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 tall, round seeds?
AaBb × Aabb
P =
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives round seeds (dominant) or wrinkled seeds; gene B gives tall plants (dominant) or short plants. Click the one cell for an offspring that is shows wrinkled 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
Where does RNA polymerase bind to start transcription of a gene?
- AAt the start codon
- BAt the terminator
- CAt the stop codon
- DAt the promoter
4.Cell biologyHard
Which checkpoint ensures all chromosomes are attached to the spindle before anaphase begins?
- AThe spindle assembly (metaphase) checkpoint
- BThe S-phase checkpoint
- CThe G1/S checkpoint
- DThe G2/M checkpoint
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(tall) = 0.75
P(round seeds) = 0.5
Multiply
P = 0.75·0.5 = 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: D · At the promoter
Why this is the answer
RNA polymerase, helped by transcription factors in eukaryotes, binds the promoter and begins making RNA from the template strand.
Answer
D · At the promoter
4.Cell biology
Answer: A · The spindle assembly (metaphase) checkpoint
Why this is the answer
The spindle checkpoint delays anaphase until every kinetochore is attached, preventing unequal chromosome distribution.
Answer
A · The spindle assembly (metaphase) checkpoint
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