(i) Human beings have 23 pairs of chromosomes out of which 22 pairs are autosomes and one pair (23^rd pair) is the sex chromosome.
(ii) The female gametes or the eggs formed are similar in their chromosome type (22+XX).
(ii) Therefore, human females are homogametic.

(iv) The male gametes or sperms produced are of two types.
(v) They are produced in equal proportions.
(vi) The sperm bearing (22+X) chromosomes and the sperm bearing (22+Y) chromosomes.
(vii) The human males are called heterogametic.
(v) It is a chance of probability as to which category of sperm fuses with the egg.
(vi) If the egg (X) is fused by the X-bearing sperm an XX individual (female) is produced.
(vii) If the egg (X) is fused by the Y-bearing sperm an XY individual (male) is produced.
(viii) The sperm, produced by the father, determines the sex of the child. The mother is not responsible in determining the sex of the child.
(ix) Now let's see how the chromosomes take part in this formation.
(x) Fertilization of the egg (22+X) with a sperm (22+X) will produce a female child (44+XX) while fertilization of the egg (22+X) with a sperm (22+Y) will give rise to a male child (44+XY).

DNA is a large molecule consisting of millions of nucleotides. Hence, it is also called a polynucleotide. Each nucleotide consists of three components.
(i) A sugar molecules - Deoxyribose sugar.
(ii) A nitrogenous base.
There are two types of nitrogenous bases in DNA.
They are
(a) Purines (Adenine and Guanine)
(b) Pyrimidines (Cytosine and Thymine)
(iii) A phosphate group
Nucleoside and Nucleotide:
Nucleoside = Nitrogen base + Sugar
Nucleotide = Nucleoside + Phosphate
The nucleotides are formed according to the purines and pyrimidines present in them.
Watson and Crick model of DNA:
(i) DNA molecule consists of two polynucleotide chains.
(ii) These chains form a double helix structure with two strands which run anti-parallel to one another.
(iii) Nitrogenous bases in the centre are linked to sugar-phosphate units which form the backbone of the DNA.
(iv) Pairing between the nitrogenous bases is very specific and is always between purine and pyrimidine linked by hydrogen bonds.
a) Adenine (A) links Thymine (T) with two hydrogen bonds (A = T)
b) Cytosine (C) links Guanine (G) with three hydrogen bonds( C ≡ G) This is called complementary base pairing.
(v) Hydrogen bonds between the nitrogenous bases make the DNA molecule stable.
(vi) Each turn of the double helix is 34 A^o (3.4 nm). There are ten base pairs in a complete turn.
(vii) The nucleotides in a helix are joined together by phosphodiester bonds.
Significance of DNA:
(i) DNA is responsible for the transmission of hereditary information from one generation to next generation.
(ii) It contains information required for the formation of proteins.
(iii) It controls the developmental process and life activities of an organism.

(i) Dihybrid cross involves the inheritance of two pairs of contrasting characteristics (or contrasting traits) at the same time.
(ii) The two pairs of contrasting characteristics chosen by Mendel were shape and colour of seeds: round-yellow seeds and wrinkled-green seeds.
(iii) Mendel crossed pea plants having round - yellow seeds with pea plants having wrinkled green seeds.
Mendel made the following observations:
(i) Mendel first crossed pure breeding pea plants having round-yellow seeds with pure breeding pea plants having wrinkled green seeds and found that only round yellow seeds were produced in the first generation (F₁).
(ii) No wrinkled-green seeds were obtained in the F₁ generation.
(iii) From this it was concluded that round shape and yellow colour of the seeds were dominant traits over the wrinkled shape and green color of the seeds.
(iv) When the hybrids of F₁ generation pea plants having round-yellow seeds were cross-breed by self pollination, then four types of seeds having different combinations of shape and colour were obtained in second generation or F₂ generation.
(v) They were round-yellow, round-green, wrinkled yellow and wrinkled-green seeds.
(vi) The ratio of each phenotype (or appearance) of seeds in the F₂ generation is 9:3:3:1. This is known as the Dihybrid ratio.
(vii) From the above results it can be concluded that the factors for each character or trait remain independent and maintain their identity in the gametes.
(viii) The factors are independent to each other and pass to the offsprings (through gametes).

Results of a Dihybrid Cross:
(i) Mendel got the following results from his dihybrid cross.
Four Types of Plants:
(i) A dihybrid cross produced four types of F₂ offsprings in the ratio of 9 with two dominant traits, 3 with one dominant trait and one recessive trait, 3 with another dominant trait and
another recessive trait and 1 with two recessive traits.
New Combination:
(i) Two new combinations of traits with round green and wrinkled yellow had appeared in the dihybrid cross (F₂ generation).

Kavitha's family members' statement is false, because the sex is determined by male.
In human being female gametes form the chromosome type 22+XX. But in male gametes two types of chromosomes are formed. One is (22+X) another one is (22+Y).
(i) If a female gametes are (22+X) fuses with a male gamete (22+X) than its forms a female child.
(ii) If a female gamete (22+X) is fuses with the male gamete (22+Y) than it forms a male child.
(iii) The sex is determined by the male gamete.

a) The F₁ generation plants are tall.
b) In the F₁ generation there were 3 tall plants and a dwarf plant. The ratio is 3 : 1.
c) The dwarf plants were missing in F₁ generation but reappeared in F₂ generation.

(i) Flowers of the garden pea are bisexual and self-pollinated.
(ii) During hybridization experiment, Mendal simply cut off anther in bud (Emasculation) and did the cross pollination of selected plant's pollen on the stigma of the flower.
(iii) So self pollination will be avoided.
(iv) And there by he followed the effects of crossing for several generation.