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 purple flowers (dominant) or white flowers; gene B gives tall plants (dominant) or short plants. Click the one cell for an offspring that is homozygous for purple flowers and shows short 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 does it mean that the genetic code is "degenerate" (redundant)?
- ASome codons are never used
- BMost amino acids are coded by more than one codon
- CThe code differs completely between species
- DEach codon codes for several amino acids
4.Cell biologyHard
A double-stranded DNA sample contains 30% adenine. What percentage is guanine?
- A40%
- B20%
- C70%
- D30%
3Solutions
1.Genetics
Answer: P = 0.0625
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) = 0.25
P(wrinkled seeds) = 0.25
Multiply
P = 0.25·0.25 = 0.0625
Answer
P = 0.0625
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 · Most amino acids are coded by more than one codon
Why this is the answer
Several codons can code for the same amino acid, often differing in the third base. This is why many point mutations are silent.
Answer
B · Most amino acids are coded by more than one codon
4.Cell biology
Answer: B · 20%
Why this is the answer
A = T = 30%, so A + T = 60%. The remaining 40% is split equally between G and C, so G = 20%.
Answer
B · 20%
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