Sexual vs asexual reproduction

• Sexual reproduction — two parents; gametes (sex cells) join at fertilisation. It mixes genes, so offspring show variation and are genetically different from their parents and each other.
• Asexual reproduction — one parent; only mitosis. Offspring are genetically identical clones of the parent, with no variation.

Trade-offs: asexual reproduction is fast and needs no mate, and is good in a stable environment. Sexual reproduction is slower but the variation it creates helps a species survive if the environment changes (raw material for natural selection).

Meiosis makes gametes

Gametes are made by meiosis, which happens only in the reproductive organs. From one diploid cell (46 chromosomes) it produces four genetically different haploid gametes, each with 23 chromosomes.

Mitosis vs meiosis: mitosis = 2 identical diploid cells (growth/repair). Meiosis = 4 different haploid gametes (sexual reproduction). At fertilisation two gametes (23 + 23) restore the diploid 46.

Gametes from meiosis

Your turn — haploid number

DNA structure & the genome

Chromosomes are made of DNA, a double helix of two strands. Each strand is a chain of nucleotides; the rungs are pairs of bases that pair by complementary base pairing:

Genome: an organism's genome is its entire set of DNA (all its genes). A gene is a short section of DNA that codes for a particular protein.

The language of genetics

• Gene — a section of DNA coding for a protein / feature.
• Allele — a different version of a gene (e.g. B or b).
• Genotype — the alleles an organism has (e.g. Bb).
• Phenotype — the characteristics you can observe (e.g. brown eyes).
• Homozygous — two same alleles (BB or bb).
• Heterozygous — two different alleles (Bb).
• Dominant allele — shows in the phenotype even if only one copy is present (written as a capital, B).
• Recessive allele — only shows when both alleles are recessive (bb; written lower case).

Watch out: genotype is the alleles (letters); phenotype is the visible feature. A heterozygous individual (Bb) shows the dominant phenotype.

Genotype word sort

Tap a genotype or term, then tap the box it belongs to.

Monohybrid crosses & Punnett squares

A Punnett square predicts the offspring of a cross. Here two heterozygous parents (Bb × Bb) for a gene where B (brown) is dominant over b (blue):

Reading ratios as probability: a 3 : 1 ratio means each offspring has a 3/4 (75%) chance of the dominant phenotype and a 1/4 (25%) chance of the recessive one.

Your turn — probability of recessive

Your turn — probability of dominant

Your turn — a different cross

Sex determination

Of the 23 pairs of human chromosomes, one pair is the sex chromosomes: XX in females and XY in males. All eggs carry an X; sperm carry either an X or a Y.

Boy or girl?

Variation

Differences between organisms of the same species are called variation. It has two sources:

• Genetic variation — caused by different alleles (from mutation and from the mixing of genes in sexual reproduction). E.g. natural eye colour, blood group.
• Environmental variation — caused by surroundings and lifestyle. E.g. a scar, or a plant grown in the shade being shorter.
• Many features (e.g. height) are affected by both genes and environment.

Mutation: a random change to the DNA base sequence. Most have little or no effect, but occasionally a mutation changes the phenotype and provides new variation for natural selection.

Match the genetics term

Tap a term on the left, then its meaning on the right.

The Human Genome Project

The Human Genome Project (HGP) worked out the order of all the bases in the human genome. Understanding the genome has several benefits:

• Predicting and preventing disease by identifying genes linked to inherited disorders.
• Developing new and better medicines and testing which drugs suit an individual.
• Understanding inherited disorders and human ancestry/evolution.

Concerns to evaluate: genetic information could be misused, for example by insurers or employers, and could cause anxiety or discrimination — so data must be handled ethically.

The big ideas to know

You've covered the core of Edexcel Topic 3 — Genetics. Press Finish to see your score.