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Inheritance and Genetics

Subject: Biology
Topic: 10
Cambridge Code: 0610 / 0970 / 5090

Basic Genetics Terms​

Gene - Section of DNA coding for protein

Allele - Alternative form of gene

  • Dominant (A) - Expressed when present
  • Recessive (a) - Only expressed homozygous

Genotype - Genetic makeup (AA, Aa, aa)

Phenotype - Visible characteristics

Homozygous - Two identical alleles (AA, aa)

Heterozygous - Two different alleles (Aa)

Mendel's First Law​

Law of Segregation - Alleles separate during gamete formation

Monohybrid Cross​

Cross involving ONE trait

Example: Pea plant height

  • P: Tall (AA) × Short (aa)
  • Gametes: A × a
  • F₁: All Aa (tall)
  • F₁ × F₁: Aa × Aa
  • F₂: 1 AA : 2 Aa : 1 aa
  • Phenotypic ratio: 3:1 (Tall:Short)
  • Genotypic ratio: 1:2:1

Punnett Square​

Aa
AAAAa
aAaaa

Result: 3 tall : 1 short

Mendel's Second Law​

Law of Independent Assortment - Alleles of different genes segregate independently

Dihybrid Cross​

Cross involving TWO traits

Example:

  • Seed color (Yellow Y, Green y)
  • Seed shape (Round R, Wrinkled r)

P: YYRR (Yellow round) × yyrr (Green wrinkled)

F₁: YyRr (all yellow round)

F₁ × F₁: YyRr × YyRr

F₂ Ratio: 9:3:3:1

  • 9 Yellow round
  • 3 Yellow wrinkled
  • 3 Green round
  • 1 Green wrinkled

Inheritance Patterns​

Simple Dominance​

One allele completely dominates over other

  • Aa shows dominant phenotype
  • aa shows recessive phenotype

Example: Brown eyes (dominant) vs blue eyes (recessive)

Incomplete Dominance​

Neither allele completely dominant

  • Heterozygote shows intermediate phenotype

Example: Red flower × White flower → Pink flower (in snapdragons)

Genotype: R^R (red), R^W (pink), W^W (white)

Codominance​

Both alleles equally expressed

  • Heterozygote shows both phenotypes

Example: ABO blood typing

  • I^A I^A or I^A i = Type A
  • I^B I^B or I^B i = Type B
  • I^A I^B = Type AB (both A and B antigens)
  • ii = Type O

Sex-Linked Inheritance​

Sex chromosome: X and Y

X-linked traits - Gene on X chromosome

  • Males (XY): One copy determines phenotype
  • Females (XX): Two copies possible

Example: Colour-Blindness (Red-Green)​

  • X^B = Normal (dominant)
  • X^b = Color-blind (recessive)

Males:

  • X^B Y = Normal
  • X^b Y = Color-blind

Females:

  • X^B X^B = Normal (homozygous)
  • X^B X^b = Normal carrier (heterozygous)
  • X^b X^b = Color-blind

Cross: Normal female (X^B X^b) × Color-blind male (X^b Y)

Offspring:

  • Female: 50% normal, 50% color-blind
  • Male: 50% normal, 50% color-blind

Test Cross​

Test cross - Individual with dominant phenotype crossed with homozygous recessive

Purpose: Determine if organism is homozygous (AA) or heterozygous (Aa)

Cross: Aa (unknown) × aa

Results:

  • If heterozygous: 50% Aa, 50% aa (1:1 ratio)
  • If homozygous: 100% Aa (all dominant)

Mutations​

Mutation - Change in DNA sequence

Types​

Point mutation: Single nucleotide changed

  • Silent: No change in protein
  • Missense: Different amino acid
  • Nonsense: Stop codon created

Insertion/Deletion: DNA segments added/removed

  • Causes frameshift
  • Usually harmful

Causes​

  • Spontaneous errors
  • UV radiation
  • Chemical mutagens
  • Ionizing radiation

Effects​

  • Beneficial: Rare (e.g., antibiotic resistance)
  • Neutral: No effect
  • Harmful: Reduce fitness (e.g., cystic fibrosis)

Natural Selection​

Natural selection - Organisms with favorable traits survive and reproduce

Process​

  1. Variation exists in population
  2. Organisms with advantageous traits survive
  3. Survivors reproduce more
  4. Favorable alleles become more common
  5. Evolution occurs over generations

Example: Antibiotic Resistance​

  • Bacteria have genetic variation
  • Antibiotics kill susceptible bacteria
  • Resistant bacteria survive
  • Resistance alleles increase in frequency
  • Population becomes resistant

Key Points​

  1. Alleles segregate during meiosis
  2. Gametes carry one allele per gene
  3. Fertilization restores diploid number
  4. Three inheritance patterns possible
  5. X-linked inheritance differs in males/females
  6. Mutations alter DNA
  7. Natural selection acts on phenotypes

Practice Questions​

  1. Complete monohybrid cross Punnett square
  2. Calculate F₂ ratios from dihybrid cross
  3. Determine genotypes from phenotypes
  4. Solve X-linked inheritance problems
  5. Explain incomplete dominance
  6. Predict mutation effects
  7. Describe natural selection

Revision Tips​

  • Master Punnett squares
  • Know dominant vs recessive
  • Learn inheritance patterns
  • Understand sex-linked traits
  • Practice cross problems
  • Know mutation types
  • Understand natural selection mechanism