NEET Biology Principle Of Inheritance And Variation Sex linked inheritance

Sex linked inheritance

Sex chromosomes of some animals and man besides having genes for sex character also possess gene for non-sexual (somatic) characters. These genes for non-sexual characters being linked with sex chromosomes are carried with them from one generation to the other. Such non-sexual (somatic) characters linked with sex chromosomes are called sex linked characters or traits, genes for such characters are called sex linked genes and the inheritance of such characters is called sex linked inheritance. The concept of sex-linked inheritance was introduced by THOMAS H. MORGAN in 1910, while working on Drosophila melanogaster.

The sex chromosomes in man and Drosophila are almost same in structure. The X and Y chromosomes, although different (non-homologous) in shape, size and structure, have atleast some similar (homologous) part which is known as homologous segment and the remaining part as non-homologous or differential segment. Genes for sex linked characters occur in both segments of X and Y chromosomes. Many sex linked characters (About 120) are found in man. Such characters are mostly recessive.

(i) Types of sex linked inheritance

(a) Diandric sex linked or X linked traits : Genes for these characters are located on non-homologous segment of X chromosome. Alleles of these genes do not occur on Y chromosome. Genes of such characters are transferred from father to his daughter and from his daughter to her sons in F₂ generation. This is known as Cris-cross inheritance. As the genes for most sex linked characters are located in X chromosome, they are called X-linked characters e.g. colour blindness and haemophilia in man and eye colour in Drosophila.

(1) Sex linked inheritance in Drosophila : Drosophila melanogaster has XX and XY sex chromosomes in the female and male respectively. Its eye colour is sex linked.

Allele of the eye colour gene is located in the X chromosome, and there is no corresponding allele in the Y chromosome. The male expresses a sex-linked recessive trait even if it has a single gene for it, whereas the female expresses such a trait only if it has two genes for it. The normal eye colour is red and is dominant over the mutant white eye colour. The following crosses illustrate the inheritance of X-linked eye colour in Drosophila.

(i) Red-eyed female ´ White-eyed male : If a homozygous red-eyed female fly is mated with a hemizygous (having a single allele for a trait) white-eyed male fly, all the F₁ flies, irrespective of their sex, are red eyed. When the red-eyed male and female flies of F₁ are intercrossed (equivalent to self-pollination in peas), the F₂ flies are in the ratio of 2 red-eyed females to 1 red-eyed male to 1 white-eyed male. Thus, the red-eyed and white-eyed flies are in the ratio of 3 : 1 in F₂ generation (Mendelian monohybrid ratio).

If X^R represents a gene for red eye and X^r that for white eye colour, the above cross may be diagramed as follows. The above cross shows that a recessive X-linked trait follows criss-cross inheritance, i.e., transmission from the father to the grandsons through the daughters. The latter are called carriers because they have a trait but do not express it.

(2) Sex linked inheritance in man. Colour blindness and Haemophila are the two main sex linked or X-linked disease are found in man.

(i) Colour blindness : Person unable to distinguish certain colours are called colour blind. Several types of colour blindness are known but the most common one is ‘red-green colour blindness’. It has been described by HORNER (1876).

The red blindness is called protanopia and the green blindness deutoranopia. X-chromosome possesses a normal gene which control the formation of colour sensitive cells in the retina. Its recessive allele fails to do its job properly and results in colour blindness. These alleles are present in X chromosome is evidenced by the following results.

(1) If a normal female is married to a colour blind man.

Results : All her sons and daughter have normal colour vision, but all daughters are carrier.

(2) But when her daughter (carrier) are married to man with normal colour vision man.

Result : Some colour blind sons are formed.

Conclusion : It means that a woman with normal colour vision whose father is colour blind gives birth to children, of which about half of the sons are colour blind and other half are normal.

(3) If a colour blind woman is married to a normal man.

Result : All her sons are colour blind whereas all the daughter have normal colour vision.

(4) But when these daughters having normal colour vision (Heterozygous) are married to colour blind man.

Result: The colourblind grandsons and grand daughters are produced with almost equal number of normal grandsons and grand daughters.

Conclusion : It means that a colour blind woman has sons all colour blind and daughters all with normal vision and a colour blind woman always has a colour blind father and her mother is a carrier.

Inheritance of colourblindness

The above results could easily be explained with the assumption that colour vision is sex linked character and its gene is present on X-chromosome, Y-chromosome lacks its allele. Always male receives its X-chromosome from mother (through ovum) and Y-chromosome from father (through sperm), whereas the female receives one X-chromosome from each parent (through ovum and sperm). From the above result following conclusions may be drawn.

(1) Colour blindness is more common in males than in females.

(2) Two recessive genes are needed for the expression of colour blindness in female, whereas only one gene gains expression in male.

(3) Males are never carriers.

(4) Colour blind women always have colour blind fathers and always produce colour blind sons.

(5) Colourblind women produce colour blind daughters only when their husbands are colour blind.

(6) Women with normal colour vision, whose fathers are colour blind, produce normal and colour blind sons in approximately equal proportion.

(ii) Haemophilia : In haemophilia the blood fails to clot when exposed to air and even a small skin injury results in continuous bleeding and can lead to death from loss of blood.

It is also called bleeder’s disease, first studied by John Cotto in 1803. The most famous pedigree of haemophilia was discovered by Haldane in the royal families of Europe. The pedigree started from Queen Victoria in the last century. In a patient of haemophilia blood is deficient due to lack necessary substrate, thromboplastin. It is of two types.

(a) Haemophilia-A : Characterized by lack of antihaemophilic globulin (Factor VIII). About four-fifths of the cases of haemophilia are of this type.

(b) Haemophilia-B : ‘Christmas disease’ (after the family in which it was first described in detail) results from a defect in Plasma Thromboplastic Component (PTC or Factor IX).

Like colour blindness, haemophilia is a well known disorder which is sex-linked recessive condition. The recessive X-linked gene for haemophilia shows characteristic Criss-cross inheritance like the gene for colour blindness. Its single gene in man results in disease haemophilia, whereas a woman needs two such genes for the same.

(iii) Defective enamel : It is a dominant X-linked trait and is inherited through a dominant X-linked gene. As X-chromosome is present in both man and woman, it is expressed in both the sexes. However, such persons have defective enamel on teeth like grey or brown unlike pure white enamel in a normal man.

Another example of dominant X-linked gene is the dimpled cheeks. Dimple may occur on one or both the cheeks.

(b) Holandric or Y-linked traits : Genes for these characters are located on non-homologous segment of Y chromosome. Alleles of these genes do not occur on X chromosome. Such characters are inherited straight from father to son or male to male e.g. hypertrichosis of ears in man.

(1) Hypertrichosis of ears: This is a condition in which excessive amount of large hair grow on the pinna in man. It is sex-linked trait controlled by a gene present on the non-homologous segment of the Y-chromosome. Hence its inheritrance is called holandric inheritance and it appears only in man. It passes directly from father to son.

(c) XY-linked inheritance: The genes which occur in homologous sections of X and Y-chromosomes are called XY-linked genes and they have inheritance like the autosomal genes.

Example of XY-linked genes are those of the inheritance of following

(1) Xeroderma pigmentosa, a skin disease characterized by the pigment patches and cancerous growth on the body.

(2) Nephritis, a kidney disease.

(ii) Sex-influenced traits: The autosomal traits in which the dominant expression depends on the sex hormones of the individual are called sex-influenced traits. These traits differ from the sex limited traits which are expressed in only one sex. It has following examples.

(1) Baldness in man : Baldness in humans is the best example of sex-influenced traits. This trait is due to a single mutant gene but the expression of the heterozygous is different in man and woman. This is a hereditary character controlled by sex-influenced gene which is dominant in men and recessive in women. The difference in expressions may be caused by varying amounts of male and female sex hormones. If autosome dominant gene ‘B’ is regarded to inherit the baldness, the homozygous (BB) dominant condition will cause baldness in man as well as women. This gene for baldness acts recessively in woman when present in heterozygous (Bb) condition, the baldness develops in males only because under such condition the phenotype expression (baldness) is influenced by androgen hormone secreted by man. A heterozygous female is normal. A homozygous recessive condition (bb) does not allow baldness to develop either in male or female.

Phenotypic expression of genotype for baldness

The different phenotypes in men and women shown in above table are sex-influenced characters and also called sex-controlled traits.

The progeny that would be obtained from the marriage of heterozygous (B/b) man and woman for baldness have been shown below.

Progeny resulting from the marriage of bald men and non-bald women both heterozygous

(2) Length of index finger : It is another example of sex-influenced trait in man. It is controlled by a gene which is dominant in male and recessive in the female. When the hand is placed on white board the tip of the fourth finger or ring finger just touches a horizontal line, it is seen that index or second finger does not reach this line in many cases. In some persons index finger extends beyond this horizontal line as shown in figure. The short index finger is inherited as a dominant trait in men and as a recessive condition in women.

(iii) Sex limited traits : Traits or characters which develop only in one sex are called sex-limited characters. They are produced and controlled by the genes which may be located on autosomes in only one sex. Such genes are responsible for secondary sexual characters as well as primary sexual characters. They are inherited according to Mendel’s laws.

Sex-limited traits in man : Beard is produced by sex-limited genes in man, which does not develop in woman. Breast development is normally limited to woman. In case of abnormalities of hormonal secretions facial hair may develop in woman and a faminine breast development may occur in man. It means that expression of sex-limited characteristics in vertebrates depend upon the secretion of sex hormones. For example genes for deep masculine voice, masculine body, masculature in man will express themselves only in the presence of male hormone. Genes for faminine voice and faminine musculature on the other hand express themselves in the absence of male hormone and will not require the secretion of female hormone. Similarly, breast development in woman requires the presence of female hormone rather than mere absence of the male hormone. It can be concluded that certain sex-limited characteristics are expressed in the absence of certain hormones and other express only in the presence of sex-hormones.

Pedigree Analysis

Pedigree Analysis

Inheritance of hundreds of characteristics such as polydactyly, haemophilia, colour blindness, attached ear lobes and tongue rolling, generation after generation in particular families of man have been studied. In order to conduct such study, a standard method has been used to represent the family pedigree in a concise, easily understood form so that one can visualize the entire pedigree (family history) at a glance of the chart.

(i) Pedigree chart and symbols: It is customary to represent men by squares and women by circles in a chart for study of pedigree analysis. Marriage is indicated by a connecting horizontal line and the children by attachment to a vertical line extending downward from the horizontal line. Individuals having particular characters to be studied are denoted by solid squares or circles while those not having them are indicated by outlines only. Twins are denoted by bifurcating vertical lines.

In such a pedigree analysis a person who is the beginner of the family history is called proband. It is called propositus, if male and poposita, if female. The children of such parents are known as sibs or siblings. So a family is constituted by such parents and their siblings. Sometimes, a very large family is formed as a result of interconnected marriages. Such a circle of large persons interconnected is called Kindred.

In order to study pedigree analysis we have taken some of the important case histories as follows :

(a) Polydactyly : The pedigree of this trait has become standard usage among the geneticists and it helps us to understand the process of transmission of this trait.

This inheritable trait was discovered when a woman brought her young daughter to a doctor for examination as she had an extra finger on one hand and an extra toe on one foot. On investigation it was found that child’s father had this characters (though his extra finger had been removed surgically) and that her brother also had the character. The other two children of this family had normal number of fingers and toes. This type of inheritance is typical of characters which are known as dominant.

(b) Attached ear lobes : This is a recessive type of inheritance and is inherited in a different way.

(1) Two parents with free ear lobes produced two children with attached ear lobe in a family of five children.

(2) In another family both parents had attached ear lobes but all the four of their children had this trait of attached ear lobes.

(c) Tongue rolling : Some persons are capable of rolling their tongue while others are not gifted with this power. A couple both of whom are tongue roller have two out of these children as tongue rollers.

(d) Crooked little fingers : This is a family pedigree of a human family where crooked little fingers are inherited through a simple dominant gene. In this pedigree a woman had two sons one of which had crooked little finger. Her husband also had same type of defective fingers. On further survey of her husbands, family it was found that her husband’s sister and mother both had crooked little fingers, as well as his grandfather also possesses this trait. The characteristic also appeared in more distant relatives.