Chapter 12

Genetics & Inheritance

High School

Why do offspring resemble their parents yet differ from them? Gregor Mendel answered this with peas and arithmetic, founding genetics before anyone knew DNA existed. His laws of inheritance still predict the traits of the next generation with startling precision.

At a glance
Core ideaMendel's laws let you predict offspring traits from the parents' alleles.
Key termGenotype vs phenotype — the alleles carried vs the trait actually shown.
You can…Build a Punnett square, including sex-linked and codominant crosses.
Watch outRatios are probabilities over many offspring, not guarantees for a few.
1 Theory

The language of genetics

  • Gene — a length of DNA coding for a characteristic. Allele — a variant form of a gene.
  • Genotype — the alleles an organism carries. Phenotype — the observable characteristic that results.
  • Homozygous — two identical alleles (e.g. AA or aa). Heterozygous — two different alleles (Aa).
  • Dominant — an allele expressed even when heterozygous (written capital, A). Recessive — expressed only when homozygous (lowercase, a).

Mendel's laws

  1. Law of Segregation — the two alleles of a gene separate during gamete formation, so each gamete carries only one. (This is meiosis at work.)
  2. Law of Independent Assortment — alleles of different genes assort independently of one another (true for genes on different chromosomes, or far apart).

Beyond simple dominance

PatternHeterozygote shows…Example
Complete dominanceThe dominant phenotypePea flower colour
Incomplete dominanceA blend of bothRed × white snapdragon → pink
CodominanceBoth alleles fully expressedHuman blood group AB
Multiple alleles>2 alleles in the populationABO blood groups (IA, IB, i)
Sex linkageGene on X (or Y) chromosomeRed–green colour blindness, haemophilia
2 Explanation

The Punnett square as a probability engine

A Punnett square is a grid that lists the possible gametes of each parent and combines them to show every equally likely offspring genotype. It is really just multiplying probabilities. If each parent is Aa, each produces ½ A and ½ a gametes. The chance of an aa child is ½ × ½ = ¼ — which is exactly the famous 3:1 ratio (¾ show dominant, ¼ show recessive) from a monohybrid cross of two heterozygotes.

Two subtleties trip up students. First, ratios are probabilities, not guarantees: a 3:1 ratio means each child independently has a ¾ chance of the dominant phenotype — you can have four dominant children in a row, just as you can flip four heads. Ratios only emerge reliably over many offspring. Second, recessive alleles can hide: a healthy heterozygous "carrier" can pass on a disease allele unseen, which is why recessive genetic conditions can appear in a child with two unaffected parents.

3 Practical

Worked example: a sex-linked cross (colour blindness)

Red–green colour blindness is caused by a recessive allele on the X chromosome (Xb). Males (XY) have only one X, so a single recessive allele makes them colour-blind. Predict the children of a carrier mother (XBXb) and a normal-vision father (XBY).

  1. Assign genotypes. Mother = XBXb (carrier, normal vision). Father = XBY (normal vision).
  2. List gametes. Mother → XB or Xb. Father → XB or Y.
  3. Build the Punnett square. Combine them:
    XB (mother)Xb (mother)
    XB (father)XBXB — normal girlXBXb — carrier girl
    Y (father)XBY — normal boyXbY — colour-blind boy
  4. Read the ratios. Offspring: ¼ normal girl, ¼ carrier girl, ¼ normal boy, ¼ colour-blind boy.
  5. Interpret by sex. All daughters have normal vision (each inherits the father's XB), though half are carriers. Sons have a 50% chance of being colour-blind, depending on which X they got from their mother.
  6. Draw the lesson. Sex-linked recessive conditions strike males far more often, because a male's single X is unmasked — a general rule that explains why haemophilia and colour blindness are much commoner in men.
4 Q&A

Test yourself

Q1 Two brown-eyed parents have a blue-eyed child. Using B (brown, dominant) and b (blue, recessive), explain and give the parents' genotypes.

Blue is recessive, so the blue-eyed child must be bb — inheriting one b from each parent. Since both parents are brown-eyed but each carried a b, both must be heterozygous Bb (carriers). The cross Bb × Bb gives offspring in a 3 brown : 1 blue ratio, so a ¼ chance per child of blue eyes. The recessive allele was hidden in each parent until two copies met.

Q2 In snapdragons, red × white flowers give all pink offspring. Name the inheritance pattern and predict the F₂ (pink × pink).

This is incomplete dominance — the heterozygote shows a blend. Let CR = red allele, CW = white. Red (CRCR) × white (CWCW) → all CRCW (pink). Crossing two pinks (CRCW × CRCW) gives 1 red : 2 pink : 1 white. Note the phenotype ratio (1:2:1) matches the genotype ratio, because each genotype has its own distinct appearance.

Q3 How can a person with blood group AB never have a group-O child, regardless of the other parent?

Group AB has genotype IAIB — it carries no i (O) allele. Group O requires genotype ii, meaning the child must inherit an i from each parent. Since the AB parent can only pass on IA or IB, never i, no child of an AB parent can be ii. Every child of an AB parent receives an A or B allele and so is group A, B, or AB. (ABO shows both codominance of A and B and multiple alleles.)

Q4 A test cross is used to determine whether a dominant-phenotype organism is homozygous or heterozygous. Explain how.

Cross the unknown (A_) with a homozygous recessive (aa). If the unknown is homozygous AA: all offspring are Aa and show the dominant phenotype (100%). If the unknown is heterozygous Aa: offspring are ½ Aa (dominant) and ½ aa (recessive), a 1:1 ratio. So the appearance of any recessive offspring reveals the unknown as heterozygous. The recessive parent contributes only a alleles, letting the unknown's hidden allele show through in the offspring.

Q5 A cross predicts a 3:1 ratio, but among 8 offspring you observe 8 dominant and 0 recessive. Is Mendel wrong?

No. A 3:1 ratio is a probability, expected on average over many offspring, not a guarantee for a small sample. Each offspring independently has a ¾ chance of the dominant phenotype; the chance of all 8 being dominant is (¾)⁸ ≈ 10% — uncommon but entirely possible, like flipping several heads in a row. With small samples, chance deviation from the expected ratio is normal. Only with large numbers does the observed ratio converge on 3:1 (this is why a chi-squared test is used to judge whether a deviation is significant).

Concept mind map

How the ideas connect

Every key idea in this chapter, branching from the core concept — use it to see the whole picture at a glance.

gene and alleledominantrecessivegenotypephenotypePunnett squaresex linkageGenetics & Inheritance
Infographic

The key facts, visualised

Allele
a version of a gene, e.g. B or b
Homozygous
two identical alleles, e.g. BB or bb
Heterozygous
two different alleles, e.g. Bb
Ratio 3:1
phenotypes from a Bb x Bb cross
Solved examples

Worked problems, step by step

Follow each solution line by line, then try to reproduce it on paper before moving on.

Example 1Cross two heterozygous tall pea plants (Tt x Tt). Give the phenotype ratio.

  1. Offspring: TT, Tt, Tt, tt
  2. Tall (T_) : short (tt) = 3 : 1

Example 2A carrier mother (XBXb) and normal father (XBY) for colour blindness. Sons affected?

  1. Sons get X from mother: XB or Xb
  2. Xb son is colour blind
Practice problem set

Now you try

Work each one out first, then tap to reveal the worked answer.

1Distinguish genotype and phenotype.
Genotype is the allele combination an organism carries; phenotype is the physical trait that shows.
2What does a Punnett square predict?
It predicts the probability of each genotype and phenotype in the offspring of a cross.
3What is codominance? Give an example.
Both alleles are fully expressed together, as in AB blood type where A and B both show.
4Why are more males colour blind than females?
The gene is on the X chromosome; a male has only one X, so a single recessive allele shows.
5State Mendel law of segregation.
The two alleles for a trait separate during gamete formation, so each gamete carries only one.
6A cross gives a 1:1 ratio. What were the likely parents?
A test cross of a heterozygote (Bb) with a recessive homozygote (bb) gives a 1:1 ratio.