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chromosomes., 5.11. SEX DETERMINATION, , 5.11.1. Sex Chromosomes, s, a specific pair of chromosomes, determine the sex of the, , In diploid organisms, with separate sexe, individual. They are called sex chromosomes, or allosomes, or heterosomes. All other chromosomes are, termed autosomal chromosomes, or autosomes. The latter carry genes which control the somatic traits and, have no bearing on the sex. The two members of each pair of homologous autosomes are similar in size and, , shape, but this may not be true of sex chromosomes., Dissimilar Sex Chromosomes. Dissimilar sex chromosomes exhibit four conditions in animals —, (a) XX-XY. In mammals, including man, and most insects, including fruitfly, one sex chromosome is, smaller than the other in the males. The larger one is known as X chromosome and the smaller one as Y, chromosome (Fig. 5.46). The female in these animals has a pair of X chromosomes. Thus, the condition in, the male may be briefly expressed as XY and that in the female as XX. The similar and dissimilar sex
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31.2. Mechanism Of Sex Determination, , The mechanism that fixes the sex of an individual as it begins life is called sex determination., , There are 3 main mechanisms of determining sex in organisms : genetic or chromosomal, envisunmental, & nongenetic and nonchromosomal. —
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A. Genetic (Chromosomal) Sex Determination, , In most animals, the sex is genetically, determined, and nearly 50% individuals are males and, 50% females. Chromosomal sex determination 1s, based on heterogamesis, i.e., formation of two kinds, of gametes in one sex. There are 5 main genetic, mechanisms of sex determination. !, , 1. XX—XY Type (Fig. 5.47). The sex is, determined in mammals, including human beings, and, certain insects, such as fruitfly, in the same manner., In both i.e, human beings and fruitfly (Drosophila),, the female has similar sex chromosomes, i.e., X X,, and the male has dissimilar sex chromosomes, i.e., X, Y. The Y chromosome is shorter than the X, chromosome in humans and is hooked in fruit fly., Though dissimilar, the X and Y chromosomes synapse, in meiosis, segitegate and pass into different gametes., Thus, in humans, the female is homozygous and male, heterogygous for the sex chromosomes. During, maturation, the sex chromosomes segregate freely like, the autosomes, so that an ovum receives an X, chromosome and a sperm gets either an X chromosome, or a Y chromosome. Thus, the eggs are all alike, each ;, with X ; and the sperms are of two types : with X or, with Y. The female is said to be homogametic as it produces similar eggs, and the male is said tot, heterogametic as it produces two types of sperms. This condition is called male digamety. The Y-containin, sperms and X-containing sperms are respectively called androsperms and gynosperms. The two are produc:, in equal proportions.The sex is determined at the time of fertilization by the kind of sperm that fuses wi, the ovum. The egg with X chromosome fertilized by a sperm with X chromosome will become a zygo, with XX sex chromosomes. This zygote will develop into a female. The egg having X chromosome fertiliz, by a sperm having Y chromosome will become a zygote with XY sex chromosomes. This zygote w, develop into a male, AUhG Y chromosome is often called an androsome as its addition in the zygote produc, , a male. FCI, , , , Dix: K, , Germ Cell, , uy, ye, , Germ Cell, , , , Human beings have 46 chromosomes. These, , Be gfe i i z Parents Phenotype: Female, are distinguishable into 1 pair of sex chromosomes ; olypes Male, : : Genotypes 44A+XY 44 A+ XX, and 22 pairs of homologous autosomes. During +, ‘ : Gametes 22A+X, 22A+Y — 22A+X, 22A#, maturation, the sex chromosomes segregate freely ;, 22A+X 22A+X, , like the autosomes. With the result, the ovum, contains 22 autosomes and an X chromosome,, whereas a sperm contains 22 autosomes and either, an X or a Y chromosome. If A represents an, autosome, the genetics of sex in human beings can, be shown as in Fig. 5.48., , a gt ie a, , Children 22A+X "44 A4XK 44A+XX Females, 22A+Y 44A+XY 44A+XY Males, , Sex Ratio. Female : Males = 1:1, , Genetics of sex in human beings. The letter A, indicates autosomes', , , , , , + acer
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3. ZW—ZZ Type (Fig. 5.50). This mechanism operates in certain insects (butterflies and moths) and, in vertebrates (fishes, reptiles and birds). The male has two homomorphic sex chromosomes (ZZ) and is, homogametic, and the female has two heteromorphic sex chromosomes (ZW) and is heterogametic. There, we, thus, two types of eggs : with Z and with W, and only one type of sperms, i.e., each with Z., , Fertilization of an egg with Z chromosome by a sperm with Z chromosome gives a zygote with ZZ, chromosomes. This zygote develops into a male. Fertilization of an egg with W chromosome by a sperm
