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
Duchenne muscular dystrophy is X-linked recessive (allele d). A carrier mother, XDXd, has children with an affected father, XdY. The mother's gametes run down the side. Click the cell for a carrier daughter.
♀ XDXd♂ XdY
- 1XD × Xd
- 2XD × Y
- 3Xd × Xd
- 4Xd × Y
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives round seeds (dominant) or wrinkled seeds; gene B gives purple flowers (dominant) or white flowers. Click the one cell for an offspring that is shows both wrinkled seeds and white flowers.
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
How do steroid hormones such as cortisol mainly act on their target cells?
- AThey bind a membrane receptor and open ion channels, changing the cell's membrane potential
- BThey activate adenylyl cyclase from outside the cell
- CThey are taken up by phagocytosis and digested
- DThey cross the membrane and bind intracellular receptors that change gene transcription
4.Cell biologyHard
Cholera toxin modifies the alpha subunit of Gs so it cannot hydrolyse GTP. Why does this cause severe watery diarrhoea?
- AThe toxin blocks aquaporins in the colon
- BcAMP falls, so gut cells stop absorbing water and sodium from the lumen
- CThe toxin destroys the villi
- DcAMP stays high in gut cells, driving chloride and water secretion
3Solutions
1.Genetics
Answer: XD × Xd
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.
Fill the square
XDXd, XDY, XdXd, XdY
Find the cell
XD × Xd = XDXd
Answer
Cell XD × Xd → XDXd
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: D · They cross the membrane and bind intracellular receptors that change gene transcription
Why this is the answer
Steroids diffuse through the membrane and bind receptors in the cytoplasm or nucleus. The hormone–receptor complex acts as a transcription factor.
Answer
D · They cross the membrane and bind intracellular receptors that change gene transcription
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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