Variation and Mutation

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 3.31 — Random fertilisation produces genetic variation in offspring
• 3.33 — Variation within a species can be genetic, environmental or both
• 3.34 — Mutation as a rare, random, heritable change in genetic material

Specification section 3(b) Inheritance — also taught in DNA, Genes and Chromosomes, Protein Synthesis, Alleles, Genotype and Phenotype, Monohybrid Inheritance and Probability.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

No two members of a species are identical, and the differences between them come from three places. Two of those places only rearrange alleles that already exist. The third is the only one that can produce an allele that has never existed before, and everything that has ever evolved traces back to it.

Source one — meiosis, which shuffles the pack

1. When gametes are made, the pairs of chromosomes separate, and each gamete receives one chromosome of each pair.
2. Which one it receives is independent for every pair, so the combinations differ from gamete to gamete.
3. With 23 pairs there are over eight million possible combinations, so no two gametes from the same person are likely to be alike.
4. Nothing new has been created. Every allele in every gamete was already present in the parent — they have simply been dealt into different hands.

Source two — random fertilisation, which deals two hands at once

1. Any one of the millions of possible sperm may fertilise any one of the possible eggs, and which does so is a matter of chance.
2. So the combination of alleles in a zygote is the product of two independent shufflings, one from each parent.
3. That is why siblings differ from each other while being recognisably related — they are different draws from the same two packs.
4. Again, nothing new. Random fertilisation multiplies the number of combinations enormously, but every allele involved already existed in one of the parents.

So where does anything new come from? A MUTATION is a rare, random change in the genetic material — a change in the sequence of bases in DNA, or in the number or structure of chromosomes. Each of those three words is doing work. RANDOM means it is not directed towards anything: a mutation does not occur because an organism needs it, and organisms in a hot place do not mutate towards heat tolerance. RARE means it happens infrequently at any particular gene, which is why populations are stable from one generation to the next. And a mutation is HERITABLE — but only if it occurs in a cell that produces gametes, because only those cells pass anything on.

That last point is worth pausing on, because it is a real and frequently examined distinction. A mutation in a skin cell affects that cell and its descendants and dies with the person. A mutation in a cell of the ovary or testis can be passed to a child and every cell in that child's body. Both are mutations; only one is inherited.

Think of it like a pack of cards and a printing press

Shuffling and dealing produce an astronomical number of different hands, and every hand is different from the last — but the pack still contains the same fifty-two cards it always did. Nothing you do at the table can produce a card that is not in the pack. To get a new card you have to go back to the press that printed them, and change what it prints. Meiosis and fertilisation are the shuffling and the dealing; mutation is the change at the press. It is much rarer, it is usually a misprint that spoils the card, and very occasionally it produces something the game has never had before.

Now the other split that runs through this topic, and it is the one to ask about first when looking at any actual difference between two organisms. Variation may be GENETIC — caused by different alleles, like blood group or eye colour. It may be ENVIRONMENTAL — caused by the conditions the organism has experienced, like a plant grown in poor soil being small, or a scar, or a language spoken. Or, most often, it is BOTH: your height is influenced by many genes and also by your diet during growth, and neither on its own determines it.

There is a useful test for telling the two apart, and it is worth knowing because it turns an opinion into an experiment. Take genetically identical individuals — clones, or identical twins — and raise them in different conditions. Any difference that appears between them cannot be genetic, because their genes are the same, so it must be environmental. Do the reverse and raise genetically different individuals in identical conditions, and any difference that remains must be genetic. Almost every real study of this question is a version of one of those two designs.

A last point about what is inherited. Environmental variation is not passed on, however dramatic it is. A weightlifter's children are not born strong, and a plant stunted by poor soil produces seeds carrying no memory of it — because nothing about the parent's experience has changed the base sequence in its gametes. Only changes to the genetic material itself are inherited, which is precisely why mutation matters so much despite being rare.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Mutation

A rare, random change in the genetic material — in the sequence of bases in DNA, or in the number or structure of chromosomes — which can be inherited if it occurs in a cell that gives rise to gametes.

The three sources of variation — and which of them creates anything new

Meiosis

The pairs of chromosomes separate independently, so each gamete gets a different combination. Rearranges existing alleles; creates nothing new.

Random fertilisation

Any sperm may fertilise any egg, so the zygote's combination is a matter of chance. Rearranges existing alleles; creates nothing new.

Mutation

A random change in the genetic material itself. The ONLY source of new alleles, and therefore the ultimate origin of all genetic variation.

Statement 3.34 — the three words in the definition, each of which is a mark

Rare

Mutations at any particular gene occur infrequently, which is why populations are stable between generations.

Random

They are not directed towards a need. An organism does not mutate because a characteristic would be useful.

Heritable

Passed on only if the mutation occurs in a cell that produces gametes. A mutation in a body cell is not inherited.

Statement 3.33 — genetic against environmental variation

Deciding whether a variation is genetic or environmental — the routine

1. Ask whether the characteristic falls into distinct categories or lies on a continuous range. Distinct categories suggest one or few genes; a continuous range suggests polygenic inheritance, environment, or both.
2. Ask whether identical twins or clones raised apart still differ in it. If they do, environment is contributing.
3. Ask whether genetically different individuals raised in identical conditions still differ. If they do, genes are contributing.
4. For most characteristics the honest answer is BOTH, and an answer that says so and explains each contribution scores more than one that picks a side.

Which mutations are inherited

A. IN A GAMETE-PRODUCING CELL: the mutation can be passed to a child, and will then be present in every cell of that child's body.

B. IN AN ORDINARY BODY CELL: the mutation affects that cell and the cells it divides into, and is not passed to any offspring.

Not this: Organisms mutate in response to their environment, developing the characteristics they need.

This: Mutations are RANDOM and undirected. An environment can make an existing variation more likely to be passed on, but it cannot cause a useful mutation to occur.

Model answer [4 marks]

Explain how meiosis and fertilisation together produce genetic variation in offspring. [4]

In meiosis the pairs of chromosomes separate independently, so each gamete receives a different combination of chromosomes and therefore of alleles. With 23 pairs there is an enormous number of possible combinations, so the gametes produced by one individual differ from one another. At fertilisation, which particular sperm fuses with which particular egg is a matter of chance. The zygote's combination of alleles is therefore the product of two independent shufflings, one from each parent, which is why offspring of the same parents differ from each other.

Model answer [3 marks]

Explain why mutation is described as the ultimate source of all genetic variation. [3]

Meiosis and random fertilisation produce new combinations of alleles, but every allele involved already existed in the parents, so neither process creates anything new. A mutation is a change in the genetic material itself and is the only way a new allele can arise. All the alleles that meiosis and fertilisation shuffle must therefore have originated as mutations at some point in the past.

Model answer [3 marks]

Two plants of the same species are grown from cuttings of the same parent plant, and one grows much taller than the other. Explain what this shows. [3]

Plants grown from cuttings of the same parent are clones, so they are genetically identical. Any difference between them cannot therefore be caused by a difference in their alleles. The difference in height must be environmental, caused by the conditions they were grown in, such as the light, water or mineral ions available.

Mark-losing trap. Mutations are RANDOM. An organism never mutates because a characteristic would be useful.

Mark-losing trap. Only mutations in gamete-producing cells are inherited. One in a skin cell is not.

Mark-losing trap. Meiosis and fertilisation SHUFFLE existing alleles. Only mutation makes a new one.

Mark-losing trap. Characteristics acquired during life are not inherited — a muscle built or a scar gained does not change the alleles in the gametes.

Mark-losing trap. Most characteristics are affected by BOTH genes and environment. Saying 'both, and here is each contribution' is usually the full-mark answer.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Define [1 mark] — What is a mutation?
2. Grade 7 · Identify [1 mark] — Which of these differences between two people is caused entirely by the environment?
3. Grade 8 · Explain [5 marks] — Explain why two children of the same parents are genetically different from each other. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [3 marks] — In a species with 4 pairs of chromosomes, meiosis separates each pair independently. Calculate the number of genetically different gametes that could be produced by the separation of the pairs alone.
5. 9+ · Analyse [5 marks] — Two genetically identical seedlings are separated. One is grown in bright light and one in dim light, and after six weeks the one in bright light is much taller and has darker green leaves. A student concludes that height and leaf colour are environmental rather than genetic characteristics. Select every statement that belongs in a full-mark evaluation.

The people behind this science

Two ways into the same idea — the one whose theory needed variation and could not explain it, and the one who worked with the thing that causes mutations. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Charles Darwin — the one whose theory needed variation and could not explain it

Darwin's argument requires a supply of inherited variation for selection to act on, and he had no account of where it came from or how it survived being inherited. He knew this was a weakness — he wrote at length about variation under domestication precisely because it was the part of his theory that rested on observation rather than explanation, and he speculated about inheritance in ways he later regretted. This page supplies what he was missing: shuffling that preserves alleles rather than diluting them, and mutation as the source of new ones.

• “Why did your theory depend so heavily on variation existing?”
• “Where did you think the differences between individuals came from?”
• “What troubled you about how characteristics were passed on?”
• “Does it matter to your argument whether variation is directed or random?”
• “What would you have done differently knowing about mutation?”

Marie Curie — the one who worked with the thing that causes mutations

The next lesson names ionising radiation as a cause of mutation, and Curie is the person who isolated the substances that produce it and who paid for it. She and Pierre described the burns their samples caused, she carried radium in her pockets and kept it by her bed, and she died of a blood disorder consistent with prolonged exposure. Her notebooks are still radioactive and are stored in lead-lined boxes. Nobody has a more direct claim on the fact that radiation changes living tissue.

• “What did you notice radium doing to skin?”
• “Did you understand at the time that it was dangerous?”
• “How can something invisible damage a living thing?”
• “Would you have worked differently if you had known?”

Carry on through the course

• Previous lesson: Mitosis and Meiosis
• Next lesson: How Mutations Affect Proteins
• Sexual and Asexual Reproduction — Two ways to make a new organism — one produces copies, the other produces something that has never existed before. Why any species would want both.
• Flowers, Pollination and Fertilisation — Two completely different solutions to one problem — a plant that cannot move has to get its male gamete to another plant's female gamete.
• Seeds and Germination — What a seed needs to start growing, what it lives on before it has a single leaf — and why light is not on the list.
• The Human Reproductive Systems — Two systems built around the same problem — making gametes and bringing them together — and the hormones that switch them on.
• The Menstrual Cycle — Four hormones, two organs and one repeating question — should the uterus lining be built up, held, or let go?
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification