Sex Determination and Sex Linkage, Biology tutorial

Sex Determination:

One of the features of living things is skill to reproduce their own kind. Method of reproduction; though, may be either of two kinds, though some organisms are able to use both kinds. Two types of reproduction are asexual or vegetative reproduction and sexual reproduction. One main characteristic of sexual reproduction is fusion of two kinds of gametes. In several organisms - all higher organisms two gametes are morphologically different. There is no fusion of gametes in asexual reproduction.

Sexually reproducing organism may be either monoecious or dioecious. In former case one individual produces both types of gametes. Both male and female parts are present in same individual. Dioecius organisms though, are characterized by fact sexes are separate; any normal individual is either male or female and hence produces only one kind of gamete. Put in a different way, among dioecious organisms, primary sex difference is in kinds of gametes produced. Addotionally, there are secondary sexual characters that distinguish sexes.

In general as a result, three kinds of chromosome can be recognized, though not all three are essentially present in all sexually reproducing organisms. The three kinds of chromosomes are AUTOSOMES, the X - and the Y - chromosomes. Number of autosomes is same in both sexes of all but the few organisms. X - and Y - chromosomes are designated SEX CHROMOSOMES as, though their number is constant (same as autosomes), for any member of the species, number and kinds of sex chromosomes present depend on sex of individual It is obvious that when sec chromosomes are the same kind they would behave like any pair of homologous chromosomes during meiosis-I, segregating to opposite poles. Though, despite the differences in their nomenclature, x (z) and Y(W) behave like pair of homlogues in meiosis-I. The net result of meiotic behavior of sex chromosomes is that individual with one pair of X- or z- chromosomes generates only X- or z- bearing gametes. On the other hand an XY or ZW individual will produce two types of gametes - X bearing and Y-bearing or Z- and W- bearing. Grasshopper male also produces two kinds of sperms X-bearing and O-bearing (pronounced zero-bearing i.e. no ex chromosome). Sex that produces only one kind of gamete is described as homogametic sex while sex that produces two types of gamestes is known as heterogametic sex.

Sex Determination in Drosophila:

Usually in meiosis-I homologous chromosomes go to opposite poles so that every gamete gets one of each pair of homologous pair. Deviations do take place though, such that both members of pair go to same pole. Condition where pair of chromosome fall to separate during cell division (meiosis an mitosis) is explained as NON-DISJUNCTION. If non-disjunction of X-chromosomes occurred during meiosis-I in female Drosophila two types of eggs can be generated. In one case egg will be XX, having 5 instead of 4 chromosomes. Other kind of egg won't have any sex chromosome, there would be only 3 autosomes. If these eggs are fertilized by normal sperm, there will be following sex chromosomal constitutions: XXX, XXY, XO, OY. OY condition is very rear and zygote dies in egg, so condition is fatal. XXX and XXY conditions give rise to females. XXXY females are fertile and are phenotypically identical from XX females. On the other hand, XXX female is frail and sterile, and dies fairly early, at times in pupil stage. XXX female is called as metafemale though some authors still use original nomenclature, superfemale. Given the features just described, term superfemale is misnomer. XO fly is phenotypically indistinguishable from normal males but it is also sterile. From results just considered mechanism of sex determination in fly may be defined as follows: In fly with a diploid number of autosome, sec is determined by number of x-chromosomes present, such that fly is female if there is more than 1 x-chromosome present. It is, though, male if there is only one x-chromosome. Y-chromosome doesn't play role in sex determination, but it carries genes that determine femaleness, fact that OY condition is fatal means that x chromosomes carries some genes that are essential for viability.

The complete theory of sex determination in Drosophila as suggested by Bridges in 1925 involves interaction between autosome and x-chromosomes, with great importance given to ratio of x chromosomes to complete sets of autosomes. Reason is that there is some proof indicating that genes for maleness are distributed among autosomes.

The validity of the above conclusion is evidenced by flies known as gynandromorphs or gynanders. Such flies are made up of male and female parts. Themale sections of the fly are samller than the female parts. The male sections of the fly are samller than the female parts since male flies are smaller than female. The male parts of the gynandromorphy are XO and the female part are XX. The proof that the chromosomal conditions are as stated comes from the expression of recessive genes on the x-chromosome. The theory is that male parts have only one x-chromosome. Therefore, any recessive gene onthat x-chromosome will be expressed. On the other hand female parts would not express such traits if they were heterozygous for the genes. Indeed, it is found when the the appropriate experiment is done, tht the parts which have the recessive phenotype are only the male parts. All the female parts have the dominant phenotype expectee for a heterozygous genotype. A heterozygous genotype would be possible only if there are two homologous chromosomes, in this case two x-chromosomes, present in the same cell.

The evidence and the ensuing theory of sex determination in Drosophila indicated that it is the x-chromosome tht is involved in sex determination. On the strength of this it is reasonable to conclude that a number of loci (genes) is involved in the determination of sex. All things being equal, that is the normal situation. However, there is evidence that single identifiable loci may play significant role in sex determination. For example, in Drosophila melanogaster. There is a recessive autosomal gene (i.e. a gene on an autosome as opposed to one on a sex chromosome) which when homozygous transforms XX-zygote i.e. female zygote into males which are sterile. The effect is only XX zygotes. The gene, transformed I symbolized as tra. From what I have said so far, there are two possible types of males which are also homozygous for tar; they are ZY, tra/tra which are sterile males. The tra/tra genotype nullifies (i.e. it is epistatic) the effects of the female determining genes on the X-chromosomes present. I should emphasise the point that normally the recessive tra allele is not present, rather it is the dominant wild type allele, tra+, which is present, and this allele has no epistatic effect on the pattern of sex determination.

Sex Determination in Man:

As in Drosphila, proof that help in clarification of pattern of sex determination in man came from cases of abnormal sex chromosomal constitution. Conditions were first described by Klinefelter's and term syndrome indicates fact that condition is illustrated by member of specific abnormalities. In this case, some of the abnormalities are that though external genitals appear normal, testes are small and there is little or no sperm production; these males are thus, sterile. Arms and legs are longer than normal. Chromosomal studies show that Klinefelter males have 47 chromosomes instead of 46. Chromosomal constitution is composed of normal complement of autosomes, i.e. 22 pairs, their sex chromosomal constitution is XXY.

Another major sex abnormally in man is Turner's Syndrome, occurring with the frequency of between one in 5,000 and one in 3,000 female births. Those affected with syndrome, first explained by Turner, are identifiable as females but they are badly developed; same is true of the secondary sexual characteristics - breasts etc. Affected females have normal complement of autosomes but only one X chromosome making total of 45 chromosomes. Sex chromosomal constitution is thus XO as opposed to XX for normal female.

The phenotype of sex chromosomal constitutions descried so far indicate that in man as in Drosophila, X-chromosome carries viability genes. Therefore, at least one should be present if embryo is to survive. Though, mechanism of sex determination in man is such that Y-chromosome carries male-determining genes but X-chromosomes also carries some genes for femaleness. These constitutions are based on fact that XY and XXY constitutions are male and XO, XX and XXX i.e. no Y present are female. There is also fact that there is development of some female secondary feature like gynecomasty, in XXY males.

There is information concerning distribution of male-determining genes along length of Y-chromosome and distribution of genes involved in sex-differentiation on both X and Y-chromosomes.

XYY individuals are males as expected and are very rare in population. They are usually above average height and in most cases are below normal in intelligence. There are no steady main abnormalities related with this constitution.

Though in normal pattern of sex determination in man, no particular gene is recognized as having specific role, there is proof that might in fact be case. In man there is condition called as testicular feminization or male pseudohermaphroditism. Examination of those affected with this feature show them to be chromosomally XY but having female features externally. Internally, though, they have testes and lack ovaries and fallopian tubes. The only known effect of gene for this feature on XX females is absence of axillary and pubic hair in some carrier women Gene also transforms XXY zygotes in sterile females. It hasn't been possible so far in man to find out whether gene responsible is on autosome or on X-chromosome. If it is autosomal characteristic, then it would have to be dominant characteristic. Therefore both carrier females and affected males would be heterozygous for gene. The gene could be either recessive or dominant gene on X-chromosome. Recessive gene would be expressed in all XY zygotes inheriting it as there is no other X-chromosome that could carry dominant gene. Whatever the case, gene is transmitted through only females, and it overrides male determining effect of Y-chromosome.

Pattern of sex determination in man is similar to most mammals. Though, in mice XO females are fertile though not as fertile as normal XX females. Also in mice gene for testicular feminization is known to be situated in X-chromosome and it is dominant.

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