Inheritance and Variation - TCE Biology (Tasmania)

Syllabus dot point

“Genetics: monohybrid crosses; use models such as Punnett squares to predict frequencies of genotypes and phenotypes for autosomal dominant or recessive, sex-linked dominant or recessive, incomplete dominance, codominance and multiple alleles (ABO blood groups only, with alleles i, IA and IB); use pedigree charts to analyse inheritance”

TCEBiologyDNA, genetics and cell division…8 min readUpdated 23 Sep 2026

Quick answer

Inheritance follows predictable patterns set out by Mendel: alleles segregate during gamete formation and assort independently, so crosses give characteristic genotype and phenotype ratios. Variation between offspring arises mainly from meiosis, random fertilisation, and mutation.

What this dot point is asking

Key genetics vocabulary

Understanding inheritance starts with precise terms.

Gene: a length of DNA coding for a characteristic.
Allele: one version of a gene (for example, a tall allele or a short allele).
Genotype: the alleles an organism carries (for example, Tt).
Phenotype: the observable characteristic (for example, tall).
Homozygous: two identical alleles (TT or tt).
Heterozygous: two different alleles (Tt).
Dominant: an allele expressed in the phenotype even when heterozygous (shown with a capital).
Recessive: an allele only expressed when homozygous (shown with a lowercase).

Mendel's laws

The law of segregation states that the two alleles for a gene separate during gamete formation so each gamete carries only one. The law of independent assortment states that alleles of different genes are distributed to gametes independently of one another (for genes on different chromosomes).

Monohybrid crosses

A monohybrid cross follows a single gene. A Punnett square predicts offspring ratios. Crossing two heterozygous tall plants (Tt x Tt) gives offspring in the ratio 1 TT : 2 Tt : 1 tt, a genotype ratio of 1:2:1. Because T is dominant, the phenotype ratio is 3 tall : 1 short.

A test cross crosses an organism showing the dominant phenotype with a homozygous recessive individual to find out whether the first organism is homozygous or heterozygous. If any recessive offspring appear, the unknown parent must have carried a recessive allele.

Predicting a monohybrid ratio

In guinea pigs, black coat (B) is dominant to white (b). Crossing a heterozygous black guinea pig (Bb) with a white guinea pig (bb): gametes from Bb are B and b, gametes from bb are b and b. The offspring are Bb, Bb, bb, bb, giving a 1:1 ratio of black to white. This is the expected outcome of a test cross on a heterozygote.

Dihybrid crosses (not examined in BIO315124)

The external assessment specifications exclude dihybrid crosses and linkage, so this section is background only.

A dihybrid cross follows two genes at once. Crossing two individuals heterozygous for both genes (for example RrYy x RrYy in peas, where R is round and Y is yellow) gives the classic 9:3:3:1 phenotype ratio: 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green. This ratio only holds when the two genes assort independently, which requires them to be on different chromosomes (or far apart on the same chromosome).

Variations on simple dominance

Not all inheritance is simple dominant and recessive.

Incomplete dominance: heterozygotes show an intermediate phenotype (for example, red x white flowers giving pink).
Codominance: both alleles are fully expressed in the heterozygote (for example, AB blood type, where both A and B antigens appear).
Multiple alleles: a gene with more than two possible alleles in the population. The only example BIO315124 uses is the ABO blood group, with alleles $I^A$ and $I^B$ (codominant) and $i$ (recessive to both). Genotypes: group A is $I^AI^A$ or $I^Ai$ ; group B is $I^BI^B$ or $I^Bi$ ; group AB is $I^AI^B$ ; group O is $ii$ .

Sex linkage

In humans, females are XX and males are XY. Genes on the X chromosome that have no matching allele on the smaller Y chromosome are sex-linked. Because males have only one X, a single recessive allele on it is expressed. This is why X-linked recessive conditions such as red-green colour blindness and haemophilia are more common in males. Write sex-linked genotypes with the chromosomes shown, for example $X^HX^h$ for a carrier female and $X^hY$ for an affected male.

The course also includes X-linked dominant inheritance. Here one copy of the allele causes the trait, so an affected father passes it to all of his daughters and none of his sons, and an affected heterozygous mother passes it to half of her children of either sex.

Sex-linked recessive inheritance

For an X-linked recessive trait, an affected father cannot pass it to his sons (he gives them his Y), but all his daughters become carriers. Carrier mothers pass the allele to half their sons, who are then affected.

Pedigrees

A pedigree is a family tree that tracks a trait across generations. Squares represent males, circles represent females, and shaded symbols are affected individuals. By analysing who is affected, you can deduce whether a trait is dominant or recessive and whether it is autosomal or sex-linked. For example, if two unaffected parents have an affected child, the trait must be recessive, because the parents both carried a hidden allele.

Sources of variation

Genetic variation within a species comes from three main sources:

Meiosis: crossing over and independent assortment shuffle alleles into new combinations in gametes.
Random fertilisation: any sperm can fertilise any egg, multiplying the possible combinations.
Mutation: changes to the DNA sequence create entirely new alleles, the ultimate source of all variation.

Environmental factors can also affect the phenotype (for example, diet affecting height), but only genetic changes are inherited.

Assuming ratios are guaranteed

A 3:1 ratio is a probability, not a certainty. With small numbers of offspring the actual results often differ from the predicted ratio by chance. Large sample sizes are needed before the observed counts approach the expected ratio.

Scope for the TASC exam

Genetics is part of criterion 8 (Section E of the exam). The external assessment specifications include monohybrid crosses, ABO multiple alleles, Mendelian inheritance, X-linked dominant and recessive inheritance, codominance and incomplete dominance, and pedigree analysis; they exclude dihybrid crosses and linkage. For identifying modes of inheritance from family trees, see pedigree analysis.

Exam-style questions

Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

Original7 marks

In pea plants, purple flower colour (

P

) is dominant over white (

p

). A heterozygous purple plant is crossed with a white plant. Using a Punnett square, determine the genotype and phenotype ratios of the offspring. Then explain why a test cross with a white plant is useful for finding the genotype of a purple-flowered plant of unknown genotype.

Show worked answer →

A 7 mark answer shows the cross with ratios and explains the test cross logic.

Cross $Pp \times pp$: The $Pp$ parent makes $P$ and $p$ gametes; the $pp$ parent makes only $p$ gametes.
Punnett square outcome: Offspring are $Pp$ and $pp$ in a $1 : 1$ genotype ratio. Phenotype ratio is $1$ purple ( $Pp$ ) to $1$ white ( $pp$ ), so $50\%$ purple and $50\%$ white.
Test cross logic: Cross the unknown purple plant with a homozygous recessive white ( $pp$ ). If the purple plant is $PP$ , all offspring are purple. If it is $Pp$ , about half the offspring are white. The appearance of any white offspring reveals the unknown plant carries a $p$ allele, so it must be $Pp$ .

Markers reward the correct $1 : 1$ ratios from the cross and the explanation that white offspring expose a hidden recessive allele.

Original6 marks

Red-green colour blindness is an X-linked recessive condition. A woman who is a carrier has children with a man who has normal colour vision. Using appropriate symbols and a Punnett square, determine the probability that a son will be colour blind and that a daughter will be colour blind, and explain why the condition is more common in males.

Show worked answer →

A 6 mark answer sets up the X-linked cross and explains the male bias.

Parents: Carrier mother $X^B X^b$ ; normal-vision father $X^B Y$ (using $B$ = normal, $b$ = colour blind).
Cross outcome: Offspring are $X^B X^B$ , $X^B X^b$ , $X^B Y$ and $X^b Y$ in equal proportions. Daughters are either normal ( $X^B X^B$ ) or carriers ( $X^B X^b$ ); none are colour blind. Sons are either normal ( $X^B Y$ ) or colour blind ( $X^b Y$ ).
Probabilities: $P(\text{son colour blind}) = \tfrac{1}{2}$ of the sons; $P(\text{daughter colour blind}) = 0$ .
Why more common in males: Males are $XY$ with only one X, so a single recessive $b$ allele is expressed. Females need two copies ( $X^b X^b$ ) to be affected, which is rarer.

Markers reward correct symbols and parental genotypes, the son/daughter probabilities, and the single-X explanation for the male bias.

Practise this

Exam-style questions 2 on this page
Flashcards Biology deck

Sources & how we know this

Biology Level 3 (BIO315124) course document (2024 onwards) — TASC