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
Two AaBb parents are crossed. Gene A gives purple flowers (dominant) or white flowers; gene B gives yellow seeds (dominant) or green seeds. Click the one cell for an offspring that is shows both white flowers and green seeds.
AaBb × AaBb
- 1Ab × AB
- 2Ab × aB
- 3Ab × ab
- 4Ab × Ab
- 5ab × AB
- 6ab × aB
- 7ab × ab
- 8ab × Ab
- 9aB × AB
- 10aB × aB
- 11aB × ab
- 12aB × Ab
- 13AB × AB
- 14AB × aB
- 15AB × ab
- 16AB × Ab
3.Cell biologyMedium
How do large proteins such as transcription factors enter the nucleus?
- AThey are made inside the nucleus by nuclear ribosomes, so they never enter
- BBy exocytosis from the endoplasmic reticulum
- CBy diffusing freely through the nuclear membrane lipids
- DThrough nuclear pore complexes, guided by a nuclear localisation signal
4.Cell biologyHard
In the intrinsic (mitochondrial) pathway of apoptosis, which event leads to caspase activation?
- ABinding of a death ligand to the Fas receptor on the cell surface
- BRelease of cytochrome c from mitochondria into the cytosol
- CFusion of lysosomes with the nucleus
- DEntry of glucose into the mitochondria
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: ab × ab
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.
Genotype asked for
aabb
Gametes
A homozygous genotype needs the same gamete from both parents, so only one cell in the 16 fits.
ab × ab = aabb
Answer
Cell ab × ab → aabb
3.Cell biology
Answer: D · Through nuclear pore complexes, guided by a nuclear localisation signal
Why this is the answer
Proteins with a nuclear localisation signal are recognised by importins and carried through nuclear pore complexes. Translation happens in the cytoplasm.
Answer
D · Through nuclear pore complexes, guided by a nuclear localisation signal
4.Cell biology
Answer: B · Release of cytochrome c from mitochondria into the cytosol
Why this is the answer
Cell stress lets pro-apoptotic Bcl-2 family proteins release cytochrome c. With Apaf-1 it forms the apoptosome, which activates caspase-9. Fas binding starts the extrinsic pathway.
Answer
B · Release of cytochrome c from mitochondria into the cytosol
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