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
red-green colour blindness is X-linked recessive (allele b). A carrier mother, XBXb, has children with an unaffected father, XBY. The mother's gametes run down the side. Click the cell for an affected son.
♀ XBXb♂ XBY
- 1Xb × XB
- 2Xb × Y
- 3XB × XB
- 4XB × Y
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives black fur (dominant) or white fur; gene B gives tall plants (dominant) or short plants. Click the one cell for an offspring that is homozygous for black fur 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 is the G0 phase of the cell cycle?
- AThe final stage of mitosis
- BA phase found only in cancer cells that have stopped responding to signals
- CA resting state in which cells have left the cycle and stopped dividing
- DThe phase in which DNA is copied
4.Cell biologyHard
A mutant Ras protein cannot hydrolyse GTP. What is the effect on cell signalling?
- ARas stays active and sends constant growth signals
- BRas stays inactive and growth signals stop
- CRas moves into the nucleus and blocks transcription
- DRas is broken down faster
3Solutions
1.Genetics
Answer: Xb × 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
XbXB, XbY, XBXB, XBY
Find the cell
Xb × Y = XbY
Answer
Cell Xb × Y → XbY
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 · 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
C · A resting state in which cells have left the cycle and stopped dividing
4.Cell biology
Answer: A · Ras stays active and sends constant growth signals
Why this is the answer
Ras is switched off by hydrolysing its GTP. A mutant that cannot do this stays permanently on, driving proliferation; this is why RAS is a common oncogene.
Answer
A · Ras stays active and sends constant growth signals
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