DNA, genes and chromosomes

1. DNA stores genetic information in its base sequence, the order of its bases.
2. In a plant or animal cell, most DNA is packaged with proteins into chromosomes in the nucleus.
3. A chromosome contains many genes, each occupying a particular position along its DNA.
4. The base sequence of a protein-coding gene determines the order of amino acids in a protein.
5. Proteins include enzymes, structural components and some hormones, so their activities help produce an organism's characteristics.
6. The genome includes both coding DNA and non-coding DNA, rather than only the genes that code for proteins.
7. Mitochondria also contain a small amount of DNA, and plant chloroplasts have DNA, so an organism's genetic material is not confined to its nuclear chromosomes.
8. Most nucleated body cells of an individual contain the same genetic information, although different cell types use different genes.

Alleles and characteristics

1. Individuals of the same species usually have the same types of genes but may carry different alleles of those genes.
2. For a gene on an ordinary chromosome pair, an individual inherits one allele from each parent.
3. A genotype describes the alleles present, whereas a phenotype describes the resulting observable or measurable characteristics.
4. A phenotype can include a physical feature, such as flower colour, or a biochemical characteristic, such as a blood group.
5. An organism's environment can also influence its phenotype, so knowing its genotype does not always predict its exact appearance or size.

Describing allele combinations

1. For a simple gene with a dominant and a recessive allele, use the same letter in upper and lower case, such as $A$ and $a$.
2. The genotypes $AA$ and $aa$ are homozygous because each contains two copies of the same allele.
3. The genotype $Aa$ is heterozygous because it contains two different alleles.
4. With complete dominance, both $AA$ and $Aa$ produce the dominant phenotype, whereas $aa$ produces the recessive phenotype.
5. A recessive allele is still present in a heterozygote and can be inherited even though it does not determine that individual's phenotype.

Gametes carry inherited information

1. Animal gametes are sperm cells and egg cells, which carry genetic information from parents to offspring.
2. The nucleus of each gamete carries one allele of each gene on an ordinary chromosome pair.
3. When gamete nuclei fuse at fertilisation, their genetic information combines in the first cell of the offspring.
4. This explains why an offspring inherits alleles from both parents while retaining its own combination of genetic information.