IGCSE Biology Inheritance notes

Chromosomes are structures made of DNA, which contains genetic information in the form of genes. A gene is a length of DNA that codes for a protein; an allele is an alternative form of a gene — for example, the gene for pea height has a tall allele and a short allele. Inheritance is the transmission of genetic information from generation to generation.

Supplement — how DNA controls the cell: the sequence of bases in a gene determines the sequence of amino acids in a protein, and different amino-acid sequences give proteins different shapes — which is what makes each enzyme, membrane carrier or neurotransmitter receptor work. How a protein is made: the gene remains in the nucleus; messenger RNA (mRNA) is made as a copy of the gene in the nucleus and moves to the cytoplasm; the mRNA passes through ribosomes, which assemble amino acids into a protein molecule in the order the mRNA bases specify.

Supplement — same genes, different jobs: most body cells contain the same genes, but a cell expresses only the genes it needs — a muscle cell makes muscle proteins, not digestive enzymes — so many genes are switched off in any particular cell. Supplement — chromosome sets: a haploid nucleus contains a single set of chromosomes (gametes); a diploid nucleus contains two sets, one from each parent — in a human diploid cell, 23 pairs of chromosomes.

Human sex is decided by the sex chromosomes: females are XX, males are XY. A Punnett square for XX × XY shows why every pregnancy is an even chance: the mother’s egg always carries an X; the father’s sperm carries either an X or a Y — so offspring are XX (female) or XY (male) in a 1:1 ratio. It is the father’s sperm that determines the sex of a child.

Supplement. Mitosis is nuclear division giving rise to genetically identical cells. Before mitosis, each chromosome replicates exactly; during mitosis the copies separate, so each daughter cell keeps the same chromosome number as the parent. Mitosis is used for growth, repair of damaged tissues, replacement of cells and asexual reproduction.

Meiosis is the division involved in the production of gametes: a reduction division in which the chromosome number is halved from diploid to haploid, and the resulting cells are genetically different from each other. Together with fertilisation, meiosis keeps the chromosome number constant between generations: diploid parent → haploid gametes → diploid zygote.

Stem cells are unspecialised cells that divide by mitosis to produce daughter cells which can become specialised for specific functions — nerve, muscle, blood cells and so on. That specialisation potential is why stem cells matter in medicine.

Convention: the dominant allele is written as a capital letter (T = tall), the recessive as the same letter lower-case (t = short). A pedigree diagram (family tree) can be read with this vocabulary: a recessive characteristic can appear in a child of two unaffected parents — both must be heterozygous carriers.

A monohybrid cross follows one gene. Worked cross — heterozygote × heterozygote (Hh × Hh), where H is dominant: each parent makes H and h gametes in equal numbers. The Punnett square fills in four equally likely combinations:

Genotypes: HH : Hh : hh = 1 : 2 : 1. Phenotypes: HH, Hh and Hh all show the dominant feature, so dominant : recessive = 3 : 1. Any heterozygote × heterozygote cross gives this ratio.

Worked cross — heterozygote × homozygous recessive (Hh × hh): gametes H, h from one parent and h, h from the other. Combinations: Hh, Hh, hh, hh — genotypes 1:1, phenotypes dominant : recessive = 1 : 1. This is also the shape of a test cross (Supplement): to identify an unknown dominant-phenotype plant, cross it with a homozygous recessive. All dominant offspring → the unknown was homozygous dominant (HH); offspring in a 1:1 mix → the unknown was heterozygous (Hh).

Supplement. Codominance is a situation in which both alleles in heterozygous organisms contribute to the phenotype — neither is dominant. Classic example: snapdragon flowers. Alleles C^R (red) and C^W (white); a C^R C^W heterozygote is pink — both alleles contribute. A cross of two pinks (C^R C^W × C^R C^W) gives genotypes C^R C^R : C^R C^W : C^W C^W = 1 : 2 : 1 and phenotypes red : pink : white = 1 : 2 : 1.

The ABO blood groups use the same idea with three alleles: Iᴬ and Iᴮ are codominant with each other, and Iᵒ is recessive to both. Phenotypes: A (IᴬIᴬ or IᴬIᵒ), B (IᴮIᴮ or IᴮIᵒ), AB (IᴬIᴮ), O (IᵒIᵒ).

This cross gives all four phenotypes equally: AB : A : B : O = 1 : 1 : 1 : 1 — a group-A and a group-B parent can have a child of any blood group if both are heterozygous.

Supplement. A sex-linked characteristic is a feature in which the gene responsible is located on a sex chromosome, making the characteristic more common in one sex than the other. Red-green colour blindness is the set example: a recessive allele on the X chromosome. Females (XX) have a second X as a backup — a carrier (X^N X^n) sees colours normally. Males (XY) have only one X, so a single recessive allele (X^n Y) is expressed — which is why colour blindness is much more common in males.

Reading the square: daughters all see normally (one is a carrier); sons split 1 normal : 1 colour-blind. Quote the sex-specific ratios rather than one blended number — a quarter of all children (half of the sons) are expected to be colour-blind.