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IGCSE Biology 0610 · Chapter 13 notes · ~24 min read

Inheritance, variation & selection

Chromosomes, genes and alleles, how mitosis and meiosis differ, how to work out monohybrid crosses with Punnett squares and pedigrees, why organisms vary, and how natural and artificial selection change populations.

Chromosomes & genesMitosis & meiosisMonohybrid crossesVariationMutationNatural & artificial selection
ASby Aiza Ma'am
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • DNA is the genetic code. A gene is a length of DNA that codes for a protein, and alleles are different versions of a gene. Humans have 46 chromosomes (23 pairs) in every body cell.
  • Mitosis makes identical cells for growth and repair. Meiosis makes gametes with half the chromosomes, all genetically different. Punnett squares predict the ratios, for example 3 : 1 and 1 : 1.
  • Offspring vary because of genes and the environment. Mutation makes new alleles. Natural selection (the environment) and selective breeding (humans) change which alleles are common.

By the end you can

  • Define chromosome, gene and allele, and describe how sex is inherited in humans.
  • Define haploid and diploid nuclei and state the chromosome number in human cells.
  • Describe mitosis and meiosis and compare them, including why each is needed.
  • Use the terms genotype, phenotype, homozygous, heterozygous, dominant, recessive and pure-breeding.
  • Predict the results of monohybrid crosses (3 : 1 and 1 : 1) with Punnett squares, and interpret a pedigree.
  • Compare continuous and discontinuous variation and say what causes variation.
  • Define mutation and name factors that increase it.
  • Describe adaptive features, natural selection and selective breeding.
  • Explain how a gene codes for a protein, and how DNA controls cell function. EXT
  • Describe stem cells. EXT
  • Predict test cross, codominance (ABO) and sex-linked inheritance results. EXT
  • Explain gene mutation, sources of variation and antibiotic resistance as natural selection. EXT
  • Describe the adaptations of xerophytes and hydrophytes. EXT

1DNA, genes and chromosomes

Chromosomes, genes and alleles

cellnucleuschromosome (made of DNA)Each body cell has 23 pairs of chromosomes (46 in all). Zoom in on one pair:Ttfrom motherfrom fathergene: a length ofDNA that codesfor one proteinalleles: differentforms of the samegene (here T and t)same position on both chromosomesEach person has two alleles of every gene: one from each parent.
Chromosomes are made of DNA. A gene is a part of a chromosome. The two alleles of a gene sit at the same place on a pair of chromosomes.

Chromosome — a thread-like structure of DNA, carrying genetic information in the form of genes.

Gene — a length of DNA that codes for a protein.

Allele — a version of a gene.

  • Chromosomes are found in the nucleus (cells). Human body cells have 46 chromosomes in 23 pairs. One chromosome of each pair came from the mother and one from the father.
  • Because the two chromosomes of a pair carry the same genes in the same places, a person has two alleles of each gene. They may be the same or different.

Haploid and diploid nuclei

SPERM23 chromosomes (n)EGG23 chromosomes (n)+fertilisationZYGOTE46 chromosomes (2n)= 23 pairsmitosisevery body cellgets 46 chromosomesHAPLOID (n) = one set of chromosomes: gametes (23).DIPLOID (2n) = two sets, one from each parent: body cells (46).
Gametes are haploid (23). Fertilisation makes a diploid zygote (46), and mitosis copies it into every body cell.

Haploid nucleus — a nucleus containing a single set of unpaired chromosomes (for example in a gamete).

Diploid nucleus — a nucleus containing two sets of chromosomes (for example in a body cell).

  • Humans: body cells are diploid (46), gametes are haploid (23). Fertilisation restores the diploid number.

How a gene makes a proteinEXTENDED

NUCLEUSTACGGATTCDNAAUGCCUAAGmRNA1The gene iscopied into mRNA2mRNA leavesthe nucleusribosomeAUGMetCCUProAAGLys3amino acids arejoined in order,3 bases at a time4protein folds intoits 3D shapeORDER of bases in the gene → ORDER of amino acids→ shape of the protein → what the protein does
EXT The sequence of bases in a gene decides the sequence of amino acids, which decides the shape and job of the protein.
  • EXT The code: DNA has four bases (A, T, C, G). Every three bases in a gene code for one amino acid. The sequence of bases therefore decides the sequence of amino acids in the protein (proteins and enzymes).
  • EXT Protein shape = job. The chain of amino acids folds into a particular 3D shape. A different amino acid sequence gives a different shape, so a different job (an enzyme, a hormone or haemoglobin).
  • EXT Protein synthesis in 4 steps: (1) the gene in the nucleus is copied into mRNA, (2) the mRNA leaves the nucleus, (3) a ribosome in the cytoplasm reads the mRNA and joins amino acids in the right order, (4) the chain folds into the protein.
  • EXT DNA controls the cell by controlling which proteins are made. Different cells use different genes (for example, only some pancreas cells make insulin), so cells become specialised.
  • EXT The diagram shows the base pairing: DNA T → A, A → U, C → G, G → C in the mRNA. You do not need to learn which codon gives which amino acid.

How sex is inherited

sperm from father (XY)XYXXeggs frommother (XX)XXgirlXYboyXXgirlXYboyEvery egg carries X. A sperm carries X or Y, so the SPERM decides the sex.Result: 1 girl (XX) : 1 boy (XY), a 1 : 1 ratio, or 50% each.
Every egg carries X. The sperm carries X or Y and decides the sex.
  • Humans have 23 pairs of chromosomes. 22 pairs are the same in both sexes. The 23rd pair are the sex chromosomes: XX in females and XY in males.
  • Females make eggs that all carry X. Males make half X sperm and half Y sperm. So the sperm decides the sex of the baby, and each baby has a 50% chance of being a girl or a boy.
Worked example

A couple already has four daughters. What is the probability that their fifth child is a boy?

  1. each fertilisation is an independent event
  2. the sperm that fertilises the egg is X or Y with equal chance
  3. probability of a boy = 1/2 = 50%
  4. the four daughters make no difference

2Cell division

Mitosis

1parent cell:2 pairs ofchromosomes2chromosomesare copied(2 copies each)3copies line upacross themiddle4copies pulledto oppositeends52 identicaldaughter cells(same genes)WHERE IS MITOSIS USED?GROWTHmore cells makethe body biggerREPAIRheals cutsand broken bonesREPLACEMENTnew skin andblood cellsASEXUALREPRODUCTIONoffspring areclonesMitosis: ONE cell → TWO identical cells, with the same number of chromosomes.
Mitosis: chromosomes are copied, lined up and pulled apart, giving two identical cells.

Mitosis — nuclear division giving rise to genetically identical cells.

  • Before the cell divides, the chromosomes are copied (replicated). Then the copies are separated into two nuclei.
  • Each daughter cell has the same number of chromosomes and the same genes as the parent cell. In a human, 46 → 46.
  • Uses of mitosis: growth (more cells), repair of damaged tissue, replacement of worn-out cells, and asexual reproduction (asexual reproduction).
  • EXT Stem cells are unspecialised cells that divide by mitosis and can become specialised (differentiate). For example, stem cells in bone marrow make new blood cells.

Meiosis

parent celldiploid (2n = 4)chromosomes copied,pairs side by sidedivision 1pairs separatecells now have2 chromosomes(haploid), eachwith 2 copiesdivision 2copies separatehaploidhaploidhaploidhaploidtype Atype BMeiosis: 1 diploid cell → 4 haploid cells. The chromosome number isHALVED and the cells are genetically different (A, B, and many more mixes).
Meiosis: one diploid cell divides twice to make four haploid cells that differ from each other.

Meiosis — nuclear division giving rise to genetically different cells, with half the number of chromosomes (a reduction division).

  • Meiosis happens in the reproductive organs (testes and ovaries, anthers and ovules) to make gametes (gametes).
  • One diploid cell makes four haploid cells. In a human, 46 → 23. Each gamete gets one chromosome from each pair, in a random mixture, so the gametes are genetically different.
  • Fertilisation then joins two haploid gametes to make a diploid zygote, so the chromosome number stays the same from one generation to the next.
MitosisMeiosis
wheremost body cellsreproductive organs only
cells made24
chromosome numberstays the same (46 → 46)halved (46 → 23)
the cells areidentical to the parentdifferent from the parent and from each other
jobgrowth, repair, replacement, asexual reproductionmaking gametes
Worked example

A fruit fly body cell has 8 chromosomes. How many chromosomes are in (a) a cell made by mitosis, (b) a gamete, (c) a zygote?

  1. (a) mitosis keeps the number: 8
  2. (b) meiosis halves it: 8 ÷ 2 = 4
  3. (c) two gametes fuse: 4 + 4 = 8
Memory trickMITOSIS = MIrror image (identical copies). MEIOSIS = MEans half, and makes gametes.

3Monohybrid inheritance

Key terms

TTTThomozygousdominantTALLTtTtheterozygousTALLtttthomozygousrecessiveSHORTGENOTYPEPHENOTYPEone pair of chromosomes, one gene, two allelesCAPITAL letter = dominant allele. Small letter = recessive allele.A dominant allele shows even when only ONE copy is present.
One gene, two alleles. A dominant allele (capital) hides a recessive allele (small letter).
TermMeaning
inheritancethe passing on of genetic information from one generation to the next
genotypethe alleles an organism has (for example Tt)
phenotypethe observable features of an organism (for example tall)
homozygoustwo identical alleles of a gene (TT or tt)
heterozygoustwo different alleles of a gene (Tt)
dominantan allele that is expressed even if only one copy is present
recessivean allele that is expressed only if no dominant allele is present
pure-breedinghomozygous for the feature, so it always breeds true
Memory trickHomo = same (TT or tt). Hetero = different (Tt). Genotype = gene letters. Phenotype = what you physically see.
Exam tipUse a capital letter for the dominant allele and the same letter in small form for the recessive allele. Choose a letter that looks different in both forms: T and t is clear, but C and c or S and s are easy to mix up.

From parents to the F1 generation

PARENTS (P): both pure-breeding (homozygous)TTtallttshortgametesTTttfertilisationF1 (first generation): ALL Ttall tall (T is dominant)
A cross between pure-breeding tall (TT) and short (tt) plants gives only Tt offspring, which are all tall.
  • P = the parents. F1 = the first generation of offspring. F2 = the second generation (offspring of the F1).
  • Each gamete carries only one allele of the gene. The TT parent makes only T gametes and the tt parent makes only t gametes, so every F1 plant is Tt.
  • All F1 plants are tall, because T is dominant. The t allele is still there, hidden.

The F1 × F1 cross (3 : 1)

pollen (gametes) from Tt plantTtTtegg gametesfrom Tt plantTTtallTttallTttallttshort1write the parents’genotypes: Tt × Tt2write the gametesalong the edges3fill in the grid: oneallele from each side4count the ratiosfrom the gridGENOTYPE ratio: 1 TT : 2 Tt : 1 ttPHENOTYPE ratio: 3 tall : 1 short (a 3 : 1 ratio)Probability of a short plant (tt) = 1 in 4 = 25%.Each offspring is an independent chance: 4 plants need not show exactly 3 : 1.
A Punnett square shows every possible combination of gametes. Tt × Tt gives 3 tall : 1 short.
  • Phenotype ratio 3 : 1. Genotype ratio 1 : 2 : 1. The short plant reappears because the F1 plants both carry t.
  • The 1 : 1 ratio: a heterozygous parent (Tt) crossed with a homozygous recessive parent (tt) gives Tt, Tt, tt, tt, which is 1 tall : 1 short.
Worked example

Cystic fibrosis is caused by a recessive allele (f). Two parents without the condition each carry one f (Ff × Ff). Find the probability that their child (a) has CF, (b) is a carrier, (c) is unaffected.

  1. gametes from each parent: F or f
  2. Punnett square gives FF, Ff, Ff, ff
  3. (a) CF = ff = 1 out of 4 = 1/4 = 25%
  4. (b) carrier = Ff = 2 out of 4 = 1/2 = 50%
  5. (c) unaffected = FF or Ff = 3 out of 4 = 3/4 = 75%
Worked example

In a Tt × Tt cross, 120 offspring are produced. How many would you expect to be short?

  1. short plants are tt = 1/4 of the offspring
  2. 120 × 1/4 = 120 ÷ 4 = 30 short
  3. the other 120 − 30 = 90 are tall
  4. real results are close to, but not exactly, the expected ratio, because fertilisation is random
Common mistake"A 3 : 1 ratio means that 3 of every 4 children will be tall." These are probabilities, not guarantees. Each child has a 3/4 chance of being tall, whatever the earlier children were.
Exam tipFour steps for every genetics question: (1) key (T = tall, t = short), (2) parent genotypes, (3) gametes then Punnett square, (4) read the ratio, then write the phenotypes in words.

Test crossEXTENDED

TEST CROSS: a tall plant of unknown genotype (T?) × a short plant (tt)Case 1: unknown is TTshort parent (tt) gametesttTTunknownparentTttallTttallTttallTttalloffspring: ALL tallso the unknown plant is TTCase 2: unknown is Ttshort parent (tt) gametesttTtunknownparentTttallTttallttshortttshortoffspring: 1 tall : 1 shortso the unknown plant is TtWhy use tt? The recessive parent can only give t, so every allelefrom the unknown parent shows up in the offspring phenotype.If ANY short offspring appear, the unknown parent must carry t.
EXT Cross an organism of unknown genotype with a homozygous recessive one and look at the offspring.

Test cross — EXT a cross between an individual showing the dominant phenotype (unknown genotype) and a homozygous recessive individual, used to find the unknown genotype.

  • EXT All offspring show the dominant feature → the unknown parent is homozygous (TT).
  • EXT Some offspring show the recessive feature (about 1 : 1) → the unknown parent is heterozygous (Tt).

Pedigrees

IIIAaAaaaAA or AaAA or AaaffectedgirlKEYmalefemaleaffectedparentschildrenTWO UNAFFECTED PARENTS HAVE AN AFFECTED CHILD,so the condition is caused by a RECESSIVE alleleand both parents must be carriers (heterozygous, Aa).The affected child is aa: one a from each parent.
In a pedigree, squares are males, circles are females and shaded shapes are affected.
  • How to read it: horizontal line = parents; vertical line down = children; shaded = affected; open = unaffected.
  • Clue for a recessive condition: two unaffected parents have an affected child. The parents must be carriers (heterozygous, Aa) and the child is aa.
  • Clue for a dominant condition: two affected parents have an unaffected child. The parents are heterozygous and the child has no dominant allele.

Codominance and the ABO blood groupsEXTENDED

BLOOD GROUPS AND THEIR GENOTYPESAIAIA or IAiBIBIB or IBiABIAIBOiiEXAMPLE CROSS: group A × group B(IAi × IBi)IBiIAiIAIBgroup ABIAigroup AIBigroup Biigroup O1 : 1 : 1 : 1 (all four groups possible)CODOMINANCE: IA and IB are BOTH expressed in the heterozygote (group AB).i is recessive: it only shows when there are two copies (group O).Neither IA nor IB is dominant over the other, so use a capital I with asuperscript letter for each allele, and a small i for the recessive allele.
EXT Alleles IA and IB are both expressed in group AB. The allele i is recessive.

Codominance — EXT when both alleles of a gene are expressed in the phenotype of a heterozygote.

  • EXT The ABO blood group gene has three alleles: IA, IB and i. IA and IB are codominant, and i is recessive to both.
  • EXT Group A = IAIA or IAi. Group B = IBIB or IBi. Group AB = IAIB. Group O = ii.
Worked example

A woman is group O and her partner is group AB. Which blood groups can their children have?

  1. mother = ii, father = IAIB
  2. mother's gametes are all i; father's are IA or IB
  3. children are IAi (group A) or IBi (group B)
  4. group A or group B in a 1 : 1 ratio, never O or AB

Sex linkageEXTENDED

The gene for colour visionis on the X chromosome.Males (XY) have only ONE X,so one recessive alleleis enough to show it.carrier mother × normal fatherXNXn × XNYXNYXNXnXNXNnormal girlXNYnormal boyXNXncarrier girlXnYcolour-blind boyN = normal vision (dominant) n = red-green colour blindness (recessive)DAUGHTERS: all have normal vision (one half are carriers).SONS: 1 normal : 1 colour-blind.Overall: 1 in 4 children is colour-blind, and it is a boy.A carrier has the recessive allele but normal vision.Colour blindness is MORE COMMON IN MALES because theyneed only one copy of the recessive allele.
EXT A recessive allele on the X chromosome shows up more often in males, who have only one X.

Sex linkage — EXT the association of a characteristic with the sex of the individual, because the gene is on a sex chromosome (usually X).

  • EXT Red-green colour blindness is caused by a recessive allele (Xn) on the X chromosome. A woman needs two copies (XnXn) to be colour-blind, but a man needs only one (XnY).
  • EXT More men than women are colour-blind. A carrier mother (XNXn) and a normal father (XNY) have daughters with normal vision and sons with a 1 in 2 chance of being colour-blind.
Common mistake"A father passes colour blindness to his sons." Sons get their X from their mother and a Y from their father, so sons inherit an X-linked allele from the mother.

4Variation and mutation

Variation

CONTINUOUS VARIATIONDISCONTINUOUS VARIATIONheight (cm)number of peopleABABOblood groupnumber of peoplea RANGE of values, withno clear groupse.g. height, body massclear CATEGORIES,nothing in betweene.g. blood group, tongue rollingContinuous: genes AND the environment (diet, exercise, sunlight).Discontinuous: genes only, the environment has little or no effect.
Continuous variation gives a range (height). Discontinuous variation gives clear categories (blood group).

Variation — differences between individuals of the same species.

  • Causes: genes (inherited alleles) and the environment (for example diet, exercise, sunlight). Many features are affected by both.
FeatureContinuous variationDiscontinuous variation
valuesa range with all values in betweendistinct categories
causegenes and environmentusually genes only
examplesheight, body mass, leaf lengthblood group, tongue rolling, sex
graphhistogram (bars touch) or line graphbar chart (gaps between bars)
  • To investigate: measure the feature in a large sample (50 or more), group the values and draw a graph. Continuous data gives a bell-shaped curve.
Worked example

The lengths of five leaves from one tree are 42, 47, 45, 50 and 46 mm. Calculate the mean and the range.

  1. mean = total ÷ number
  2. total = 42 + 47 + 45 + 50 + 46 = 230
  3. mean = 230 ÷ 5 = 46 mm
  4. range = largest − smallest = 50 − 42 = 8 mm

Mutation

ORIGINAL GENETACGGATTCDNAAUGCCUAAGmRNAMetProLysproteinone base changes: G → TMUTATED GENETACGTATTCDNAAUGCAUAAGmRNAMetHisLysproteina different amino acid (Pro → His)MUTATIONa random change inthe DNA of a geneCAUSESionising radiation(UV, X-rays, gamma)some chemicals(e.g. tar in tobacco smoke)RESULTa NEW ALLELEusually no effect,some harmful,a few beneficialA new allele can make a different protein, and so a different feature.
Mutation is a random change in the DNA. A new allele can make a different protein.

Mutation — genetic change.

  • Mutation is the source of new alleles. The rate of mutation is increased by ionising radiation (UV rays, X-rays, gamma rays) and by some chemicals (for example tar in tobacco smoke).
  • EXT A gene mutation is a random change in the sequence of DNA bases. It may change the amino acids in a protein and so its shape and function. Most are harmless, some are harmful (for example sickle-cell anaemia, caused by a one-base change in the haemoglobin gene) and a few are beneficial.
  • EXT Sources of variation: mutation, meiosis (a new mix of chromosomes in each gamete), random mating and random fertilisation.
Exam tipMutations are random. The environment does not cause a helpful mutation to appear. It only selects among the variation that is already there.

5Selection and adaptation

Adaptive features

Adaptive feature — an inherited feature that helps an organism survive and reproduce in its environment.

  • EXT Fitness is the probability of an organism surviving and reproducing in the environment where it lives.
OrganismAdaptive featureHow it helps
polar bearthick fur and a fat layer; white furinsulation against the cold; camouflage in snow
camellarge flat feet; hump of fat; long eyelashesdo not sink into sand; food store; keep sand out of the eyes
cactusspines; thick waxy stemless water lost; stores water
water lilylarge floating leavescatch lots of light at the surface
Exam tipIn answers use the chain feature → how it helps → survival and reproduction. For example: "White fur hides the bear in the snow, so it can catch more prey and survive to breed."

XerophytesEXTENDED

XEROPHYTE (e.g. cactus): a plant of dry placesspines instead of leaves:small area, less water lostthick waxy cuticle:reduces evaporationfleshy stem:stores waterlong and wide roots:reach water in dry soilOther xerophytes also have:sunken stomata (trap moist air),hairs, and leaves that roll up.extensive roots reach waterdeep down or spread out wideto catch rain quickly
EXT Xerophytes are adapted to save water and to find water.

Xerophyte — EXT a plant adapted to survive in dry habitats.

  • EXT Less water lost: thick waxy cuticle, small leaves or spines (small surface area), sunken stomata and hairs (trap moist air), and rolled leaves.
  • EXT Water stored in fleshy stems. Long or widely spread roots reach water.

HydrophytesEXTENDED

HYDROPHYTE (e.g. water lily): a plant of waterlarge flat leaf floats:catches lots of lightstomata on the UPPERsurface (in the air)air spaces in the stem:keep it afloat andcarry oxygensmall root system:water is all aroundthin cuticle, or none:no need to save waterstem cross-section:big air spaces
EXT Hydrophytes are adapted to life in water, where water loss is no problem but oxygen and light are.

Hydrophyte — EXT a plant adapted to live partly or wholly in water.

  • EXT Large, flat, floating leaves catch light and have stomata on the upper surface (in the air).
  • EXT Air spaces in the leaves and stems make the plant float and carry oxygen to the roots. Thin (or no) cuticle, small roots and a flexible stem.

Natural selection

1 GENERATION 12 BIRDS EAT THE EASY ONES3 GENERATION 25 green + 5 brown(inherited colour)green ones show up onbrown soil and are eatensurvivors breed:2 green + 8 brown1VARIATION: individuals differ (here, in colour) because of their genes2OVERPRODUCTION: more offspring are born than can survive3STRUGGLE: competition for food and escaping predators4SURVIVAL OF THE FITTEST: the best adapted survive and breed5INHERITANCE: survivors pass the helpful allele to their offspringOver many generations the helpful allele becomes MORE COMMON.
Natural selection: the environment selects the best adapted individuals, which pass on their alleles.

Natural selection — the process in which organisms that are better adapted to their environment survive and produce more offspring, passing on the alleles for their adaptive features.

  • Five steps: (1) there is variation, (2) more offspring are produced than can survive, (3) there is competition (food, mates, predators), (4) the best adapted survive and breed, (5) they pass the helpful alleles to their offspring, so these alleles become more common over generations.
  • EXT This is how populations become adapted to their environment, and over a very long time how new species can arise.
Common mistake"Giraffes stretched their necks to reach leaves, so their babies had longer necks." No. Features gained during life are not inherited. The alleles for long necks were already in the population and were selected.
Common mistake"Organisms change because they need to." Organisms do not choose to change. Variation already exists and selection acts on it.

Antibiotic resistanceEXTENDED

1one bacterium has amutation: it isresistant2antibiotic kills thenormal bacteria; theresistant one lives3the survivor divideswith no competitionfrom the others4the whole populationis resistant: theantibiotic now failsThis is NATURAL SELECTION: the antibiotic is the selection pressure.Slow it down: finish every course of antibiotics, and do not use themfor viral infections such as colds and flu (antibiotics do not work on viruses).Resistant bacteria such as MRSA are very hard to treat.
EXT The antibiotic kills the normal bacteria and selects the resistant one, which then multiplies.
  • EXT A random mutation makes a bacterium resistant to an antibiotic. When the antibiotic is used, non-resistant bacteria die and resistant bacteria survive and reproduce. The allele for resistance becomes common: natural selection.
  • EXT Reduce the problem: complete the full course of antibiotics, and do not use them for viral infections. (antibiotics)

Selective breeding

1choose parents with thefeature you want(e.g. high milk yield)2breed the chosenparents together3choose the best offspring(highest yield) to bethe next parents4repeat for manygenerationsaverage milk yield per cowyield123456generationARTIFICIAL selection: humans, not the environment, choose the parents.
Selective breeding: choose the best parents, breed them, choose the best offspring, repeat for many generations.

Selective breeding — the intentional breeding of individuals with desired features, so that the features become more common in the population (artificial selection).

  • Examples: cattle for more milk or meat, wheat and rice for higher yield and disease resistance, flowers for colour and scent.
  • It usually takes many generations. A disadvantage is that the variation in the population falls, so one disease may harm all of them.
Natural selectionArtificial selection
who chooses?the environmenthumans
what is selected?features that help survival and reproductionfeatures that are useful to people
who benefits?the organismusually the farmer or customer

6Test yourself

Test yourself — cover the answers first

Quick check

  1. Define a gene.
  2. What is an allele?
  3. How many chromosomes are in a human body cell?
  4. Which chromosomes make a human male?
  5. What is mitosis?
  6. Give two uses of mitosis.
  7. How does meiosis change the chromosome number?
  8. Define phenotype.
  9. What does heterozygous mean?
  10. What is the phenotype ratio of a Tt × Tt cross?
  11. What is the phenotype ratio of a Tt × tt cross?
  12. Give one example of continuous variation.
  13. Name two factors that increase the rate of mutation.
  14. What is an adaptive feature?
  15. Define selective breeding.
  16. EXT What do three bases in a gene code for?
  17. EXT What is a stem cell?
  18. EXT Why is a test cross done with a homozygous recessive?
  19. EXT Which blood group has the genotype IAIB?
  20. EXT Why are more men than women colour-blind?
  21. EXT Name two adaptations of a xerophyte.

Exam-style

  1. Describe how the sex of a baby is inherited. [3]
  2. State three differences between mitosis and meiosis. [3]
  3. Two heterozygous tall plants (Tt) are crossed. Use a Punnett square to show the genotypes and phenotype ratio of the offspring. [4]
  4. Describe the difference between continuous and discontinuous variation. [3]
  5. Describe how natural selection leads to a population becoming better adapted. [4]
  6. Describe how selective breeding could be used to produce a cow that gives more milk. [4]
  7. EXT Explain how a gene controls the production of a protein. [4]
  8. EXT Explain how bacteria become resistant to an antibiotic. [4]
  9. EXT A woman who is a carrier of colour blindness has children with a man with normal vision. Explain the chance that a son is colour-blind. [4]
Show answers
  1. a length of DNA that codes for a protein
  2. a version of a gene
  3. 46 (23 pairs)
  4. X and Y
  5. nuclear division giving genetically identical cells
  6. growth, repair, replacement of cells, asexual reproduction
  7. it halves it (diploid to haploid)
  8. the observable features of an organism
  9. having two different alleles of a gene
  10. 3 dominant : 1 recessive
  11. 1 dominant : 1 recessive
  12. height, body mass
  13. ionising radiation (UV, X-rays), some chemicals such as tar
  14. an inherited feature that helps survival and reproduction
  15. the intentional breeding of organisms with desired features
  16. one amino acid
  17. an unspecialised cell that divides by mitosis and can become specialised
  18. its gametes all carry the recessive allele, so the offspring show the unknown genotype
  19. group AB (codominance)
  20. the allele is recessive and on the X chromosome, and men have only one X
  21. thick waxy cuticle, spines or small leaves, water-storing stem, long or wide roots
  22. females have XX and males have XY
    eggs all carry X; sperm carry X or Y
    the sperm that fertilises the egg decides the sex (50% chance of each)
  23. mitosis makes 2 cells and meiosis makes 4
    mitosis keeps the chromosome number and meiosis halves it
    mitosis gives identical cells and meiosis gives genetically different cells
  24. gametes T and t from each parent
    offspring TT, Tt, Tt, tt
    1 TT : 2 Tt : 1 tt
    3 tall : 1 short
  25. continuous has a range of values with no clear categories (e.g. height)
    discontinuous has distinct categories (e.g. blood group)
    continuous is caused by genes and environment; discontinuous mostly by genes only
  26. there is variation in the population
    more offspring are born than can survive
    the best adapted survive and reproduce
    they pass on their alleles, so the allele becomes more common
  27. choose a cow and a bull from parents with high milk yield
    breed them together
    choose the offspring that give the most milk
    repeat over many generations
  28. the base sequence of the gene is copied into mRNA
    mRNA moves to a ribosome
    ribosome joins amino acids in the order set by the mRNA
    the amino acid sequence decides the shape of the protein
  29. a random mutation makes a bacterium resistant
    the antibiotic kills the non-resistant bacteria
    the resistant bacterium survives and reproduces
    the resistance allele becomes common (natural selection)
  30. mother is XNXn, father is XNY
    the son gets Y from the father and an X from the mother
    the X from the mother is XN or Xn with equal chance
    so there is a 1 in 2 (50%) chance that a son is colour-blind
Practise this chapter
Chromosomes, genes & proteinsMitosis & meiosisMonohybrid inheritanceVariationAdaptive featuresSelection
Next chapterEcology & human influences →
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