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 an unaffected son.
♀ XDXd♂ XdY
- 1Xd × Y
- 2Xd × Xd
- 3XD × Y
- 4XD × Xd
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives black fur (dominant) or white fur; gene B gives yellow seeds (dominant) or green seeds. Click the one cell for an offspring that is homozygous for black fur and shows green 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
Which molecules can cross a phospholipid bilayer most easily by simple diffusion?
- AIons such as Na⁺ and K⁺
- BProteins such as albumin
- CLarge polar molecules such as glucose
- DSmall non-polar molecules such as O₂ and CO₂
4.Cell biologyHard
Tumour suppressor genes such as p53 normally:
- ASpeed up cell division so that damaged cells are quickly replaced
- BProduce growth factors
- CMake cells immortal
- DStop the cell cycle or trigger apoptosis when DNA is damaged
3Solutions
1.Genetics
Answer: XD × Y
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
XdY, XdXd, XDY, XDXd
Find the cell
XD × Y = XDY
Answer
Cell XD × Y → XDY
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 · Small non-polar molecules such as O₂ and CO₂
Why this is the answer
Small non-polar molecules dissolve in the hydrophobic core and diffuse through easily. Ions and large polar molecules need channels or carriers.
Answer
D · Small non-polar molecules such as O₂ and CO₂
4.Cell biology
Answer: D · Stop the cell cycle or trigger apoptosis when DNA is damaged
Why this is the answer
p53 arrests the cycle for repair or triggers apoptosis; mutations that inactivate it let damaged cells keep dividing.
Answer
D · Stop the cell cycle or trigger apoptosis when DNA is damaged
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