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
Duchenne muscular dystrophy is X-linked recessive (allele d). A carrier mother, XDXd, has children with an unaffected father, XDY. The mother's gametes run down the side. Click the cell for a carrier daughter.
♀ XDXd♂ XDY
- 1XD × XD
- 2XD × Y
- 3Xd × XD
- 4Xd × Y
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives tall plants (dominant) or short plants; gene B gives black fur (dominant) or white fur. Click the one cell for an offspring that is shows both short plants and white fur.
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
Which cell junction lets ions and small molecules pass directly between neighbouring cells?
- ATight junction
- BGap junction
- CDesmosome
- DAdherens junction
4.Cell biologyHard
In the most common cystic fibrosis mutation (ΔF508), the CFTR protein is made but is missing from the cell membrane. Why?
- AThe misfolded protein is held in the ER and degraded
- BThe mRNA cannot leave the nucleus
- CThe gene is not transcribed at all
- DThe protein is secreted out of the cell instead of being inserted
3Solutions
1.Genetics
Answer: Xd × XD
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.
Fill the square
XDXD, XDY, XdXD, XdY
Find the cell
Xd × XD = XdXD
Answer
Cell Xd × XD → XdXD
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: B · Gap junction
Why this is the answer
Gap junctions are channels formed by connexons. They connect cytoplasm of adjacent cells, allowing electrical coupling as in cardiac muscle.
Answer
B · Gap junction
4.Cell biology
Answer: A · The misfolded protein is held in the ER and degraded
Why this is the answer
Loss of one phenylalanine makes CFTR misfold. ER quality control retains it and sends it for proteasomal degradation, so little reaches the membrane.
Answer
A · The misfolded protein is held in the ER and degraded
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