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
haemophilia is X-linked recessive (allele h). A carrier mother, XHXh, has children with an unaffected father, XHY. The mother's gametes run down the side. Click the cell for an unaffected son.
♀ XHXh♂ XHY
- 1Xh × XH
- 2Xh × Y
- 3XH × XH
- 4XH × Y
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives round seeds (dominant) or wrinkled seeds; gene B gives yellow seeds (dominant) or green seeds. Click the one cell for an offspring that is shows both wrinkled seeds 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
What does it mean that the genetic code is "degenerate" (redundant)?
- AMost amino acids are coded by more than one codon
- BSome codons are never used
- CThe code differs completely between species
- DEach codon codes for several amino acids
4.Cell biologyHard
A signal molecule that cannot cross the membrane, such as adrenaline, acts by:
- ABeing converted into ATP
- BDiffusing into the nucleus and binding DNA to change gene transcription
- CDissolving the membrane
- DBinding a surface receptor that triggers a second-messenger cascade
3Solutions
1.Genetics
Answer: XH × Y
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
XhXH, XhY, XHXH, XHY
Find the cell
XH × Y = XHY
Answer
Cell XH × Y → XHY
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 · Most amino acids are coded by more than one codon
Why this is the answer
Several codons can code for the same amino acid, often differing in the third base. This is why many point mutations are silent.
Answer
A · Most amino acids are coded by more than one codon
4.Cell biology
Answer: D · Binding a surface receptor that triggers a second-messenger cascade
Why this is the answer
Hydrophilic signals bind membrane receptors (e.g. G-protein-coupled receptors), activating intracellular messengers; steroid hormones instead cross the membrane.
Answer
D · Binding a surface receptor that triggers a second-messenger cascade
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