Revision: Heredity and Variation Science SSC (English Medium) 9th Standard Maharashtra State Board

The nucleus contains most of the cell's DNA, which is organised into discrete units called chromosomes.

After duplication, a chromosome has two identical sections. A chromosome is formed during cell division by the union of two chromatids.

Aneuploidy is the addition or deletion of one or two chromosomes in a diploid chromosomal pair.

Aneuploidy refers to the chromosomal variation due to a loss or a gain of one or more chromosomes deviating from the normal genome number for that species due to nondisjunction of the homologous chromosome.

One vertical half of a duplicated chromosome is called a chromatid.

Two identical chromatids that are joined by a centromere are called sister chromatids which eventually get separated during anaphase. 

A pair of corresponding chromosomes of the same shape and size, one obtained from each parent.

The nucleus contains most of the cell's DNA which is organized into discrete units called chromosomes.

or

Chromosomes are highly coiled, ribbon-like structures formed by the condensation of chromatin fibres during cell division. 

Each chromosome contains one long DNA molecule associated with many proteins. This complex of DNA and proteins is called the chromatin.

Each chromosome in its condensed form as visible during the start of cell division, consists of two sister chromatids joined at some point along the length. This point of attachment is called centromere, and it appears as a small constricted region.

Mendel's first experiments were with the varieties of garden pea that differed in only one visible character. These are known as monohybrid experiments.

Mendel investigated not only those crosses in which the parent differed in single pair of characters, but also others in which the parent differed in two pairs. Such a cross which involves two pairs of contrasting characters simultaneously is called dihybrid cross.

or

A genetic cross involving two pairs of contrasting characters simultaneously is called a dihybrid cross.

Non-disjunction occurs when chromosomes fail to split during cell division, resulting in aberrant chromosomal combinations.

1. Discovered by Walther Flemming in 1882 during his study of cell division in salamanders.
2. Chromatin (DNA + proteins) condenses to form chromosomes during cell division.
3. Chromosomes are ribbon-like, highly coiled structures that become visible when stained.
4. Each chromosome has two identical sister chromatids joined at a point called the centromere.
5. The centromere attaches to spindle fibres and helps separate sister chromatids during cell division.
6. After division, chromatids decondense into chromatin fibres in the nucleus.
7. Chromosomes ensure the equal distribution of genetic material to daughter cells.
8. Each chromosome contains one long DNA molecule that carries hereditary information.

1. DNA structure was first studied by Rosalind Franklin (1953); later explained by Watson and Crick, who proposed the double helix model (Nobel Prize, 1962).

2. DNA is a macromolecule made of two complementary strands twisted into a double helix.

3. Each strand is made up of nucleotides, which include phosphate, sugar (pentose), and a nitrogenous base.

4. There are four nitrogenous bases:

• Adenine (A) pairs with Thymine (T) (2 hydrogen bonds)
• Guanine (G) pairs with Cytosine (C) (3 hydrogen bonds)

5. The two strands form a ladder-like structure, with bases as rungs and sugar-phosphate as the backbone.

• Nature of RNA: RNA is usually a single-stranded nucleic acid molecule.
• Composition: RNA is composed of nucleotides containing ribose sugar, phosphate group, and a nitrogenous base.
• Nitrogenous Bases: RNA contains four bases—adenine (A), guanine (G), cytosine (C), and uracil (U); uracil replaces thymine.
• Base Pairing: Adenine pairs with uracil, while guanine pairs with cytosine.
• Backbone Structure: The RNA backbone consists of alternating ribose sugar and phosphate groups.

• Gregor Johann Mendel (1822–1884), an Austrian monk, is known as the Father of Genetics for his pioneering work on heredity.
• He studied science and mathematics at the University of Vienna, which helped him apply a quantitative approach to biological problems.
• Mendel conducted systematic hybridization experiments on garden pea (Pisum sativum) from 1856 to 1863.
• From these experiments, he formulated the fundamental Laws of Inheritance, explaining how traits are transmitted across generations.
• Although his work was ignored during his lifetime, it was rediscovered in 1900, leading to widespread recognition and the foundation of modern genetics.