Meiosis — making gametes

Meiosis makes gametes (sex cells such as sperm and egg cells). It differs from mitosis:

• It produces four cells, each with half the chromosome number (they are haploid).
• The cells are genetically different from each other (mitosis makes identical cells).
• At fertilisation two gametes fuse, restoring the full (diploid) chromosome number.

Mitosis vs meiosis: mitosis = 2 identical diploid cells (growth & repair); meiosis = 4 genetically different haploid gametes.

How is meiosis different?

Genetics — the language

• Gene — a section of DNA coding for a protein/characteristic.
• Allele — a different version of a gene.
• Dominant allele (capital letter) — shows even if only one copy is present.
• Recessive allele (lower case) — only shows if both alleles are recessive.
• Homozygous — two of the same allele (BB or bb). Heterozygous — two different (Bb).
• Genotype — the alleles present. Phenotype — the characteristic you can see.

Watch: a recessive characteristic only appears when the genotype is homozygous recessive (e.g. bb).

Which genotype shows the recessive trait?

Predicting inheritance with Punnett squares

A Punnett square predicts the ratio of offspring genotypes from a cross. For a cross Bb × Bb:

• Each parent passes on one allele to each gamete.
• Read the four boxes to get the genotype ratio, then the phenotype ratio.
• A 3:1 ratio is the classic result of crossing two heterozygotes.

Probability: a ratio of 3:1 means each offspring has a ¾ (75%) chance of the dominant trait and ¼ (25%) chance of the recessive trait.

Your turn — probability from a cross

How can a child show a feature neither parent has?

What made the bacteria resist the antibiotic?

Variation — genetic & environmental

Differences between organisms are called variation:

• Genetic variation — from the alleles you inherit (e.g. blood group).
• Environmental variation — caused by surroundings (e.g. a scar, a plant grown in the shade).
• Most variation is a combination of both (e.g. height).

New alleles arise from mutations — random changes to DNA. Most have no effect, some are harmful, and very rarely one gives an advantage — the raw material for evolution.

Mutations are the ultimate source of all new genetic variation.

Genetic or environmental?

Evolution by natural selection

Darwin and Wallace's theory of natural selection explains how species change over time:

• Individuals show variation due to mutations and sexual reproduction.
• Those with advantageous characteristics are more likely to survive and reproduce.
• They pass the beneficial alleles to their offspring.
• Over many generations the advantageous characteristics become more common — the species evolves.

Evidence: fossils and antibiotic-resistant bacteria (resistant bacteria survive antibiotics and pass on the resistance) show natural selection happening.

Antibiotic resistance

What made the bacteria resist the antibiotic?

Selective breeding

Selective breeding (artificial selection) is when humans choose which organisms breed to develop useful features:

• Choose parents with the desired characteristic (e.g. high milk yield, disease resistance).
• Breed them together; from the offspring, again select those with the best feature.
• Repeat over many generations.

Risk: selective breeding reduces the gene pool (less variation), so populations can become more vulnerable to disease or inherited defects.

Spec note: OCR lists selective breeding of food plants and animals under B6.2 Feeding the human race — it is revised here because it is the human-driven version of selection.

Sort each into its group

Tap each statement, then tap whether it describes natural selection, selective breeding, or both.

The big ideas to know

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