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
In a population of 500 in Hardy–Weinberg equilibrium, 1 in 1600 show a recessive trait. How many individuals N are expected to be homozygous dominant?
N = 500q² = 0.000625
N =
3.Cell biologyMedium
How do steroid hormones such as cortisol mainly act on their target cells?
- AThey cross the membrane and bind intracellular receptors that change gene transcription
- BThey bind a membrane receptor and open ion channels, changing the cell's membrane potential
- CThey are taken up by phagocytosis and digested
- DThey activate adenylyl cyclase from outside the cell
4.Cell biologyHard
Brown fat contains thermogenin (UCP1), which lets protons leak back across the inner mitochondrial membrane. What is the effect?
- AThe electron transport chain stops working because protons can no longer be pumped
- BMore ATP is made per glucose
- CEnergy from the proton gradient is released as heat instead of making ATP
- DThe cell switches to fermentation
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: N = 475.3
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 = √0.000625 = 0.025
p = 0.975
Homozygous dominant
p² = 0.9506
N = 500·0.9506 = 475.3
Answer
N = 475.3
3.Cell biology
Answer: A · They cross the membrane and bind intracellular receptors that change gene transcription
Why this is the answer
Steroids diffuse through the membrane and bind receptors in the cytoplasm or nucleus. The hormone–receptor complex acts as a transcription factor.
Answer
A · They cross the membrane and bind intracellular receptors that change gene transcription
4.Cell biology
Answer: C · Energy from the proton gradient is released as heat instead of making ATP
Why this is the answer
Uncoupling proteins let protons return to the matrix without passing ATP synthase. Electron transport continues, but the energy is released as heat, which is useful in newborns.
Answer
C · Energy from the proton gradient is released as heat instead of making ATP
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