Heredity

Module 5 is worth about a quarter of the HSC paper (25.5 marks a year on average from 2020 to 2025). Genetic variation (Punnett squares, pedigrees, inheritance patterns) is the most heavily examined part, at about 7–8 marks a year, followed by reproduction (about 6) and DNA and polypeptide synthesis (about 5–6).

The syllabus has five inquiry questions:

• Reproduction: asexual and sexual reproduction in animals, plants, fungi, bacteria and protists; fertilisation, implantation and the hormonal control of pregnancy and birth; how humans manipulate plant and animal reproduction in agriculture.
• Cell replication: mitosis and meiosis, DNA replication, and why accurate copying matters for the continuity of a species.
• DNA and polypeptide synthesis: how DNA is packaged in prokaryotes and eukaryotes, transcription and translation, protein structure, and how genes and the environment together produce a phenotype.
• Genetic variation: Punnett squares and pedigrees for autosomal, sex-linked, codominant, incompletely dominant and multiple-allele inheritance; recording and analysing frequency data such as single-nucleotide polymorphisms (SNPs).
• Inheritance patterns in a population: how DNA sequencing and profiling are used, and how population genetics data help in conservation, disease studies and human evolution.

Asexual reproduction produces offspring genetically identical to one parent. It is fast and needs no mate, but gives little variation, so a population can be wiped out by one disease or environmental change. Sexual reproduction combines gametes from two parents, so offspring are genetically different. It is slower and costs more energy, but the variation helps a species survive change.

Fertilisation

External fertilisation happens outside the body, usually in water (most fish, frogs, corals). Huge numbers of gametes are released because most are lost, and offspring get little protection. Internal fertilisation (mammals, birds, reptiles) places sperm inside the female, so fewer gametes are needed and the embryo is protected, either inside the mother or in a shelled egg.

In flowering plants, pollination (transfer of pollen from anther to stigma, by wind or animals) must happen first. A pollen tube then grows down the style and the male gamete fertilises the egg in the ovule. The ovule becomes the seed and the ovary becomes the fruit.

Hormones in pregnancy and birth

• FSH (from the pituitary) stimulates a follicle to develop; the follicle releases oestrogen, which thickens the endometrium.
• A surge of LH triggers ovulation. The empty follicle becomes the corpus luteum, which secretes progesterone to maintain the endometrium.
• After fertilisation, the embryo implants in the endometrium about a week later. It releases hCG, which keeps the corpus luteum alive (this is what pregnancy tests detect).
• During the first trimester (by about weeks 8–10) the placenta takes over making progesterone and oestrogen. High progesterone keeps the uterus lining in place and stops contractions.
• Near birth progesterone falls and oestrogen rises, making the uterus sensitive to oxytocin. Oxytocin from the posterior pituitary causes contractions, which trigger more oxytocin release: a positive feedback loop that ends with delivery. Prolactin then stimulates milk production.

The placenta lets oxygen and glucose diffuse, and antibodies (IgG) be actively transported, from mother to fetus, and lets wastes (CO₂, urea) move the other way, without the two blood supplies mixing.

Manipulating reproduction in agriculture

• Plants: selective breeding, artificial pollination (hand-transferring pollen to control the cross), grafting, cuttings and tissue culture (cloning a high-yielding plant).
• Animals: selective breeding, artificial insemination (one prize bull can sire thousands of calves), IVF and embryo transfer, and cloning.

These boost yield and consistency but reduce genetic diversity, which leaves crops and herds vulnerable to disease. That trade-off is a favourite "evaluate" question and links to Module 6.

Mitosis and meiosis

Meiosis produces variation in three ways:

1. Crossing over in prophase I: homologous chromosomes swap sections, giving new combinations of alleles on each chromatid.
2. Independent assortment in metaphase I: each homologous pair lines up independently, so a human can produce $2^{23}$ (over 8 million) chromosome combinations without crossing over.
3. Random fertilisation of one gamete by another multiplies this further. (Fertilisation is not part of meiosis, but it is examined alongside it.)

Be able to draw or model chromosomes through each stage, showing homologous pairs separating in meiosis I and sister chromatids separating in meiosis II.

DNA replication

DNA replication happens in the S phase of interphase, before a cell divides, so each daughter cell gets a complete copy.

1. Helicase unwinds the double helix and breaks the hydrogen bonds between bases.
2. DNA polymerase adds free nucleotides to each exposed template strand by complementary base pairing (A with T, C with G). It can only add to the 3' end, so new DNA grows 5' to 3'.
3. The leading strand is made continuously; the lagging strand is made in short Okazaki fragments.
4. DNA ligase joins the fragments into a continuous strand.

Replication is semi-conservative: each new molecule has one original strand and one new strand (shown by Meselson and Stahl in 1958). DNA polymerase proofreads, so errors are rare. Accurate copying keeps traits stable from one generation to the next, while the few uncorrected errors (mutations) are a source of new alleles.

DNA in prokaryotes and eukaryotes

• Prokaryotes: one circular chromosome in the nucleoid (no nucleus), little non-coding DNA, plus small circular plasmids.
• Eukaryotes: several linear chromosomes in the nucleus, wound around histone proteins to form chromatin; lots of non-coding DNA, and genes contain introns. Mitochondria (and chloroplasts) also contain their own small circular DNA.

Transcription and translation

A gene is a section of DNA that codes for a polypeptide.

Transcription (nucleus): RNA polymerase binds to the promoter, unwinds the DNA and builds an mRNA strand complementary to the template strand, using U instead of T. In eukaryotes the pre-mRNA is then processed: introns are removed and exons joined, and a cap and tail are added. The mRNA leaves through a nuclear pore.

Translation (ribosome in the cytoplasm): the ribosome reads mRNA one codon (three bases) at a time, starting at AUG. Each tRNA carries a specific amino acid and has an anticodon that pairs with the codon. The ribosome joins neighbouring amino acids with peptide bonds until it reaches a stop codon (UAA, UAG or UGA), then releases the polypeptide.

Worked example: from gene to polypeptide

A section of template DNA reads 3'-TAC AAG ACC GTG-5'. Use the genetic code to find the mRNA and amino acids.

mRNA (complementary to the template, U for T): 5'-AUG UUC UGG CAC-3'.

Amino acids: AUG = Met, UUC = Phe, UGG = Trp, CAC = His.

tRNA anticodons: UAC, AAG, ACC, GUG. Notice the anticodons have the same sequence as the template DNA, with U for T.

Proteins

The sequence of amino acids is the protein's primary structure. Hydrogen bonds fold it into α-helices and β-pleated sheets (secondary), the chain folds into a specific 3D shape (tertiary), and some proteins combine several polypeptides (quaternary, eg haemoglobin). Shape determines function: enzymes, hormones such as insulin, antibodies, structural proteins (collagen, keratin) and transport proteins. That is why polypeptide synthesis matters: a change to the gene can change the shape and so the function.

Genes and environment

The genotype sets the possible range of a trait; the environment affects how it is expressed (the phenotype). Examples: Himalayan rabbits and Siamese cats have a heat-sensitive pigment enzyme, so only the cooler extremities are dark; hydrangea flowers are blue in acidic soil and pink in alkaline soil; human height depends on genes and nutrition. Environmental factors can also switch genes on or off.

Key terms

• Allele: a version of a gene. Homozygous: two identical alleles; heterozygous: two different alleles.
• Dominant alleles show in heterozygotes; recessive alleles only show when homozygous.
• Codominance: both alleles fully expressed in heterozygotes (AB blood group; roan coats).
• Incomplete dominance: heterozygote is an intermediate blend (red × white snapdragons give pink).
• Multiple alleles: more than two alleles exist in the population, eg ABO blood groups with $I^A$, $I^B$ and $i$.
• Sex-linked: the gene is on the X chromosome, so males (XY) show a recessive X-linked trait with only one copy. Examples are haemophilia and red-green colour blindness.

Worked example: two genes

In a plant, tall (T) is dominant to short (t) and round seeds (R) are dominant to wrinkled (r). The genes are on different chromosomes. What fraction of offspring from $TtRr\times Ttrr$ are tall with round seeds?

Treat each gene separately and multiply:

The full 4 × 2 Punnett square gives the same answer: 3 tall round, 3 tall wrinkled, 1 short round and 1 short wrinkled out of 8. Multiplying is quicker and is accepted, as long as you state that the genes are inherited independently.

Worked example: multiple alleles

A person with blood group A (genotype $I^{A}i$) has children with a person with blood group B ($I^{B}i$). The gametes are $I^A$ or $i$, and $I^B$ or $i$, so the four equally likely offspring genotypes are $I^A{I}^B$, $I^{A}i$, $I^{B}i$ and $ii$. Each blood group (AB, A, B, O) has a probability of $\frac{1}{4}$. Two parents can therefore have a child with a blood group neither of them has.

Reading pedigrees

Squares are males, circles are females, shaded symbols are affected. Work through these tests and quote the individuals that prove each point:

• Two unaffected parents with an affected child: the trait is recessive (both parents are carriers).
• Two affected parents with an unaffected child: the trait is dominant.
• An affected daughter of an unaffected father: the trait is not X-linked recessive (her father would have to be affected).
• Mostly males affected, passed from carrier mothers to sons, never father to son: suspect X-linked recessive.

When asked for genotypes, write both alleles for females and one X allele plus Y for males, eg $X^H{X}^h$ and $X^{h}Y$.

SNP and frequency data

A single-nucleotide polymorphism is a single base that varies between individuals at one position in the genome (the less common version occurs in at least about 1% of a population). SNPs are used as genetic markers to compare individuals and populations, trace ancestry, and find links with disease. The syllabus asks you to collect and present frequency data and identify trends, so practise turning genotype counts into frequencies and graphs.

Worked example: allele frequencies

At one SNP, 250 people were genotyped: 100 are CC, 100 are CT and 50 are TT. Each person has two alleles, so there are 500 alleles.

Hardy–Weinberg is not named in the syllabus; if a question uses it, the formula is given. If the population were in Hardy–Weinberg equilibrium (random mating, no selection, migration or mutation, large size), the expected heterozygote frequency would be $2pq=2(0.6)(0.4)=0.48$, or 120 people. Only 100 (0.40) are observed, so there are fewer heterozygotes than expected. The difference could be chance in a small sample, or a sign of non-random mating or mixed populations.

Technologies

• DNA sequencing determines the exact order of bases (Sanger sequencing, now faster next-generation methods). It finds disease-causing mutations, identifies carriers and compares species. The Human Genome Project was declared complete in 2003.
• DNA profiling compares regions of non-coding DNA that vary greatly between people, especially short tandem repeats (STRs). Samples are copied by PCR, then fragments are separated by size using electrophoresis. Profiles are used in forensics, paternity testing and checking relatedness in breeding programs. The technique details are covered in Module 6.

Uses of population genetics

• Conservation: low genetic diversity reduces a population's ability to adapt and increases inbreeding. Tasmanian devils have very low diversity, which is one reason a transmissible facial tumour spread so easily. Genetic data help managers choose which animals to breed or move between populations.
• Disease: comparing allele frequencies in affected and unaffected groups (including SNP studies) identifies genes linked to conditions, and pedigree data across families reveal how a disorder is inherited.
• Human evolution: mitochondrial DNA (inherited from the mother) and the Y chromosome (father to son) change slowly and do not recombine, so they trace lineages. They support a recent African origin of modern humans, and genome studies show most non-African people carry a small amount of Neanderthal DNA.

• Confusing chromosomes, chromatids and homologous pairs when describing meiosis.
• Writing that mitosis produces variation, or that meiosis produces two cells.
• Using T in mRNA, or reading the coding strand instead of the template strand.
• Saying "the codon codes for the tRNA". Codons are on mRNA; anticodons are on tRNA.
• Calling codominance "blending". Blending is incomplete dominance.
• Giving a ratio when a probability is asked for (or the reverse), or forgetting the probability resets for every child.
• For sex linkage, giving males two alleles or forgetting to write X and Y.
• Concluding a pedigree pattern without naming the individuals that prove it.

• Explain answers need a cause and effect: "crossing over swaps sections between homologous chromosomes, so each gamete has a new combination of alleles".
• Compare needs similarities and differences; a table is fine if every row covers both items.
• Describe a process (replication, protein synthesis): use the correct enzyme and molecule names in the right order. Markers look for helicase, DNA polymerase, complementary base pairing and semi-conservative.
• Genetics calculations: define your allele symbols, show parental genotypes, gametes, the Punnett square and the final ratio or probability. Most marks are for this setting-out.
• Pedigrees: state the pattern and justify it with specific individuals (eg "II-3 is affected but both parents are not, so the trait is recessive").
• Data questions often give SNP or allele counts; remember each diploid individual has two alleles.

• Module 6: mutations change DNA sequences and so polypeptides; PCR, electrophoresis and recombinant DNA build on this module; genetic drift and gene flow change the allele frequencies you calculate here.
• Module 7: antibodies are proteins whose shape depends on their amino acid sequence; antibody transfer across the placenta gives passive immunity.
• Module 8: genetic diseases such as cystic fibrosis follow the inheritance patterns here; insulin and ADH are proteins made by polypeptide synthesis.