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
Duchenne muscular dystrophy is X-linked recessive (allele d). A carrier mother, XDXd, has children with an affected father, XdY. The mother's gametes run down the side. Click the cell for an affected son.
♀ XDXd♂ XdY
- 1XD × Xd
- 2XD × Y
- 3Xd × Xd
- 4Xd × Y
2.GeneticsHard
Two AaBb parents are crossed. Gene A gives round seeds (dominant) or wrinkled seeds; gene B gives yellow seeds (dominant) or green seeds. Click the one cell for an offspring that is shows wrinkled seeds and is homozygous for yellow 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
A codon consists of:
- AOne nucleotide
- BA whole gene
- CThree amino acids
- DThree nucleotides on mRNA that code for an amino acid or a stop signal
4.Cell biologyHard
In Tay–Sachs disease, a ganglioside builds up in nerve cells because of a missing enzyme. In which organelle does the build-up occur?
- ASmooth ER
- BPeroxisome
- CMitochondrion
- DLysosome
3Solutions
1.Genetics
Answer: Xd × 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
XDXd, XDY, XdXd, XdY
Find the cell
Xd × Y = XdY
Answer
Cell Xd × Y → XdY
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: D · Three nucleotides on mRNA that code for an amino acid or a stop signal
Why this is the answer
Each triplet of bases specifies an amino acid or stop; the genetic code is redundant (several codons per amino acid).
Answer
D · Three nucleotides on mRNA that code for an amino acid or a stop signal
4.Cell biology
Answer: D · Lysosome
Why this is the answer
Tay–Sachs is a lysosomal storage disease. Lack of hexosaminidase A means GM2 ganglioside cannot be broken down and accumulates in neuronal lysosomes.
Answer
D · Lysosome
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