Reproduction & Inheritance

• 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.