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 a carrier daughter.
♀ XBXb♂ XBY
- 1XB × Y
- 2XB × XB
- 3Xb × Y
- 4Xb × XB
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives yellow seeds (dominant) or green seeds; gene B gives tall plants (dominant) or short plants. Click the one cell for an offspring that is shows green seeds and is homozygous 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
How do steroid hormones such as cortisol mainly act on their target cells?
- AThey cross the membrane and bind intracellular receptors that change gene transcription
- BThey activate adenylyl cyclase from outside the cell
- CThey are taken up by phagocytosis and digested
- DThey bind a membrane receptor and open ion channels, changing the cell's membrane potential
4.Cell biologyHard
Knudson's "two-hit" hypothesis, based on retinoblastoma, explains that tumour suppressor genes:
- AUsually need both alleles to be inactivated before a tumour develops
- BAre always inherited in mutated form from both the mother and the father
- CAre only found in children
- DCause cancer when one allele is overactive
3Solutions
1.Genetics
Answer: Xb × XB
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
XBY, XBXB, XbY, XbXB
Find the cell
Xb × XB = XbXB
Answer
Cell Xb × XB → XbXB
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 · 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
A · They cross the membrane and bind intracellular receptors that change gene transcription
4.Cell biology
Answer: A · Usually need both alleles to be inactivated before a tumour develops
Why this is the answer
Loss of one copy of RB1 is not enough; the second allele must also be lost. Children who inherit one mutant copy need only one more hit, so they often get bilateral tumours early.
Answer
A · Usually need both alleles to be inactivated before a tumour develops
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