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
red-green colour blindness is X-linked recessive (allele b). A carrier mother, XBXb, has children with an unaffected father, XBY. The mother's gametes run down the side. Click the cell for a carrier daughter.
♀ XBXb♂ XBY
- 1XB × XB
- 2XB × Y
- 3Xb × XB
- 4Xb × Y
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 shows both short plants and 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
Why does inserting a single nucleotide into a coding sequence usually have a large effect?
- AIt changes the sugar in the DNA backbone
- BIt always changes one amino acid only
- CIt shifts the reading frame of all following codons
- DIt stops the gene from being transcribed into messenger RNA
4.Cell biologyHard
Why can translation in bacteria begin before transcription of the mRNA is finished?
- ABacterial ribosomes can read the DNA template directly in the cytoplasm
- BThere is no nuclear envelope separating DNA from ribosomes
- CBacterial mRNA is spliced faster
- DBacteria do not use mRNA
3Solutions
1.Genetics
Answer: Xb × XB
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
XBXB, XBY, XbXB, XbY
Find the cell
Xb × XB = XbXB
Answer
Cell Xb × XB → XbXB
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: C · It shifts the reading frame of all following codons
Why this is the answer
An insertion that is not a multiple of three causes a frameshift. Every codon downstream is read differently, often soon reaching a stop codon.
Answer
C · It shifts the reading frame of all following codons
4.Cell biology
Answer: B · There is no nuclear envelope separating DNA from ribosomes
Why this is the answer
In prokaryotes DNA lies in the cytoplasm, so ribosomes can attach to mRNA while it is still being made. In eukaryotes mRNA must be processed and exported first.
Answer
B · There is no nuclear envelope separating DNA from ribosomes
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