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Sex Determination Full Notes.Pdf DETERMINATION OF SEX The word “sex” is derived from the Latin word “sexus” means sections. The sexually reproducing organisms are classified into two types viz, monoecious (hermaphrodite) and dioecious. In monoecious organisms, both male and female gametes (sex cells) are produced by a single individual. The organisms in which both male and female gametes are produced by different individuals are called dioecious. The sex cells and reproductive organs form the primary sexual characters of both sexes. Besides this, male and female sexes differ from each other in many somatic characters known as secondary sexual characters. The phenomenon of molecular, morphological physiological or behavioral differentiation between male and female sexes is called sexual dimorphism. Sex determination is recognized as a process in which signals are initiated for male or female developmental patterns. During sex differentiation, events occur in definite pathways leading to the development of male and female phenotypes and secondary sexual characters. Significant progress has been made in understanding the mechanism of sex in human beings and other mammals and new genes have been identified. The primary function of such a sexual differentiation of a species into male and female sexes is to prevent combination of gametes derived from the same parent and the maintenance of a high degree of heterozygosity and genetic exchange. Theories of Sex Determination The problem of sex determination has been one of the most important biological puzzle up to the year 1900. A number of theories were postulated from time to time by the biologists to explain this critical phenomenon. Chromosome Theory of Sex Determination: Sex determination in higher animals is controlled by the action of one or more genes. The testis determining factor (TDF) gene is the dominant sex determining factor in human beings. Hemking a German biologist identified a particular nuclear structure throughout the spermatogenesis in of squash bug, Pyrrhocoris. He named it as “X-body” and showed that sperm differed by its presence or absence. After three years, Miss Steven and Wilson succeeded in understanding oogenesis and spermatogenesis in Protenor bugs. The X body was later found to be a chromosome that determined sex. It was identified in several insects and is known as the sex or X chromosome. In males, all the chromosomes are paired, but the chromosome analogues to X chromosome is smaller and is called as Y chromosome. Thus, the chromosome theory of sex determination states that female and male individuals differ in their chromosomes. In majority of sexually reproducing animals two types of chromosomes are found: (i) Autosomes: The chromosomes which have no relation with sex and contain the genes which determine the somatic characters of the individuals are called as autosomes (A). They are found in all cells. The two members of this pair are similar in shape, ie, homologous pair (homomorphic). (ii) Sex Chromosomes or Allosomes: The chromosomes which carry genes for sex determination are called allosomes. A pair of them determines the sex. They are variously named as X and Y chromosomes (Man and Drosophila), Z and W chromosomes (Birds and Moth), odd chromosomes, idiosoines, heterosomes or allosomes. The two members of this pair are often dissimilar in male and are represented as X and Y chromosomes or as Z and W chromosomes. Types of chromosomal mechanisms of sex determination In dioecious diploidic organisms following two systems of sex chromosomal mechanisms of sex determination have been recognized 1. Heterogametic male In this type of sex determination female has two X chromosomes, hence produces similar type of gametes and called as homogametic sex, But male has only one X chromosomes, hence during gametogenesis it produces two types of gametes, 50% gamete carry X chromosomes while rest either lacks X chromosomes (XO) or has a Y chromosome (XY) a. XX Female - XO Male Type: Mc Clung and Wilson described this type of sex mechanism in insects especially grasshopper. In male there is no mate for X chromosome, hence the name Xo is given, there is no Y chromosome They produce sperm of two types, 50% with X chromosome and 50% without X. In females there are two similar or homomorphic sex chromosomes XX (homogametic) (Fig 18.3) b. XX Female - XY Male Types: This type of sex mechanism is found in Drosophila (fruitfly) and majority of mammals including man. In this type the female is homogametic (XX) and male is heterogametic (XY) consisting of two dissimilar chromosomes X and Y. The females produce ova all of one type having X chromosome (homogametic. Males produce two types of sperms: -50% with X-chromosome and remaining 50% with Y-chromosome. Thus, the sex chromosomes in female are homomorphic and those of male are heteromorphic or heterogametic (Fig. 5.13). Fig: 5.13 XX-XY type of sex determination 2. Heterogametic females In this type of sex chromosomal sex determination, the male possess two homomorphic X chromosomes, thus is homogametic and produces gametes with a single X chromosome. The female either, has only one X chromosomes, or has one X and one Y, hence during gametogenesis it produces two types of gametes, 50% gamete carry X chromosomes while rest either lacks X chromosomes or has a Y chromosome. To avoid confusion with that of XX-XO and XX-XY methods of sex determination, instead of X and Y Z and W alphabets are used respectively a. ZO Female -ZZ Male Type This system of sex determination is found in moths and butterflies. Feale hence ZO and is heterogametic producing 50 % gametes with Z and 50 % without Z. Males possess two Z chromosomes hence homogametic and produces single types of gametes(with Z chromosome) and are called as homogametic. The sex of offspring depends on egg as shown below b. ZW Female - ZZ Male Type In certain insects, fishes , reptiles and birds, the female is heterogametic; having dissimilar Z and W chromosomes, whereas the male is homogametic having similar ZZ chromosomes (It is a convention to designate female as ZW instead of XY and male as ZZ instead of XX). The situation here is just reverse to first type. SEX DETERMINATION IN HUMANS The sex of a human being in decided at the moment of conception. Sex is determined by the sort of spermatozoon, X-bearing or Y-bearing, which happens to fertilize the ovum. In human beings, sex is determined by genetic inheritance. Genes inherited from the parents determine whether an offspring will be a boy or a girl. We have a total of 46 chromosomes. Half of them come from the mother and the rest, from the father. Out of these 46 chromosomes, 44 are autonomies and 2 are sex chromosomes. The sex chromosomes are not always a perfect pair. In females there are 44 autonomies and two X chromosomes (44 + XX), in males there are 44 autonomies, one X chromosome and one Y chromosome (44 + XY). The human female produces only one sort of ovum —each containing 22 autosomes and an X chromosome (22A + X). The human male, on the other hand, produces two sorts of spermatozoon, those containing 22 autosomes and an X chromosome (22A + X); and other those containing 22 autosomes and a Y-chromosome(22A + Y). There are two possibilities when fertilization takes place: 1. An ovum (bearing an X- chromosome) may be fertilized by a spermatozoon bearing an X chromosome; the result is an individual bearing 44 autosomes and two X-chromosome—that is a female. 2. An ovum bearing an X-chromosome may be fertilized by a spermatozoon bearing a Y- chromosome; the result is an individual bearing 44 autosomes and one X- chromosome and one Y-chromosome that is a male. Let us see the inheritance pattern of X and Y chromosomes. During gamete formation, the normal diploid chromosome number is halved. This is called the haploid condition. All the eggs of a female have 22 + X chromosomes. A male produces two types of sperms—one type bears the 22 + X composition and the other, 22 + Y. Therefore, in every 100 sperms, 50 have Y chromosomes and 50 have X chromosomes (Fig 7.5). Any one of the two types of sperms can fertilize the egg. If a Y-bearing sperm fertilizes the egg, the zygote has the 44 + XY composition, and the resulting embryo grows to be a boy. When an X-bearing sperm fertilizes the egg, the resulting zygote has the 44 + XX composition. This embryo develops into a girl. All the children inherit one X chromosome from the mother. Therefore, sex is always determined by the other sex chromosome that they inherit from the father. One who inherits the X chromosome of the father is a girl, while one who inherits the Y chromosome of the father is a boy (Fig 7.5). SRY GENE AND ITS ROLE A small region of the Y chromosome, perhaps a single gene, is responsible for initiating the sequence of events that lead to testis formation and, hence, to male development. This gene is known as TDF (Testis determining factor).Testis-determining factor (TDF), also known as Sex-determining Region Y (SRY) protein, is a DNA-binding protein (also known as gene- regulatory protein/transcription factor) encoded by the SRY gene that is responsible for the initiation of male sex determination in humans. SRY is an intronless sex- determining gene located on the short arm of Y chromosome in therians (placental mammals and marsupials). In the absence of TDF gene female development ensures. The absence of TDF in the XY females prevented them from developing testes. These observations show that a particular segment of the Y chromosome was required for the development of the male (Fig.
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