SEX LINKAGE

Sex Linkage

• Content

Introduction

Characteristics

Inheritance of white eye in drosophila

Nondisjunction of X chromosome

Attached X chromosome in drosophila

Sex linkage in human and other organisms

Partial sex linkage

• Introduction:
• In Mendel’s crosses F1 and F2 from crosses show same results unaffected by whether it is a male or female both shows identical ratios.
• But this rule is may not be applicable sometimes due to some exception produced by the sex linkage phenomenon.
• Sex linkage is the phenomenon of gene being located on X or Y chromosome.
• Inheritance of trait is determined by this gene on sex chromosomes.
• First known case of sex linkage is haemophilia in human beings. In haemophilia, affected person bleed profusely even from minor cuts because their blood doesn’t clot on exposure to air.
• There are many more exa mple of sex linked inheritance but it is clearly explained by Morgan in 1910( white  eye gene in drosophila).

• Characteristics
• Heterogametic sex has more no of individual showing recessive sex linked character than the homogametic sex.
• Genes that control sex linked traits are not getting transferred from male parents to their male progeny directly.
• Sex linked gene are transmitted from male parent to its entire daughter (daughter receives its half X chromosome from father) then daughter transmit this gene to half of its male progeny. As a result the gene transferred from male to female progeny and then to the half of male progeny. This transfer of male to female and back to the male is known as criss-cross inheritance.
• Y chromosomes not carry sex linked genes. Heterogametic sex is present in hemizygous condition which lead to the expression of the recessive alleles of sex linked genes and in homogametic sex they have to be present in homozygous condition.

• Inheritance of white eye in drosophila

Morgan in pedigree culture of normal dull red eye drosophila observed a single white eye male. He mate white eye male with red eye female and found F1 flies had all red eyes and in F2 3 red eyes and one white eye showing that white eye is due to a recessive gene.

• But when he classified F2 flies on the basis of sex he found that all the female had red eyes and half of the male is red eyed and remaining half is white eyed. It seems as  if the eye colour related to the sex of the flies.
• After that Morgan mated white eyed female with red eyed male he observed that half of them were red eyed and other half were white eyed, in F2 generation the ratio of red eyed and white eyed is 1:1, which same as the F1.
• Later Moran reasoned that X chromosome carry white eye gene of drosophila,  and Y chromosome doesn’t carry any allele of this gene.

Nondisjunction of X chromosome

• In 1916 Bridges, studying the inheritance of vermilion colour gene in drosophila. Observe that vermilion eye colour gene v (sex linked recessive gene) show the same pattern of inheritance as w
• Cross between the vermilion females and red males, among the progeny of F1 some females were vermilion eyed and some males were red eyed. However, majority of females were red eyed and males were vermilion eyed.
• Bridges explained these data and postulated that the X chromosome failed to separate during oogenesis of some oocytes of vermilion eyed females and both moved to the same pole and the opposite pole did not receive X chromosome such irregular distribution is known as nondisjunction.

• Attached X chromosome in drosophila
• Morgan in 1922, obtained all females with yellow body and all males with grey body when he mated yellow body female with grey body male drosophila
• Yellow is recessive to the normal grey body located on X chromosome and determined by y.
• Morgan reasoned that X chromosome nondisjunction is regularly takes place in oocytes and produced the following X^vX^v  and O unusual eggs
• Such highly regular disjunction only takes place when the two X chromosomes shared the same centromere and behave as a single chromosome this fused form is known as attached X chromosome.

• Sex linkage in human and other organisms
• In addition to drosophila sex linked inheritance is also known in mice, cat , insects, cattle , in man
• Drosophila had more than 150 sex linked genes  and over 200 genes in humans most of them cause genetic diseases. Example haemophilia ( failure in blood clotting ability), colour blindness ( inability to see one or more colour), optic atrophy (optic nerve degeneration), myopia(short-sightedness), etc.
• For example like colour blindness it is a sex linked disorder occurs in 5-10% males and only 1% to the females. Human has there protein for red green and blue colour present on cone cells genes encoding for red and green light are located on x chromosome  and gene for blue light is placed on autosome.

Partial sex linkage

X and Y chromosome in human aur morphologically different. But during meiosis they pair in male cell

The paring  in two telomeric region is called as pseudoautosomal regions

1. PAR1 major region of 2.6 Mb long, located at the tip of short arm of X and Y chromosomes, it is 70 times more than the normal recombination frequency. Example SHOX, WE7, Tramp
2. PAR2 minor region of 320 kb located at the tip of the long arms of X and Y chromosomes, crossing over is not so frequent. Example IL9R and SYBL1.

X chromosomes contain PAR1 and PAR2 but this genes don’t show inheritance pattern of sex linkage because Y Chromosome but they resemble of autosomal genes.it  is known as partial sex linkage  gene X chromosome show  autosomal pattern