A. Sex Chromosome mechanism The chromosomal basis for the determination of sex was first proposed by Clarance Mc clung in 1902. He observed gametogenesis in grasshopper (Xiphidium fasciatum).He reported that while the somatic cells of the females had 24 chromosomes the males had 23 chromosomes. Similar observations were made in bugs and beetles by several other workers. By further studies it was concluded that in dioecious organisms there are two types of chromosomes. Among them the chromosomes concerned with body or somatic characters were named as autosomes (A). The other type of chromosomes concerned with sex determination were called sex chromosomes (X and Y). Later it was found that sex chromosomes (X and Y) had structural differences. The cytological studies have shown that X-chromosomes in most cases are straight, rod-like and slightly longer. The Y-chromosome was found to be smaller with one end curved or bent to one side (as in Drosophila).
Based on sex chromosomal determination of sex, there may be two types of organisms, namely 1. Heterogametic males 2. Heterogametic females. Heterogametic males :- In this type of sex determination the female has two x-chromosomes. The males have only one x-chromosome. During gametogenesis the males could produce two types of gametes ie., one type having ‘X-chromosomes’ and the other without ‘X-chromosome’. Hence the males could be called the heterogametic sex. The females can produce only one type of gametes ie., all eggs having one ‘X’-chromosome each. Hence the female could be called the homogametic sex. Each type of sex determination has subtypes as shown in the table.
B. Genic balance mechanism Further studies on sex determination showed that sex determination was not the inheritance of genes by the sex chromosomes alone. Studies on intersex and supersex indicated the operation of still more a complex mechanism for the determination of sex. Thus the genic balance mechanism of sex was discovered. It was first described by C.B Bridges in 1921. In Drosophila it was discovered that the sex of an individual depends upon the ratio of x chromosomes to the autosomes. For sex determination, each haploid set of autosomes carry factors with a male determining value equal to one (1). Each x chromosome carries factors with a female determining value of one and a half (1 .5 ). Hence in a normal male (AAXY), the male female determinants are in the ratio of 2 : 1.5(‘A’ represents a haploid set of autosomes). Thus the genic balance is in favour of maleness. A normal female (AAXX) has the male female determination ratio of 2:3. Therefore the balance is in favour of femaleness
Sex determination in Human beings The human sex determination mechanism to a larger extent resembles XX – XY type of Drosophila. However, the Y chromosome contains male determining genes and it is the determiner of fertility and sex of male individual. Thus in human beings, the presence of Y chromosome determines maleness and its absence determines femaleness. Evidences to establish this viewpoint had been provided by certain abnormal conditions called syndromes.
Sex anomalies in Human beings
1. Turner’s syndrome (XO Females) : In this abnormality the females are sterile and have short stature. They have webbed neck, broad shield-shaped chest, low intelligence, under developed breasts and poorly developed ovaries. These conditions result due to the presence of 44 autosomes and only one X chromosome in her body cells. This abnormality is known as Turner’s syndrome.
2. Klinefelter’s syndrome :- This syndrome is caused due to the presence of an extra X chromosome in males. This happens when an abnormal egg with XX chromosome is fertilized by a sperm carrying Y chromosome. The zygote will have three sex chromosomes (XXY). The resulting young one is an abnormal sterile male. The symptoms of this syndrome are the presence of small testicles, mental retardation, longer arms and high pitched voice.
3. Super females :- These females are also known as Poly X females. They possess an extra X-chromosome (44 autosomes +3 x chromosomes). The poly X females are mentally retarded and sterile.
4. XYY males :- Such males will have an extra Y chromosome (XYY). This condition results in mental retardation and criminal attitudes
5. Hermaphroditism :- A hermaphroditic person will have one extra X and Y chromosome. The person will have both ovarian and testicular tissues. The external genitalia will not be well defined.
C. Male haploidy or Haplo-diploidy mechanism. This mechanism is also known as arrhenotokus parthenogenesis. It is a common mechanism in several insects such as ants, bees and wasps. In these insects, fertilized eggs develop into diploid females and unfertilized egg into haploid males. In a honey bee colony a queen bee can lay two types of eggs. They are the fertilized and unfertilized eggs. It happens by controlling the sphincter in the sperm receptacle of the female. The diploid female zygote can develop either into a queen or a sterile female worker bee. The sterile nature of the worker bee is due to poor nourishment. The haploid zygote develops into a male. This mechanism of sex determination helps to maintain the polymorphic nature in a honey bee colony. Apart from genetical systems, the sexuality can also be controlled by various factors such as metabolism, environment and hormones
Sex in Bonellia Bonellia viridis is a marine worm. Its sex determination was studied by F. Baltzer(1935). The adult female worm is about 2.5 cm long. It has a well defined anatomical organization
The male is very small and microscopic. Its body organs are rudimentary. The males normally live as parasites attached to females. All larvae of Bonellia are genetically similar. However a larva settling on the proboscis of an adult female becomes a male individual. If a larva develops in isolation (ie., in the absence of a female) it develops into female. If a developing male is detached from the proboscis of female, it becomes an intersex. From these observations it could be inferred that the proboscis of adult female secretes some hormone like substance and that substance suppresses femaleness and induces maleness in the larvae which remains attached.
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