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 round seeds (dominant) or wrinkled seeds; gene B gives tall plants (dominant) or short plants. Click the one cell for an offspring that is homozygous for round seeds and for tall plants.
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
What separates during anaphase I of meiosis?
- ACentromeres of each chromatid
- BHomologous chromosomes
- CSister chromatids
- DWhole nuclei
4.Cell biologyHard
A signal molecule that cannot cross the membrane, such as adrenaline, acts by:
- ADissolving the membrane
- BDiffusing into the nucleus and binding DNA to change gene transcription
- CBeing converted into ATP
- DBinding a surface receptor that triggers a second-messenger cascade
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: B · Homologous chromosomes
Why this is the answer
In anaphase I, homologous chromosomes move to opposite poles, halving the chromosome number. Sister chromatids separate in anaphase II.
Answer
B · Homologous chromosomes
4.Cell biology
Answer: D · Binding a surface receptor that triggers a second-messenger cascade
Why this is the answer
Hydrophilic signals bind membrane receptors (e.g. G-protein-coupled receptors), activating intracellular messengers; steroid hormones instead cross the membrane.
Answer
D · Binding a surface receptor that triggers a second-messenger cascade
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