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
haemophilia is X-linked recessive (allele h). A carrier mother, XHXh, has children with an unaffected father, XHY. The mother's gametes run down the side. Click the cell for an affected son.
♀ XHXh♂ XHY
- 1Xh × Y
- 2Xh × XH
- 3XH × Y
- 4XH × XH
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives yellow seeds (dominant) or green seeds; gene B gives round seeds (dominant) or wrinkled seeds. Click the one cell for an offspring that is shows both green seeds and wrinkled seeds.
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
Apoptosis is:
- AProgrammed cell death
- BCell fusion
- CCell growth
- DUncontrolled cell division
4.Cell biologyHard
Knudson's "two-hit" hypothesis, based on retinoblastoma, explains that tumour suppressor genes:
- AUsually need both alleles to be inactivated before a tumour develops
- BAre only found in children
- CAre always inherited in mutated form from both the mother and the father
- DCause cancer when one allele is overactive
3Solutions
1.Genetics
Answer: Xh × Y
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
XhY, XhXH, XHY, XHXH
Find the cell
Xh × Y = XhY
Answer
Cell Xh × Y → XhY
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 · Programmed cell death
Why this is the answer
Apoptosis removes damaged or unneeded cells in a controlled way via caspases; necrosis is uncontrolled death with inflammation.
Answer
A · Programmed cell death
4.Cell biology
Answer: A · Usually need both alleles to be inactivated before a tumour develops
Why this is the answer
Loss of one copy of RB1 is not enough; the second allele must also be lost. Children who inherit one mutant copy need only one more hit, so they often get bilateral tumours early.
Answer
A · Usually need both alleles to be inactivated before a tumour develops
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