• A gene locus is a gene's fixed address on a chromosome; genes on the same chromosome are linked (autosomal or sex-linked).
• Meiosis makes genetically varied haploid gametes via independent assortment and crossing over.
• Sperm and egg cells are specialised for their jobs — sperm for delivery, eggs for nourishment and protection.
• Fertilisation in mammals is single-sperm-only, enforced by the cortical reaction; in plants it's double fertilisation (zygote + endosperm).
• The cell cycle (interphase → mitosis → cytokinesis) is tightly regulated; mitosis produces genetically identical daughter cells for growth, repair, and asexual reproduction.
• Mitotic index quantifies what proportion of a cell sample is actively dividing.
• Stem cells vary in potency: totipotent → pluripotent → multipotent, each with different medical uses and ethical baggage.
• Cells specialise via differential gene expression, controlled by transcription factors.
• Post-transcriptional splicing (and alternative splicing) lets one gene code for multiple proteins.
• Epigenetics (DNA methylation, histone acetylation) switches genes on/off without changing the DNA sequence — and can be inherited.
• Polygenic inheritance (many genes, additive effects + environment) produces continuous variation like height.

Every chromosome is basically a very long piece of DNA carrying hundreds or thousands of genes, each coding for a different protein. The locus (plural: ) of a gene is simply its fixed physical address on the chromosome — like a house number on a street. That gene for eye colour, or blood clotting, or whatever, is found at that same spot on that same chromosome, in every person, in every generation.

Key distinction to nail down:

• Gene = the type of information (e.g. "eye colour gene")
• Locus = the physical location of that gene on the chromosome
• Allele = a specific version of that gene (e.g. "blue" vs "brown")

Genes that sit on the same chromosome are said to be linked — because during meiosis, whole chromosomes get shuffled and separated, not individual genes. If two genes are physically stuck together on the same chromosome, they tend to be inherited as a package deal, rather than assorting independently like Mendel's classic peas.

Autosomal linkage

This is linkage that occurs on the autosomes — any of the 22 human chromosome pairs that the sex chromosomes. Two genes on the same autosome don't assort independently during meiosis; instead they stay together in their original parental combination and get passed on to offspring as a unit (through gametes).

Sex linkage

Humans have two sex chromosomes: X and Y. Females are XX, males are XY. Because the X chromosome is much longer than the tiny Y, most sex-linked genes are actually found on the X chromosome only.

A female who carries one recessive disease allele (masked by a dominant normal allele) is called a carrier — she's not affected herself, but has a 50% chance of passing the recessive allele to each child. If that child is male, he have the condition (no second X to protect him).

Historical example: Thomas Hunt Morgan & fruit flies

Morgan bred and noticed a strange pattern: crossing a homozygous white-eyed female with a red-eyed male gave all red-eyed females and all white-eyed males in the offspring. This sex-biased pattern only makes sense if the eye-colour gene sits on the X chromosome — proving sex linkage experimentally for the first time.

Meiosis is the type of cell division used to make gametes (sex cells). Unlike mitosis, it deliberately creates cells that are genetically different from each other and from the parent cell. This variation matters hugely for natural selection — genetically diverse offspring are more likely to include some individuals who can survive a changing environment.

Meiosis takes one diploid (2n) parent cell nucleus and puts it through two rounds of division:

• Meiosis I — homologous chromosome are separated → 2 haploid (n) nuclei, each chromosome still made of 2 chromatids
• Meiosis II — the of each chromosome are separated → 4 haploid (n) nuclei, each chromosome now a single chromatid

During metaphase I, homologous chromosome pairs line up at the equator of the spindle. Here's the key bit: for each pair, it's completely random which chromosome (maternal or paternal) ends up facing which pole. Crucially, the orientation of one pair has zero effect on the orientation of any other pair — they assort .

The second big source of variation happens during prophase I. Homologous chromosomes pair up tightly, forming structures called bivalents. While paired, non-sister chromatids can physically tangle at points called chiasmata (singular: chiasma). This tangling puts stress on the DNA, and a section of one chromatid can break off and rejoin with the equivalent section of the other chromosome's chromatid.

The result: a brand new combination of alleles on each chromatid that didn't exist in either parent originally. There's usually at least one chiasma per bivalent, and crossing over happens more often further from the centromere.

Sperm and egg cells are both haploid (23 chromosomes each in humans) but look completely different, because they have completely different jobs. Sperm need to ; eggs need to .

1. Sperm are ejaculated near the cervix and follow a chemical trail released by the egg, travelling through the uterus into the oviduct.
2. A sperm cell releases enzymes from its acrosome to digest through the zona pellucida — the acrosome reaction.
3. The sperm's nucleus enters and fuses with the egg's nucleus.
4. The egg immediately releases cortical granules, which cause the zona pellucida to rapidly harden — the cortical reaction. This ensures only one sperm can fertilise the egg.
5. The fused nuclei form a zygote with the full 46 chromosomes (23 from each parent).
6. The zygote divides by mitosis repeatedly to form an embryo, whose cells eventually specialise.

Plants don't have sperm swimming through fluid — instead, pollination (transfer of pollen from anther to stigma) is followed by the growth of a pollen tube down the style towards the ovary.

Inside the pollen tube travel two haploid male nuclei (produced when the generative nucleus divides by mitosis). When the tube reaches the ovule, something unique to plants happens — double fertilisation:

• Male nucleus #1 fuses with the egg cell nucleus → forms the diploid zygote (2n)
• Male nucleus #2 fuses with two polar nuclei → forms the triploid endosperm nucleus (3n), which becomes the endosperm — a food store for the germinating embryo

The cell cycle is the regulated sequence of events between one cell division and the next, controlled by chemical signals called cyclins. It has three main phases:

• Interphase — cell grows, replicates DNA, prepares for division (by far the longest phase)
• Nuclear division (Mitosis / "M phase") — the nucleus splits into two genetically identical nuclei
• Cell division (Cytokinesis) — the whole cell physically splits into two daughter cells

Interphase has three sub-stages: G₁, S, G₂

Prophase

• Chromosomes condense (now visible when stained), each made of 2 sister chromatids joined at the centromere
• Centrosomes move to opposite poles
• Spindle fibres begin to form from the centrosomes
• The nuclear envelope breaks down into small vesicles

Metaphase

• Centrosomes reach opposite poles
• Chromosomes line up at the equator (metaphase plate)
• Each sister chromatid attaches to a spindle fibre from an pole

Anaphase

• Sister chromatids separate at the centromere
• Spindle fibres shorten, pulling the separated chromatids (now called chromosomes) to opposite poles

Telophase

• Chromosomes arrive at poles and start to decondense
• Nuclear envelopes reform around each set of chromosomes
• Spindle fibres break down
• Followed by cytokinesis — the whole cell splits (constriction in animal cells; new cell wall in plant cells)

Mitosis produces two daughter cells genetically identical to the parent and to each other (clones). This underpins three big biological processes:

• Growth — a single-celled zygote divides repeatedly by mitosis to become a multicellular organism
• Repair & replacement — damaged or dying cells (e.g. skin, gut lining) are continually replaced by identical new ones; some animals (zebrafish, axolotls) can even regenerate whole body parts
• Asexual reproduction — unicellular organisms (e.g. ) reproduce by cell division; multicellular organisms bud off genetically identical offspring (e.g. , yeast, strawberry runners)

Growth in plants is concentrated in meristems (e.g. just behind the root tip cap), where lots of cells are actively dividing — making it the perfect place to observe mitosis under a microscope.

Method summary

1. Cut off root tips (~1 cm) from onion/garlic and place in warmed (60°C) 1M HCl for 5 minutes — this softens the tissue and helps separate cells
2. Rinse in cold water, blot dry
3. Cut off ~2 mm of the very tip and place on a slide
4. Add a stain (e.g. acetic orcein — stains chromosomes deep purple)
5. Gently squash using a blunt tool or coverslip to spread cells into a thin single layer
6. View under the microscope and identify cells in different mitotic stages

The mitotic index tells you what proportion of cells in a sample are actively undergoing mitosis (i.e. have visible condensed chromosomes) versus in interphase.

Worked Example: A sample has 64 cells total: 36 in interphase, 14 in prophase, 5 in metaphase, 3 in anaphase, 6 in telophase.

Mitotic index = (14 + 5 + 3 + 6) ÷ 64 = 28 ÷ 64 = 0.44

A stem cell can divide by mitosis an unlimited number of times. Each new cell can either stay a stem cell, or differentiate into a specialised cell type. Potency describes how many different cell types a stem cell is capable of becoming.

Stem cells can replace damaged tissue — already used to treat leukaemia via bone marrow transplant, and researched for spinal cord injuries and heart disease.

Here's a genuinely mind-bending fact: every cell in your body contains the exact same DNA (the same genome). Yet a neuron looks and behaves nothing like a skin cell or a muscle cell. How? Because different cells switch on (express) different subsets of that shared genome — this is called differential gene expression.

The basic steps of differentiation:

1. Certain genes in a stem cell are activated, others inactivated
2. mRNA is transcribed only from the active genes
3. mRNA is translated into proteins
4. These proteins modify the cell's structure and function
5. The cell becomes increasingly specialised — and this process is irreversible once complete

A transcription factor is a protein that controls gene transcription by binding to a specific region of DNA (often the promoter region, just "upstream" of a gene). About 10% of human genes actually code for transcription factors — that's how central they are to controlling everything else.

Eukaryotic genes are full of non-coding sections called introns, interspersed between the coding sections called exons. During transcription, the whole thing — introns AND exons — gets copied into a pre-mRNA molecule.

Before this pre-mRNA leaves the nucleus, a process called splicing removes the introns and joins the exons together, producing mature mRNA that contains only coding sequence.

Alternative splicing — one gene, multiple proteins

Here's the clever bit: exons don't to be spliced together in the same combination every time. A particular exon might be included in one mature mRNA and skipped in another. This means a single gene can code for several different proteins, depending on which exons end up in the final mRNA — which is a huge part of why the human proteome (all proteins) is far larger than the human genome (all genes).

An organism's observable characteristics (its phenotype) aren't determined by genes alone. Environmental factors — diet, light, temperature, nutrient availability — can significantly influence how a genotype is actually expressed.

Classic examples:

• Drosophila diet: normally-grey fruit flies fed a diet high in silver salts develop yellow colouring regardless of their genotype
• Plant chlorosis: plants grown without enough light or magnesium turn yellow (can't make chlorophyll) even though genetically they "should" be green
• Etiolation: plants grown in the dark develop long, spindly stems with small curled leaves

Epigenetics is the control of gene expression by factors the DNA sequence itself — switching genes on and off without changing the actual genetic code. This happens by chemically modifying the chromatin — the structure formed when DNA wraps around proteins called histones.

Two main mechanisms

The reverse of acetylation — deacetylation — returns lysine to its positive charge, which binds DNA more tightly again and switches the gene back off.

Remarkably, epigenetic tags (collectively called the epigenome) can sometimes survive gamete formation and be passed on to offspring. Normally DNA gets de-methylated during gamete production, but not always completely — meaning epigenetic changes triggered by a parent's environment can, in some cases, affect the next generation too.

Discontinuous variation = individuals fall into clear, separate categories with nothing in between (you're blood group A, B, AB, or O — never "a bit of both"). Continuous variation = a smooth range of values exists between two extremes, with no natural breaks — you can be any height between very short and very tall.

When multiple genes affect the same characteristic, and each gene contributes a similar small effect, these effects can add together — this is called an additive effect. A large number of genes contributing this way to one phenotype are together called polygenes.

Because there are several possible genotype combinations that give intermediate heights (not just "tall" or "short"), you get a smooth spread of possible phenotypes — continuous variation — especially once you also factor in environmental effects on top.

• I can explain what a gene locus is and distinguish it from a gene and an allele
• I can explain autosomal linkage and sex linkage, and why males show more sex-linked recessive conditions
• I can describe Meiosis I and II and state exactly when the chromosome number halves
• I can explain independent assortment and calculate combinations using 2ⁿ
• I can explain crossing over and the role of chiasmata
• I can describe the adaptive features of sperm and egg cells
• I can explain the acrosome reaction and cortical reaction in mammalian fertilisation
• I can explain double fertilisation in flowering plants and the resulting ploidy of the zygote vs endosperm
• I can describe the three phases of the cell cycle and the sub-phases of interphase (G₁, S, G₂)
• I can describe all four stages of mitosis (PMAT) in order with correct detail
• I can explain the biological significance of mitosis (growth, repair, asexual reproduction)
• I can calculate mitotic index from cell count data
• I can distinguish totipotent, pluripotent, and multipotent stem cells
• I can evaluate the benefits, risks, social and ethical issues of embryonic vs adult stem cell use
• I can explain differential gene expression and the role of transcription factors (activators/repressors)
• I can explain splicing and alternative splicing, and why one gene can make multiple proteins
• I can explain epigenetics, including DNA methylation and histone acetylation, and why it's different from mutation
• I can explain how environmental factors affect phenotype and why this variation isn't inherited
• I can distinguish monogenic/discontinuous from polygenic/continuous variation
• I can calculate phenotype outcomes using the additive effect of polygenes