Chromosomes: The Basis of Heredity

Chromosomes are thread-like structures found in the nucleus of cells. They contain genes responsible for passing traits from parents to offspring.

Structure of Chromosomes

Chromosomes are made of proteins and nucleic acids, primarily DNA and RNA.

1. DNA (Deoxyribonucleic Acid): Found in the nucleus, it's the main hereditary material.
2. RNA (Ribonucleic Acid): Present in some viruses.

DNA is composed of repeating units called nucleotides, each containing:

• Deoxyribose sugar
• Phosphate group
• Nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).

The two strands of nucleotides form a double helix, with:

• A pairs with T (A-T)
• G pairs with C (G-C)

Chromosomes and Sex Determination

In humans, there are 23 pairs of chromosomes (46 total):

• 22 pairs of autosomes: Not involved in sex determination.
• 1 pair of sex chromosomes: Determines sex—XX for females, XY for males.

During fertilization, the combination of sperm (X or Y chromosome) with an egg (X chromosome) determines the baby’s sex:

• XX = Female
• XY = Male

Role of Chromosomes in Heredity

Gametes (sperm and eggs) are formed through meiosis, reducing the chromosome number by half. This allows for genetic recombination and variation.

Crossing over during meiosis leads to new combinations of genes, contributing to genetic diversity.

Chromosomal Disorders

Down’s Syndrome (Trisomy 21)

Caused by an extra copy of chromosome 21 (47 chromosomes).

Symptoms: Intellectual disability, distinctive facial features, and poor skeletal development.

Turner’s Syndrome (XO)

Affects females who have only one X chromosome (44 autosomes + XO).

Symptoms: Short stature, webbed neck, underdeveloped ovaries, and low intelligence.

Klinefelter’s Syndrome (XXY)

Affects males with an extra X chromosome (44 autosomes + XXY).

Symptoms: Tall stature, underdeveloped testis, and infertility.