2.2Unit 2: Cells, Development, Biodiversity and Conservation

SNAB Reproduction & Inheritance — IAL Edexcel Linear Revision Notes & Topical Questions

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2.2 Reproduction and Inheritance — IAL EDEXCEL LINEAR notes: Describe mammalian gametes and fertilisation, gene linkage, and how meiosis produces genetic variation.Gametes — they are haploid sex cells made by meiosis that fuse at fertilisation to form a zygote with DNA from both …

Sperm cell adaptations
Egg cell adaptations
After fertilisation
Autosomal linkage
Sex chromosomes
Sex linkage

Part 1 of 8Mammalian gametes

Gametes

Gametes they are haploid sex cells made by meiosis that fuse at fertilisation to form a zygote with DNA from both parents.
Human sperm and egg cells each contain 23 single chromosomes.

Sperm cell adaptations

Sperm cell adaptations a flagellum for swimming, many mitochondria to supply energy for the flagellum, and an acrosome with digestive enzymes.
Acrosome enzymes break down the zona pellucida around the egg.

Egg cell adaptations

Egg cell adaptations a much larger cell, because the cytoplasm stores food to nourish the embryo.
Follicle cells form a protective coating.
The jelly-like glycoprotein zona pellucida forms a barrier after fertilisation to stop other sperm entering.

Part 2 of 8Fertilisation in mammals

Semen is ejaculated high in the vagina near the cervix.

Sperm follow a chemical trail from the egg through the cervix and uterus into the oviduct.

Fertilisation is most likely 1–2 days after ovulation, when the egg is released from the ovary into the oviduct.

In the acrosome reaction the sperm head releases enzymes that digest a path through the zona pellucida.

In the cortical reaction the egg releases cortical granules from vesicles into the space under the zona pellucida.

The chemicals make the zona pellucida thicken and harden, so only one sperm can fertilise the egg.

The sperm nucleus enters the egg and fuses with the egg nucleus to form a zygote with 46 chromosomes, half from each parent.

After fertilisation
- the zygote divides by mitosis, and cells later specialise to form all the tissues of the offspring.

Part 3 of 8Gene loci and alleles

Gene

Gene a length of DNA that codes for a single polypeptide, and a chromosome holds hundreds or thousands of genes.

Locus

Locus the fixed position of a gene on a chromosome, so the gene for a characteristic is always at the same position.

Allele

Allele an alternative form of a gene with a slightly different nucleotide sequence at the same locus.

Part 4 of 8Autosomal and sex linkage

Autosomal linkage

Autosomal linkage genes on the same autosome do not assort independently in meiosis.
- (a non-sex chromosome)
Linked genes stay together in their parental combination and pass to gametes together.

Sex chromosomes

Sex chromosomes females are XX and males are XY, with a shorter Y chromosome.

Sex linkage

Sex linkage some genes are present on only one sex chromosome, most often the longer X chromosome.
Males have one copy of an X-linked gene and females have two.
Males are more likely to show X-linked recessive conditions such as red-green colour blindness and haemophilia.
Females usually inherit a dominant allele that masks the recessive allele.

Example

Example a carrier mother (X^HX^h) and an unaffected father (X^HY) have a probability of 0.5 that a son has haemophilia.

Part 5 of 8Meiosis

Meiosis

Meiosis nuclear division that produces four genetically different haploid nuclei from one diploid nucleus, used to make gametes.

Chromosomes replicate before meiosis I, so each consists of two chromatids.
In meiosis I, homologous chromosomes are separated, halving the number from 2n to n, and each chromosome still has two chromatids.
In meiosis II, the chromatids of each chromosome separate to give four haploid nuclei with single-chromatid chromosomes.

Importance

Importance genetically different offspring are advantageous for natural selection.

Part 6 of 8Crossing over

Crossing over exchange of alleles between non-sister chromatids of homologous chromosomes during meiosis I.

Homologous chromosomes pair up closely as bivalents.

Non-sister chromatids become entangled at crossing points called chiasmata.

The entanglement stresses the DNA, so a section of one chromatid breaks and rejoins with the other chromatid.

New combinations of alleles result on the two chromosomes.

Frequency
- there is usually at least one chiasma per bivalent, and crossing over is more likely further from the centromere.

Part 7 of 8Independent assortment and other sources of variation

Independent assortment

Independent assortment production of different allele combinations because homologous pairs align randomly at the spindle equator in meiosis I.

Each pair can face either way, and the orientation of one pair does not affect another.
The number of possible chromosome combinations is 2ⁿ, where n is the haploid number.
For humans, 2²³ gives 8 388 608 combinations.

Mutation

Mutation a random change in a gene during DNA replication produces new alleles and more genetic variation.

Part 8 of 8More from the specification

Key terms

Term	Meaning
Gamete	A haploid sex cell that fuses at fertilisation.
Acrosome	Sperm head structure containing enzymes that digest the zona pellucida.
Cortical reaction	Release of cortical granules that hardens the zona pellucida after one sperm enters.
Locus	The position of a gene on a chromosome.
Bivalent	A pair of homologous chromosomes lying together in meiosis I.
Chiasma	A point where non-sister chromatids cross over.
Independent assortment	Random alignment of homologous pairs giving different allele combinations in gametes.

Avoid these mistakes

Saying chromosome number halves in meiosis II — it halves in meiosis I when homologous chromosomes separate.

Saying crossing over occurs between sister chromatids — it occurs between non-sister chromatids of homologous chromosomes.

Saying the cortical reaction digests the zona pellucida — the acrosome reaction digests it and the cortical reaction hardens it.

Saying females cannot be affected by sex-linked recessive conditions — they can if they are homozygous recessive.

Last updated: 15 October 2026