10th Science Unit 18 Genetics Book Back Answers

Chromosomes are thin, long, thread-like structures in the nucleus, consisting of two identical strands called sister chromatids held together by the centromere.

Each chromatid is made of a spirally coiled thin thread called chromonema, which bears bead-like structures called chromomeres along its length.

Primary constriction (centromere): the point where the two arms meet; spindle fibres attach here during cell division.

Secondary constriction: present in some chromosomes; known as the nucleolar organiser, as the nucleolus is formed here.

Telomere: the end of the chromosome; it has polarity, prevents joining with adjacent chromosomes and gives stability.

Satellite: an elongated knob-like appendage at one end; such chromosomes are called sat-chromosomes.

Chemically a chromosome is made up of DNA, RNA, chromosomal proteins (histones and non-histones) and certain metallic ions.

Labels of the diagram: (a) phosphate group (small circles of the backbone), (b) deoxyribose sugar (pentagons), (c) nitrogenous bases projecting inwards, (d) hydrogen bonds (dotted lines) between the paired bases, and the two sugar–phosphate backbones of the two polynucleotide chains.

DNA is a polynucleotide; each nucleotide has a deoxyribose sugar, a nitrogenous base and a phosphate group (Nucleoside = base + sugar; Nucleotide = nucleoside + phosphate).

According to the Watson and Crick model, DNA consists of two polynucleotide chains forming a double helix, the two strands running anti-parallel to each other.

The sugar–phosphate units form the backbone of the DNA and the nitrogenous bases lie in the centre; nucleotides in a chain are joined by phosphodiester bonds.

Base pairing is specific, always between a purine and a pyrimidine: Adenine pairs with Thymine by two hydrogen bonds ($A=T$) and Cytosine pairs with Guanine by three hydrogen bonds ($C\equiv G$). This is complementary base pairing.

These hydrogen bonds make the DNA molecule stable. Each turn of the helix is $34\,\mathring{A}$ (3.4 nm) and contains ten base pairs.

Meaning: A dihybrid cross involves the inheritance of two pairs of contrasting characters at the same time.

Characters chosen by Mendel: Shape and colour of seeds — round-yellow seeds and wrinkled-green seeds.

Parental cross: Pure breeding round-yellow seeded plants ($RRYY$) were crossed with pure breeding wrinkled-green seeded plants ($rryy$).

F1 generation: All the seeds produced were round and yellow ($RrYy$). No wrinkled-green seeds appeared. Hence round shape and yellow colour are dominant over wrinkled shape and green colour.

F2 generation: When F1 plants were self-pollinated, four types of seeds appeared — round-yellow, round-green, wrinkled-yellow and wrinkled-green in the ratio $9:3:3:1$. This is the dihybrid ratio.

Results: 9 plants with two dominant traits, 3 with one dominant and one recessive trait, 3 with the other dominant and other recessive trait, and 1 with two recessive traits.

New combinations: Two new combinations — round-green and wrinkled-yellow — appeared in F2, showing that the factors of one pair assort independently of the other pair (Law of Independent Assortment).

Difference 1 (number of traits): A monohybrid cross involves only one pair of contrasting characters (e.g. tall × dwarf), while a dihybrid cross involves two pairs at the same time.

Difference 2 (ratios): Monohybrid F2 gives a phenotypic ratio of $3:1$ and a genotypic ratio of $1:2:1$, whereas dihybrid F2 gives a phenotypic ratio of $9:3:3:1$.

Difference 3 (law demonstrated): Monohybrid cross demonstrates the Law of Dominance and the Law of Segregation; the dihybrid cross demonstrates the Law of Independent Assortment and produces new combinations of traits.

Nature of DNA: DNA is the hereditary material and the most important constituent of a chromosome. It is a large molecule made of millions of nucleotides, so it is also called a polynucleotide.

Nucleotide: Each nucleotide has three components — a deoxyribose sugar, a nitrogenous base and a phosphate group. (Nucleoside = nitrogen base + sugar; Nucleotide = nucleoside + phosphate.)

Bases: The nitrogenous bases are of two kinds — purines (Adenine and Guanine) and pyrimidines (Cytosine and Thymine).

Watson–Crick model: DNA consists of two polynucleotide chains forming a double helix, with the two strands running anti-parallel to one another.

Backbone: The nitrogenous bases lie in the centre and are linked to sugar-phosphate units, which form the backbone of the DNA.

Base pairing: Pairing is specific, always between a purine and a pyrimidine linked by hydrogen bonds — Adenine links Thymine by two hydrogen bonds ($A=T$) and Cytosine links Guanine by three hydrogen bonds ($C\equiv G$). This is complementary base pairing, and these hydrogen bonds make the molecule stable.

Dimensions: Each turn of the double helix is $34\,\text{Å}$ ($3.4$ nm) and contains ten base pairs. The nucleotides in a helix are joined by phosphodiester bonds.

Significance 1: DNA is responsible for the transmission of hereditary information from one generation to the next generation.

Significance 2: It contains the information required for the formation of proteins.

Significance 3: It controls the developmental process and life activities of an organism.

Chromosome number: Human beings have 23 pairs of chromosomes — 22 pairs of autosomes and one pair of sex chromosomes (allosomes).

Female gametes: All eggs formed by the mother are alike in chromosome type, $(22+X)$. Therefore human females are homogametic.

Male gametes: The father produces two types of sperms in equal proportion — $(22+X)$ and $(22+Y)$. Therefore human males are heterogametic.

Fusion 1: Egg $(22+X)$ + sperm $(22+X)$ → zygote $(44+XX)$, which develops into a female child.

Fusion 2: Egg $(22+X)$ + sperm $(22+Y)$ → zygote $(44+XY)$, which develops into a male child.

Matter of chance: It is purely a matter of probability which category of sperm fuses with the egg, so the sex of the child cannot be predicted or controlled.

Who decides: Since the mother can give only an X chromosome, it is the sperm produced by the father that determines the sex of the child; the mother is not responsible for determining the sex of the child.

Choice of plant: Mendel chose the garden pea (Pisum sativum) because, though its flowers are bisexual and self-pollinating, they can easily be cross-pollinated by hand.

Emasculation: To stop self-pollination, he removed the immature anthers (male part) from the flower of the plant chosen as the female parent, before the pollen matured.

Bagging: The emasculated flower was then covered with a bag so that no unwanted pollen from other plants could fall on its stigma.

Artificial (hand) pollination: He collected mature pollen grains from the desired male parent plant and dusted them on the stigma of the emasculated flower, and re-bagged it.

Raising F1 plants: The seeds so formed were collected and sown; the plants grown from them formed the first filial (F1) generation of the monohybrid cross (tall × dwarf) and dihybrid cross (round-yellow × wrinkled-green).

Raising F2 plants: The F1 hybrids were allowed to self-pollinate naturally, and the seeds obtained gave the F2 generation, in which he got the 3:1 (monohybrid) and 9:3:3:1 (dihybrid) ratios.

Other helpful features: The pea plant is an annual with a short life span, has clearly defined pairs of contrasting characters and is naturally self-pollinating, so pure-breeding lines were easy to maintain — these made his experiments successful.

The statement made by the family members is not true.

In humans, of the 23 pairs of chromosomes, the 23rd pair is the sex chromosome pair. The mother has XX and the father has XY.

The mother produces only one type of egg (22+X), so human females are homogametic; she can never give a Y chromosome.

The father produces two types of sperms in equal proportion — (22+X) and (22+Y); so human males are heterogametic.

If the egg (22+X) is fertilised by an X-bearing sperm, the child is a female (44+XX); if it is fertilised by a Y-bearing sperm, the child is a male (44+XY).

Thus the sex of the child depends purely on chance — on which type of sperm from the father fuses with the egg. The mother is in no way responsible.

So Kavitha cannot be blamed, and there is an equal (50%) chance of her having a male child in future.

The law (textbook statement): "In case of inheritance of two or more pairs of characters simultaneously, the factors or genes of one pair assort out independently of the other pair." Mendel framed it from his dihybrid cross between round-yellow and wrinkled-green seeded pea plants.

Condition 1 – Two or more pairs of contrasting characters must be studied at the same time. The law applies only to a dihybrid (or polyhybrid) cross, e.g. seed shape (round/wrinkled) together with seed colour (yellow/green), because independence can be observed only when two pairs are followed simultaneously.

Condition 2 – The parents must be pure breeding (homozygous). Mendel crossed pure round-yellow (RRYY) plants with pure wrinkled-green (rryy) plants, so that the F1 was a uniform dihybrid (RrYy) receiving one factor of each pair from each parent.

Condition 3 – The two pairs of genes must lie on different (non-homologous) chromosomes, i.e. they must not be linked. Only then can the factors of one pair enter the gametes without being tied to the factors of the other pair; linked genes would travel together and the law would not hold.

Condition 4 – The factors must remain pure and segregate independently during gamete formation. As the law of segregation states, the two factors of an allelic pair remain together without mixing and separate at gamete formation, so each gamete receives either R or r and, independently, either Y or y. This produces four kinds of gametes — RY, Ry, rY and ry — in equal numbers.

Condition 5 – Fertilization must be random and the offspring number large. All four types of male gametes must have an equal chance of fusing with all four types of female gametes, and a large F2 population is needed for the expected proportions to appear.

Why these conditions give the result: When the four gamete types combine at random, 16 combinations are formed in the Punnett square and four phenotypes appear — round-yellow 9, round-green 3, wrinkled-yellow 3 and wrinkled-green 1, i.e. the dihybrid ratio 9:3:3:1.

Proof of independence: Two entirely new combinations, round-green and wrinkled-yellow, appear in the F2 generation. Such new combinations can arise only if the factor for seed shape passed into the gamete independently of the factor for seed colour, showing that each character maintains its own identity.