genic balance theory

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What Is Genic Balance Theory?

C.B. Bridges in 1922 proposed the Genic Balance Theory for sex determination while working on his experimental model, Drosophila.

Like mammals, XX sex chromosome determines female character and XY as males in Drosophila. However, unlike humans, Y chromosome is not responsible for maleness in Drosophila, as Y chromosome is heterochromatic in nature and thus not active in sex determination.

According to Bridges, sex determination in Drosophila depends on the X/A sex index ratio. Following are the sex index ratios for maleness and female characters:

If X/A = 1.0 it means normal female
If X/A = 0.5, it means normal male
If X/A = <0.5, it means super male
If X/A = 0.5 to 1.0, it means intersex
If X/A = >1.0, it means super female

For example, 3A+XXX: X/A = 3/3 = 1.0 => Normal female
2A+XO: X/A = 1/2 = 0.5 => Normal male

The theory of Bridges that the sex of a fruit fly is determined by the relative number of X chromosomes and autosomal sets.

Modern Theories of Sex Determination (with Diagrams)

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The problem of sex determination has been one of the most important biological puzzle up to the year 1900. A number of theories were postulated from time to time by the biologists to explain this critical phenomenon.
Hippocrates and other theorists believed that the age or vigour of the parents was responsible for determining the sex of the offspring. The older or more vigorous parent tries to impress its sex upon the offspring.
According to some philosophers if an egg is fertilized soon after ovulation it gives rise to female but if it remains in the oviduct for sometime before fertilization it produces male. Galen and various others claim that germ cells from the right ovary produce males while the germ cells from the left ovary produce females.
Professor Shenk of Vienna put forward the Nutrition Theory. He assumed that high degree of nourishment to the mother brings about female offspring while less nutrition causes male offspring’s. These speculations of the early biologists are now replaced by more genetic and scientific theories.
Modern Theories of Sex Determination:

The Modern Theories Given For Sex Determination Are As Follows:
(1) Chromosomal theory or Theory of Heterogamy
(2) Genic balance theory
(3) Hormonal theory
(4) Theory of environmental factors.
(1) Chromosomal Theory or Theory of Heterogamy:

The complete account of chromosomal sex determination was at first worked out by Stevens (1905). This view was later supported by other scientists such as Wilson, Bridge, Goldschmidt and Whitings.
In majority of sexually reproducing animals two types of chromosomes are found:
(i) Autosomes:
They are found in all cells. The genes present on autosomes are responsible for determining the somatic characters but sometime influence the sex of the organism. The two members of each homologous pair are similar in shape and size (homomorphic).
(ii) Sex Chromosomes or Allosomes:
They carry genes for sex. A pair of them determines the sex. They are variously named as X and V chromosomes (Man and Drosophila), Z and W chromosomes (Birds and Moth), odd chromosomes, idiosoines, heterosomes or allosomes. The genes which determine the sex are being located on these chromosomes. The two members of this pair arc often dissimilar in male and are represented as X and Y chromosomes or as Z and W chromosomes.
(a) XX Female and XY Male Types:
This type of sex mechanism is found in Drosophila (fruitfly) and majority of mammals including man. In this type the female is homogametic (XX) and male is heterogametic (XY) consisting of two dissimilar chromosomes X and Y. The females produce ova all of one type having X chromosome. Males produce two types of sperms: -50% with X-chromosome and remaining 50% with Y-chromosome. Thus, the sex chromosomes in female are homomorphic and those of male are heteromorphic (Fig. 5.13).
http://www.biologydiscussion.com/wp-...02_thumb53.jpg
(b) ZW Female and ZZ Male Type:
In butterflies and birds, the female is heterogametic having dissimilar Z and W chromosomes whereas the male is homogametic having similar ZZ chromosomes (It is a convention to designate female as ZW instead of XY and male as ZZ instead of XX). The situation here is just reverse to first type.
(c) XX Female and XO Male Type:
Mc Clung and Wilson (1903) described this type of sex mechanism in insects especially in grasshopper. In male there is no mate for X chromosome, hence the name XO is given, there is no Y chromosome. They produce sperm of two types, 50% with X chromosome and 50% without X. In females there are two similar or homomorphic sex chromosomes XX.
2. Genic Balance Theory:

This theory was proposed by Calvin Bridges (1921). From his study on Drosophila he suggested that the X chromosomes carry factor for femaleness whereas autosomes “A” carry genes for maleness. Y chromosome does not take part in sex determination. From the Fig. 5.15 it is clear that the genic balance is governed by the ratio of the number of X-chromosomes to the number of sets of autosomes in the zygote at fertilization.
http://www.biologydiscussion.com/wp-...04_thumb27.jpg II the ratio is 1.0 the offspring develops into female, but if it is 0.5, then the offspring develops into male. If the ratio is intermediate between 1.0 and 0.5, the resulting individual is neither a male or nor a female but an inter sex. Super females have a ratio of 1.5 and super males have a ratio 0.33.