sex linked inheritance in drosophila and man
TRANSCRIPT
Patterns of Inheritance: Sex Linked Inheritance in
Drosophila
DR. ASIM MASOOD
Patterns of Inheritance
It is a manner in which a particular genetic trait or disorder is passed from one generation to another.
Why is it important to study patterns of inheritance?
The reason to study patterns of inheritance of disorders within families is to enable advice to the family members regarding the likelihood of their developing that particular disorder or passing it to their children.
Two kinds of patterns are generally studied based on types of chromosomes:
1. Autosomal dominant inheritance
2. Sex linked inheritance
Autosomal Dominant Inheritance
Both males and females are affected.
Transmission of disease can be from both the sexes.
Mutation in one allele is enough to express the disease.
The offspring have 50% chance to have been affected.
Genes for autosomal inheritance are located on the autosomes.
Examples : Huntingston disease, polycystic kidney disease.
All dominant parent type were produced
Female Male
RR rr
R R r r
F1
Rr Rr Rr Rr
Autosomal Dominant Inheritance
1. Monohybrid Cross
75% dominant type and 25% recessive type were produced
Ratio--- 3:1
Female Male
Rr Rr
R r R r
F1
RR rr Rr Rr
2. F1 selfing (Inbreeding)
50% dominant type and 50% recessive type were produced
Ratio--- 1:1
Female Male
Rr rr
R r r r
F1
Rr rr rr Rr
3. Test Cross
All dominant types were produced
Female Male
rr RR
r r R R
F1
Rr Rr Rr Rr
4. Reciprocal Cross
Sex Linked Inheritance
Refers to the pattern of inheritance shown by genes that are located on either of the sex chromosomes.
Genes carried by X-chromosome referred to as X-linked genes.
Genes carried by Y-chromosome referred to as Y-linked or Holandric genes.
Example: Eye color in Drosophila is carried by X-chromosome.
Four cross: Monohybrid cross, Selfing, Test Cross and Reciprocal cross are genrally done to study the inheritance pattern in a population.
All dominant parent type (Red eyed) were produced.
Females produced however are all carriers.
Note: Red eye is dominant over white eye and the gene is carried by X chromosome
Sex Linked Inheritance
1. Monohybrid Cross: Red eyed female (WW) crossed with white eyed male (w-)
Female (XX) Male (XY)
WW w-
W W w -
F1
Ww W- Ww W-
75% dominant type (Red eyed) and 25% recessive type (white eyed) were produced with ratio
All females are red eyed. While, 50 % males are red eyed and 50% males are white eyed.
Sex Linked Inheritance
2. Selfing: Red eyed carrier female (Ww) crossed with red eyed male (W-)
Female (XX) Male (XY)
Ww W-
W w W -
F1
WW w- Ww W-
50 % females are red eyed and 50% females are white eyed. Red eyed females are carriers.
50 % males are red eyed and 50% males are white eyed.
Test Cross ratio = 1:1.
Sex Linked Inheritance
3. Test Cross: Red eyed carrier female (Ww) crossed with white eyed male (w-)
Female (XX) Male (XY)
Ww w-
W w w -
F1
Ww w- ww W-
All females are red eyed (carrier).
All males are white eyed.
Sex Linked Inheritance
4. Reciprocal Cross: White eyed female (ww) crossed with red eyed male (W-)
Female (XX) Male (XY)
ww W-
w w W -
F1
Ww w- Ww w-
Conclusion
An X-linked recessive trait is one determined by gene carried on the X-chromosome and usually only manifest the male population.
In case of autosomal inheritance males and females are equally affected as observed in autosomal crosses we made.
While in Sex-linked inheritance number of males are more affected than females, since the -gene for white/red eye are present on X-chromosome.
Note: In all four crosses only one female produced is white eyed while 4 males are white eyed. Monohybrid cross (none), Selfing ( 1 male), Test Cross ( 1male and 1female) and Reciprocal cross (2 males) are generally done to study the inheritance pattern in a population.