Genetics & Heredity
Uncover how traits are inherited from parents to offspring using Mendel’s laws, Punnett squares, and the language of dominant and recessive alleles.
Key Genetic Terminology
Genetics uses precise language. Before exploring Mendel’s laws, it is important to understand these fundamental terms:
Gene
A segment of DNA that codes for a specific protein and ultimately a particular trait (e.g., eye colour, blood type).
Allele
A variant form of a gene. Organisms typically have two alleles for each gene — one inherited from each parent.
Dominant allele
An allele whose trait is expressed even when only one copy is present. Written as a capital letter (e.g., B).
Recessive allele
An allele whose trait is only expressed when two copies are present. Written as a lowercase letter (e.g., b).
Genotype
The genetic makeup of an organism (the actual alleles it carries), e.g., BB, Bb, or bb.
Phenotype
The observable physical characteristics of an organism resulting from its genotype and environment, e.g., brown eyes.
Homozygous
Having two identical alleles for a gene (e.g., BB or bb). Also called “true breeding”.
Heterozygous
Having two different alleles for a gene (e.g., Bb). Also called a “carrier” for recessive conditions.
Gregor Mendel and His Laws
Gregor Mendel (1822–1884), an Austrian monk, discovered the principles of inheritance by crossing pea plants over many generations and carefully counting the offspring. His findings established two fundamental laws:
Law of Segregation
Each organism carries two alleles for each trait. These alleles separate (segregate) during gamete (egg/sperm) formation, so each gamete carries only one allele. When gametes combine at fertilisation, the offspring receives one allele from each parent.
Law of Independent Assortment
Alleles for different traits are distributed to gametes independently of one another — the inheritance of one trait does not affect the inheritance of another (for genes on different chromosomes). This explains the variation seen in offspring.
Punnett Squares
A Punnett square is a grid used to predict the possible genotypes (and therefore phenotypes) of offspring from a cross between two parents. The alleles of one parent are placed along the top, and the other parent’s alleles down the side.
Monohybrid Cross: Brown eyes (B) × Blue eyes (b)
Parents: Bb (brown) × bb (blue)
BB
Homozygous dominant
Phenotype: Brown eyes
Bb
Heterozygous
Phenotype: Brown eyes (B is dominant)
bb
Homozygous recessive
Phenotype: Blue eyes
Key Vocabulary
| Term | Definition |
|---|---|
| Genotype | The combination of alleles an organism carries for a particular gene (e.g., BB, Bb, bb). |
| Phenotype | The observable characteristics of an organism, determined by its genotype and influenced by the environment. |
| Dominant | An allele that is expressed in the phenotype when at least one copy is present (uppercase letter). |
| Recessive | An allele that is only expressed in the phenotype when two copies are present (lowercase letter). |
Worked Examples
Predicting offspring from a monohybrid cross (Tt × Tt).
Given: Tall (T) is dominant over short (t). Both parents are heterozygous (Tt).
Step 1: Draw the Punnett square with T and t across the top, T and t down the side.
Step 2: Fill in: TT, Tt, Tt, tt.
Genotype ratio: 1 TT : 2 Tt : 1 tt
Phenotype ratio: 3 tall (TT or Tt) : 1 short (tt) → 3:1 phenotype ratio
Identifying carriers of a recessive condition.
Given: Cystic fibrosis (CF) is caused by a recessive allele (f). Normal function is F (dominant). A mother has genotype Ff and the father has genotype Ff.
Step 1: Punnett square gives: FF, Ff, Ff, ff.
Result: 25% chance of FF (unaffected, not carrier), 50% chance of Ff (unaffected carrier), 25% chance of ff (affected by CF).
Answer: Each pregnancy has a 1 in 4 (25%) chance of producing a child with CF. Two Ff parents are both phenotypically normal but are carriers.
Determining a parent’s genotype from offspring ratios.
Given: A black-furred rabbit (B = black, b = white) is crossed with a white rabbit (bb). Half the offspring are black and half are white.
Step 1: The white parent must be bb (homozygous recessive).
Step 2: A 1:1 ratio of black:white offspring suggests one parent is Bb (heterozygous).
Answer: The black parent’s genotype is Bb. A Bb × bb cross produces 50% Bb (black) and 50% bb (white).
Knowledge Check
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Key Concepts Summary
- •Traits are determined by genes; each gene has alternative forms called alleles (dominant = uppercase, recessive = lowercase).
- •Genotype = the alleles an organism has; phenotype = the observable trait expressed.
- •Mendel’s Law of Segregation: alleles separate during gamete formation; offspring receive one from each parent.
- •Punnett squares predict offspring genotype and phenotype ratios; Tt × Tt gives 3:1 phenotype ratio.
- •A heterozygous (Bb) carrier shows the dominant phenotype but can pass the recessive allele to offspring.