Genic Balance Theory

Genic Balance Theory

DR. ROHAN GAVANKAR

Multiple Choice

What is the sex determination mechanism in Humans

• XX- XY

• XX- XO

• ZZ-ZW

• ZZ-ZO

Multiple Choice

In Humans and Fruit flies the males are heterogametic and females are homogametic

• True

• False

Sex determination in Humans

• In Humans, Y chromosomes determines the maleness irrespective the number of variants in X chromosomes.

• So XX are females and XY are males

• But if the Cytogenetics is XXY or XXXY then ​

  What will be these individuals phenotypically ?​

• Is this same method followed in Drosophila ??????

Genic Balance Theory of Sex determination

• Chromosomal sex determination: Individual will possess specialized chromosomes called SEX CHROMOSOMES

• Genic Balance Theory: Presence or absence of Sex chromosomes or relative ratio of these sex chromosomes will determine the sex of the organism.​

• Example: Drosophila

• In fruit fly Females are XX and males are XY but the mechanism of sex determination in Drosophila is different from Humans​

Genic Balance Theory of Sex determination

• Drosophila has 8 chromosomes (n =4 )​

  Three pair of autosomes and one pair of allosomes (Sex chromosomes)​

• XX- females and XY- males but Y doesn't determine maleness like it does in humans​

• Sex in drosophila is determined by ratio of ​

  X chromosomes : Autosomes.  

• BINGO- Genic Balance Theory​

Genic Balance Theory

• Sex determination in Drosophila is based by striking the balance between the female determinants on the X chromosome and male determinants on the autosomes.​

• This theory is called as Genic balance theory, was proposed by Calvin Bridges in 1926​

• This theory also explains about the variants like intersex, meta males and meta females in fruit flies.​

• 
  

Multiple Choice

Genic balance theory is based on the ratio of _______

• Autosomes : Sex chromosomes

• Sex chromosomes : Autosomes

• One autosome : two sex chromosome

• X Chromosome : Autosome

Multiple Choice

Who proposed Genic balance theory

• Mendel

• Calvin Bridges

• Morgan

• Albert

Genic Balance Theory of Sex determination

• It is based on X/A ratio where X is number of X chromosomes and A is no of autosome sets​

• If ratio =1 it is a females​

  Files will be Males when the ratio is 0.5​

• Between 0.5 to 1 are inter-sex flies (shows both male and female characteristics)​

• If the ratio is greater than 1 than the flies are meta females (several developmental problems​)

• If the ratio is lesser than 0.5 than the flies are meta males (sterile and weak)​

Genic Balance Theory of Sex determination

• HOW DO WE CALCULATE........?​

• Generally the flies may have one X or two X chromosomes and Two sets of autosomes ​

• So if there are 2 X then 2/2 (X/A) = 1 ….... Females​

• And If there is 1 X then ½ (X/A) = 0.5 ......... Males

• 
  

Multiple Choice

Drosophila males have ratio ____

• 0.5

• 1

• less than 1

• more than 1

Multiple Choice

Drosophila Females have ratio ____

• 0.5

• 1

• less than 1

• more than 1

image source:- https://www.slideshare.net/EasyBiologyClassEBC/genic-balance-theory-of-sex-determination-in-drosophila-melanogaster-ppt-by-easybiologyclass

Multiple Choice

SOLVE: IF THERE IS ONE X CHROMOSOME AND ONE Y CHROMOSOME WITH 2 SETS OF AUTOSOMES IN FRUIT FLY, THE FLY IS ______

• MALE

• FEMALE

• INTERSEX

• META MALE

Multiple Choice

SOLVE: IF THERE ARE TWO X CHROMOSOME AND ONE Y CHROMOSOME WITH 2 SETS OF AUTOSOMES IN FRUIT FLY, THE FLY IS ______

• MALE

• FEMALE

• INTERSEX

• META MALE

Genic Balance Theory of Sex determination

So when we talk about this theory there are many inputs but exact mechanism is still unclear​

It is said that X chromosomes has female determinants and the autosomes have male determinants​

The Y chromosomes has no role to play in DETERMINATION OF MALENESS--- In SEX DETERMINATION in fruit flies​

IS THIS CONCEPT CLEAR​

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