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 unaffected 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 round seeds (dominant) or wrinkled seeds; gene B gives yellow seeds (dominant) or green seeds. Click the one cell for an offspring that is homozygous for round seeds and for yellow 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 cell junction anchors intermediate filaments and gives tissues such as skin mechanical strength?
- ADesmosome
- BPlasmodesma
- CTight junction
- DGap junction
4.Cell biologyHard
With 23 chromosome pairs, how many different chromosome combinations can independent assortment alone produce in human gametes?
- A2 × 23
- B46
- C23²
- D2²³ (about 8 million)
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: A · Desmosome
Why this is the answer
Desmosomes link the keratin intermediate filaments of neighbouring cells via cadherins, so tissues under stress do not tear apart.
Answer
A · Desmosome
4.Cell biology
Answer: D · 2²³ (about 8 million)
Why this is the answer
Each of the 23 pairs can orient two ways, giving 2²³ ≈ 8.4 million combinations, even before crossing over adds more variation.
Answer
D · 2²³ (about 8 million)
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