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 × XB
- 2XB × Y
- 3Xb × XB
- 4Xb × Y
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives purple flowers (dominant) or white flowers; gene B gives yellow seeds (dominant) or green seeds. Click the one cell for an offspring that is shows both white flowers and 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
The contractile ring that pinches an animal cell in two during cytokinesis is made of:
- AMicrotubules and kinesin
- BKeratin filaments
- CActin and myosin
- DCellulose fibres
4.Cell biologyHard
In the Hershey–Chase experiment, phages labelled with ³²P and ³⁵S infected bacteria. What was found inside the bacteria?
- AMostly ³⁵S, showing protein enters the cell
- BNeither label, because phages stay outside
- CEqual amounts of both labels
- DMostly ³²P, showing DNA enters the cell
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
XBXB, XBY, XbXB, XbY
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: C · Actin and myosin
Why this is the answer
A ring of actin filaments and myosin II contracts under the membrane, forming the cleavage furrow that separates the two daughter cells.
Answer
C · Actin and myosin
4.Cell biology
Answer: D · Mostly ³²P, showing DNA enters the cell
Why this is the answer
The ³²P-labelled DNA entered the bacteria while most ³⁵S-labelled protein coats stayed outside. This supported DNA as the genetic material of phage.
Answer
D · Mostly ³²P, showing DNA enters the cell
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