Chromosomes and genes

1. The nucleus of a cell contains thread-like structures called chromosomes.
2. Each chromosome is made from one very long molecule of DNA that is tightly coiled up.
3. A gene is a small section of that DNA that carries the instructions for one feature.
4. The genes sit along a chromosome in a fixed order, and the fixed position of a gene is called its locus.
5. A single chromosome carries many hundreds or thousands of genes, one after another.

Alleles

1. Body cells contain chromosomes in matching pairs, called homologous pairs.
2. In humans there are $23$ pairs, giving $46$ chromosomes in each body cell.
3. One chromosome of each pair comes from the mother and one from the father.
4. The two chromosomes in a pair carry the same genes at the same loci.
5. A gene can exist in different versions called alleles, and the two alleles at a locus may be the same or different.

Dominant and recessive alleles

1. A dominant allele shows its effect even if only one copy is present.
2. A recessive allele shows its effect only when two copies are present.

Homozygous and heterozygous

1. Homozygous means the two alleles for a gene are the same, such as $BB$ or $bb$.
2. Heterozygous means the two alleles are different, such as $Bb$.
3. A homozygous organism is pure-breeding for that gene because every gamete receives the same allele.

Genotype and phenotype

1. The genotype is the alleles an organism has for a gene.
2. The phenotype is the observable feature that results.
3. The phenotype is set by the genotype together with the environment.

Gametes and zygotes

1. A gamete contains one allele for each gene because it is haploid and carries one chromosome from each homologous pair.
2. At fertilisation, a male gamete and a female gamete fuse. The resulting zygote is diploid, so it contains two alleles for each gene, one inherited from each parent.
3. Different combinations of parental alleles give offspring different genotypes, which can produce different inherited phenotypes.