Genetics & Heredity

**1. Introduction & Core Concept**

Why do you look like your parents? Why do some diseases run in families? Why do children inherit some features from mum and others from dad? The answers lie in genetics — the study of heredity and DNA.

Your body contains about 37 trillion cells, and almost every one has a complete copy of your DNA — the instruction manual for building and running you. That manual has 3.2 billion "letters" (base pairs) arranged in 23 pairs of chromosomes. Understanding how genes are passed from parent to child is one of biology's greatest achievements.

**2. Core Theory**

2.1 — Key Terms

• Gene: a section of DNA coding for a specific protein or characteristic.
• Allele: a version of a gene (e.g., T = tall, t = short for pea height).
• Dominant allele: expressed even when only one copy present (written as capital letter).
• Recessive allele: only expressed when two copies present (lowercase).
• Homozygous: two identical alleles (TT or tt).
• Heterozygous: two different alleles (Tt).
• Genotype: the alleles an organism carries (e.g., Tt).
• Phenotype: the observable characteristic (e.g., tall).

2.2 — Monohybrid Inheritance (Punnett Square)

Cross between two heterozygous tall pea plants (Tt × Tt):

| | T | t |

|---|---|---|

| T | TT | Tt |

| t | Tt | tt |

Results: 3 tall : 1 short (phenotype ratio); TT : 2Tt : tt (genotype ratio)

2.3 — Sex Determination

Humans: 23 pairs of chromosomes. 22 pairs = autosomes. 1 pair = sex chromosomes.

• Females: XX
• Males: XY

All eggs carry X chromosome. Sperm carry either X or Y.

If X sperm fertilises egg → XX (female). If Y sperm fertilises egg → XY (male).

So sex is determined by the father's sperm.

2.4 — X-Linked Inheritance

Some genes are only on the X chromosome (X-linked). e.g., colour blindness, haemophilia.

• Males (XY) have only ONE X chromosome → if they carry the recessive allele, they show the trait.
• Females (XX) need TWO recessive alleles to show the trait → females are usually carriers.
• X-linked conditions are more common in males.

Example: Haemophilia (X^H = normal clotting; X^h = haemophilia)

• Carrier female: X^H X^h (phenotypically normal, carries haemophilia allele)
• Affected male: X^h Y (haemophilia)

2.5 — Codominance

Neither allele is dominant — both are expressed simultaneously.

Example: ABO blood groups (I^A, I^B, I^O)

• I^A I^B → blood group AB (both A and B antigens expressed)
• I^A I^O → blood group A
• I^B I^O → blood group B
• I^O I^O → blood group O

**3. Worked Examples**

Example 1: A woman is a carrier for cystic fibrosis (Ff). Her husband is also a carrier (Ff). Cystic fibrosis is recessive. What is the probability their child has cystic fibrosis?

Punnett square: Ff × Ff → FF : 2Ff : ff

Probability = 1/4 = 25%

Example 2: A colour-blind man (X^b Y) marries a woman with normal vision who is a carrier (X^B X^b). What fraction of their sons will be colour-blind?

Sons get Y from father, X from mother. Mother's gametes: X^B or X^b (50% each).

Sons: X^B Y (normal) or X^b Y (colour-blind). 50% of sons will be colour-blind.

Example 3: Two parents have blood groups A (I^A I^O) and B (I^B I^O). List the possible blood groups of their children.

I^A I^B (AB), I^A I^O (A), I^B I^O (B), I^O I^O (O) — all four blood groups possible.

**4. Pakistan Angle**

Thalassaemia is the most common genetic disorder in Pakistan — affecting 5,000+ babies per year. It is a recessive blood disorder: children inherit defective haemoglobin genes from both carrier parents. Pakistan has the world's second highest prevalence of thalassaemia carriers (~6% of population are carriers). The Thalassaemia Federation of Pakistan runs carrier screening programmes — couples where both partners are carriers (analogous to Ff × Ff) are counselled about the 25% risk with each pregnancy. This is direct application of Punnett square genetics.

**5. Exam Strategy**

• Always draw a Punnett square — even for "obvious" questions. Show the gametes clearly above and to the left.
• State genotype AND phenotype in your answers. Give probability as a fraction or percentage.
• X-linked: boys only need one recessive X allele to show the trait. Girls need two.
• Codominance: BOTH alleles are expressed — neither is dominant. Blood groups I^A and I^B are codominant; I^O is recessive.
• Carrier: heterozygous for a recessive allele — phenotypically normal but can pass the allele to offspring.