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, wrinkled seeds?
AaBb × aaBb
P =
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives tall plants (dominant) or short plants; gene B gives black fur (dominant) or white fur. Click the one cell for an offspring that is shows short plants and is homozygous 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
A point mutation changes a codon for an amino acid into a stop codon. What type of mutation is this?
- ASilent mutation
- BNonsense mutation
- CFrameshift mutation
- DMissense mutation
4.Cell biologyHard
In the Meselson–Stahl experiment, bacteria grown in ¹⁵N were moved to ¹⁴N. After one round of replication all DNA formed one band of intermediate density. Which model did this rule out?
- ABoth semi-conservative and dispersive models
- BThe conservative model
- CThe semi-conservative model
- DThe dispersive model
3Solutions
1.Genetics
Answer: P = 0.125
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(wrinkled seeds) = 0.25
Multiply
P = 0.5·0.25 = 0.125
Answer
P = 0.125
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 · Nonsense mutation
Why this is the answer
A nonsense mutation creates a premature stop codon, giving a shortened protein that usually does not work.
Answer
B · Nonsense mutation
4.Cell biology
Answer: B · The conservative model
Why this is the answer
Conservative replication would give separate heavy and light bands. The single hybrid band ruled it out; after the second generation, a light band appeared, which ruled out the dispersive model.
Answer
B · The conservative model
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