Genes and alleles

1. A gene is a section of DNA carrying the instructions for one protein, and it sits at a fixed position on a chromosome called its locus.
2. Because chromosomes come in homologous pairs, a body cell carries two copies of every gene, one on each chromosome of the pair.
3. The two copies may be the same version or two different versions, and each version is called an allele.
4. New alleles arise when a mutation changes the base sequence of a gene, which changes the instruction and often changes the protein made from it.
5. A gene may have many alleles across a whole population, but any one diploid individual carries only two of them.
6. Alleles are written as letters, and both alleles of the same gene are written with the same letter, for example $T$ and $t$.

Dominant and recessive alleles

1. A dominant allele determines the phenotype whenever it is present, so one copy is enough for its effect to show.
2. A recessive allele determines the phenotype only when no dominant allele of that gene is present, so two copies are needed.
3. Dominant alleles are written as capital letters and recessive alleles as the matching lower case letter, so $T$ and $t$ are two alleles of one gene.
4. Writing $T$ alongside $s$ would suggest two separate genes rather than two versions of the same gene, so the letters must always match.
5. When a dominant and a recessive allele are present together, the protein coded by the dominant allele is enough to produce the dominant characteristic.
6. A recessive characteristic can therefore disappear for a generation and reappear later, whenever two recessive alleles meet in the same individual.

Genotype and phenotype

1. The genotype is the pair of alleles an individual carries for a gene, written as two letters such as $TT$, $Tt$ or $tt$.
2. The phenotype is the characteristic that can be seen or measured, such as a tall stem or a short stem.
3. The genotype decides which proteins the cell can make, and those proteins, together with the conditions the organism grows in, produce the phenotype.
4. Two individuals with different genotypes can share one phenotype, because $TT$ and $Tt$ both give a tall stem.
5. The recessive phenotype has only one possible genotype, because a short stem can only come from $tt$.

Homozygous and heterozygous

1. An individual is homozygous for a gene when its two alleles are identical, written $TT$ for homozygous dominant and $tt$ for homozygous recessive.
2. An individual is heterozygous when its two alleles are different, written $Tt$.
3. A homozygous individual can pass on only one kind of allele, so it is described as pure-breeding for that characteristic.
4. A heterozygous individual can pass on either of its two alleles, so its offspring are not all alike.
5. A heterozygous individual that shows the dominant characteristic while carrying a recessive allele is a carrier of that allele.
6. A carrier passes the recessive allele on unseen, which is how a recessive characteristic can appear in offspring whose parents do not show it.

How alleles pass to offspring

1. Meiosis separates the two alleles of a gene, because the homologous chromosomes carrying them are pulled into different gametes.
2. Every gamete therefore carries one allele of the gene and never both.
3. A homozygous $TT$ parent makes only $T$ gametes, and a homozygous $tt$ parent makes only $t$ gametes.
4. A heterozygous $Tt$ parent makes $T$ gametes and $t$ gametes in equal numbers.
5. At fertilisation one allele carried by the male gamete joins one allele carried by the female gamete, which rebuilds the pair in the zygote.
6. A cross that follows one gene through this route is a monohybrid cross, and it is set out as a genetic diagram.
7. A genetic diagram is written in a fixed order, and every line of it earns credit.
   1. Line 1: the phenotypes of the two parents, described in words.
   2. Line 2: the genotypes of the two parents, written as pairs of allele letters.
   3. Line 3: the gametes each parent can make, written as single alleles.
   4. Line 4: the offspring genotypes produced by combining those gametes.
   5. Line 5: the offspring phenotypes that follow from those genotypes.

Crosses between the genotypes

1. A cross between two homozygous parents, $TT\times tt$, gives every offspring the genotype $Tt$, so all of them show the dominant characteristic.
2. A cross between two heterozygous parents, $Tt\times Tt$, gives offspring of genotype $TT$, $Tt$, $Tt$ and $tt$.
3. Three of those four offspring carry at least one $T$ and show the dominant characteristic, while the fourth is $tt$ and shows the recessive characteristic.
4. A cross between a heterozygote and a homozygous recessive parent, $Tt\times tt$, gives offspring of genotype $Tt$, $Tt$, $tt$ and $tt$.
5. Half of those offspring show the dominant characteristic and half show the recessive characteristic.
6. Any cross involving a homozygous dominant parent, such as $TT\times Tt$, gives no recessive offspring at all, because every offspring receives at least one $T$.
7. The recessive characteristic appears only in an offspring that receives a recessive allele from both parents at once.