What this chapter is about
This chapter explores how biological traits pass from parents to offspring and why offspring are not exact copies of their parents. You will study the foundational work of Gregor Mendel, who experimented with garden pea plants and discovered the basic laws governing inheritance. His observations, made in the 1860s, form the bedrock of classical genetics.
Building on Mendelian genetics, you will learn how chromosomes carry genetic information and how the behaviour of chromosomes during meiosis explains Mendel's laws. The chapter also covers situations where inheritance patterns deviate from simple Mendelian ratios, such as incomplete dominance, co-dominance, multiple alleles, and polygenic inheritance. You will understand how genes are linked on chromosomes and how crossing over creates new combinations of alleles.
By the end of this chapter, you should be able to predict the outcome of genetic crosses using Punnett squares, explain why certain traits appear in predictable ratios, understand sex determination in humans, and recognise common genetic disorders and their inheritance patterns.
Key ideas
- Mendel's Law of Dominance: When two contrasting alleles are present, one (dominant) expresses itself while the other (recessive) remains hidden in the phenotype of the heterozygote.
- Law of Segregation: During gamete formation, the two alleles of a gene separate so that each gamete receives only one allele; this occurs during anaphase I of meiosis when homologous chromosomes move to opposite poles.
- Law of Independent Assortment: Alleles of different genes located on different chromosomes assort independently during gamete formation, producing new allele combinations in offspring.
- Chromosomal Theory of Inheritance: Genes are located on chromosomes; the behaviour of chromosomes during meiosis (separation and independent assortment) parallels Mendel's laws.
- Linkage and Recombination: Genes on the same chromosome tend to be inherited together (linkage), but crossing over during meiosis can separate them, creating recombinant offspring at a frequency proportional to the distance between genes.
- Sex Determination: In humans, females are XX and males are XY; the Y chromosome carries the SRY gene that triggers male development. Sex-linked traits are often carried on the X chromosome.
- Deviations from Mendelian Ratios: Incomplete dominance (heterozygote intermediate), co-dominance (both alleles express equally, as in ABO blood groups), and multiple alleles (more than two allelic forms in a population) modify expected phenotypic ratios.
- Genetic Disorders: Some conditions follow Mendelian inheritance: autosomal recessive (phenylketonuria, sickle-cell anaemia), autosomal dominant (Huntington disease), and sex-linked recessive (haemophilia, colour blindness).
Formulas and facts to remember
1. Genotypic ratio in a monohybrid cross (F₂): 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive (1 : 2 : 1).
2. Phenotypic ratio in a monohybrid cross (F₂): 3 dominant phenotype : 1 recessive phenotype (3 : 1).
3. Phenotypic ratio in a dihybrid cross (F₂): 9 : 3 : 3 : 1 when genes assort independently.
4. Test cross: Crossing an individual of dominant phenotype with a homozygous recessive individual reveals whether the dominant individual is homozygous or heterozygous.
5. Recombination frequency = (Number of recombinant offspring / Total offspring) × 100 %; a value of 50 % indicates independent assortment (genes on different chromosomes or far apart).
6. ABO blood group inheritance: Three alleles (Iᴬ, Iᴮ, i); Iᴬ and Iᴮ are co-dominant to each other and both are dominant over i. Genotypes: Iᴬ Iᴬ or Iᴬ i → Type A; Iᴮ Iᴮ or Iᴮ i → Type B; Iᴬ Iᴮ → Type AB; ii → Type O.
7. Sex-linked inheritance pattern: A trait carried on the X chromosome (e.g., colour blindness) passes from a carrier mother to half her sons, who then express the trait because they have only one X chromosome.
8. Pedigree symbols: Square = male, circle = female; filled symbol = affected individual; horizontal line = mating, vertical line = offspring.
Worked examples
### Example 1: Monohybrid cross predicting F₂ ratio
A plant with round seeds (RR) is crossed with a plant with wrinkled seeds (rr). Predict the phenotypic ratio in the F₂ generation.
Step 1: P generation cross → RR × rr. Step 2: F₁ offspring are all Rr (heterozygous, round seeds because R is dominant). Step 3: Self-fertilise F₁ → Rr × Rr. Step 4: Punnett square:
- **R** · R: RR · r: Rr
- **r** · R: Rr · r: rr
Step 5: Genotypic ratio = 1 RR : 2 Rr : 1 rr. Step 6: Phenotypic ratio = 3 round : 1 wrinkled.
---
### Example 2: Determining blood group possibilities
A woman with blood group A (genotype Iᴬ i) marries a man with blood group B (genotype Iᴮ i). What blood groups can their children have?
Step 1: Mother's gametes → Iᴬ or i. Father's gametes → Iᴮ or i. Step 2: Punnett square:
- **Iᴬ| Iᴬ Iᴮ** · Iᴮ: Iᴬ i
- **i** · Iᴮ: Iᴮ i · i: ii
Step 3: Possible genotypes and phenotypes:
- Iᴬ Iᴮ → Type AB
- Iᴬ i → Type A
- Iᴮ i → Type B
- ii → Type O
Conclusion: Children can have blood group A, B, AB or O, each with probability 1/4.
---
### Example 3: Sex-linked inheritance of colour blindness
A woman who is a carrier for colour blindness (Xᶜ X) marries a man with normal vision (X Y). Predict the probability that their son will be colour blind.
Step 1: Mother's genotype Xᶜ X produces gametes Xᶜ or X. Father's genotype X Y produces gametes X or Y. Step 2: Sons inherit Y from father, so their X must come from mother. Step 3: If a son receives Xᶜ from mother, genotype is Xᶜ Y → colour blind. If he receives X, genotype is X Y → normal vision. Step 4: Probability = 1/2 that a son is colour blind.
(Daughters, receiving X from father, will be either carrier Xᶜ X or normal X X, none colour blind.)
Common mistakes
- Confusing genotype (genetic makeup, e.g., Rr) with phenotype (observable trait, e.g., round seeds) → genotype is the allele combination; phenotype is what you see or measure.
- Assuming a 3 : 1 ratio always applies → this ratio holds only when one allele is completely dominant; incomplete dominance gives 1 : 2 : 1 phenotypic ratio.
- Forgetting that linked genes do not assort independently → genes on the same chromosome travel together unless crossing over separates them.
- Thinking all daughters of a colour-blind father will be colour blind → daughters receive one X from each parent; if mother is not a carrier, daughters are carriers, not affected.
- Mixing up homozygous (two identical alleles) and heterozygous (two different alleles) → careful reading of genotype symbols prevents this error.
Quick revision
- Mendel's three laws: Dominance, Segregation, Independent Assortment.
- Monohybrid F₂ phenotypic ratio: 3 : 1; dihybrid F₂ ratio: 9 : 3 : 3 : 1.
- A test cross reveals whether a dominant-phenotype individual is homozygous or heterozygous.
- Co-dominance: both alleles express equally (ABO blood groups); incomplete dominance: heterozygote shows intermediate phenotype.
- Sex-linked recessive traits appear more often in males because they have only one X chromosome.
- Recombination frequency measures how often linked genes separate; 50 % indicates independent assortment.