The menstrual cycle is the cyclic change in the reproductive system of a human female, of about 28 days. Its phases are: (1) Menstrual phase (days 1-5) - the uterine lining breaks down and is shed as menstrual flow. (2) Follicular (proliferative) phase (days 6-13) - under FSH the ovarian follicle matures and the uterine lining is rebuilt, and oestrogen rises. (3) Ovulation (about day 14) - a surge of LH causes the release of the ovum from the ovary. (4) Luteal (secretory) phase (days 15-28) - the corpus luteum secretes progesterone, which maintains the uterine lining; if fertilisation does not occur, it degenerates and the next cycle begins.

In a monohybrid cross, a pure tall plant (TT) is crossed with a pure dwarf plant (tt). All the F1 plants are Tt and are tall, because T (tall) is dominant over t (dwarf). When the F1 plants (Tt) are self-pollinated, the gametes T and t combine in all possible ways to give the F2 generation: TT, Tt, Tt and tt. This gives a phenotypic ratio of 3 tall : 1 dwarf and a genotypic ratio of 1 TT : 2 Tt : 1 tt.

In a dihybrid cross, two pairs of contrasting characters are studied together (for example, seed shape and seed colour in peas: round yellow RRYY crossed with wrinkled green rryy). All F1 plants are round and yellow (RrYy). When the F1 are self-pollinated, the F2 generation shows four phenotypes in the ratio 9 (round yellow) : 3 (round green) : 3 (wrinkled yellow) : 1 (wrinkled green). This 9 : 3 : 3 : 1 ratio illustrates Mendel's law of independent assortment.

F2 dihybrid ratio = 9 : 3 : 3 : 1