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 16% of a population in Hardy–Weinberg equilibrium. What fraction f of the population are carriers (heterozygous)?
q² = 0.16
f =
3.Cell biologyMedium
Transcription produces:
- ADNA from DNA
- BmRNA from a DNA template
- CProtein from mRNA
- DDNA from RNA
4.Cell biologyHard
A double-stranded DNA sample contains 30% adenine. What percentage is guanine?
- A40%
- B20%
- C30%
- D70%
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(wrinkled seeds) = 0.5
Multiply
P = 0.75·0.5 = 0.375
Answer
P = 0.375
2.Genetics
Answer: f = 0.48
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 = √0.16 = 0.4
p = 1 − 0.4 = 0.6
Carriers
2pq = 2·0.6·0.4 = 0.48
Answer
f = 0.48
3.Cell biology
Answer: B · mRNA from a DNA template
Why this is the answer
RNA polymerase transcribes a DNA template strand into RNA; translation then makes protein at ribosomes.
Answer
B · mRNA from a DNA template
4.Cell biology
Answer: B · 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
B · 20%
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