Inheritance Patterns

In short: Inheritance patterns show how traits pass from parents to offspring through alleles, the different versions of a gene. A dominant allele shows with one copy and a recessive allele needs two, so two Tt parents give a 3 : 1 ratio of dominant to recessive phenotypes.

Starting point: what you already know

Body cells have pairs of chromosomes, one from each parent (see DNA, Chromosomes and Cell Division).
Gametes are haploid and carry one of each pair.
Genes code for features.

Key words

Term	Meaning
Allele	A different version of a gene (for example, tall or short)
Dominant	An allele that shows its effect even if only one copy is present. Capital letter: T
Recessive	An allele that shows only if two copies are present. Lower case: t
Genotype	The alleles an organism has: TT, Tt or tt
Phenotype	The feature you can see: tall or short
Homozygous	Two identical alleles: TT or tt
Heterozygous	Two different alleles: Tt (a "carrier" of the recessive allele)

Phenotype depends on genotype and the environment. A pea plant with tall alleles will still be short if it has poor soil.

Monohybrid crosses with Punnett squares

Worked exampleCrossing two heterozygous tall plants

Parents: Tt × Tt. Each parent makes gametes T and t.

	T	t
T	TT	Tt
t	Tt	tt

Genotype ratio: 1 TT : 2 Tt : 1 tt. Phenotype ratio: 3 tall : 1 short. The chance of a short plant is $\frac{1}{4}$ or 25%.

Test cross. To find out if a tall plant is TT or Tt, cross it with a short (tt) plant. If any offspring are short, the tall parent must be Tt.

When neither allele is dominant

Codominance: both alleles show fully in the heterozygote. A roan cow, genotype $C^{R} C^{W}$ , has both red hairs and white hairs mixed.

Incomplete dominance: the heterozygote is a blend. Red snapdragon × white snapdragon gives pink flowers. Pink × pink gives 1 red : 2 pink : 1 white.

Multiple alleles: some genes have more than two alleles. Human ABO blood groups have three: $I^{A}$ and $I^{B}$ are codominant, and $i$ is recessive.

Blood group	Possible genotypes
A	$I^{A} I^{A}$ or $I^{A} i$
B	$I^{B} I^{B}$ or $I^{B} i$
AB	$I^{A} I^{B}$
O	$i i$

Sex-linked inheritance

Genes on the X chromosome are sex-linked. Males (XY) have only one X, so a single recessive allele on it will always show. This is why red-green colour blindness and haemophilia are much more common in males.

Worked exampleA carrier mother

Mother: $X^{B} X^{b}$ (normal vision, carrier). Father: $X^{B} Y$ (normal).

	$X^{B}$	$Y$
$X^{B}$	$X^{B} X^{B}$	$X^{B} Y$
$X^{b}$	$X^{B} X^{b}$	$X^{b} Y$

Daughters: all normal vision (half are carriers). Sons: half normal, half colour blind.

Pedigrees

A pedigree is a family tree showing a trait. Squares are males, circles are females, shaded shapes show the trait.

Inheritance

Words

Allele, dominant, recessive
Genotype and phenotype
Homozygous and heterozygous

Monohybrid cross

Punnett square
Tt × Tt gives 3 : 1
Test cross with tt

Other patterns

Codominance: both show
Incomplete: blend
Multiple alleles: ABO

Sex-linked

On the X chromosome
More common in males

Pedigrees

Recessive: skips generations
Dominant: in every generation

Year 11Dihybrid crosses

A dihybrid cross follows two genes at once. If the genes are on different chromosomes, they are inherited independently.

Pea seeds: Round (R) is dominant to wrinkled (r). Yellow (Y) is dominant to green (y). Cross RrYy × RrYy. Each parent makes four gametes: RY, Ry, rY, ry. A 4 × 4 Punnett square gives 16 boxes and the famous ratio:

$9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green$

If results are far from 9 : 3 : 3 : 1, the genes may be linked (close together on the same chromosome) and are inherited together unless crossing over separates them.

Recombination. If two genes on the same chromosome are separated by crossing over in 8% of gametes, they are linked and fairly close together. The further apart two genes are, the more often crossing over separates them.

Multiple alleles in a dominance series. Rabbit coat: agouti > chinchilla > Himalayan > albino. Two agouti rabbits can have an albino kit if both carry the albino allele hidden underneath.

Year 12Probability shortcuts for the HSC

For independent genes, multiply probabilities. What is the chance that a child of two parents who are both Aa Bb has genotype aa bb?

$P (aa) \times P (bb) = \frac{1}{4} \times \frac{1}{4} = \frac{1}{16}$

Each child is an independent event. If two carrier parents already have one child with cystic fibrosis, the chance that the next child has it is still $\frac{1}{4}$ .

Carrier chance when unaffected. Two carriers (Aa × Aa) have an unaffected child. Of the unaffected outcomes (AA, Aa, Aa), two are carriers, so the chance is $\frac{2}{3}$ .

Exactly one of two children. Affected then unaffected, or unaffected then affected: $2 \times \frac{1}{4} \times \frac{3}{4} = \frac{3}{8}$ .

Dominant disease. A heterozygous parent with Huntington's disease and an unaffected partner: Hh × hh gives $\frac{1}{2}$ for each child.

X-linked patterns.

X-linked dominant: an affected father passes it to all his daughters and none of his sons.
A carrier mother and an affected father ( $X^{H} X^{h} \times X^{h} Y$ ): half the daughters and half the sons are affected.
Tortoiseshell cats: $X^{B} X^{O} \times X^{O} Y$ gives tortoiseshell or orange daughters, and black or orange sons.
In birds, females are ZW and males ZZ, so females have only one Z and show sex-linked recessive traits more often.

Small samples mislead. One family where only males are affected is too few results to prove X-linkage; it could happen by chance.

Quick summary

Alleles are versions of a gene. Dominant alleles show with one copy, recessive alleles need two.
Genotype is the alleles. Phenotype is what you see, shaped by genes and environment.
Tt × Tt gives 3 dominant : 1 recessive phenotypes.
Codominance shows both alleles. Incomplete dominance blends them. ABO blood groups have multiple alleles.
X-linked recessive traits are more common in males.
Pedigrees show how a trait passes through a family.

Quiz

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Different versions of the same gene are called
1. A alleles
2. B chromosomes
3. C phenotypes
4. D gametes
Show answer
A: alleles
Alleles are gene variants.
A plant has genotype Tt. It is
1. A homozygous dominant
2. B heterozygous
3. C homozygous recessive
4. D haploid
Show answer
B: heterozygous
Two different alleles.
Which describes a phenotype?
1. A Tt
2. B TT
3. C Tall
4. D An allele
Show answer
C: Tall
The phenotype is the visible feature.

Starting point: what you already know

Key words

Monohybrid crosses with Punnett squares

When neither allele is dominant

Sex-linked inheritance

Pedigrees

Quick summary

Quiz

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