3bInheritance

The words, in order of size

• Genome — the entire DNA of an organism.
• Chromosome — a long molecule of DNA found in the nucleus of a cell; genes are located on chromosomes.
• Gene — a section of a molecule of DNA that codes for a specific protein.

So: nucleus → chromosomes → genes → sequence of bases.

Human body cells contain 46 chromosomes (23 pairs). Each chromosome carries hundreds of genes, always in the same position on that chromosome.

Learn the definition of a gene word-for-word. "A section of DNA" alone is not enough — you must say it codes for a specific protein. Do not write "codes for a characteristic".

DNA structure (separate Biology only — not in Science (Double Award))

A DNA molecule is two strands coiled to form a double helix. The two strands are held together by a series of paired bases:

This is complementary base pairing. So if one strand reads A–C–G–T, the other strand reads T–G–C–A.

The order of the bases in a gene is the code — it determines which protein is made.

RNA (separate Biology only — not in Science (Double Award))

In RNA, uracil (U) pairs with adenine (A) instead of thymine.

Quick check

A DNA strand reads A T G C. The complementary RNA made from it reads U A C G.

Examiner tip. A small number of candidates each year state it the wrong way round — that RNA contains thymine and DNA contains uracil. Fix it with one sentence: T is for DNA, U is for RNA. Also, when asked to describe a DNA molecule, you must mention both the double helix and the specific base pairs (A–T, C–G); naming one of these only will not gain full marks.

Separate Biology only — not in Science (Double Award).

A gene's base sequence has to be turned into a chain of amino acids. This happens in two stages, in two different places.

Stage 1 — Transcription (in the nucleus)

1. The DNA double helix unwinds/unzips at the gene being used.
2. One strand is used as a template. Free RNA nucleotides line up by complementary base pairing (A–U, C–G) to build a molecule of mRNA (messenger RNA).
3. The mRNA is a single stranded copy of the gene, containing uracil in place of thymine.
4. The mRNA leaves the nucleus through a pore and travels to a ribosome in the cytoplasm.

Transcription produces mRNA — not protein.

Stage 2 — Translation (at the ribosome)

1. The ribosome reads the mRNA in groups of three bases called codons. Each codon codes for one amino acid.
2. tRNA (transfer RNA) molecules each carry a specific amino acid and each has a three-base anticodon.
3. The tRNA anticodon pairs with the complementary codon on the mRNA, bringing the correct amino acid into place.
4. The amino acids are joined together in order, forming a polypeptide chain, which folds into a protein with a specific shape.

Translation produces protein — not RNA.

Summary table

Why the base sequence matters

The order of bases in the gene → order of codons on the mRNA → order of amino acids in the protein → shape of the protein. Change the bases and you can change the protein.

Examiner tip. The single most common error reported here is swapping the products round: writing that transcription produces a protein or that translation produces RNA. Say the product out loud with each stage as you revise. A second reported error: when a question gives you a short strand of RNA and asks what it would do, candidates describe protein synthesis beautifully but forget to say that the complementary RNA would bind (base pair) to the mRNA — read what the question actually asks.

Alleles

Genes exist in alternative forms called alleles. Different alleles of the same gene give rise to differences in inherited characteristics (e.g. the gene for eye colour has an allele for brown and an allele for blue).

The vocabulary you must be able to define

Codominance (separate Biology only — not in Science (Double Award))

In codominance, neither allele is recessive — both alleles are expressed in the phenotype of a heterozygote. Use capital letters with superscripts, e.g. flower colour: C^R C^R = red, C^W C^W = white, C^R C^W = pink (or red and white patches).

A cross of two pink plants (C^R C^W × C^R C^W) gives 1 red : 2 pink : 1 white — three phenotypes, not two.

Polygenic inheritance

Most phenotypic features are not controlled by a single gene. They are the result of polygenic inheritance — many genes acting together (e.g. human height, body mass, skin colour). Single-gene examples exist but are the exception.

Monohybrid cross — how to set out a genetic diagram

Cross a heterozygous tall pea plant with a dwarf plant. T = tall (dominant), t = dwarf (recessive).

• Parental phenotypes: tall × dwarf
• Parental genotypes: Tt × tt
• Gametes: T or t × t or t

• Offspring genotypes: 2 Tt : 2 tt
• Offspring phenotypes: 1 tall : 1 dwarf (50% chance of each)

Always: state the letters you use and what they mean, give parental genotypes, gametes, the Punnett square, then the phenotype ratio or probability.

Probabilities

From Tt × Tt you get 3 dominant : 1 recessive, i.e. a 1 in 4 (25%) chance of the recessive phenotype. This is a probability, not a guarantee — real offspring numbers rarely match exactly because fertilisation is a random process (and some seeds may not germinate).

Family pedigrees

• Squares = males, circles = females; shaded = has the characteristic.
• If two unaffected parents have an affected child, the condition must be recessive and both parents are heterozygous carriers.
• Anyone showing a recessive condition must be homozygous recessive.
• Work out the genotypes you can be certain of first, then use them to deduce the others. Where two genotypes are possible, write both (e.g. TT or Tt).

Sex determination

Sex is controlled by one pair of chromosomes: XX in a female, XY in a male.

• Parents: female XX × male XY
• Gametes: eggs all X; sperm X or Y

Offspring: 2 XX : 2 XY = 50% female : 50% male. Sex is determined at fertilisation, by whether the sperm carries an X or a Y chromosome.

Examiner tip. Marks are routinely lost on genetic diagrams for symbols, not biology: use the same letter for both alleles, with a capital for the dominant and the same letter in lower case for the recessive — never two different letters. Codominance appears on Paper 2 and is often poorly handled, so learn the superscript notation and remember heterozygotes show both alleles. Finally, if asked why real offspring numbers do not fit a 3:1 ratio, say that fertilisation is random — don't just describe the numbers.

Chromosome numbers in humans

• Diploid number = 46 (23 pairs) — body cells.
• Haploid number = 23 — gametes (sperm and egg).

Mitosis

A diploid cell divides by mitosis to produce two cells that contain identical sets of chromosomes — genetically identical to each other and to the parent cell, and still diploid.

Before division, the chromosomes are copied, so each new cell receives a full set.

Mitosis occurs during:

• growth
• repair (replacing damaged cells)
• cloning
• asexual reproduction

Meiosis

A cell divides by meiosis to produce four cells, each with half the number of chromosomes (haploid). The four cells are genetically different from each other — they are the gametes.

In humans: one diploid cell (46) → four haploid gametes (23 each).

Random fertilisation

Gametes are already genetically different from one another. At fertilisation, any one sperm can fuse with any one egg — which combination occurs is random. This produces an enormous number of possible allele combinations, so offspring of the same parents are genetically different from each other and from their parents. This is a source of genetic variation.

Types of variation

Variation within a species can be genetic, environmental, or a combination of both.

Examiner tip. Two reported errors, both easy to avoid. First, when describing growth or repair, candidates say the division is meiosis — in growth, repair, cloning and asexual reproduction it is always mitosis, and you should state that the nucleus is diploid. Marks are specifically lost for not referring to a diploid nucleus. Second, when a question asks about mitosis or about producing genetically identical cells, some candidates drift into describing taking cuttings or other methods of asexual reproduction — answer the question that was asked.

Mutation

A mutation is a rare, random change in the genetic material (the DNA base sequence) that can be inherited.

If the mutation occurs in a gamete, it is passed on to the offspring.

Common mistake: vague definitions such as "a change in an organism" or "an inherited change" score nothing. You need change in DNA / change in the base sequence of a gene, plus random and rare.

How a mutation can change the phenotype (separate Biology only — not in Science (Double Award))

A change in the DNA base sequence changes the codons on the mRNA, so it can alter the sequence of amino acids in the protein. A different amino acid sequence means the protein folds into a different shape.

For an enzyme: if the shape of the active site changes, the substrate no longer fits, so the enzyme cannot catalyse its reaction — the reaction slows or stops, which changes the phenotype.

Size of effect:

• Most mutations have no effect on the phenotype.
• Some have a small effect.
• Rarely do they have a significant effect.

Mutagens (separate Biology only — not in Science (Double Award))

The incidence of mutations is increased by:

• ionising radiation — gamma rays, x-rays, ultraviolet rays
• some chemical mutagens — for example, chemicals in tobacco

Darwin's theory of evolution by natural selection

Learn it as five linked steps — each one is a mark:

1. There is genetic variation within a population (caused by mutation).
2. Organisms compete for limited resources (food, mates, space) and many do not survive.
3. Individuals with alleles that make them better adapted to the environment are more likely to survive.
4. The survivors reproduce and pass on the advantageous alleles to their offspring.
5. Over many generations the proportion of individuals carrying the advantageous allele increases, so the species changes — it evolves.

Antibiotic resistance in bacteria

1. A random mutation in a bacterium produces an allele giving resistance to an antibiotic.
2. When the antibiotic is used, the non-resistant bacteria are killed but the resistant ones survive.
3. The resistant bacteria have no competition and reproduce rapidly, passing on the resistance allele.
4. The proportion of resistant bacteria in the population increases — this is natural selection.

Why this makes infections difficult to control: the antibiotic no longer works, so the infection cannot be treated with it; resistant strains (e.g. MRSA) spread easily, especially in hospitals; new antibiotics must be developed, which is slow and expensive. Resistance is slowed by not over-prescribing antibiotics and by patients completing the full course.

Examiner tip. Watch the command word. Describe = say what happens; explain = give the reason why. Examiners repeatedly report candidates describing a pattern (e.g. "the number of resistant bacteria went up") when asked to explain it — you must name mutation, survival, reproduction and passing on the allele. Similarly, when explaining a mutation's effect, candidates stop at "the amino acid sequence changes" and lose the marks for continuing to the effect on the enzyme's shape/active site and its function.