Genetics
- Monohybrid cross (Aa × Aa)
- genotypes 1 : 2 : 1, phenotypes 3 : 1
- Dihybrid cross
- phenotypes 9 : 3 : 3 : 1
- Hardy–Weinberg
- p + q = 1, p² + 2pq + q² = 1
- Independent genes
- multiply the probabilities
Biology: Genetics and Cell Biology: cram pack
Formula sheets, the key method steps for every topic and 4 practice problems, with full solutions at the back.
1Formulas and methods
2Practice problems
1.GeneticsMedium
Tall (A) is dominant over short (a), and round seeds (B) over wrinkled (b). The genes are on different chromosomes. For the cross AaBb × aaBb, what is the probability P that an offspring is tall, round seeds?
AaBb × aaBb
P =
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives yellow seeds (dominant) or green seeds; gene B gives tall plants (dominant) or short plants. Click the one cell for an offspring that is homozygous for yellow seeds and for tall plants.
AaBb × AaBb
- 1aB × Ab
- 2aB × AB
- 3aB × aB
- 4aB × ab
- 5ab × Ab
- 6ab × AB
- 7ab × aB
- 8ab × ab
- 9AB × Ab
- 10AB × AB
- 11AB × aB
- 12AB × ab
- 13Ab × Ab
- 14Ab × AB
- 15Ab × aB
- 16Ab × ab
3.Cell biologyMedium
What role does cholesterol play in animal cell membranes?
- AIt makes the membrane completely rigid at every temperature
- BIt forms channels for ions
- CIt helps keep membrane fluidity stable across temperatures
- DIt is the main carrier of glucose
4.Cell biologyHard
Why does colchicine arrest dividing cells in mitosis?
- AIt destroys the nuclear envelope permanently
- BIt breaks down actin filaments, so cytokinesis cannot happen
- CIt stops tubulin polymerising, so the spindle cannot form
- DIt stops DNA replication in S phase
3Solutions
1.Genetics
Answer: P = 0.375
Mendelian genetics: each parent passes on one allele of each gene, chosen at random. A Punnett square lists every combination of gametes, each equally likely. A dominant allele (capital letter) shows whenever it is present; a recessive one only in the homozygous form. Genes on different chromosomes are inherited independently, so their probabilities multiply.
Each gene on its own
P(tall) = 0.5
P(round seeds) = 0.75
Multiply
P = 0.5·0.75 = 0.375
Answer
P = 0.375
2.Genetics
Answer: AB × AB
Punnett square: one parent's gametes run along the top, the other's down the side. Each cell joins the gamete above it with the gamete beside it, and every cell is equally likely. A recessive trait only shows when both alleles are recessive; for an X-linked trait, sons get their only X from the mother.
Genotype asked for
AABB
Gametes
A homozygous genotype needs the same gamete from both parents, so only one cell in the 16 fits.
AB × AB = AABB
Answer
Cell AB × AB → AABB
3.Cell biology
Answer: C · It helps keep membrane fluidity stable across temperatures
Why this is the answer
Cholesterol reduces fluidity at high temperatures and prevents tight packing at low temperatures, buffering membrane fluidity.
Answer
C · It helps keep membrane fluidity stable across temperatures
4.Cell biology
Answer: C · It stops tubulin polymerising, so the spindle cannot form
Why this is the answer
Colchicine binds tubulin and prevents microtubule assembly. Without a working spindle, chromosomes cannot attach properly and the spindle checkpoint holds cells in metaphase.
Answer
C · It stops tubulin polymerising, so the spindle cannot form
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