Genetics

🟢 Lite — Quick Review (1h–1d)

Rapid summary of genetics for NABTEB biology.

Genetics is the study of heredity — how traits are passed from parents to offspring.

Key Definitions:

• Gene: A section of DNA that codes for a specific trait
• Allele: Different versions of a gene (e.g., allele for tall vs allele for short)
• Dominant allele: Expressed when present (capital letter: e.g., T)
• Recessive allele: Only expressed when no dominant allele is present (lowercase: e.g., t)
• Homozygous: Two identical alleles (TT or tt)
• Heterozygous: Two different alleles (Tt)
• Genotype: Genetic makeup (e.g., Tt)
• Phenotype: Physical appearance (e.g., tall)
• Carrier: Heterozygous individual with a recessive allele but showing normal phenotype

Mendel’s Laws:

1. Law of Segregation: Each organism has two alleles for each trait; during gamete formation, these alleles separate so each gamete receives one allele.

2. Law of Independent Assortment: Alleles for different traits are distributed to gametes independently of one another.

Monohybrid Cross:

Cross between two individuals differing in one trait.

Example: Tall (T) × Short (t) $$P: \text{Tall (TT)} \times \text{Short (tt)}$$ $$F_1: \text{All Tall (Tt)}$$ $$F_2: \genfrac{}{}{0pt}{}{\text{T}}{\text{T}} \genfrac{}{}{0pt}{}{}{} \Rightarrow 3 \text{ Tall} : 1 \text{ Short}$$

⚡ NABTEB Exam Tip: In genetics questions, always determine the parental genotypes from the phenotypic ratios given. Work systematically: write the P generation, gametes, F₁, then F₂.

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🟡 Standard — Regular Study (2d–2mo)

For NABTEB students who want thorough understanding of genetics.

Punnett Square:

A grid used to predict offspring genotypes and phenotypes.

Example — Dihybrid Cross (Two Traits):

Cross between round-yellow (RRYy) and wrinkled-green (rryy) peas.

Gametes: RY, Ry, rY, ry (from RRYy) and ry, ry, ry, ry (from rryy)

	ry	ry	ry	ry
RY	RrYy	RrYy	RrYy	RrYy
Ry	Rryy	Rryy	Rryy	Rryy
rY	rrYy	rrYy	rrYy	rrYy
ry	rryy	rryy	rryy	rryy

F₂ Ratio (when crossing RrYy × RrYy):

• 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green

Incomplete Dominance:

Neither allele is completely dominant — the heterozygote shows an intermediate phenotype.

Example: Snapdragon flowers

• Red (RR) × White (WW) → Pink (RW)

Codominance:

Both alleles are expressed fully.

Example: AB blood group — both A and B antigens are expressed on red blood cells.

Blood Groups:

Genotype	Blood Group
$I^A I^A$ or $I^A i$	A
$I^B I^B$ or $I^B i$	B
$I^A I^B$	AB
$ii$	O

Sex Determination:

In humans:

• Females: XX (one X from mother, one X from father)
• Males: XY (X from mother, Y from father)
• Probability of male = 50%, female = 50%

Sex-Linked Inheritance:

Genes located on sex chromosomes (usually X chromosome):

Example — Haemophilia:

• $X^H$ = normal allele (dominant)
• $X^h$ = haemophilia allele (recessive)
• A carrier female ($X^H X^h$) has 50% chance of passing the allele to each child
• Males only have one X chromosome — if it carries $X^h$, they have haemophilia

Sex-linked example cross:

• Carrier mother ($X^H X^h$) × Normal father ($X^H Y$)
• Daughters: $X^H X^H$ (normal), $X^H X^h$ (carrier)
• Sons: $X^H Y$ (normal), $X^h Y$ (haemophiliac)

⚡ NABTEB Exam Tip: For sex-linked questions, always write the sex chromosomes first, then the trait. Males are never carriers — they are either affected or normal because they only have one X chromosome.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage of genetics for thorough NABTEB preparation.

DNA Structure — The Double Helix:

DNA is composed of:

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

Base Pairing:

• A pairs with T (2 hydrogen bonds)
• G pairs with C (3 hydrogen bonds)

The double helix was discovered by Watson and Crick in 1953, with crucial data from Rosalind Franklin’s X-ray crystallography.

DNA Replication:

DNA copies itself before cell division:

1. The double helix unwinds (helicase enzyme)
2. Each strand serves as a template
3. Free nucleotides pair with complementary bases (DNA polymerase)
4. Two identical DNA molecules result (semi-conservative — each new DNA has one old and one new strand)

From Gene to Protein:

Transcription:

• DNA → mRNA in the nucleus
• RNA polymerase reads one DNA strand and builds a complementary mRNA
• mRNA leaves nucleus through nuclear pores

Translation:

• mRNA → Protein at ribosome
• mRNA codons (3 bases) are read
• Each codon specifies a particular amino acid
• tRNA brings the correct amino acid (anticodon pairs with codon)
• Amino acids are linked by peptide bonds to form a protein

The Genetic Code:

• 64 codons (4³) code for 20 amino acids
• AUG is the start codon (also codes for methionine)
• UAA, UAG, UGA are stop codons

Gene Mutations:

Changes in DNA sequence:

• Substitution: One base replaced (e.g., sickle cell anaemia — GAG → GTG)
• Insertion: Extra base added (frameshift)
• Deletion: Base removed (frameshift)

Mutations and their effects:

• Silent mutation: Same amino acid produced (due to codon degeneracy)
• Missense mutation: Different amino acid (may or may not affect function)
• Nonsense mutation: Stop codon created (shortened protein)
• Frameshift mutation: Altered reading frame (severe effects)

Chromosomal Mutations:

Changes in chromosome structure or number:

• Deletion: Part of chromosome lost
• Duplication: Part copied and inserted
• Inversion: Section reversed
• Translocation: Part attaches to another chromosome
• Aneuploidy: Wrong chromosome number (e.g., Down syndrome — trisomy 21: 47 chromosomes instead of 46)
• Polyploidy: Whole extra set of chromosomes (common in plants)

Dominant vs Recessive Traits in Humans:

Dominant	Recessive
Freckles	No freckles
Curly hair	Straight hair
Huntington’s disease	Normal
Marfan syndrome	Normal
Normal (AA or Aa)	Albinism (aa)

Hardy-Weinberg Equilibrium:

When a population is not evolving, allele and genotype frequencies remain constant: $$p^2 + 2pq + q^2 = 1$$ $$p + q = 1$$

Where:

• $p$ = frequency of dominant allele
• $q$ = frequency of recessive allele
• $p^2$ = frequency of homozygous dominant
• $2pq$ = frequency of heterozygotes
• $q^2$ = frequency of homozygous recessive

Conditions for Hardy-Weinberg:

• No mutation
• No natural selection
• Random mating
• Large population
• No gene flow (migration)

Probability in Genetics:

For independent events:

• AND = multiply probabilities
• OR = add probabilities (for mutually exclusive events)

Example: Probability of two children both being girls = $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

⚡ NABTEB Quick Reference:

• Gene: DNA segment coding for a trait
• Alleles: different gene versions
• Dominant = expressed in heterozygote (TT, Tt)
• Recessive = only expressed in homozygote (tt)
• Genotype = genetic makeup; Phenotype = physical appearance
• Punnett square: predict offspring ratios
• Dihybrid F₂ ratio: 9:3:3:1
• Incomplete dominance: intermediate phenotype (e.g., pink flowers)
• Codominance: both alleles expressed (e.g., AB blood)
• Sex-linked: genes on X chromosome (e.g., haemophilia)
• DNA: double helix; A-T, G-C base pairing
• Replication: semi-conservative
• Transcription: DNA → mRNA
• Translation: mRNA → protein at ribosome
• Mutations: substitution, insertion, deletion, frameshift
• Hardy-Weinberg: $p^2 + 2pq + q^2 = 1$