Episode 5 — Module 5 Heredity: Comparing Genetic Similarities & Differences

Learning Intentions

• Define and clarify all key vocabulary related to genetic variation and inheritance.
• Accurately construct and analyse Punnett squares for:
  • Autosomal dominant/recessive crosses.
  • Sex-linked (X-linked) dominant and recessive crosses.
  • Co-dominant interactions.
  • Incomplete dominance interactions.
  • Traits governed by multiple alleles.
• Read and interpret pedigree charts, distinguishing autosomal vs. sex-linked and dominant vs. recessive inheritance.

Core Terminology & Concepts

• Gene – discrete section of DNA that encodes a trait.
• Allele – alternative form of a gene (e.g. eye-colour alleles: brown, blue, green).
  • Multiple alleles: more than two alternatives exist for a single gene.
• Dominant allele – expressed in the phenotype whenever present; more common in a population.
• Recessive allele – expressed only when two copies are present; less common.
• Genotype – the allele combination an organism carries.
• Phenotype – the observable, expressed trait.
• Homozygous – genotype contains two identical alleles (e.g. $YY$ or $yy$).
• Heterozygous – genotype contains two different alleles (e.g. $Yy$).
• Autosomal chromosomes – chromosome pairs $1!\text{–}!22$ (numbered 1$–$44 when counted individually).
• Sex chromosomes – pair $23$; designated $XX$ (female) or $XY$ (male).
• Punnett square – grid used to predict genotypic & phenotypic ratios of offspring.
• Co-dominance – two dominant alleles are both fully expressed (no blending).
• Incomplete dominance – heterozygote displays an intermediate (blended) phenotype.

Autosomal Punnett Squares (Dominant & Recessive)

• Use same letter for both alleles; uppercase = dominant, lowercase = recessive.
  • Choose letters with clear case distinction (A, B, Y).
• Example: Pea colour
  • Alleles: $Y$ (yellow, dominant), $y$ (green, recessive).
  • Cross: homozygous recessive $yy$ (green) × heterozygous $Yy$ (yellow).
  • Punnett grid → offspring genotypes: $Yy, Yy, yy, yy$.
  • Genotypic ratio $Yy:yy = 1:1$ (or $50\%$ heterozygous, $50\%$ homozygous recessive).
  • Phenotypic ratio 2 yellow : 2 green ($1:1$).
  • Rule: a single uppercase letter guarantees expression of the dominant phenotype.

Sex-Linked (X-Linked) Punnett Squares

• Traits reside on the X chromosome; males = $XY$, females = $XX$.
• Alleles written as superscripts on the X (e.g. $X^{h}$).
• Males have only one X allele → express whatever is present (no carrier state).
• Example: Hemophilia (recessive)
  • Alleles: $X^{h}$ = hemophilia, $X^{H}$ = normal.
  • Cross: affected male $X^{h}Y$ × unaffected female $X^{H}X^{H}$.
  • Offspring: 1 affected daughter ($X^{h}X^{H}$ carrier), 1 affected son ($X^{h}Y$), 1 unaffected daughter ($X^{H}X^{H}$), 1 unaffected son ($X^{H}Y$).

Co-Dominance

• Both alleles are dominant; heterozygote shows both traits simultaneously (spots, stripes, speckles).
• Notation: two different uppercase letters (e.g. $C^{R},C^{W}$).
• Example: Cattle coat colour
  • $C^{W}C^{W}$ (white) × $C^{R}C^{R}$ (red) → all $C^{W}C^{R}$ (roan: red & white hairs intermixed).
• Human blood groups: $I^{A}$ & $I^{B}$ are co-dominant, $i$ is recessive.

Incomplete Dominance

• Heterozygote phenotype is a blend (intermediate).
• Use uppercase for dominant allele, lowercase for recessive.
• Example: Snapdragon flowers
  • $R$ = red (dominant), $r$ = white (recessive).
  • $RR$ × $rr$ → all $Rr$ (pink flowers).
  • Heterozygous (pink) self-cross $Rr × Rr$ → genotype ratio $1 RR : 2 Rr : 1 rr$; phenotype ratio $1\,\text{red} : 2\,\text{pink} : 1\,\text{white}$.

Multiple Alleles

• More than two allele options in the gene pool.
• Blood type (ABO system)
  • Alleles: $I^{A}, I^{B}, i$.
  • $I^{A}, I^{B}$ are co-dominant; $i$ is recessive.
  • Example cross: parent with type A (genotype $I^{A}i$) × parent with type O ($ii$).
  • Offspring genotypes: $50\%\;I^{A}i$ (type A), $50\%\;ii$ (type O).
  • Phenotypic ratio $1\,\text{type A} : 1\,\text{type O}$.
• Rabbits: four alleles (full colour, chinchilla, Himalayan, albino) create complex dominance hierarchy.

Pedigree Charts – Symbols & Conventions

• Square = male; circle = female.
  • Shaded = affected; unshaded = unaffected.
• Generation levels labeled with Roman numerals (I, II, III…).
• Individuals within a generation numbered left → right.
• Horizontal line = mating; vertical bracket = siblings.
• Key/legend explains symbol meaning; essential for accurate interpretation.

Autosomal Dominant Pedigree

• Trait appears every generation; affected individuals in each generation > unaffected.
• One dominant allele (
  $H$) sufficient to express trait.
• Example: Huntington’s disease
  • Parental cross $Hh × Hh$ → offspring ratio $3\,\text{affected} : 1\,\text{unaffected}$.

Autosomal Recessive Pedigree

• Trait often skips generations; fewer affected individuals.
• Parents may be carriers (heterozygous) and phenotypically normal.
• Example: Cystic fibrosis
  • Carrier parents $Ff × Ff$ → $1/4$ chance each child will be affected ($ff$).

X-Linked Dominant Pedigree

• Affected fathers transmit allele to ALL daughters but NO sons.
• Affected mothers pass allele to $50\%$ of sons & $50\%$ of daughters.
• Usually more females than males affected.
• Example: Fragile X syndrome; single dominant copy causes phenotype in females.

X-Linked Recessive Pedigree

• More males than females affected (males need only one recessive allele).
• Affected mothers pass allele to ALL sons; daughters inherit allele & may be carriers or affected depending on second X.
• Carrier (unaffected) mother × unaffected father gives sons a $50\%$ risk of being affected, daughters unaffected but $50\%$ carriers.
• Example: Red-green colour-blindness.

Practical, Ethical & Real-World Relevance

• Understanding inheritance patterns guides genetic counselling (e.g. risk calculations for prospective parents).
• Pedigrees aid diagnosis & management of hereditary diseases (Huntington’s, cystic fibrosis, haemophilia).
• Blood-type genetics underpins safe blood transfusions & organ matching.
• Livestock breeders exploit co-dominance & incomplete dominance for desirable traits (e.g. roan cattle, flower colour).
• Ethical considerations: privacy of genetic information, potential discrimination, informed consent for genetic testing.

Quick Reference Equations & Ratios

• Expected Mendelian monohybrid phenotypic ratio (dominant × recessive heterozygote self-cross): $3:1$.
• Genotypic ratio for heterozygous self-cross: $1\,\text{homozygous dominant} : 2\,\text{heterozygous} : 1\,\text{homozygous recessive}$.
• Probability formula: $P(\text{event}) = \frac{\text{favourable outcomes}}{\text{total outcomes}} \times 100\%$.

Study Tips

• Always list parental genotypes before drawing a Punnett square.
• Label axes with gametes; fill grid systematically.
• For sex-linked traits, never write a bare allele—attach to $X$ or $Y$.
• In pedigrees, work from known individuals backward to infer unknown genotypes.
• Ratios can be converted to percentages for clearer understanding in real-world contexts (e.g. medical risk communication).