Structure of a Chromosome:
(i) The chromosomes are thin, long and thread like structures consisting of two identical strands called sister chromatids.
(ii) They are held together by the centromere.
(iii) Each chromatid is made up of spirally coiled thin structure called chromonema.
(iv) The chromonema has number of bead-like structures along its length which are called chromomeres.
(v) The chromosomes are made up of DNA, RNA, chromosomal proteins (histones and non-histones) and certain metallic ions.
(vi) These proteins provide structural support to the chromosome.
A chromosome consists of the following regions:
Primary constriction:
(i) The two arms of a chromosome meet at a point called primary constriction or centromere.
(ii) The centromere is the region where spindle fibres attach to the chromosomes during cell division.
Secondary constriction:
(i) Some chromosomes possess secondary constriction at any point of the chromosome.
(ii) They are known as the nuclear zone or nucleolar organizer (formation of nucleolus in the nucleus).
Telomere:
(i) The end of the chromosome is called telomere. Each extremity of the chromosome has a polarity and prevents it from joining the adjacent chromosome.
(ii) It maintains and provides stability to the chromosomes.
Satellite:
(i) Some of the chromosomes have an elongated knob-like appendage at one end of the chromosome known as satellite.
(ii) The chromosomes with satellites are called as the sat-chromosomes

(i) Euploidy is the condition in which the individual bears more than the usual number of diploid (2n) chromosomes.
(ii) Tetraploidy plants are advantageous as they often result in increased fruit and flower size.

(i) Allosomes are chromosomes which are responsible for determining the sex of an individual.
(ii) They are also called as sex chromosomes or hetero-chromosomes.
 

(i) Pea plant is naturally self-pollinating and so is very easy to raise pure breeding individuals.
(ii) It has a short life span as it is an annual and so it was possible to follow several generations.
(iii) It is easy to cross-pollinate.
(iv) It has deeply defined contrasting characters.
(v) The flowers are bisexual.

(i) In the dihybrid cross the law of independent assortment holds good because two factors making up a pair of contrasting characters are called alleles or allelomorphs.
(ii) Mendel crossed two pairs of contrasting characteristics at the same time. He chose shape and colour of seeds, round-yellow seeds and wrinkled-green seeds.
(iii) The F₁ generation round-yellow seed was formed.
(iv) In the F₂ generation 9: 3: 3: 1 plants were formed.
(v) From the above results it can be concluded that the factors for each character or trait remains independent and maintains their identity in the gametes.
(vi) The factors are independent to each other and pass on to the offspring (through gametes).