Updated by: Arabinda Naik
Sex chromosomes
Every cell in human body has DNA that tightly packed into compact structure called chromosomes. They are present inside the nucleus of the cell there are 23 pairs of chromosomes of which 22 pairs are called autosomes and the 23rd pair is called allosomes or sex chromosomes.
Autosomes
Autosomes are homologous pairs, which transfer genetic information from the parents to the offspring. Each pair of chromosome contains all the genes that are responsible for determining the somatic characters. Although autosomes are not sex chromosome, they contain a few genes that determine sex male or female. There are 22 pairs of autosomes in every cell. In each pair, the chromosomes are of same length with the centromere placed in the same location. During mitosis, these chromosomes are duplicated and transferred to two daughter cells. So, the new daughter cells get a complete copy of chromosomes containing the genetic information from the parent cell. Autosomes are labelled from one to 22, based on number of base pairs present within them. Autosomal genetic disorders occur due to either the non-disjunction in parent’s chromosome aneuploidy during gametogenesis or the mendelian inheritance of deleterious alleles. An example of for aneuploidy is Down’s syndrome which possesses three copies of chromosomes 21 per cell. Disorders with mendelian inheritance can either be dominant or recessive examples sickle cell anaemia. In men 23 pairs of chromosomes. 22 pairs are autosomes. In drosophila 4 pairs of chromosomes present, out of four three pairs are autosomes.
Allosomes
Those chromosomes of an individual organisms that are involved in the determination of sex of that individual or called sex chromosomes. The 23rd pair of chromosomes present in human is called the allosomes. They differ in size and function when compared to autosomes. They determine the sex of an individual. They are labelled using letter X and Y. Allosomes in female comprise two XX chromosomes, and in male it comprises of one X chromosome and one Y chromosome. These chromosomes have a different location of centromere.The Y chromosome contains few genes only while the X chromosome contains more than 300 genes. In allosomes, the Y chromosome is slightly shorter than the X chromosome. The Y chromosome contains a gene, SRY which triggers embryonic development as a 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.
XX-X0 type
XX-XO type (XX female-X0 Male)
XX-X0 type (XX female-XO Male) (bug, grasshopper, cockroach) The female has two sex chromosomes XX while the males have only one X chromosome and produce two type of sperms. X chromosome and without X chromosome. Therefore, the males are XO. The females are homogametic because they produce only one type of eggs (A + X). The males are heterogametic with half the male gametes gymnosperm 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.
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)
Genic balance Theory
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 1/2 = 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 3/4 = 0.75 INTER SEX
XXXX + 4A 4/4 = 1.0 FEMALE.
Sex determination in Honeybee
Honeybees use a haplodiploid sex determination mechanism. In honeybees, the male is haploid while the female is diploid Queens and worker bees are produced from fertilised honeybee eggs, which are diploid and have two sets of chromosomes. Male insects are haploid because they develop partheno-genetically from unfertilized eggs. The male progeny of honeybees develops naturally from unfertilised eggs, which are haploid and have only one set of chromosomes. The phenomenon is called arrhenotoky. Meiosis does not occur during the formation of sperms. Females grow from fertilized eggs and are hence diploid. Queen bee picks up all the sperms from the drone during nuptial flight and stores the same in her seminal vesicle. Formation of worker bees (diploid females) and drones (haploid males) depends upon the brood cells visited by the queen. While visiting the smaller brood cells, the queen emits sperms from its 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. 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 male has half number of chromosomes that a female has and are diploid.
A male has no father and cannot have sons, but he has a grandfather and can have grandsons.
In honeybee, 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. 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 derived from the father. Those female larvae which are throughout fed on a richer diet of royal jelly develops into Queen while others develop into workers.
Environmental control of sex determination
In some animals, the environment plays an important role in the differentiation of sex or in the expression of genes encoding male and female characters.
A worm (Bonella), the larvae are potentially.
hermaphrodite.If a newly hatched worm is reared from a single egg in isolation, it develops as a female. If newly hatched larvae are reared in water containing mature females, some larvae adhear to the proboscis. These are transformed into males, which eventually migrate into the reproductive tract, where they become parasitic. It has been established that the proboscis of the mature female Bonellia secretes a chemotactic substance, which induces the larve to differentiate as males
Temperature-dependent sex determination:
This is a type of environmental sex determination, in which the temperature experienced during embryonic development determines the sex of the offspring.
Bonella a marine worm, the swimming larva has no sex. It is settles down alone and it develops into a large female. If it lands on or near an existing female a chemical secretes from female and the larva develop into a tiny male. In turtles, alligators a temperature below 28C produces more males, above 33C produces more females and between 28C-33C produces males and females in equal proportion.
Hormonal theory
Hormones are secretion of the endocrine glands which modify the sex. They are responsible for the expression of secondary sexual characters. An example, a hen laid fertile eggs, but if it lost its ovary or artificially removed ovary it stopped laying eggs and developed male characters such as comb and male plumage and become a cock. This is because the removal of ovary led to stoppage of production of ovarian hormones. But the rudiment of testis become functional by the loss of ovary and produced male hormone which is responsible for appearance of male secondary sexual characters.
Sex reversal
It is observed in fishes, amphibians, birds and even in some mammals. Artificial removal of gonads of either sex before puberty in mammals and even in human (castration or ovariectomy) results in the development of secondary sexual characters of the opposite sex reversal in chick.
Free martin
In cattle when the twins of the opposite sex are born the male is normal, but the female is sterile with many male characteristic features. Such sterile female is called free martin. During the development foetal membrane of the twins have a common blood circulation. Since the male hormone appears earlier in the development it passes into body of the underdeveloped female through blood circulation and influence the female foetus to become sterile and causes partial sex reversal of the female.