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
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 shows white 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
Which molecules can cross a phospholipid bilayer most easily by simple diffusion?
- ASmall non-polar molecules such as O₂ and CO₂
- BIons such as Na⁺ and K⁺
- CProteins such as albumin
- DLarge polar molecules such as glucose
4.Cell biologyHard
In the Meselson–Stahl experiment, bacteria grown in ¹⁵N were moved to ¹⁴N. After one round of replication all DNA formed one band of intermediate density. Which model did this rule out?
- AThe dispersive model
- BBoth semi-conservative and dispersive models
- CThe conservative model
- DThe semi-conservative model
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(short) = 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: A · Small non-polar molecules such as O₂ and CO₂
Why this is the answer
Small non-polar molecules dissolve in the hydrophobic core and diffuse through easily. Ions and large polar molecules need channels or carriers.
Answer
A · Small non-polar molecules such as O₂ and CO₂
4.Cell biology
Answer: C · The conservative model
Why this is the answer
Conservative replication would give separate heavy and light bands. The single hybrid band ruled it out; after the second generation, a light band appeared, which ruled out the dispersive model.
Answer
C · The conservative model
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