Give the account of the chromosomal basis of sex determination in animals      

                                                                                            

Sex determination in human                                                                     

In case of man and the total number of chromosomes is 23 pairs. Out of 46, 44 chromosomes are autosomes (A) and two other chromosomes are sex chromosomes XX female XY male. In male diploid cells contain 44A +XY chromosome and in female 44A + XX. The female produces one type of eggs (22 + X). The male produce gamete are two types. (22+X) and (22 + Y). Sperm with 22A +X unites with egg having 22A +X chromosomes the resultant zygote will be 44A +XX and it will develop into a female baby. But if a sperm with 22A +Y fertilize an egg with 22A + X chromosome the zygote will be 44A + XY and the baby will be male.  

XX female and XY male types:This type is present in certain insects like Drosophila and mammals including human. In this type of the female is homogametic XX and male is heterogametic XY consisting of two dissimilar chromosomes X and Y. The female produces ova all of one type having X chromosome. Male produce two types of sperm X and Y. Thus, in this type of sex determination the presence of Y chromosomes determines the maleness. 

Sex determination in Drosophila: In drosophila 8 numbers of chromosomes are present. Among which 6 autosomes and 2 sex chromosomes. So, the male has three pairs of autosomes and XY chromosomes (6 + XY) and female have three pairs of autosomes and XX chromosomes (6 + XX). The male is heterogametic and produce two types of sperm (3+X and 3+Y) and the female are homogametic and all ova contain one type of ova (3+X and 3+X)  

3 + X 3 + Y 

3 + X 6 + XX 6 + XY 

3 + X 6 + XX 6 + XY 

It is found that male or female daughter formation chance is always 50%. The ovum fertilized by the X carrying sperm develops into a female while ovum fertilized by the Y carrying sperm develops into a male. 

Genic balance theory of sex: This theory was proposed by Calvin Bridges. From his study on Drosophila the X chromosomes carry factor for femaleness whereas autosomes A carry genes for maleness. Y chromosome does not take part in sex determination. If X/A ratio is 1.0 the offspring develops into female. But if it is 0.5 then the offspring develops into male. If the ratio intermediate between 1.0 and 0.5 the resultant individual is neither a male nor a female. If ratio is 1.5 it is super female and if ratio is 0.33 it is super male. 

Chromosome complement X/A ratio Sexual morphology 

XX + 2A 2/2 = 1.0 FEMALE 

XX + 3A 2/3 = 0.67 INTER SEX 

XY + 2A 1/2 = 0.5 MALE 

XO + 2A  ½ = 0.5 MALE 

XY + 3A 1/3 = 0.33 META MALE OR SUPER 

MALE 

XXX + 2A 3/2 = 1.5 SUPER FEMALE OR 

META FEMALE 

XXX +3A 3/3 = 1.0 FEMALE 

XXX + 4A ¾ = 0.75 INTER SEX 

XXXX + 4A 4/4 = 1.0 FEMALE. 

 

Gynandromorphy in Drosophila: In drosophila occasionally flies are obtained in which a part of the body exhibits female characters while the other part male characters. These develop due to failure of segregation. Of X chromosome. The zygote starts with 2A + 2X chromosome. During first cleavage one of the X chromosomes is lost. As a result, one of the blastomeres acquires 2A + 2X while the other 2A + X. at the end of development 2A + XX differentiate as female phenotype while 2A + X differentiate as male phenotype. Thus, half of the body is female while the other half is male. 

It is three types. Bilateral gynandromorph: Male character on one lateral side of body and female on the other. Anterior posterior gynandromorphy: In this type of male character present on anterior side and female character on the posterior side. Sex piebald: In this type of individual is male or female with patches of tissue of the other sex scattered on it. 

                                                                                                                                                                

XX-X0 type (XX female-X0 Male) (bug, grasshopper, cockroach) 

The female has two sex chromosomes XX while the males have only one X chromosome and produce two types of sperms. X chromosome and without X chromosome. Therefore, the males are X0. The females are homogametic because they produce only one type of eggs (A + X). The males are heterogametic with half the male gametes gymnosperms carrying X chromosome (A + X) while the other half androsperms (A + 0). The produced sex ratio in progeny is 1:1. Ova fertilized by a sperm carrying X chromosome develop into females while ova fertilized by a sperm carrying no X chromosomes develop into males.  

 A + X A + 0 

A + X AA + XX AA + X0 

A + X AA + XX AA + X0 

 

AA + XX female     AA + X0 Male 

 

ZZ-ZW type                                                                                    

ZZ-ZW type (bird, butterfly, moths reptiles) 

It is just reverse of the method of man and drosophila. In birds the female is heterozygous whereas the male is homozygous. A male bird has two like sex chromosomes which are ZZ chromosomes, and a female bird has two unlike sex chromosomes which are named as ZW chromosomes. In male bird it produces only one kind of sperm carrying Z chromosome but in female it produces two kind of eggs half contain Z chromosome and other half has W chromosome. During fertilization if a sperm fertilizes an egg with Z chromosome the resulting individual will be male ZZ. If the egg carries a W chromosome the individual produced will be a female bird. (ZW) 

 A + Z A + Z 

A + Z AA + ZZ AA + ZZ 

A + W AA + 

ZW AA + ZW 

 

AA + ZZ male bird 50% AA + ZW female bird 50% 

ZZ-ZO type                                                                                               

(butterfly,moths) This mechanism occurs in certain butterfly and moths. The female is heterogametic and produce two types of eggs half with Z chromosome and half without Z chromosome. The males have homomorphic sex chromosomes and is homogametic. It forms only one kind of sperms each with Z chromosome.  

 A + Z A + 0 

A + Z AA + ZZ AA + Z0 

A + Z AA + ZZ AA + Z0 

 

Haplo-diploidy mechanism (honeybee, wasp)                                          

It is a type of sex determination in which the male is haploid while the female is diploid. Males are haploid because they develop partenogenetically from unfertilized eggs. The phenomenon is called arrhenotoky. Meiosis does not occur during formation of sperms. Females grow from fertilized eggs and have diploid chromosome. Queen bee picks up all the sperms from drone during nuptial flight and store the sperms in her seminal receptacles. While queen visited smaller brood cells, the queen emits sperms from the seminal receptacle over the eggs. As it visits the larger brood cells it lays the eggs, but the seminal receptacles fail to emit the sperms due to low pressure.  In this system, sex is determined by the number of sets of chromosomes an individual receives. An offspring formed from the union of a sperm and an egg develops as a female, and an unfertilized egg develops as a male. This means that the males have half the number of chromosomes that a female has and are haploid. The haplodiploid sex-determination system has several peculiarities. For example, a male has no father and cannot have sons, but he has a grandfather and can have grandsons.  

In honeybees, the drones (males) are entirely derived from the queen, their mother. The diploid queen has 32 chromosomes, and the haploid drones have 16 chromosomes. Drones produce sperm cells by mitosis that contain their entire genome, so the sperm are all genetically identical. The male bees' genetic makeup is therefore entirely derived from the mother, while the genetic makeup of the female worker bees is half derived from the mother, and half from the father Those female larvae which are throughout fed on a richer diet of royal jelly develops into queens while others develop into workers.