Biology · Chapter 08

🧬 Genetics & DNA

DNA, genes, Mendel's laws, heredity.

💡 The Code of Life

DNA (Deoxyribonucleic Acid) is a long molecule that carries all the genetic instructions for life. It's shaped like a twisted ladder — the double helix, discovered by Watson and Crick in 1953.

Structure:

• Two strands twisted around each other
• Each "rung" is a pair of bases: A-T (adenine-thymine) and G-C (guanine-cytosine)
• Bases ALWAYS pair: A with T, G with C (complementary base pairing)

A section of DNA that codes for a specific trait is called a gene. Humans have ~20,000-25,000 genes. All your genes together = your genome.

DNA is packed tightly into structures called chromosomes. Humans have 23 pairs of chromosomes (46 total). 22 pairs are autosomes, 1 pair is sex chromosomes (XX = female, XY = male).

🧬 Mendel's laws (NEET high-yield)

Gregor Mendel experimented with pea plants. He found:

• Law of Dominance — when two different alleles meet, one (dominant) shows; the other (recessive) hides. Example: T (tall) dominant over t (short).
• Law of Segregation — gametes get only ONE allele from each parent.
• Law of Independent Assortment — different traits are inherited independently (e.g., seed color and seed shape).

🔬 Monohybrid cross — the 3:1 ratio

Cross two heterozygous parents (Tt × Tt):

Offspring possible combinations: TT, Tt, Tt, tt = 1:2:1
Phenotype (visible trait): 3 tall : 1 short = 3:1 ratio
This is the most famous genetics ratio.

⚡ Sex determination in humans

• Mother: XX (gives X always)
• Father: XY (gives X or Y, 50-50)
• If sperm carries X → girl (XX). If sperm carries Y → boy (XY).

So the father's sperm determines the sex of the baby, not the mother. (This historical misunderstanding caused much grief in Indian families — please share this fact.)

🎬

The DNA Double Helix

Animation
DNA — TWISTED LADDER OF A-T AND G-C PAIRS BASE PAIRS A (Adenine) T (Thymine) G (Guanine) C (Cytosine) A ↔ T · G ↔ C DID YOU KNOW Length unraveled in 1 cell: ~2 metres All your DNA combined: ~64 billion km (Earth to Sun and back 215 times) Yet packed inside microscopic nuclei. GENE → CHROMOSOME → GENOME A gene is a section of DNA. Chromosomes are bundles of genes. Humans have 23 pairs = 46 chromosomes total.

Notice: A always pairs with T, G always with C. This rule lets DNA copy itself accurately.

🔬

Mendel's Monohybrid Cross

Interactive

Pick a cross between two parents. See offspring ratios.

Genotype ratioAll Tt
Phenotype ratio100% Tall
When pure tall (TT) meets pure short (tt), all offspring get one T and one t → all show the dominant tall trait.
Practice (NCERT): If a tall pea plant (Tt) is crossed with a short pea plant (tt), what fraction of offspring will be short?
Cross: Tt × tt
Possible gametes: T,t from first parent; t,t from second.
Offspring combinations: Tt, Tt, tt, tt = 2 Tt : 2 tt = 1:1 ratio
So 50% (1/2) will be short (tt) and 50% will be tall (Tt). This is the test cross ratio.
Practice (NEET): In a Tt × Tt cross of pea plants, what is the probability that the offspring is HOMOZYGOUS dominant?
Cross: Tt × Tt
Gametes: T,t from each parent.
Punnett square:
TT (tall, homozygous dominant)
Tt (tall, heterozygous)
Tt (tall, heterozygous)
tt (short, homozygous recessive)
So genotype ratio = 1 : 2 : 1
Homozygous dominant (TT) probability = 1/4 = 25%.
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