What is DNA, and how is genetic information passed from parents to offspring?
DNA, genes and chromosomes, the genome, sexual and asexual reproduction, meiosis and the production of gametes, and the key genetic terms.
A focused answer to Edexcel GCSE Combined Science Topic 3 (CB3), covering DNA, genes and chromosomes, the genome, sexual and asexual reproduction, meiosis and the production of gametes, and the key genetic terms used in inheritance.
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What this dot point is asking
Edexcel wants you to describe DNA, genes and chromosomes and what the genome is, compare sexual and asexual reproduction, explain how meiosis produces gametes with half the number of chromosomes, and use the key genetic terms correctly.
DNA, genes and chromosomes
The genome is the entire genetic material of an organism. Understanding the human genome helps scientists search for genes linked to diseases, understand inherited disorders, and trace human migration and ancestry. DNA is found in the nucleus of eukaryotic cells, wound tightly around proteins so that very long molecules fit inside a tiny space. Each chromosome carries hundreds of genes, and the order of the genetic code along the DNA determines which proteins, and therefore which characteristics, a cell can make.
Sexual and asexual reproduction
- Sexual reproduction involves two parents and the joining (fertilisation) of two gametes (sex cells: sperm and egg in animals, pollen and ovule in plants). It produces offspring that are genetically different, giving variation.
- Asexual reproduction involves one parent and mitosis only. The offspring are genetically identical clones of the parent, with no variation. Many plants, bacteria and some animals reproduce this way.
Meiosis and gametes
Meiosis is the type of cell division that produces gametes. It happens only in the reproductive organs and:
- Starts with one cell and produces four gametes.
- Halves the chromosome number, so each gamete is haploid (one chromosome from each pair).
- Produces gametes that are all genetically different from each other.
At fertilisation, a male and female gamete fuse, restoring the full diploid chromosome number and combining genes from both parents.
Key genetic terms
- Gene: a section of DNA coding for a protein.
- Allele: a different version of the same gene.
- Dominant allele: shown even if only one copy is present (written as a capital letter).
- Recessive allele: shown only if two copies are present (written as a lower-case letter).
- Genotype: the alleles an organism has.
- Phenotype: the characteristics that are shown.
- Homozygous: two identical alleles; heterozygous: two different alleles.
Try this
Q1. Define a gene. [1 mark]
- Cue. A small section of DNA that codes for a protein.
Q2. State one advantage of asexual reproduction. [1 mark]
- Cue. It is fast and needs only one parent (offspring are identical clones).
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksCompare sexual and asexual reproduction in terms of the number of parents, the cell division used, and the genetic make-up of the offspring.Show worked answer →
A 4-mark compare question needs linked points across both types.
Sexual reproduction involves two parents, whereas asexual reproduction involves only one (1 mark). Sexual reproduction uses meiosis to make gametes, whereas asexual reproduction uses mitosis (1 mark). In sexual reproduction the offspring are genetically different from each other and the parents (showing variation), whereas in asexual reproduction the offspring are genetically identical clones of the parent (2 marks).
Markers reward two parents versus one, meiosis versus mitosis, and variation versus identical clones.
Edexcel 20213 marksExplain why gametes produced by meiosis have half the number of chromosomes, and why this is important for sexual reproduction.Show worked answer →
A 3-mark explain question on meiosis.
Meiosis halves the chromosome number so that each gamete has one chromosome from each pair (it is haploid) (1 mark). This is important because when two gametes join at fertilisation, the full (diploid) number of chromosomes is restored (1 mark). If the number were not halved, it would double every generation, which would not produce a viable organism (1 mark).
Markers reward halving to haploid, restoring the diploid number at fertilisation, and the consequence of keeping a constant chromosome number.
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Sources & how we know this
- Edexcel GCSE (9-1) Combined Science (1SC0) specification — Pearson (2016)