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, round seeds?
AaBb × aaBb
P =
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives round seeds (dominant) or wrinkled seeds; gene B gives purple flowers (dominant) or white flowers. Click the one cell for an offspring that is homozygous for round seeds 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
The Golgi apparatus mainly:
- AModifies, sorts and packages proteins for transport
- BReplicates DNA
- CProduces ATP
- DBreaks down old organelles and engulfed bacteria with enzymes
4.Cell biologyHard
Some antibiotics, such as tetracyclines, block bacterial protein synthesis with little effect on human cytoplasmic ribosomes. Why?
- ABacteria use DNA instead of mRNA as the template for translation on ribosomes
- BBacterial 70S ribosomes differ in structure from eukaryotic 80S ribosomes
- CHuman cells do not make proteins
- DHuman ribosomes are inside the nucleus
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(short) = 0.5
P(round seeds) = 0.75
Multiply
P = 0.5·0.75 = 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: A · Modifies, sorts and packages proteins for transport
Why this is the answer
The Golgi modifies proteins (e.g. glycosylation) and sorts them into vesicles for secretion, membranes or lysosomes.
Answer
A · Modifies, sorts and packages proteins for transport
4.Cell biology
Answer: B · Bacterial 70S ribosomes differ in structure from eukaryotic 80S ribosomes
Why this is the answer
Bacterial ribosomes (70S: 30S + 50S) differ from cytoplasmic eukaryotic ribosomes (80S: 40S + 60S), so drugs can target them selectively.
Answer
B · Bacterial 70S ribosomes differ in structure from eukaryotic 80S ribosomes
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