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, wrinkled seeds?
AaBb × aabb
P =
2.GeneticsHard
A recessive condition affects 9% of a population in Hardy–Weinberg equilibrium. What fraction f of the population are carriers (heterozygous)?
q² = 0.09
f =
3.Cell biologyMedium
Which enzyme makes the second messenger cyclic AMP when activated by a Gs protein?
- AProtein kinase A
- BPhospholipase C
- CAdenylyl cyclase
- DPhosphodiesterase
4.Cell biologyHard
A double-stranded DNA sample contains 30% adenine. What percentage is guanine?
- A20%
- B40%
- C30%
- D70%
3Solutions
1.Genetics
Answer: P = 0.25
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) = 0.5
P(wrinkled seeds) = 0.5
Multiply
P = 0.5·0.5 = 0.25
Answer
P = 0.25
2.Genetics
Answer: f = 0.42
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 = √0.09 = 0.3
p = 1 − 0.3 = 0.7
Carriers
2pq = 2·0.7·0.3 = 0.42
Answer
f = 0.42
3.Cell biology
Answer: C · Adenylyl cyclase
Why this is the answer
Activated Gs stimulates adenylyl cyclase to convert ATP into cAMP. cAMP activates protein kinase A, and phosphodiesterase breaks cAMP down.
Answer
C · Adenylyl cyclase
4.Cell biology
Answer: A · 20%
Why this is the answer
A = T = 30%, so A + T = 60%. The remaining 40% is split equally between G and C, so G = 20%.
Answer
A · 20%
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