xᴺ (mother) | Xᶜ (mother) | |
| XN (father) | XᴺXᴺ | XNXᶜ |
| (daughter — non-carrier, normal vision) | (daughter — carrier, normal vision) | |
| Y (father) | XᴺY | XᶜY |
| (son — normal vision) | (son — colour-blind) |
Daughters: all normal (½ non-carrier XᴺXᴺ, ½ carrier XᴺXᶜ)
Sons: 1 normal (XᴺY) : 1 colour-blind (XᶜY)
Overall phenotypic ratio (all children): 3 normal : 1 colour-blind (25% affected).
8. Why is X-linked inheritance termed criss-cross inheritance?
The type of inheritance of a recessive sex linked character from father to daughter and then from the daughter to her sons is known as criss-cross inheritance or sex linked or X-linked inheritance.
9. Give an account of genetic disorders caused by abnormal chromosomal number.
- Mongolism or Down’s syndrome
The individual has 47 chromosomes because of one extra chromosome in the 21st pair (Trisomy of chromosome 21). The outcome of this defect is the following characters or features:
- Mentally retarded
- Have a thick tongue
- Drooping (false expression of pleasure) face.
The possibility of giving birth to a Mongolian child is far greater in pregnant mothers above the age of forty.
- Klinefelter’s syndrome
The individual is a male with 47 chromosomes with one extra X chromosome. (44 autosomes + XXY). Typical features of Klinefelter’s syndrome are:
Tall, mentally retarded male;
Sterile and shows breast development or gynaecomastia (gynae: female; massere: mammary glands).
- Turner’s syndrome
An individual is a female with 45 chromosomes and with only one X chromosome (22 pairs of autosomes +XO). The characteristic features of this syndrome are
- Mentally retarded
- Web-like skin on the neck.
- Incompletely developed breasts.
10. What is amniocentesis? How and for what is it carried out?
Amniocentesis is a technique by which hereditary disorders due to defects in genes can be detected in the fetus. In this technique
- (i) A small sample of amniotic fluid, which surrounds the foetus, is syringed out.
- (ii) This fluid has cells that break off from the skin of the foetus.
- (iii) Foetal cells are picked up and cultured.
- (iv) Chromosomes in the dividing cells are analysed for genetic defects.
11. In what way is chromosomal sex determination of humans different from that of birds?
Sex Determination in Humans
Sex chromosomes in males are morphologically dissimilar (i.e., XY). Such individuals produce two types of gametes (one containing X and the other containing Y) and are called heterogametic. For example, a human male produces two kinds of sperm, X-bearing and Y-bearing sperms. When the human egg is fertilised by an X-bearing sperm, a girl is born, and if the human egg is fertilized by a sperm having “Y chromosome, a boy is born.
Sex Determination in Birds
In birds, both sexes (male and female) possess two sex chromosomes, but unlike human beings, the female has the heteromorphic, morphologically different sex chromosomes (ZW) while the
males bear homomorphic (condition, the sex chromosomes (ZZ). Thus, the females are heterogametic and produce two types of eggs: A+Z and A+W (‘A’ stands for autosomes). The male gamete is only of one type: A+Z. This type of sex determination is called the ZW-ZZ type or WZ-ZZ type of sex determination. The letters Z and W are used to distinguish these types of sex chromosomes from X and Y chromosomes found in the X-Y type of sex determination.
12. From which kind of eggs do males and females of honeybees emerge?
In honey bees, fertilised eggs emerge as females and unfertilised eggs develop into males. Here, the sex is determined by the number of sets of chromosomes an individual receives, and thus, males are haploid. It is very interesting in honey bees that males have no father and cannot have sons, but have a grandfather and can have grandsons.
