遗传

Chromosomes 染色体 are made of DNA, which carries the genetic information in units called genes 基因.

• A gene is a length of DNA that codes 编码 for one protein 蛋白质.
• An allele 等位基因 is an alternative form of a gene. For example, a gene for eye colour may have a brown allele and a blue allele.

Sex chromosomes

Sex is inherited through the sex chromosomes 性染色体. Females are XX and males are XY. An egg always carries an X; a sperm carries either an X or a Y, so the sperm decides the baby's sex. This gives a 1:1 ratio 比例 of girls to boys.

The base sequence 碱基序列 of a gene sets the order of amino acids 氨基酸 in a protein. A different order of amino acids folds the protein into a different shape, and the shape decides its job. So DNA controls the cell by controlling which proteins are made — including enzymes 酶, membrane carriers and receptor proteins 受体蛋白 for neurotransmitters 神经递质.

Most body cells hold the same genes, but each cell only expresses 表达 (switches on) the genes it needs, so it makes only the proteins for its own job.

In the eukaryotic cell model here, the gene stays in the nucleus 细胞核, but proteins are made in the cytoplasm 细胞质. The link between them is messenger 信使 RNA (mRNA):

1. mRNA is made in the nucleus as a copy of the gene.
2. the mRNA moves out into the cytoplasm and passes through a ribosome 核糖体.
3. the ribosome reads the mRNA's bases 碱基 and joins amino acids in the matching order to build the protein.

Body cells are diploid 二倍体 (two sets of chromosomes; humans have 23 pairs). Gametes are haploid 单倍体 (one set). Cells make new cells by nuclear division 分裂, of two kinds.

Mitosis 有丝分裂:

• makes two cells that are genetically identical to the parent cell.
• the chromosomes are copied exactly (replication 复制) before division, so each daughter cell 子细胞 keeps the full chromosome number.
• it is used for growth, repair of tissues, replacing old cells, and asexual reproduction 无性生殖.
• stem cells 干细胞 are unspecialised cells that divide by mitosis; their daughter cells can then become specialised.

Meiosis 减数分裂:

• makes gametes 配子.
• it is a reduction division: the chromosome number is halved, from diploid to haploid.
• the cells it makes are genetically different from one another.

Inheritance 遗传 is the passing of genetic information from one generation to the next. Learn these words:

To predict the offspring of a cross 杂交 involving one gene, use a genetic diagram or a Punnett square 庞纳特方格:

• write each parent's genotype, then the gametes (each gamete carries one allele).
• combine them in a grid to find the possible offspring genotypes and phenotypes.

Two heterozygous parents (Bb × Bb) give a 3:1 ratio of dominant to recessive. A heterozygous crossed with a homozygous recessive (Bb × bb) gives a 1:1 ratio. You can also read a pedigree diagram 系谱图 (a family tree) to follow a feature and work out genotypes.

(Supplement) A test cross 测交 finds an unknown genotype: cross the unknown with a homozygous recessive. If any offspring show the recessive feature, the unknown must have been heterozygous.

Worked example. Brown eyes (B) are dominant to blue (b). Two brown-eyed parents have a blue-eyed child. Give the parents' genotypes, and the chance that their next child has blue eyes. Start from the child: blue is recessive, so the child must be bb, and it got one b from each parent. So each parent carries a b. But both parents show brown eyes, so each must also carry a B: both parents are Bb. The Punnett square for Bb × Bb gives BB, Bb, Bb and bb, so the chance of a blue-eyed child is 1 in 4 (25%). Always work backwards from the recessive individual - a recessive phenotype pins a genotype exactly, and the rest follows. The 25% applies to each child on its own: three brown-eyed children do not make the fourth blue.

In codominance 共显性, both alleles in a heterozygous organism show in the phenotype.

The ABO blood groups 血型 are an example. The alleles are $I^A$, $I^B$ and $I^o$. The alleles $I^A$ and $I^B$ are codominant, while $I^o$ is recessive:

• $I^A I^A$ or $I^A I^o$ → group A
• $I^B I^B$ or $I^B I^o$ → group B
• $I^A I^B$ → group AB (both alleles shown)
• $I^o I^o$ → group O

A sex-linked 伴性遗传 characteristic is controlled by a gene on a sex chromosome (usually the X). Because males have only one X chromosome, a recessive allele on it always shows in males, so the feature is more common in males than in females.

Red-green colour blindness 色盲 is an example: the recessive allele is on the X chromosome, so it is much more common in boys than in girls.

Common mistakes.

• Mixing up gene and allele. A gene is the length of DNA for one protein (eye colour); an allele is one form of it (blue or brown).
• Writing a Punnett square without the gametes. Show each parent's genotype, then the gametes (one allele each), then the offspring.
• Reading a 3:1 ratio as a promise. It gives the chance for each child: a 1 in 4 chance of blue eyes does not mean exactly one child in four will have them.
• Using different letters for the two alleles, such as B and r. Use the same letter for one gene: capital for dominant, small for recessive (B and b).
• Thinking a dominant allele is more common or "stronger". Dominant only means it shows with one copy; a dominant allele can be rare.
• Saying mitosis makes gametes. Mitosis makes identical diploid cells for growth and repair; meiosis makes the haploid gametes, which are all different.
• Saying the mother decides a baby's sex. Every egg carries an X; the sperm carries X or Y, so it decides.
• Calling group AB "a blend" of A and B. In codominance both alleles show fully in the phenotype: $I^A I^B$ is group AB, not something in between.

• Gene = a length of DNA coding for a protein. Allele = one form of a gene.
• Genotype = the alleles present; phenotype = what you see. Homozygous = two same; heterozygous = two different.
• Dominant shows with one copy; recessive needs two copies.
• Learn to draw a Punnett square. Bb × Bb → 3:1; Bb × bb → 1:1.
• (Supplement) Codominance: both alleles show (blood group AB). Sex linkage: a gene on the X, more common in boys (colour blindness).