Revision notes · Inheritance, variation and evolution
Reproduction
Sexual and asexual reproduction4.6.1.1
Definition: Sexual reproduction involves the fusion of male and female gametes, whereas asexual reproduction involves only one parent and produces genetically identical offspring.
Organisms reproduce either sexually by combining genetic material or asexually by making identical clones.
| Sexual reproduction | Asexual reproduction | |
|---|---|---|
| Parents | Two | One |
| Cell division | Fusion of gametes, formed by meiosis | No fusion — mitosis |
| Offspring | Genetically varied | Genetically identical (clones) |
⚠️ Common mistake: Assuming plants only reproduce sexually; many plants can also reproduce asexually using runners or tubers.
🧠 Remember: Sexual = Special cells (gametes) + Sort of different (variation). Asexual = Alone (one parent) + Alike (clones).
Advantages and disadvantages of sexual and asexual reproduction4.6.1.3
Sexual and asexual reproduction offer distinct evolutionary advantages depending on environmental stability.
| Sexual | Asexual | |
|---|---|---|
| Speed | Slower — needs a mate | Faster — no mate needed |
| Advantage | Variation gives a survival edge if the environment changes (natural selection) | Efficient — many offspring quickly in favourable conditions |
| Risk | Costs time & energy to find a mate | No variation — vulnerable if conditions change |
- •Some organisms reproduce both ways; for example, malaria parasites reproduce asexually in human hosts and sexually in mosquitoes.
⚠️ Common mistake: Stating that asexual reproduction is always superior because it is faster, ignoring that a lack of genetic variation can cause species extinction if conditions change.
🧠 Remember: Sex brings survival in changing environments; Asex brings speed in stable environments.
DNA and the genome4.6.1.4
Definition: The genome of an organism is the entire genetic material of that organism.
All human genetic material is contained within DNA molecules enclosed inside the cell nucleus.
- •DNA (deoxyribonucleic acid) is a polymer made of two strands forming a double helix.
- •DNA is contained in structures called chromosomes inside the cell nucleus.
- •A gene is a small section of DNA on a chromosome that codes for a specific sequence of amino acids to make a specific protein.
- •Understanding the whole human genome helps scientists search for genes linked to inherited diseases and trace human migration patterns from the past.
⚠️ Common mistake: Confusing a gene with a chromosome; a single chromosome contains hundreds or thousands of individual genes.
🧠 Remember: Genome = the whole genetic book; Gene = one recipe for a protein.
DNA structure4.6.1.5
Definition: A nucleotide is the repeating monomer unit of DNA, consisting of a sugar, a phosphate group, and one of four bases.
DNA is a polymer made from four different nucleotide units twisted into a double helix.
- •Each nucleotide consists of a sugar molecule, a phosphate group, and a base (A, C, G, or T).
- •DNA strands are held together by complementary base pairs: A always pairs with T, and C always pairs with G.
- •A sequence of three bases (a triplet code) codes for a specific amino acid in a protein chain.
- •Long chains of amino acids fold into specific shapes to form proteins such as enzymes, hormones, and structural components.
- •Mutations occur continuously and change the base sequence of DNA, which may alter the protein produced.
⚠️ Common mistake: Thinking that complementary bases pair up randomly; A must pair with T, and C must pair with G.
🧠 Remember: Apple in the Tree (A-T), Car in the Garage (C-G).
Genetic inheritance4.6.1.6
Definition: An allele is a different version of a specific gene.
Characteristics are controlled by combinations of alleles inherited from both parents.
| Term | Meaning |
|---|---|
| Dominant allele | Always expressed, even with one copy (e.g. B) |
| Recessive allele | Only expressed with two copies (e.g. b) |
| Homozygous | Two identical alleles (BB or bb) |
| Heterozygous | Two different alleles (Bb) |
| Genotype | The combination of alleles present |
| Phenotype | The physical characteristic observed |
- •Punnett squares use probability to predict the possible genotypes and phenotypes of offspring.
⚠️ Common mistake: Confusing genotype (the genetic code, e.g., Bb) with phenotype (the physical appearance, e.g., brown eyes).
🧠 Remember: Homo = Same alleles; Hetero = Different alleles.
Inherited disorders4.6.1.7
Definition: An inherited disorder is a genetic condition caused by faulty alleles passed down from parents.
Certain medical conditions are passed through families via dominant or recessive alleles.
| Polydactyly | Cystic fibrosis | |
|---|---|---|
| Caused by | Dominant allele | Recessive allele |
| Effect | Extra fingers or toes | Disorder of cell membranes |
| Inherited if... | Only one parent carries it | Both parents pass on the faulty allele (a carrier has one copy and no symptoms) |
- •Embryo screening allows parents to test embryos during IVF for genetic disorders before implantation.
- •Screening raises economic, social, and ethical issues regarding potential embryo destruction and discrimination.
⚠️ Common mistake: Thinking a person can be a 'carrier' of a dominant disorder like polydactyly; if you inherit one dominant allele, you will have the disorder.
🧠 Remember: Polydactyly needs 1 allele (dominant); Cystic fibrosis needs a Couple of alleles (recessive).
Sex determination4.6.1.8
Definition: Sex determination is the biological process that determines the biological sex of an organism based on inherited sex chromosomes.
Human body cells contain 23 pairs of chromosomes, with one pair determining biological sex.
- •Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
- •All egg cells carry an X chromosome, whereas sperm cells carry either an X or a Y chromosome.
- •Fertilisation results in a 50% chance of producing a female offspring (XX) and a 50% chance of a male offspring (XY).
- •Genetic cross diagrams show that the expected sex ratio of offspring in humans is always 1:1.
⚠️ Common mistake: Believing that the mother determines the sex of the child; the sperm cell from the father contains either an X or Y chromosome, determining sex.
🧠 Remember: XX = Female, XY = Male (Y makes a guy).
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