Thus, in Humans, male and not the female determine the sex of the offspring.

LIVINGS

In

BIOLOGY

FOR

GENERAL SECONDARY CERTIFICATE

Unit (II)

Chapter (3)

The Inheritance in Livings

Sex determination in livings

Sex determination

The sex on an individual is determined by a special type of chromosomes called sex chromosomes. Usually one or two of these sex chromosomes exist in each cell of the individual, and the rest of the chromosomes are called autosomes.

There is one system of sex determination in Man, mammals, Drosophila, and other animals.

- In the female cells there are two identical sex chromosomes; each is donated by (X). Thus, the sex chromosomal structure in the female is XX.

- In the male cells there are one (X) chromosome, and another one which differs in shape, and size and is donated by (Y). Thus, the sex chromosomal structure of the male is XY.

The chromosome (Y) is a main constituent of sex determination. If the chromosome Y is present, the individual will be a male, if it is absent, the individual will be a female.

Sex determination in fruit fly (Drosophila):

- The female cells contain four identical pairs of chromosomes, one of the is straight which is the sex chromosomes XX. The chromosomal structure of the female Drosophila is (6+XX). During meiosis, the female produces one type of gametes, which is (3+X).

- In male cells, one sex chromosome is X, while the other one has a bent end and is called Y chromosome. The chromosomal structure of the male Drosophila is (6+XY). During meiosis, the male produces two different types of gametes in equal ratio which are (3+X), and (3+Y).

Upon fertilization, if the ovum is fertilized by a sperm containing X chromosome, a zygote containing XX that grows to a female develops. Fertilization by a sperm containing Y chromosome results in a male XY.

Sex determination in Man:

There are 46 chromosomes in each human somatic cell.

- The chromosomal structure of the female somatic cell is (44+XX). When meiosis occurs during gamete formation, The chromosomal structure of the ova produced is (22+X)

- The chromosomal structure of the male somatic cell is (44+XY). When meiosis occurs during gamete formation, half of the sperms produced has the chromosomal structure (22+X), while the other half has the chromosomal structure (22+Y).

Thus, in Humans, male and not the female determine the sex of the offspring.

There are other systems of sex determination in livings, as that shown in the following table:

There is no one single case in Man or other mammals that lack the sex chromosome X, since life requires at least one of such chromosome. The chromosome Y is not essential for life, since females don’t have it.

Action of genes responsible for sex determination:

The genes responsible for sex determination and carried on the sex chromosomes X, and Y act only in the first months of Human embryonic development:

- After six weeks of pregnancy, the embryo begins the production of male hormones if the Y chromosome is present. These hormones induce the gonad tissues, not yet developed until that time, to produce testes. The genital organs differentiate and the other secondary sexual signs begin to develop under the direct influence of the male sexual hormones formed by the testes.

- If the embryo does not contain the Y chromosome, his genital organs begin to differentiate to a female by the twelfth week, and continue growth which is followed afterwards by the development of the secondary sexual organs under the effect of hormones, and the direction of the genes.

N.B.: