Cells, genes and DNA
Four ideas that carry most of this unit. Nearly every question is really asking you to keep track of chromosomes.
Mitosis and meiosis
The idea: mitosis makes two identical copies for growth and repair. Meiosis makes four different gametes with half the chromosomes, so fertilisation lands back at the full number.
Humans have 46 chromosomes in a body cell — 23 pairs. Gametes have 23. If gametes had 46, every generation would double, so meiosis has to halve the count.
The two things people mix up
- Anaphase I pulls whole homologous pairs apart — that is the step that halves the count. Anaphase II pulls sister chromatids apart, like mitosis does.
- Crossing over happens in prophase I, when homologues pair up as tetrads. That pairing is what makes the exchange possible, and it is a major source of variation.
Watch out: after meiosis I, a cell with 2n = 12 has 6 chromosomes, but each is still two sister chromatids. Halving the chromosome number is not the same as halving the DNA content.
Punnett squares
The idea: write one parent's possible gametes across the top and the other's down the side, then fill in the boxes. A dominant allele shows whenever it is present.
Capital letter means dominant, lowercase means recessive. TT and tt are homozygous (two the same), Tt is heterozygous. Tall (T) beats short (t), so both TT and Tt look tall — only tt is short.
Worked example: Tt × Tt
Notice the genotype ratio is 1 TT : 2 Tt : 1 tt, but the phenotype ratio is 3 : 1, because TT and Tt look the same.
Ratios worth memorising
- Tt × Tt → 3 : 1 (the classic monohybrid)
- Tt × tt → 1 : 1 (a testcross)
- TtYy × TtYy → 9 : 3 : 3 : 1 (dihybrid, both heterozygous)
- TtYy × ttyy → 1 : 1 : 1 : 1 (dihybrid testcross)
Two genes at once, without a 16-box grid
For a dihybrid question you rarely need the whole square. Handle each gene separately and multiply the fractions. For AaBb × aaBb, the chance of aaBB is (Aa × aa gives ½ aa) × (Bb × Bb gives ¼ BB) = ⅛.
Watch out: giving the genotype ratio when the question asked for the phenotype ratio. Read which one it wants.
When it is not simple dominance
The idea: not every pair of alleles has one that wins outright. Three exceptions cover almost everything you will be asked.
Incomplete dominance
The heterozygote is a blend. A red snapdragon (RR) crossed with a white one (WW) gives all pink (RW). Cross two pinks and you get 1 red : 2 pink : 1 white — the phenotype ratio now matches the genotype ratio, because every genotype looks different.
Codominance
Both alleles show fully, side by side rather than blended — think a red-and-white patched flower, or blood type AB where both the A and B markers are present.
Multiple alleles: blood type
Three alleles exist: IA, IB and i. IA and IB are codominant with each other and both dominant over i. A type A father (IAi) and a type B mother (IBi) can have children of all four types — IAIB (AB), IAi (A), IBi (B) and ii (O), each with probability ¼.
Sex linkage
Genes on the X chromosome behave differently in each sex. A son gets his only X from his mother, so a recessive allele there has nothing to mask it. That is why colour blindness and haemophilia show up far more often in males.
For a carrier mother (XHXh) and an unaffected father (XHY), the four outcomes are XHXH, XHXh, XHY and XhY. So half the sons are affected, which is a quarter of all children.
Watch out: "half the sons" and "a quarter of the children" are both right — but they answer different questions. Check whether the question says sons or children.
From DNA to protein
The idea: DNA is transcribed into mRNA, and mRNA is translated into a chain of amino acids. Three bases (one codon) code for one amino acid.
DNA pairs A–T and C–G. RNA has no thymine — it uses uracil, so when transcribing, A on the template becomes U on the mRNA.
Counting amino acids
Divide the number of bases by three. A template of 9 bases gives 3 codons, so 3 amino acids. It really is that direct.
The enzymes worth knowing
- Helicase unwinds the double helix and breaks the hydrogen bonds between the strands.
- DNA polymerase builds the new complementary strands.
- Ligase seals the gaps between fragments on the lagging strand.
- RNA polymerase does transcription, building mRNA from the DNA template.
Watch out: writing thymine into an mRNA sequence. If you see a T in something labelled RNA, it is wrong — it should be U.