SEX DETERMINATION

In humans and other mammals, gonadal sex (testes or ovaries) is _________________________ determined at fertilization by _________________________________________.

Human Sex Chromosomes:

• Humans have _____ chromosomes in each cell (except for the gametes and red blood cells), consisting of ________ matched-pairs, an equal half from each parent. The first 22 chromosome pairs are numerically labelled according to size, with 1 being the largest and 22 the smallest. The final pair consists of the sex chromosomes: ______ and ______.  Females normally possess two X chromosomes (______), and males possess one X and one Y chromosome (________).
• Most of the chromosomes contain 800-2000 genes, including the X chromosome, which contains about _______ genes; however, the Y chromosome appears to possess only about ______ genes.
• In females, each egg cell possesses ____________ chromosome (along with the 22 non-sex chromosomes).
• In males, however, ________________________ of sperm cells are produced: ~50% of the sperm possess an X chromosome and ~50% possess a Y chromosome (along with the 22 non-sex chromosomes)
• The sex of an offspring is determined by the ___________________________ that fertilizes the egg.  If the sperm that fertilizes the egg carries an X chromosome, the resulting offspring will be female (X from sperm + X from egg = XX).  If, on the other hand, a Y-bearing sperm fertilizes the egg, the XY combination results in the development of a male offspring. (See Figure 1)
• Thus, in humans (as in other mammals), it is the ____________ parent that determines the sex of the offspring.

If a Y Chromosome is inherited:

• The Y chromosome contains a gene referred to as _________ (sex-determining region), which has been shown to code for a protein called ________ (testis-determining factor).
• At 6-7 weeks gestation, TDF acts like an internal switch by triggering the "_______________________________" embryonic gonad to develop into testes, thereby initiating male development.
• Other genes on the ___________________ appear to code for proteins involved in spermatogenesis and spermiogenesis.
• The remainder of male sexual differentiation and development is governed by genes that are _______________ chromosome, and also requires __________________________ secretion from the testes, as well as reception and assimilation by target tissues.

If a Y Chromosome is not inherited:

·  Then there is no SRY gene, so TDF is not produced.  In this case, the "________________________________" embryonic gonad automatically begins developing as an ovary at about the 10th week of gestation, resulting in female development.   (Exceptions can occur when the SRY region of the Y has been translocated to another chromosome in the father's sperm, in which case the resulting embryo could develop as a male in the absence of a Y chromosome.)

·  Current research indicates that _________________ is NOT needed to initiate or complete female sexual development in the fetus. 

If an X Chromosome is not inherited (see lecture on nondisjunction):

·  Absence of an X chromosome is fatal - it results in a nearly immediate spontaneous __________________ of the fertilized egg.

·  Thus, an X chromosome is a necessary prerequisite for life, whereas a Y chromosome appears to be needed only for ______________________________________  and _____________________________________________.

Mechanisms of Sex Determination Vary Among Animals:

• Mammals & Some Lizards: male determines the sex of the offspring (sperm contain X or Y sex chromosome, eggs contain only X sex chromosome)
• Birds & Snakes: ___________________ determines the sex of the offspring (opposite of mammals) (See Figure 2)
• Some Insects: females are XX, males are X (sex is determined by # of chromosomes, not by type)
• Turtles, Crocodiles, Some Lizards & Most Fishes: sex is determined by the ______________________________(physical or social), not by chromosomes.  
• Is the human Y chromosome shrinking in size and will it eventually disappear altogether?  Click here for more info

Human Sex Ratio:

• Since there are equal numbers of X and Y sperm cells, there should be an ___________ ______________________ of conceiving a male or female offspring. However, for unknown reasons, the ratio of males to females at birth is 105:100.
• The male-biased sex ratio is even greater at conception, since more male fetuses are lost through miscarriage than female fetuses.
• New biomedical technology has the potential to skew the natural ratio. (See http://www.microsort.com/)

Definitions of Sex and Gender are not Absolute
There are several factors that need to be considered when defining the sex and gender of an individual:

• ______________________ _________: possession or absence of a Y sex chromosome (typically XX or XY)
• ______________________ _________: possession of ovaries or testes
• ______________________ _________: predominance of estrogens or testosterone
• _________________ ________________________ ___________________: e.g. uterus and vagina, or prostate and seminal vesicles
• _________________ ________________________ ____________________: e.g. clitoris and vaginal opening, or penis and scrotum
• ____________________ _____________________: the gender in which the child is reared – the gender the parents and society believe the child to be, based primarily on appearance of external genitalia
• ____________________ ______________________: one’s private, internal sense of femaleness or maleness, expressed in personality and behavior (prenatal brain development is sensitive to testosterone, which might affect gender identity)

Click for “sex determination” question review