Sex linked Inheritance,Linkage with gene and chromosome

LINKAGE :
The persistent association of genes located in same chromosome which are inherited together in same position from one generation to another is called Linkage.Gene present in the same chromosome is called linked gene.
The main significance of linkage is that it reduces the possibility of variation in gametes unless crossing over takes place.
CHROMOSOME THEORY OF LINKAGE:
Morgan and castle formulated the Chromosome theory of Linkage according to which he listed the following points:
1)Gene which show phenomenon of linkage are located in same gene.
2)They are arranged in linear fashion in the chromosome.
3)The strength of linkage depends upon the distance between the linked genes in the chromosomes.The closely located genes show strong linkage while the widely located genes show weak linkage.
4)The number of genes per individual is far more than the number of chromosome pair.Thus,each chromosome bears many gene.
5)Linked gene remain in their original combination during the course of inheritance.
TYPES OF LINKAGE:
1)COMPLETE LINKAGE:
The linkage in which character are inherited together in their original combination for two or more generation in continuous and regular fashion is called complete linkage.There is always parental combination's and no any recombination.Margon reported complete Linkage in Drosophila.
Morgan crossed a wild Drosophila having grey body and normal wings(BBVV)with a black body and vestigial wing(bbvv).All the hybrids in F1 generatio have grey body and normal wings(BbVv).when F1 male hybrids are crossed with a double recessive female having black body and vestigeal wing(bbvv),in F2 generatio the offspring of 2 types in equal proportions are produced.These offspring's resembled their grandparents.
Parents Grey,Normal * Black,Vestigeal
BBVV bbvv
Gametes BV * bv
F1 generation BbVv

Test cross F1 male grey,normal * female black,vestigeal
BbVv bbvv
gametes Bv bv * bv

Test cross Grey ,normal * Black,Vestigeal
ratio BbVv bbvv
50% 50%
The result indicates that the grey body character is inherited along with normal wings.Its Suggest that the genes are linked together.

2)INCOMPLETE LINKAGE:
Incomplete linkage is defined as the separation of linked gene due to crossing over.In this type of linkage,genes are widely located in the chromosome. Incomplete linkage was demonstrated by Hutchinson in maize plant.He crossed a maize plant having coloured and full seeds(CCSS)with Colourless and shrunken seeds(ccss).The F1 hybrids appeared with coloured and full seed(CcSs).When the F1 hybrids are crossed with double recessive parents
(ccss),they showed four types of progeny in the following ratio:

Parents Coloured,Full * Colourless,Shrunken
CCSS ccss
Gametes CS * cs
F1 hybrids CcSs

Test cross F1 coloured,full * Colourless,Shrunken
CcSs ccss
Gametes CS Cs cS cs * cs
Progeny Coloured full coloured shrunken colourless full colourless shrunken
CCSS CcSs ccSs ccss
48% 2% 2% 2%

SEX LINKED INHERITANCE:
There are two types of chromosomes in living organism.They are:
1)Autosomes:
Chromosomes bearing genes for somatic organs are autosomes.There are 22 pairs of autosomes in human beings.genes linked with the autosome shows autosomal Linkage.
2)Sex chromosome/Allosome:
Chromosome that determine the sex of the species is called sex chromosome.Human has one pair of sex chromosome.Sex chromosome in female is X and in male is Y.Gene linked with sex chromosome show sex linkage.In most of the animal including human beings,female have two alike rod shaped chromosome"XX" while male has one rod shaped chromosome"X" and another hooked shaped chromosome"Y".
Genes located in X chromosome is sex linked gene and the pattern of inheritance of such gene is called sex linked inheritance.These genes are inherited along with X chromosome.In most of the animals,Y chromosome is genetically inert.it may or may not contain genes.Genes present in Y chromosome are called holandric genes.

CRISS CROSS INHERITANCE:
The inheritance in which parents transmits their sex linked traits to their grandchildren of same sex through their offspring of opposite sex is called criss cross inheritance.For eg:Father transmits their sex linked traits to their grandson through daughters.

SEX LINKED INHERITANCE IN DROSOPHILA:
Sex linked genes were reported for the first time by American Zoologist T.H morgan while studying colour in Drosophilla melanogaster.Morgan,while working in Drosophilla found a male fly with white eye colour in the culture of red eye Drosophilla.The normal eye colour of Drosophilla is red.Actually the gene for white eye arose as a mutation of gene for red eye.Morgan and his associates,from numbers of crosses found that the inheritancce of eye colour in Drosophila was associated with sex chromosome of the parents.Drosophila has a pair of sex chromosome.Female has alike rod shaped chromosome XX whle male has One rod shaped chromosome X and another hook shaped chromosome Y.
Morgan crossed a red eyed female and white eyed male.The F1 hybrid were red eyed out of which half were male and half were female.When the red eyed male and red eyed female of F1 generation were intercrossed then all the F2 females were red eyed and out of two F2 males half were red eyed and half white eyed.

Parents Red eyed female (RR) * White eyed male(WW)
Gametes XR XR XW Y

F1 hybrids XRXW XRXW * XRY XRY

Red eyed female Red eyed male

F1 hybirds Red eyed male(XRY) Red eyed female(XRXW)
Gametes XR Y * XR xW
f2 Hybrids XRXR XRXW XRY xwY
Red eyed female Red eye male white eyed male

Thus,this shows that the white eye character is transmitted from grandfather to grandson through daughter.

CHARACTERISTICS OF SEX LINKED INHERITANCE:
1)Criss cross inheritance:
It is type of sex linked inheritance in which parents passes their sex linked traits to their grand child of same sex through offspring but of opposite sex.
2)Sex linked traits are recessive.
3)Males suffer more from sex linked disorders.
4)Recessive gene can express in female only in homozygous state otherwise female are only carriers.
5)Traits governed by sex linked recessive gene:
a)Produce disorders in male.
b)Express when represented by single allelle because Y-chromosome don't carry any allele.
c)Seldom occurs in father or son.
d)Fail to appear in female unless their father also posses the same and mother is carrier.
6)Traits governed by sex linked dominant genes:
a)Produces more disorder in female.
b)All the femal offspring will exhibit if father produces the same.

SEX LINKED INHERITANCE IN HUMAN BEINGS:
1)COLOUR BLINDNESS:
It is a condition in which a person is unable to distinguish between red,green or both colours from other colours.It is also known as Red Green colour blindness.or Daltanism.THe gene for this red green colour blindness is present on X chromosome.
eg.
When a colour blind man(XcY) marries a normal woman(XX) then all their male and female children are normal but the female children are carrier.If carrier female marries a normal male than two normal visioned female,one normal visioned male and one colour blind male is produced.
Parents Colour blind man(XCY) * Normal woman(XX)
Gametes XC Y * X X
F1 hybids XCX XCX XY XY
carrier females normal male

Carrier females(XCX) Normal male(XY)
Gametes XC X * X Y
F2 hybrids XCX XCX XCY XY
Carrier female colour blind Normal male
male
2)HAEMOPHILIA(BLEEDERS DISEASE):
A disease in which suffering people donot have ability for blood clotting is called Haemophilia.As Queen Victoria of England was its carrier it is also known as Royal disease.The pattern of inheritance is criss cross.
If a normal woman marries a haemophilic man then both the F1 sons are normal whereas F1 daughters are normal but carrier.When carrier female marries normal male then the F2 daughters are normal whereas among two F2 sons,one son is normal and the other is Haemophilic.
Parents Normal woman(XX) Haemophilic man(XHY)
Gametes X X * XH Y
F1 hybrids XHX XHX XY XY
Carrier female normal male

Parents Carrier female (XHX) Normal male(XY)
Gametes XH X * X Y
F2 hybrids XHX XX XHY Xy
Carrier female Normal female Haemophilic maleNormal Male