Sex linkage may be classified into the following categories, depending upon which chromosome carries the genes. They are-
1. XY linked characters (genes)
2. Y linked characters (genes)
3. X linked characters (genes)
Inheritance of XY linked genes:
The genes which are XY linked are present on the homologous sections of X and Y chromosomes. They can undergo pairing and consequently can cross over from X to Y and vice versa.
They are not linked permanently either to X or Y and hence called incompletely linked (XY linked) genes. Their inheritance pattern is like that of the autosomes as they undergo cross over.
One of the XY linked character in Drosophila melanogaster is the bobbed condition of bristles. The gene for bobbed, bb is present on the X chromosome and its allele BB is present on the short arm of the Y chromosome.
These two segments of X and Y are homologous and can undergo cross over. A male with the bobbed gene b on the X chromosome may be normal when it has the normal B gene in the Y chromosome.
When such a normal (but heterozygous) male is crossed with a homozygous bobbed female (b and b present on both the X chromosomes), among the progeny male are normal (because they get the X chromosome from the mother b and Y from the fatehr B) while female are bobbed (X chromosome from father has b and X. from the mother also has b). The inheritance pattern on the bobbed gene has been worked out by Sybena (1972).
Several XY linked characters have been identified in human beings also. Total color blindness Xeroderma pigmentosum (a skin disease characterized by dry patches on the skin upon exposure of light).
Epidermalysis bullosa, Retinitis pigmentosa (a progressive degeneration of retina, accompanied by pigment deposition in the eye) certain forms of nephritis, spastic paraplegia etc., are known to the XY linked genes.
Inheritance of Y linked Character:
As has already been pointed out, sorne genes are exclusively linked to Y chromosome. A few instances of these in roan are –
(a) Porcupine man in whom the entire surface of the skin will be warty Coarse (Ichthyosis hystrix)
(b) Hypertrichosis hair on the lobes of the ear,
(c) Keratoma dissipatum skin disorder affecting the hand and feet
(d) Occurrence of Webbed toes in some individuals etc.
Be due to the sex linked character of the gene for eye color. The gene for normal eye color (red) W is present on the X chromosome. White eye color is a recessive gene occupying the same locus (w).
Hence for white eye color to appear in the females the genes should be double homozygous (should be present on both the X chromosomes). In the heterozygous condition in female (Ww) one X carries the W (red) gene, while the other X carries the w (white) gene.
Phenotypically the heterozygous females are red eyed, but they are called carriers as they carry the recessive gene.
The males however cannot be carriers, for the gene for red or white being present on the only one X chromosome they have, their genotype and phenotype are one and the same.
Even a single dose of the recessive gene (w) is enough to produce the white eye color. This can be understood by the reciprocal crosses as follows.
Red eyed male X White eyed female: When a red eyed male (Wo) is crossed to a white eyed female (ww), in the F, generation all the females would be red eyed and all the male individuals are white eyed.
This is because the white eyed recessive gene is present on both the X chromosomes in the female parent. The F, male offspring gets its X (w) from the mother and Y from the father. Since Y chromosome does not carry the gene for eye color the individual would be white eyed.
The female offspring (F,) having got one X (W) from the father and one X (w) from the mother would be heterozygous (Ww) genetically, but phenotypically show red eye color.
When the F, heterozygous female is crossed with the white eyed F, male, in the F2 generation both red and white eyed (1:1) individuals are found in the female individuals, while in the male progeny also-red and white eyed individuals appear in equal proportion. Red eyed female X White eyed male
When a pure red eyed female (WW) is crossed with a white eyed male (wo), the Fj offspring will have both females and males red eyed. The females however are carriers (heterozygous) as they have both Ww genes on their two X chromosomes.
When these two F, individuals are crossed (heterozygous red eyed female x white eyed male), among the female (F2) progeny (1) 50% will be red eyed (heterozygous) (2)50% will be white eyed (homozygous double recessive) among the male progeny (3) 50% will be red eyed and (4) 50% white eyed. The genotype of these flies will be as follows.
(1) Red eyed female (Ww)
(2) White eyed female (ww)
(3) Red eyed male (Wo indicates Y chromosome which has no gene for eye color)
(4) White eyed male (wo)
Sex linked lethal in Drosophila:
Several crosses have indicated the presence of a lethal gene on the X chromosome of Drosophila. This recessive lethal can be tolerated in the female in the heterozygous condition, but the males with the lethal gene do not survive.
When a female heterozygous for lethal (Li) is crossed to a normal male (Lo), among the F progeny of females and males, all females survive (50% females are completely normal and 50% heterozygous).
Among the male progeny however 50% are normal (Lo) and survive, while the other 50% (lo) getting their X (/) chromosome from the mother die hence the ratio would be 3:1 females to males.
Morgan has indicated that at least about 140 characters in Drosophila may be sex linked, whose, genes are to be found on the X chromosome.
Sex linked inheritance in cats:
The coat color inheritance in tortoise shell cats is known to be sex linked. The tortoise shell coat color is found only in females and is characterized by the presence of patches of black, yellow and white pigments in the skin.
Tortoise shell cats maybe produced by crossing black homozygous female (BB), with yellow male cats (bo), the F, progeny will be heterozygous (Bb) and has tortoise shell coat. Here the gene for black (B) is not completely dominant over yellow (b).
In a reciprocal cross also involving a yellow female (bb) and a black male (Bo), the F, progeny females have tortoise shell coat (Bb). Occasionally male cats are also found to have tortoise shell coat. But these males are sterile and they apparently have two X chromosomes and one Y chromosome (XXY = Bbo).
The presence of two X chromosomes may be explained on the basis of non disjunction of X chromosomes during division. As a result an egg carries one set of autosomes and two X chromosomes.
Sex linked inheritance in Man:
In human beings sex linked inheritance follows the same pattern as explained for Drosophila. Generally sex linked deficiencies appear more in males than in females the reasons for this are not far to seek.
All the sex linked mutant genes being recessive and present on the X chromosome, the female gets the protection readily in the form of the normal dominant allele present on the other X chromosomes.
This protection is not available to the male as it has only one X chromosome. Either the male is completely normal or expresses the recessive gene phenotypically, whereas the female may be completely normal or may appear to be normal, while carrying the defective gene in one of the X chromosomes (heterozygous condition).
Nearly 20 sex linked characters have been reported in man. Some of these are –
(1) Red-green color blindness first reported in 1777
(2) Hemophilia (bleeder’s disease) first recorded as a heritable character in 1793
(3) Optic atrophy (degenerating disease of the optic nerve).
(4) Juvenile glaucoma (hardening of the eye ball)
(5) Myopia (short sight)
(6) Tective iris
(7) Juvenile muscular dystrophy (degenerating disease of muscles)
(8) Distichiasis (double eye lashes)
(9) Mitral stenosis (Abnormal mitral valve of the heart
(10) Mystagmus (involuntary oscillation of the eye ball
(II) Baldness and
(12) Epidermal cysts. We shall study the inheritance pattern of a few of these traits.
Inheritance of Hemophilia:
Commonly called bleeder’s disease, this refers to a condition, where blood on exposure to air will not clot; as a result hemophilic individuals may bleed to death even at the slightest injury Hemophilia is a sex linked trait often associated with males, females being very rare, (but genetically possible).
The failure of the blood to clot is due to the absence of a protein antihemophilia globulin, which is normally present in the blood of man hemophilic individuals lack this, as a result bleeding, when occurs may prolong for 1/2 hour to 20 hrs., leading to the death of the individual if proper measures are not taken.
The production of the coagulating substance is governed by a gene present on the X chromosome in Hemophilics, this wild gene would have undergone mutation resulting in a hemophilic gene.
Hemophilia gained some notoriety in the past as the Royal disease, as some members of British royalty were hemophilics. One of the well known victims of the disease was the great grandson of Queen Victoria who died due to an injury and subsequent non stop bleeding.
As has already been pointed out, hemophilia is found predominantly in males, but the inheritance is criss cross-from father to daughter to grandson.
Interesting feature is the females generally do not suffer, but they will be earners. Hemophilia can manifest in females only in double recessive condition, an individual dies before adolescence.
The chances of random mating using a hemophilic are extremely rare (in males it is about 1 in 10,000); it is the hemophilic males survive and undergo random mating, squar13111611103′ Probability of a homozygous female would be equal to the genre of 10.000 or 1 in 1,00,000,000. The genetic manifestation of hemophilic is as follows.
The hemophilic gene is a recessive allele of the wild gene situated on the X chromosome only. There is no allele for this on the Y chromosome. Hence no cross over is possible between X Y for hemophilia. If we denote the normal gene as H and hemophilic gene as h
The genetic constitution of different individuals will be as follows all female progeny will be normal; but 50% are homogyzous and 50% carriers. Among the male progeny 50% are normal and 50% are hemophilics.
Color blindness in Human beings:
The red and green colorblindness is another sex linked recessive trait in man. Individuals suffering from this defect will not be able to distinguish between various shades of red and green and hence are colouring blind.
Individually red color blindness is called Protanopia and green color blindness is called Deuteranopia.
Ishihara has designed a color chart to identify these defects. It has been shown that the gene for color blindness is a recessive allele of normal vision and is situated on the X chromosome.
Like any sex linked color blindness also is more prevalent among males than among females chromosome does not carry any allele for vision.
A survey for color blindness has not been conducted in our country, but data available from USA give a figure of 8% for male and 0.5% for female in the total population. The following chart gives all the possible mating types and the possible progeny for color blindness among human beings.
In all individuals where sex determining mechanism follows that of man or Drosophilia (male being heterogametic and sex linked, recessive gene being located only on the X chromosome) the following are the salient features of its manifestation.
1. Sex linked recessive traits are phenotypically expressed more often in males than in females.
2. The trait appears among the female progeny only when it is found in the male parent.
3. The trait seldom is found both in father and son. It may be the case, when mother is homozygous.
4. The inheritance of the sex linked recessive gene is from father to daughter to grandson.
Dominant sex linked character in Man:
These characters are very few. One of the dominant sex linked genes producing a defective enamel of the teeth has been found in man. The gene for this is also located on the X chromosome. Sex linked characters in Poultry
In birds (including poultry), moths, butterflies and certain other animals where (females are the heterogametic sex, a different kind of sex linkage exists. The mechanism is exactly opposite to what is seen in Drosophila, Man etc., where male is the heterogametic sex.
The inheritance of barred plumage in poultry birds is a classical example of sex linked inheritance where males are heterogametic. In poultry birds, females have only one X chromosome (XO -O indicates absence of another X chromosome), whereas males have 2 X chromosomes.
The barred plumage feathers are banded with, bars of black on a white background. The gene for barred character is a dominant allele and is dominant over black or non barred plumage. Let us study the results of reciprocal crosses (Barred male X Black female and Barred female X black male). Barred male X Black female
When a barred male (BB) is crossed with a black female (bo), the F) progeny will have only barred individuals both male and female.
The males however are heterozygous for barred condition (Bb) and the females have only one barred gene as they have one X chromosome Bo). When these two? Birds are crossed among themselves the following progeny are obtained.
1. Barred male (Homozygous BB)
2. Barred male (Heterozygous Bb)
3. Barred female (Bo)
4. Black female (bo)
Among the male progeny all are barred, however 50% of them are homozygous and the rest heterozygous. Among the female progeny 50% are barred and 50% black.
Barred female x Black male:
When a black homozygous male bird (bb) is crossed with a barred female (Bo), among the F, progeny, the males are barred (heterozygous Bb), while the females are black as they get their only X chromosome from the male parent which is black (b).
When these F birds are bred among themselves, the following gametes are obtained. The barred male F produces two types of sperms B and b, while the F, female also produces two types of eggs – one carrying the lone -X chromosome carrying the b gene and the other carrying no sex chromosome (o).
On random mating the progenies shown below are obtained.
1. Unlike in Drosophila, man etc. the female is the heterogametic sex, but it does not have XY but only X, designated as XO.
2. The barred gene is dominant and is not a recessive mutant of the normal as is the case in most of the sex linked traits in Drosophila.
3. Male is a carrier for the barred gene (heterozygous) while the female has to phenotypically express whether a barred or black gene as it has only one X chromosome.
Significance of sex linked characters in birds:
When the phenotype and genotype of the parents are known, the sex of the newly hatched can be determined by looking at their plumage pattern.
Sex influenced genes (traits):
A particular type of baldness called pattern baldness is quite common among human males. Here the hair is removed from the centre of the head, while the hair growth is normal on the sides.
This was first thought to be a sex linked recessive character as males are more prone to this, while pattern baldness in females is extremely rare. But investigations showed clearly that the pattern of inheritance of baldness does not follow the sex linked pattern, for often a son can inherit pattern baldness from the father.
The gene for pattern baldness is not situated on the X chromosome but on the autonomies. However the same gene behaves as recessive in females, but as dominant in males hence while only one gene is enough to produce baldness in males, two such genes are required for baldness in females.
It seems a single gene can function only in the presence of the male hormone. In females, the absence of the male hormone suppresses the single gene from functioning. However when two genes are there they find the phenotypic expression.
Such genes, which find expression only in one sex, but not linked to the sex chromosomes are called sex influenced genes (traits). Sex influenced genes should not be mistaken with sex linked genes for they are not situated on the X chromosome.
They are also not holandric (situated on the Y chromosome) even though they are expressed predominatly in males. They are autosomal genes, whose phenotypic expression is aided by the male hormone. The genotype and phenotype of males and females with reference to pattern baldness is as follows.
In both males and females, there are two alleles for the gene for hair growth. N-normal, n – bald gene sex linked
An interesting case of pattern baldness in man being influenced by male hormone is that of an under sexed man, who took injections of male hormones to regain masculinity, but lost his hair. Evidently this man was heterozygous for the bald gene.
In the-under sexed condition, the single gene could not express itself in the absence of enough quantity of male hormone with the injection of the male hormone, the man regained his masculinity, but the single bald gene found phenotypic expression in the new environment and he lost his hair.
Another instance of sex influenced gene in man is that of length of the index finger. In normal cases the index finger is as long as or longer than the fourth (ring) finger. But in some cases the index Finger is shorter than the forth Finger. This is due to mutant gene, which again is dominant in male and recessive in female.
Sex limited genes:
They are genes responsible for the production of traits in any one sex; which are apparently secondary sexual characters. They are different from sex linked characters, because they are found on the autosomes and not on the sex chromosomes.
They are also different from sex influenced characters in that, they (sex limited) are found’ in. only one sex (either male or female), and there is no genetic possibility of finding expression in both the sexes.
Sex influenced characters on the other hand can find expression in both the sexes under suitable genetic conditions (homozygous in females; homozygous or heterozygous in males) even though they are predominantly found in one sex.
Sex limited characters are exclusively limited to one sex (male or female). The antlers in deer are a typical example of sex limited character. Females do not have the antlers. We shall study some well known examples of sex limited characters.
Development of beard in males, breast development in females are typical examples of sex limited characters in man.
A woman normally does not have a beard but genes for beard growth must be present, because a man can inherit these and can get facial hair.
In rare instances, abnormalities in hormone secretion may influence these genes to express themselves.
But here again, these are different from sex influenced genes, because in the latter, if suitable genes are present in female, even though there is no hormonal abnormality sex influenced character may appear.
In sex limited characters even though genes are present, only a hormonal abnormality can produce an abnormal secondary sexual character (facial hair in women and breasts in man).
Another well known example of sex limited character is the high milk Producing trait in cattle. Investigations have shown that the bull also carries genes for high milk production just like the cattle, but it does not find expression, his can be tested by a cross in which the same cow is bred with two bulls.
The spring in one case had a high yielding capacity, while in the other it was less, nose two cases, the mother was same, but the fathers were different indicating a they also carry genes for milk production. But the genes for milk production expression only in female
Sex limited genes in invertebrates:
Color pattern in male and female butterflies is an example of sex limited genes. In clover butterfly, the males are always yellow, but the females may be either yellow or white. The gene for white is dominant, but it can express itself only in females.
The females are YY or Yy (white homozygous or heterozygous) when white and yy when yellow. However all the three genotypes are yellow in males?
Sex limited genes in birds:
Sex limited traits produce a variety of sexual dimorphism in birds, so much so they can be distinguished merely on their morphology in the male pheasant or pea cock, the plumage is brilliantly colored, as a contrast to the drab plumage of females.
In some domestic fowls, the female plumage pattern may be exhibited in the male when the gene combination for male plumage is absent. The dominant gene H produces the female pattern plumage in males, while the male’s plumage is due to two recessive allele’s hh.
In females however irrespective of the genotype H or h, female plumage is produced. A female feathered cock (Hh) can be produced by crossing a male (hh) with a heterozygous (Hh) female. Non disjunction of sex chromosomes
During gametogenesis in diploid organisms haploid gametes are produced and consequently the male gametes have one set of autosomes and one X or Y (1 A + X, 1 A+ Y) and the female gametes ie., the eggs have 1 set of autosomes and one X chromosome.
During oogenesis, of the 2 X chromosomes of the mother cell, the egg receives only one the 2 X chromosomes separate after pairing and move to opposite poles. But in some rare instances the two X chromosomes fail to separate during meiosis-I so that an egg will carry 1 set of autosomes and 2 X chromosomes instead of one. This phenomenon in which the two X ‘chromomomes fail to separate during meiosis is called non disjunction.
In individuals showing non disjunction of X chromosomes 3 kinds of eggs are formed. These are
(a ) One set of autosomes + one X chromosome (Normal)
(b) One set of autosomes + two X chromosomes (non disjunction)
(c) One set of autosomes + no X chromosome.
The phenomenon of non disjunction was first observed in Drosophila melanogaster by Bridges (1916) when he was studying sex linked characters viz. red and white eye color.
When red eyed males were crossed with white eyed females, he observed that the expected, white eyed males and red eyed females did not appear in the F Instead there were white eyed males, red eyed males, and white eyed females etc., produced in different proportions.
Bridges explained the results on the basis of non disjunction of X chromosome. Non disjunction of X chromosome is of two types’ Primary non disjunction and Secondary non disjunction.
When the two X chromosomes in a normal female fail to separate it is called primary non disjunction; on the other hand, if the two X chromosomes fail to separate in an exceptional female (XXY) it is called secondary non disjunction.
Primary non disjunction:
When a white eyed (homozygous) female is crossed with a red eyed male, the progeny will be as follows, if there is no disjunction of X chromosome in some flies.
Among the progeny that survive both male and female will have 50% red eye and 50% white eye? But 50% of the males are exceptional in that they do not have the Y chromosome and 50% of the females have 2 X and one Y chromosome.
Secondary non disjunction:
The exceptional females obtained in the previous cross are used as the female parent and crossed with a normal red eyed male.
The cross and the progeny are as follows. In the four kinds of gametes mentioned above, the first two kinds are produced during meiosis-I when the X would go to one pole and XY go to pother pole.
Dung meiosis-II, in the case of XY dyad after duplication XX go of pole and Y goes to another pole, as a result the third and the fourth kind foil gametes are Produced- When these mate with the normal male gametes in the 8 progeny are obtained.
Non disjunctions of automsomes as well as sex chromosomes have also been reported in man causing various types of phenotypic abnormalities.
Many of the abnormalities in human beings both sexual and other characteristics have been now known to be due to non disjunction of chromosomes, when gametes are formed during meiosis.
For some reason or other with respect to one or more chromosome pairs, the gametes may be unreduced i.e. they carry both the complements of the homologous chromosomes.
When such gametes are fertilized by other normal gametes, the resultant offspring will have trilogy (three complements for a particular homologous pair) leading to abnormalities.
One of the best known trisomy of an autosome in man is the one causing the Down’s syndrome or Mongolism.
The causes of non disjunction of the chromosomes during meiotic division are not known. However exposure to X radiation in Drosophila might bring about non disjunction (Mayor).
Sex linked characters and chromosome theory of inheritance:
Although chromosomes were thought to be vehicles of genes much before the discovery of sex linked characters, the identification of a gene or a trait following the inheritance pattern of a particular chromosome provided conclusive evidence for the theory of inheritance through chromosomes. From this point of view sex linked characters are very significant.