遗传

A diploid 二倍体 cell (2n) has two full sets of chromosomes 染色体 — one set from each parent. A haploid 单倍体 cell (n) has just one set.

Chromosomes come in homologous chromosomes 同源染色体 pairs: the two chromosomes in a pair are the same length and carry the same genes 基因 (though they may carry different versions of them).

Gametes 配子 (sex cells) must be haploid. If they were diploid, the chromosome number would double at every generation. So gametes are made by a special division, meiosis 减数分裂, which halves the chromosome number.

Meiosis has two divisions, one after the other, so there are eight named stages: prophase I, metaphase I, anaphase I and telophase I, then prophase II, metaphase II, anaphase II and telophase II. As in mitosis, the nuclear envelope 核膜 breaks down, a spindle 纺锤体 forms, and the chromosomes are moved by the spindle.

The stages of meiosis, step by step

Before meiosis starts, every chromosome is copied in interphase, so it is made of two identical sister chromatids 姐妹染色单体 joined at the centromere 着丝粒.

So meiosis I separates the homologous chromosomes (this is where the number halves), and meiosis II separates the chromatids, as in mitosis.

Worked example. A cell has a diploid number of 8 ($2n = 8$). How many chromosomes and chromatids are there in each cell at (a) metaphase I, (b) metaphase II, (c) the end of telophase II?

(a) Metaphase I: nothing has separated yet, so there are 8 chromosomes, each of two chromatids: 16 chromatids. (b) Metaphase II: meiosis I has sent one chromosome of each pair to each cell, so 4 chromosomes, still with two chromatids each: 8 chromatids. (c) After telophase II the chromatids have separated, so each cell has 4 chromosomes of one chromatid each.

Meiosis makes the gametes genetically different from each other in two ways:

• crossing over 交叉互换 — homologous chromosomes swap matching pieces, mixing the alleles.

• independent assortment 自由组合 — the pairs line up in a random order, so each gamete gets a random mix of the parent's chromosomes.

Then at fertilisation 受精, any gamete can fuse with any other. This random fusion makes every new individual genetically different.

You must know these terms exactly:

• a gene is a length of DNA that codes for a protein 蛋白质. Its position on a chromosome is its locus 基因座.
• an allele 等位基因 is one version of a gene.
• a dominant 显性 allele shows its effect even when only one copy is present; a recessive 隐性 allele only shows when two copies are present.
• codominant 共显性 alleles both show their effect together.
• the genotype 基因型 is the alleles an organism has; the phenotype 表现型 is the features you can see.
• homozygous 纯合 means the two alleles are the same; heterozygous 杂合 means they are different.
• a test cross 测交 crosses an organism with the recessive homozygote to find its unknown genotype.

You show a cross with a genetic diagram, often using a Punnett square 庞纳特方格 — a grid that shows all the ways the gametes can combine.

• a monohybrid cross 单基因杂交 follows one gene; a dihybrid cross 双基因杂交 follows two genes at once.
• some genes have multiple alleles 复等位基因 (more than two versions in the population), such as the alleles for human blood groups.
• in sex linkage 伴性遗传, the gene is on the X chromosome, so the result is different for males and females.
• in linkage 连锁, genes on the same autosome 常染色体 (a non-sex chromosome) tend to be inherited together.
• in epistasis 上位性, one gene affects how another gene is shown.

The chi-squared test 卡方检验 compares the results you actually counted (the observed numbers, $O$) with the results you expected from the genetic diagram (the expected numbers, $E$). It tells you whether the difference is small enough to be due to chance, or large enough to mean something else is going on.

You add up, for every group, the squared difference between observed and expected, divided by the expected:

Then you compare this $\chi^2$ value with a critical value from a table. The row you use is set by the degrees of freedom (the number of groups minus 1). If $\chi^2$ is less than the critical value, the difference is not significant — the results fit the expected ratio, and any difference is just chance. If $\chi^2$ is greater than the critical value, the difference is significant, so some other factor is involved.

Worked example. A $\text{Tt} \times \text{Tt}$ cross gives $160$ offspring: $114$ tall and $46$ short. Test whether this fits the expected $3:1$ ratio.

The expected numbers are $\tfrac{3}{4} \times 160 = 120$ tall and $\tfrac{1}{4} \times 160 = 40$ short. Then:

There are $2$ groups, so degrees of freedom $= 2 - 1 = 1$. The critical value at $p = 0.05$ is $3.84$. Because $1.20 < 3.84$, the difference is not significant: the results fit a $3:1$ ratio, and the small difference is due to chance.

A gene codes for a protein, and that protein affects the phenotype. If the gene is faulty, the protein is faulty, and the phenotype changes:

Gibberellin and stem height

In pea plants, the dominant allele Le codes for a working enzyme that makes gibberellin 赤霉素, so the plant grows tall by stem elongation 伸长. The recessive allele le codes for a broken enzyme, so little gibberellin is made and the plant is short.

Not all genes are switched on all the time. Cells control which proteins they make.

• structural genes 结构基因 code for useful proteins such as enzymes; regulatory genes 调节基因 control whether other genes are switched on.
• an inducible enzyme 可诱导酶 is made only when it is needed; a repressible enzyme 可抑制酶 is normally made but can be switched off.

The lac operon

In a prokaryote 原核生物 such as a bacterium, a group of genes called the lac operon 乳糖操纵子 controls the digestion of lactose:

• when there is no lactose, a repressor 阻遏物 protein binds to the operator (a short control sequence within the operon) and blocks transcription 转录, so the lactose-digesting enzymes are not made.
• when lactose is present, it binds to the repressor and pulls it off. Transcription can now happen, and the enzymes are made. The enzymes are therefore inducible.

Control in eukaryotes

In eukaryotes, transcription factors 转录因子 are proteins that bind to DNA and control gene expression 基因表达, by increasing or decreasing the rate of transcription.

Gibberellin switches genes on in this way: it causes the breakdown of DELLA protein repressors. These DELLA proteins normally block the factors that turn on transcription, so removing them lets those genes be expressed.

Common mistakes.

• Saying the chromosome number halves in meiosis II. It halves in meiosis I, when the homologous pairs separate; meiosis II separates sister chromatids.
• Mixing up homologous chromosomes and sister chromatids. Homologous chromosomes come from different parents and may carry different alleles; sister chromatids are identical copies joined at a centromere.
• Saying crossing over happens between sister chromatids. It happens between chromatids of the two homologous chromosomes in a bivalent; swapping identical pieces would change nothing.
• Writing a gene and an allele as the same thing. The gene is for a feature (eye colour); the alleles are its versions (blue, brown).
• In a chi-squared test, using percentages or ratios instead of the numbers of offspring, or comparing with 3.84 when there is more than one degree of freedom.

• Set out a cross fully: parental genotypes → gametes (in circles) → Punnett square → offspring ratio and phenotypes.
• Use the chi-squared test to compare observed with expected; degrees of freedom $=$ classes $- 1$, compare with $3.84$ at $p = 0.05$.
• Recognise codominance, multiple alleles, sex linkage, epistasis and linkage — each alters the expected ratio.
• Meiosis creates variation by crossing over and independent assortment — state both.