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 short, wrinkled seeds?
AaBb × Aabb
P =
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives yellow seeds (dominant) or green seeds; gene B gives round seeds (dominant) or wrinkled seeds. Click the one cell for an offspring that is shows both green seeds and wrinkled seeds.
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 is the G0 phase of the cell cycle?
- AThe final stage of mitosis
- BA resting state in which cells have left the cycle and stopped dividing
- CA phase found only in cancer cells that have stopped responding to signals
- DThe phase in which DNA is copied
4.Cell biologyHard
Cholera toxin modifies the alpha subunit of Gs so it cannot hydrolyse GTP. Why does this cause severe watery diarrhoea?
- AcAMP falls, so gut cells stop absorbing water and sodium from the lumen
- BThe toxin destroys the villi
- CThe toxin blocks aquaporins in the colon
- DcAMP stays high in gut cells, driving chloride and water secretion
3Solutions
1.Genetics
Answer: P = 0.125
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(short) = 0.25
P(wrinkled seeds) = 0.5
Multiply
P = 0.25·0.5 = 0.125
Answer
P = 0.125
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: B · A resting state in which cells have left the cycle and stopped dividing
Why this is the answer
Cells in G0 have exited the cycle and do not prepare to divide. Some can return to G1 when stimulated, such as liver cells after injury.
Answer
B · A resting state in which cells have left the cycle and stopped dividing
4.Cell biology
Answer: D · cAMP stays high in gut cells, driving chloride and water secretion
Why this is the answer
Permanently active Gs keeps adenylyl cyclase on. High cAMP activates PKA, which opens CFTR chloride channels, and water follows into the gut lumen.
Answer
D · cAMP stays high in gut cells, driving chloride and water secretion
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