Male or Female
The testes of the bull and the ovaries of the cow produce reproductive cells by a special series of cellular divisions that halve the normal number of chromosomes in a cell. The spermatozoa and the ova contain only one member of a chromosome pair. Thus the cells of cows and bulls contain 60 chromosomes (2n = 60), but the spermatozoa in the semen and the ova in the ovaries contain only 30 chromosomes (n = 30, Figure 2). The two basic principles of the transmission of a trait (e.g., sex) are as follows
1) Separation of the paired chromosomes during the formation of reproductive cells;
2) Union of a spermatozoon with an ovum to create a new cell with aunique set of chromosomes.For 29 of the chromosome pairs, both members are visually identical. However, for one of the pairs, one member is much longer; it is called the X chromosome, and the shorter member is called the Y chromosome. All the ova carry the X chromosome, but the spermatozoa can carry either the X or the Y chromosome.
During cellular division to form the reproductive cells, each member of a chromosome pair goes into a separate cell. As a result, 50% of the spermatozoa will carry the X chromosome and the other 50%, the Y chromosome. If by chance a spermatozoon carrying a Y chromosome fertilizes an ovum, the offspring will be a male. However, an offspring that receives two X chromosomes develops into a female (Figure 2). It is important to realize that it is impossible to predict the sex of an offspring at the time of mating (insemination); however, we can predict that, on the average, 50% of all the offspring will be male and 50% will be female.
