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
Tall (A) is dominant over short (a), and round seeds (B) over wrinkled (b). The genes are on different chromosomes. For the cross AaBb × aaBb, what is the probability P that an offspring is short, round seeds?
AaBb × aaBb
P =
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives round seeds (dominant) or wrinkled seeds; gene B gives black fur (dominant) or white fur. Click the one cell for an offspring that is homozygous for round seeds and for black fur.
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
In which direction do DNA and RNA polymerases add new nucleotides?
- AIn both directions equally
- B5′ to 3′
- CFrom the centromere outwards
- D3′ to 5′
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: P = 0.375
Mendelian genetics: each parent passes on one allele of each gene, chosen at random. A Punnett square lists every combination of gametes, each equally likely. A dominant allele (capital letter) shows whenever it is present; a recessive one only in the homozygous form. Genes on different chromosomes are inherited independently, so their probabilities multiply.
Each gene on its own
P(short) = 0.5
P(round seeds) = 0.75
Multiply
P = 0.5·0.75 = 0.375
Answer
P = 0.375
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: B · 5′ to 3′
Why this is the answer
Polymerases only add nucleotides to the 3′ end of the growing strand, so synthesis always runs 5′ to 3′. This causes the leading and lagging strands.
Answer
B · 5′ to 3′
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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