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 affected son.
♀ XHXh♂ XHY
- 1Xh × XH
- 2Xh × Y
- 3XH × XH
- 4XH × Y
2.GeneticsHard
A recessive condition affects 4% of a population in Hardy–Weinberg equilibrium. What fraction f of the population are carriers (heterozygous)?
q² = 0.04
f =
3.Cell biologyMedium
A codon consists of:
- AOne nucleotide
- BA whole gene
- CThree nucleotides on mRNA that code for an amino acid or a stop signal
- DThree amino acids
4.Cell biologyHard
In E. coli, under which conditions is the lac operon most strongly transcribed?
- ALactose present and glucose absent
- BLactose absent and glucose present
- CBoth lactose and glucose present
- DBoth lactose and glucose absent
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: f = 0.32
Hardy–Weinberg: in a large, randomly mating population with no selection, the allele frequencies p (dominant) and q (recessive), p + q = 1, give genotype frequencies p², 2pq and q².
Allele frequencies
q = √0.04 = 0.2
p = 1 − 0.2 = 0.8
Carriers
2pq = 2·0.8·0.2 = 0.32
Answer
f = 0.32
3.Cell biology
Answer: C · Three nucleotides on mRNA that code for an amino acid or a stop signal
Why this is the answer
Each triplet of bases specifies an amino acid or stop; the genetic code is redundant (several codons per amino acid).
Answer
C · Three nucleotides on mRNA that code for an amino acid or a stop signal
4.Cell biology
Answer: A · Lactose present and glucose absent
Why this is the answer
Lactose (as allolactose) removes the repressor. Low glucose raises cAMP, which activates CAP to boost transcription. Both are needed for maximal expression.
Answer
A · Lactose present and glucose absent
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