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
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
Where in the cell does the citric acid (Krebs) cycle take place?
- ACytoplasm
- BIntermembrane space
- CInner mitochondrial membrane
- DMitochondrial matrix
4.Cell biologyHard
Why is DNA replication called semi-conservative?
- AOnly half of the DNA is copied
- BMutations are conserved in half of the daughter cells after every division
- CEach new double helix contains one original strand and one new strand
- DThe new DNA is completely new
3Solutions
1.Genetics
Answer: P = 0.0625
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) = 0.25
P(wrinkled seeds) = 0.25
Multiply
P = 0.25·0.25 = 0.0625
Answer
P = 0.0625
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: D · Mitochondrial matrix
Why this is the answer
The citric acid cycle runs in the mitochondrial matrix, producing NADH, FADH₂ and CO₂. The electron transport chain sits in the inner membrane.
Answer
D · Mitochondrial matrix
4.Cell biology
Answer: C · Each new double helix contains one original strand and one new strand
Why this is the answer
Meselson and Stahl showed that after one replication the DNA had intermediate density, consistent with one old and one new strand.
Answer
C · Each new double helix contains one original strand and one new strand
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