From factors to genes
Mendel said a characteristic is controlled by a pair of factors, without being able to say what a factor was. Half a century later it had a name, a location and a chemical identity.
Gene
What Mendel called a factor is now called a gene — a unit that controls a characteristic and is passed from parent to offspring.
Alleles
The two forms a gene can take are called alleles of each other. T and t are not two genes; they are two alleles of the one gene for stem height.
Where the pair comes from
Genes sit on chromosomes, and chromosomes come in homologous pairs — which you met in the chapter on cell division. That is why genes come in pairs at all:
- Each homologous pair of chromosomes carries the gene for a given characteristic at the same position on both chromosomes.
- One chromosome of the pair came from the mother, the other from the father, so one allele came from each parent.
- The two alleles may be the same or they may be different. That is the only choice there is, and the next page gives the two cases their names.
Mendel's pair was the chromosome pair all along
Gametes carry one factor of each pair because meiosis puts one chromosome of each homologous pair into each gamete. Mendel deduced the behaviour from counting peas, decades before anyone watched chromosomes do exactly that under a microscope.
Two independent lines of evidence, one answer
Mendel counted plants and concluded that something comes in pairs and separates when gametes form. Cytologists stained cells and watched chromosomes come in pairs and separate when gametes form. Neither knew of the other's work at first — and they were describing the same thing.