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, round seeds?
AaBb × Aabb
P =
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives tall plants (dominant) or short plants; gene B gives purple flowers (dominant) or white flowers. Click the one cell for an offspring that is homozygous for tall plants and for purple flowers.
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
Active transport differs from facilitated diffusion because it:
- AMoves substances down their gradient faster by using carrier proteins
- BOnly moves water
- CUses energy to move substances against their concentration gradient
- DNeeds no proteins
4.Cell biologyHard
How does a cell mark a damaged or short-lived protein for destruction by the proteasome?
- ABy removing all its phosphate groups
- BBy attaching a chain of ubiquitin molecules
- CBy adding a signal sequence for the ER
- DBy adding mannose-6-phosphate
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(round seeds) = 0.5
Multiply
P = 0.75·0.5 = 0.375
Answer
P = 0.375
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: C · Uses energy to move substances against their concentration gradient
Why this is the answer
Active transport uses ATP (or another gradient) to move molecules against their gradient; facilitated diffusion is passive.
Answer
C · Uses energy to move substances against their concentration gradient
4.Cell biology
Answer: B · By attaching a chain of ubiquitin molecules
Why this is the answer
Enzymes attach a polyubiquitin chain to the target protein. The proteasome recognises this chain, unfolds the protein and breaks it into short peptides.
Answer
B · By attaching a chain of ubiquitin molecules
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