models of inheritance that explain phenotype expression, including dominant and recessive autosomal patterns, codominance, incomplete dominance, multiple alleles and sex-linked genes, using Punnett squares to predict outcomes

What this dot point is asking

VCAA wants the five main patterns of single-gene inheritance: dominant/recessive on autosomes, codominance, incomplete dominance, multiple alleles, and sex-linked. For each you should be able to write the genotypes, predict the phenotypes (using a Punnett square) and explain the underlying molecular basis.

The answer

Autosomal dominant and recessive (classical Mendelian)

Dominant allele: produces its phenotype in the heterozygote. Conventionally written with a capital letter (P).

Recessive allele: only produces its phenotype in the homozygote. Written with a lowercase letter (p).

Example: pea flower colour. P = purple (dominant), p = white (recessive).

A monohybrid cross Pp times Pp gives 3 purple : 1 white in the offspring.

Molecular basis: P produces a functional enzyme for pigment; p produces a non-functional version. One functional copy (Pp) is enough to make the pigment.

Examples in humans:

• Autosomal dominant: Huntington's disease, achondroplasia, polydactyly.
• Autosomal recessive: cystic fibrosis, sickle cell anaemia (heterozygotes have advantage; see codominance), phenylketonuria, Tay-Sachs disease.

Codominance

In a heterozygote, both alleles are fully expressed. Neither allele masks the other; both phenotypes show side by side.

Example 1: ABO blood groups. The ABO gene has three alleles: IA, IB and i.

IA and IB are codominant to each other; both i is recessive to both.

Example 2: MN blood groups. Heterozygotes (LM LN) express both M and N glycoproteins.

Example 3: Sickle cell trait. Hb-A and Hb-S are codominant: heterozygotes (Hb-A Hb-S) have both normal and sickle haemoglobin in red blood cells, giving partial malaria resistance with mild symptoms.

Molecular basis: each allele codes for a distinct protein product; both are made simultaneously and both are visible.

Incomplete dominance

In a heterozygote, neither allele dominates. The phenotype is intermediate between the two homozygotes, as if the traits had blended.

Classic example: snapdragon flower colour.

Crossing red times white gives all pink. Crossing two pinks gives 1 red : 2 pink : 1 white (the original colours reappear in the F2 generation, distinguishing incomplete dominance from a true blend).

Another example: familial hypercholesterolaemia, where homozygotes have very high cholesterol, heterozygotes have moderately raised cholesterol, and normal homozygotes have typical levels.

Molecular basis: one functional copy of the gene produces only half the protein product, giving a partial phenotype.

Difference: codominance vs incomplete dominance

Multiple alleles

A gene can have more than two alleles in a population, although any one diploid individual still carries only two.

Example: the ABO blood-group locus has three common alleles (IA, IB, i) and many rare variants.

Example: the MHC (HLA) genes have hundreds of alleles each, which is why finding a tissue-typing match for transplantation is hard.

Example: coat colour in rabbits is controlled by four alleles (C, c-ch, c-h, c) at one locus, ordered by dominance.

Multiple alleles are detected at the population level. Within an individual, the rules still apply: two alleles per locus, with dominance relationships determining phenotype.

Sex-linked inheritance

A sex-linked gene is one whose locus is on a sex chromosome.

X-linked genes are the most important in humans, because the X carries many genes (about 800) while the Y carries very few. A female has two X chromosomes; a male has only one.

X-linked recessive disorders have a characteristic pattern:

• Males are affected more often than females. A male needs only one copy of the recessive allele to be affected (he has no second X to mask it). A female needs two.
• Affected males inherit the allele from their mother (he gets his Y from his father).
• A carrier mother (X-H X-h) passes the trait to about 50% of her sons.
• An affected father cannot pass an X-linked trait to his sons (sons get Y from father), but all his daughters become carriers.
• Affected females usually have an affected father and a carrier mother.

Examples of X-linked recessive disorders: haemophilia (blood clotting), red-green colour blindness, Duchenne muscular dystrophy (DMD).

X-linked dominant disorders are rarer. Both males and females can be affected; an affected father passes the trait to all daughters but no sons. Example: fragile X syndrome (with complications), some forms of rickets.

Y-linked traits are passed father to all sons, never to daughters. Few medically important examples beyond male sexual development.

Sex-linked Punnett square example

A carrier mother (X-H X-h) and an unaffected father (X-H Y) for haemophilia:

Daughters: 50% unaffected (X-H X-H), 50% carriers (X-H X-h). Sons: 50% unaffected (X-H Y), 50% affected (X-h Y).

Notation note: always include the X (and Y) in your genotypes so the sex is explicit. Writing just "H" and "h" loses information.

Worked example

A man with blood type AB marries a woman with blood type O. The man's genotype is IA IB; the woman's is ii. The Punnett square:

All children: 50% IA i (type A) and 50% IB i (type B). Note that no child can be AB or O. This is a useful example of codominance combined with multiple alleles.

Common traps

Calling everything dominant or recessive. Codominance and incomplete dominance do not have a "dominant" allele in the classical sense.

Confusing codominance with incomplete dominance. Codominance: both traits visible together (blood AB). Incomplete dominance: intermediate blend (pink snapdragon).

Forgetting sex notation in X-linked crosses. Always include the X superscript, otherwise you lose track of which sex each genotype refers to.

Saying "the X-linked gene is on the Y too". Most X-linked genes have no Y counterpart. That is why males are hemizygous and recessive disorders show up so much more often in males.

Calling colour-blind women "carriers". A homozygous X-c X-c female is affected, not a carrier. Carriers are heterozygous.

Forgetting multiple alleles exist at the population level. Any one person has two alleles per locus, but the population may have many alleles total.

In one sentence

Single-gene inheritance follows several patterns: autosomal dominant or recessive (one allele masks the other completely), codominance (both alleles fully expressed together, as in blood type AB), incomplete dominance (heterozygote is intermediate, as in pink snapdragons), multiple alleles (more than two alleles exist in the population, as in ABO), and sex-linked (locus on a sex chromosome, usually X, so males express recessive alleles more often, as in haemophilia and colour blindness).