1. to understand what is meant about being able to determine a ratio of occurrence of traits in offspring The expression of traits in offspring can be predicted The expression is determined by paired factors or alleles or genes located at homologous locations on paired chromosomes The exact location of the alleles for many traits has been determined by researchers and the process is called gene mapping.
Mendel did not know about genes andalleles but he did work out ratios representing the expression of traits. He determined that traits are represented by pairs and that in the germ cells of each parent only half of the pair is found because of meiosis where half of the homologous pair or chromosomes is present. For each trait he assigned a pair of letters, for each gamete—a single letter. He then devised a scheme to show all of the possibilities of combinations of germ cells from two parents.
The same scheme can be used for human traits that come in pairs. However some traits are represented by more than a pair of alleles such as blood type which has three loci. In humans there are 22 pairs of autosomes and one pair of sex chromosomes. The twenty-third pair of chromosomes in females show XX in the karyotype and in the male—XY.
The germ cell of the female can therefore only be X but the germ cell of the male can be X or Y. For each mating therefore there is a 50/50 chance that the offspring will be male and a 50/50 chance of a female. To have two of the same sex consecutively the ratio is represented as 1/2 x 1/2 = 1/4 possibility.
(figure available in print form)