OxfordAQA International GCSE Biology: Genetic variation, inheritance, DNA and disorders (9201)

This guide teaches sections 3.5.3 (Genetic variation) and 3.5.4 (Genetic disorders) of the OxfordAQA International GCSE Biology (9201) specification, Version 5.2, for exams May/June 2018 onwards. The qualification is untiered, so everything here is for every candidate. Use the revision notes to condense it and the practice questions to test it. Course pages: the OxfordAQA IGCSE Biology hub, the printable checklist, the exam preparation guide and the free 10-minute diagnostics.

What this unit covers

Section	What you must be able to do
3.5.3 a	Separate genetic, environmental and combined causes of variation
3.5.3 b, c	Explain that genes on chromosomes are passed on in gametes
3.5.3 d	Describe sex determination (XX and XY)
3.5.3 e, f, g	Use allele, homozygous, heterozygous, dominant, recessive, genotype, phenotype; construct and interpret monohybrid crosses and family trees; know Mendel’s principles
3.5.3 h to k	Relate DNA, genes, proteins, the double helix and base triplets
3.5.4 a	Interpret data on inherited disorders (polydactyly, cystic fibrosis, sickle cell anaemia)
3.5.4 b	Explain that an extra chromosome causes Down’s syndrome

Causes of variation (3.5.3 a)

Individuals of the same kind differ. The differences have three possible causes:

• Genetic: the genes they inherited, such as blood group.
• Environmental: the conditions they developed in, such as a scar or a plant bent towards light.
• Both: most characteristics, such as height or body mass.

Genetically identical individuals (clones, identical twins) let you separate the two causes. Any difference between them must be environmental.

Worked example. A gardener grows cuttings from one tomato plant. Three grow in full sun and three in shade. After six weeks the mean heights are 42 cm (sun) and 27 cm (shade).

The cuttings are genetically identical, so the 15 cm difference (42 - 27) is environmental. The shaded plants are 15 ÷ 42 × 100 = 35.7% shorter than the sunny ones. This does not show that genes play no part in height. It shows that light affected height in these plants.

Genes, chromosomes and gametes (3.5.3 b, c)

Offspring resemble their parents because genes pass on in the gametes (sex cells). The nucleus holds chromosomes, which carry genes, and chromosomes are normally found in pairs. Each gamete receives one chromosome of every pair, so fertilisation restores pairs: one chromosome of each pair comes from each parent.

Sex determination (3.5.3 d)

Humans have 23 pairs of chromosomes. One pair carries the genes that determine sex. Females are XX. Males are XY.

All eggs contain an X. Sperm come in two equal kinds, X-bearing and Y-bearing. The baby’s sex is set by which kind fertilises the egg.

	Sperm X	Sperm Y
Egg X	XX (girl)	XY (boy)
Egg X	XX (girl)	XY (boy)

The ratio is 1 girl : 1 boy, so each baby has a probability of 1/2 of being a boy. Fertilisation is random each time. Three girls in a row do not change the chance for the next baby.

Worked example. Probability that a family of three has two boys and one girl. There are three orders (BBG, BGB, GBB) and each has probability (1/2)³ = 1/8. So the answer is 3 × 1/8 = 3/8.

Alleles, genotype and phenotype (3.5.3 e, f, g)

Different genes control different characteristics. Some characteristics are controlled by a single gene. A gene can have different forms called alleles.

Term	Meaning
Allele	A different form of a gene
Homozygous	Both chromosomes of a pair carry the same allele (AA or aa)
Heterozygous	The two chromosomes carry different alleles (Aa)
Dominant allele	Controls the characteristic when present on only one chromosome
Recessive allele	Controls the characteristic only when the dominant allele is absent
Genotype	The alleles an individual has (for example Aa)
Phenotype	The characteristic you can observe

Use a capital letter for the dominant allele and the same letter in lower case for the recessive. A heterozygous individual shows the dominant phenotype. The recessive allele is still there and can be passed on.

Genes work at a molecular level: an allele affects which protein is made, and the proteins in a cell help to develop the visible characteristic.

Monohybrid crosses

A monohybrid cross follows one gene. The method has five steps.

1. Choose letters and state which allele is dominant.
2. Write the parents’ genotypes.
3. Write the possible gametes (one allele each).
4. Combine them in a Punnett square.
5. Read off genotypes, phenotypes, ratio and probability.

Worked example 1: two heterozygous parents. In tomato plants a purple stem (A) is dominant to a green stem (a). Cross two heterozygous plants, Aa × Aa.

	A	a
A	AA	Aa
a	Aa	aa

Genotypes: 1 AA : 2 Aa : 1 aa. Phenotypes: 3 purple : 1 green. Probability of green = 1/4. For 160 seedlings, expect 160 × 3/4 = 120 purple and 160 × 1/4 = 40 green. Real counts vary a little because fertilisation is random.

Worked example 2: pure-breeding parents. AA × aa gives only Aa, all purple. Crossing two of these Aa plants gives the 3 : 1 result above. A recessive characteristic can vanish for a generation and then return.

Worked example 3: finding an unknown genotype. A purple plant could be AA or Aa. Cross it with a green plant (aa), which can only give a.

• If the purple plant is AA, every seedling is Aa: all purple.
• If it is Aa, half the seedlings are Aa and half aa: about 1 purple : 1 green.

Any green seedling proves the unknown parent was Aa.

Genetic diagrams are models. They predict probabilities. They do not say what will happen in one family.

Mendel and monohybrid inheritance

Gregor Mendel studied inheritance in garden peas. His method had these principles:

• He used pure-breeding lines, so each parent had a known, stable characteristic.
• He studied one characteristic at a time, each with two clear forms (round or wrinkled seeds).
• He controlled the crosses by moving pollen between chosen plants.
• He counted large numbers of offspring.

Crossing round-seeded and wrinkled-seeded parents gave only round seeds in the first generation. In the next generation he counted 5474 round and 1850 wrinkled seeds. The ratio is 5474 ÷ 1850 = 2.96, close to 3 : 1. He explained this with inherited “factors” that stay separate and can be dominant or recessive.

His work came before other scientists linked his “inherited factors” to chromosomes. Chromosomes and genes were not understood when he published, so his explanation was a model without a known physical basis.

Family trees

A family tree shows who has a characteristic across generations. Squares are males, circles are females, shaded symbols show the characteristic.

Rule 1. Neither parent shows a characteristic but a child does: the characteristic is recessive, and both parents are carriers.

Rule 2. Both parents show a characteristic but a child does not: the characteristic is dominant, and each parent is heterozygous.

Worked example. In one family the characteristic is polydactyly (extra fingers or toes), which here is caused by a dominant allele, D.

Generation	Person	Phenotype
I	1 (man), 2 (woman)	Both affected
II	3 (daughter)	Unaffected
II	4 (son)	Affected

Persons 1 and 2 are both affected but have an unaffected child, so the allele is dominant (rule 2). Person 3 is dd, so each parent gave a d: persons 1 and 2 are Dd. Person 4 is DD or Dd; from this tree alone you cannot tell which.

DNA, genes and proteins (3.5.3 h to k)

• Chromosomes are made of very long DNA molecules. DNA holds the coded information for inherited characteristics.
• A gene is a short section of DNA. It codes for a particular combination of amino acids, which make a specific protein.
• DNA is made of two very long strands twisted into a double helix. The strands contain four different compounds called bases. You do not need their names, or how base pairing allows DNA to replicate.
• Each run of three bases is the code for one amino acid. The base sequence decides the sequence in which amino acids are assembled into the protein.

Worked example. A protein has 150 amino acids. Each needs three bases, so the coding sequence has at least 150 × 3 = 450 bases. Changing the order of the bases changes the order of the amino acids, so a different protein may be made.

Invented code for practice, using bases W, X, Y and Z: WXY = amino acid 1, ZZW = amino acid 2, XWY = amino acid 3. The sequence WXYZZWXWY is read as WXY | ZZW | XWY, which is amino acids 1, 2, 3. Always split into threes before you read it.

There are 4 × 4 × 4 = 64 different triplets of four bases, enough to code for the amino acids used by living things.

Genetic disorders (3.5.4)

Some disorders are inherited. Be sensitive when you write about them: describe the condition and the genetics, and avoid judging people. You may be given data on these three.

Disorder	Cause	Inheritance
Polydactyly	Extra fingers or toes	Dominant allele (as used in the data you meet)
Cystic fibrosis	Affects the lungs and digestive system (thick sticky mucus)	Recessive allele
Sickle cell anaemia	Abnormal haemoglobin makes red blood cells sickle-shaped, which can block blood vessels	Recessive allele; carriers (heterozygous) usually do not have the disease

Worked example. Two unaffected parents who both carry the cystic fibrosis allele (Cc × Cc) give: 1 CC : 2 Cc : 1 cc. Probability of an affected child = 1/4. An unaffected child has a 2/3 probability of being a carrier, because 2 of the 3 unaffected boxes are Cc.

Reading data. Compare observed counts with the predicted 3 : 1 or 1 : 1. Small samples vary a lot. A table of 16 children might show 3 affected where 4 were predicted, which is consistent with the model.

Down’s syndrome (3.5.4 b). Some conditions come from an abnormal number of chromosomes, not from alleles. Down’s syndrome is caused by an extra copy of chromosome 21, so body cells contain 47 chromosomes instead of 46 (24 in the affected gamete instead of 23).

Common errors

• Saying a recessive allele “disappears” in a heterozygote. It is present but not expressed.
• Answering with a ratio where a probability is wanted: 3 : 1 is not 3/4.
• Crossing whole genotypes instead of gametes in a Punnett square.
• Using different letters for the two alleles.
• Reading a family tree without stating the parents’ phenotypes first.
• Confusing a gene (a section of DNA) with a chromosome.

Next, try the practice questions.

Official syllabus

OxfordAQA International GCSE Biology (9201), Version 5.2, for exams May/June 2018 onwards, published by OxfordAQA. Sections 3.5.3 Genetic variation and 3.5.4 Genetic disorders.