NEET Biology Principle Of Inheritance And Variation Sex Determination

Sex Determination

Fixing the sex of an individual as it begins life is called sex determination. The various genetically controlled sex-determination mechanisms have been classified into following categories

(i) Chromosomal theory of sex determination: The X-chromosome was first observed by German biologist, Henking in 1891 during the spermatogenesis in male bug and was described as X-body. The chromosome theory of sex determination was worked out by E.B. Wilson and Stevens (1902-1905). They named the X and Y chromosomes as sex-chromosomes or allosomes and other chromosomes of the cell as autosomes.

Sex chromosomes carry genes for sex. X-chromosomes carries female determining genes and Y-chromosomes has male determining genes. The number of X and Y chromosomes determines the female or male sex of the individual, Autosomes carry genes for the somatic characters. These do not have any relation with the sex.

(a) XX-XY type or Lygaeus type : This type of sex-determining mechanism was first studied in the milk weed bug, Lygaeus turcicus by Wilson and Stevens. Therefore, it is called Lygaeus type. These are two different patterns of sex determination in Lygaeus type.

(1) Female homogametic XX and male heterogametic XY : The homogametic sex (XX) is female and produces ova all of one type, i.e. having X-chromosome. The male is heterogametic-XY and produces sperm of two types. 50% of which possess X-chromosome and other 50% Y-chromosome. This is simple XX-XY type and is found in man, Drosophila and certain insects.

Example : In Drosophila total number of chromosomes is eight, of which six are autosomes, common to both male and female. The fourth pair is of sex chromosomes. In male this is represented by XY i.e. Karyotype of male Drosophila 6+XY and in female XX i.e. 6+XX. Ova produced by female are all similar possessing 3+X chromosomes, whereas the sperm produced by male are 3+X and 3+Y in equal numbers.

(2) Female heterogametic and male homogametic : In fowl, other birds and some fishes, certain moths and butterflies, the female sex is heterogametic, with X and Y chromosome often represented by Z and W and laying two types of eggs, one half with X or Z chromosome and the other half with Y or W chromosome. The male sex is homogametic having XX or ZZ chromosomes. It produces sperm all of one type.

(b) XX-XO type or Protenor type : Mc clung in male squash bug (Anasa) observed 10 pairs of chromosomes and an unpaired chromosome. Their females have eleven pairs of chromosomes (22). Thus all the eggs carry a set of eleven chromosomes but the sperm are of the two types: fifty percent with eleven chromosomes and the other fifty percent with ten chromosomes. The accessory chromosome was X-chromosomes. Fertilization of an egg by a sperm carrying eleven chromosomes results in a female, while its fertilization by a sperm with ten chromosomes produces male. It is said to be evolved by the loss of Y-chromosome.

(c) Haploid-diploid mechanism of sex determination: Hymenopterous insects, such as bees, wasps, saw flies, and ants, show a unique phenomenon in which an unfertilized egg develops into a male and a fertilized egg develops into a female. Therefore, the female is diploid (2N), and the male is haploid (N). eggs are formed by meiosis and sperms by mitosis. Fertilization restores the diploid number of chromosomes in the zygote which gives rise to the female. If the egg is not fertilized, it will still develop but into a male. Thus, the sex is determined by the number of chromosomes.

In honeybee, the quality of food determines whether a diploid larva will become a fertile queen or a sterile worker female. A larva fed on royal jelly, a secretion from the mouth of nursing workers, grows into a queen, whereas a larva fed on pollen and nectar grows into a worker bee. Thus, the environment determines fertility or sterility of the bee but it does not alter the genetically determined sex. The sex ratio of the offspring in the hive is controlled by the queen. She lays more fertilized eggs that produce worker females and fewer unfertilized eggs which produce haploid males. The queen mates only once in her life time, keeps a store of sperms in the seminal receptacle, and can control fertilization of eggs by releasing or not releasing sperms.

Different types of chromosomal mechanisms of sex-determination in animals

(ii) Quantitative or ratio theory of sex determination: C.B Bridges worked out ratio theory of sex determination in Drosophila. According to this theory the ratio of chromosomes to autosomes is the determining factor for the sex. Single dose of X-chromosome in a diploid organism produces male, whereas 2X-chromosomes produce a female. If a complete haploid set of autosomes is designated by A then 2A : X will give rise to male and 2A : 2X to female.

(a) Intersexes in Drosophila and ratio theory of sex determination: Bridges hypothesis was supported by studies of flies arising after abnormal distribution of chromosomes on account of non-disjunction. Due to abnormal meiosis during oogenesis both the X-chromosomes fail to separate and move to one pole of meiotic spindle. Thus few eggs are formed with single autosomal genome but with 2X chromosomes, i.e. (AXX) and other with single autosomal genome but no sex chromosome (A). when such abnormal eggs are fertilized with normal sperm, the following result are obtained.

Results of fertilization of abnormal female gametes

AAXXY             -           Female

AAXXX             -           Super female

AAX     -           Sterile male

AAY     -           Nonviable

Out of this progeny 1/4th males with no X are nonviable; the other 1/4 are without Y-chromosome and sterile. 1/4th females have an extra Y-chromosome while rest 1/4th females with 3X are super females. These are sterile with under developed sexual characteristics.

(ii) Triploid intersexes and balance theory: The triploid flies with (3A + 3X) are much like the normal diploid females both in appearance as well as in fertility. On mating to diploid males their progeny consisted of following types.

(1) AAAXXX      -           Triploid females

(2) AAXX          -           Dilpoid females

(3) AAXXY        -           Diploid females

(4) AAAXX        -           Intersexes

(5) AAAXXY      -           Intersexes

(6) AAXY          -           Normal males

(7) AAXXX        -           Super females

(8) AAAXY        -           Super males

The intersexes are sterile and intermediate between females and male, because the sex balance ratio in the intersexes comes to 2 : 3.

(2) Gyandromorphs in Dorsophila and ratio theory of sex determination: In Drosophila occasionally flies are obtained in which a part of the body exhibits female characters and the other part exhibits male characters. Such flies are known as gynandromorphs. These are formed due to misdivision of chromosomes and start as female with 2A+2X-chromosomes. One of the X-chromosomes is lost during the division of the cell with the result that one of the daughter cells possesses 2A+2X chromosomes and the other 2A+X. If this event happens during first zygotic division, two blastomeres with unequal number of X-chromosomes are formed. The blastomere with 2A+2X-chromosomes develops into female half, while the second blastomere with 2A+X chromosomes produces male half and the resultant fly is a bilateral gynandromorph. The occurrence of gynandromorphs clearly indicates that the number of X-chromosomes determines the sex of the individual.

(iii) Genic balance theory : Based upon the observations of ratio theory Bridges put forward genic balance theory in which he suggested that every individual whether male or female possesses in its genotype genes for both male and female characteristics. Which sex will actually develop is decided by the preponderance of that type of genes.

According to the genic balance theory of Bridges in Drosophila melanogaster, sex is determined by the ratio of the X-chromosomes and the set of autosomes. The Y-chromosomes play no part in sex determination it only governs male fertility. The XO flies are male, but sterile. Sex is governed by the ratio of the number of X chromosomes to sets of autosomes. The table given below indicates how the ratio of X/A help to determine the sex.

Ratio of X-chromosome to autosomes and the corresponding phenotype in Drosophila

Genes for maleness are carried on the autosomes, those for femaleness on the X-chromosomes. The sex index ratio of female is 1.0 while in males is 0.50. If X/A ratio is greater than 1.0 produces super females (meta females) and less than 0.50 produces super males. The X/A ratio lesser than 1.0 but greater than 0.5 (for example 0.66) result in intersexes. The degree of femaleness is greater where the X/A ratio is closer to 1.0 and the degree of maleness is greater where that ratio is closer to 0.5.

Human sex determination : The genic balance theory of sex determination is not universally accepted. Unlike Drosophila X : A does not influence sex determination. The key to sex determination in humans is the SRY (for sex region on the Y) gene located on the short arm of the Y-chromosome. In the male, the testis-determining factor (TDF) is produced by SRY on the Y-chromosome. TDF induces the medulla of the embryonic gonads to develop into testes. In the absence of SRY on Y, no TDF is produced. The lack of TDF allows the cortex of the embryonic gonads to develop into ovaries.

(iv) Hormonal theory of sex determination : The sex determination theories of chromosomes and genic balance successfully apply to the lower animals but in higher vertebrates and under certain conditions in invertebrates, the embryo develops some characters of the opposite sex together with the characters of its own sex-chromosome. It means, the sex changes under specific circumstances. This is due to the hormones secreted by the gonads of that animal.

(a) Free martinism : The influence of hormones on sex determination comes from free-martins often found in cattles. LILLIE and others found that where twins of opposite sex (one male and other female) are born, the male is normal but female is sterile with many male characteristics. Such sterile females are known as free martins.

The scientific explanation for the formation of free martins is the effect of hormones of the male sex on the female. In cattle the foetal membranes of the twins are fused in such a manner that they have a common circulation of blood. The female hormone is produced at a slightly later stage in the development and guides its development towards female side. But since the twins have a common circulation and blood passes from one twin into the body of other twin, the male hormone which is produced slightly in advance of female hormone, enters the body of female twin and before the female hormone onsets the development of female characteristics it is already differentiated in the guidance of male hormone. As a result the developing female is sterile.

(v) Environmental theory of sex determination : In some animals, there is environmental determination of sex.

(a) In Bonellia, a marine worm, the swimming larva has no sex. If it settles down alone, it develops into a large (2.5 cm) female. If it lands on or near an existing female proboscis, a chemical secreted from her proboscis causes the larva to develop into a tiny (1.3 mm) male. Male lives as a parasite in the uterus of the female.

(b) In turtles, a temperature below 28°C produces more males, above 33°C produces more females, and between 28°C to 33°C produces males and females in equal proportion, while in crocodile male sex is predominant at high temperature.

(vi) Barr body in sex determination : Murray Barr (1949), a geneticist noticed a small body in the nucleus of the nerve cells of female cats which stained heavily with nuclear stains. Further investigations showed that not only nerve cells, but many other cells from female cats only, had these bodies, now known as sex chromatin or Barr bodies. It was soon learnt that such bodies can be found in females of many mammals including human. In women the Barr body lies against the nuclear membrane like a round disc in the neutrophil blood cells, skin cells, nerve cells, cells of mucous membrane, cells of lining in vagina and urethra. They are absent in man. These bodies are thus named after the discover Barr.

Barr bodies are used to determine the sex of unborn human embryos. In this technique called amniocentesis sample of the amniotic fluid is examined for Barr bodies. The sex is determined by the presence or absence of Barr bodies in epithelial cells of embryo present in the amniotic fluid sample. Studies from the cells of aborted embryos show that Barr bodies can be distinguished at about 15 or 16^th day after conception that means several weeks before the formation of gonads. Whereas sex of embryo is determined soon after fertilization, sex differentiation can be noticed in third week stage of pregnancy.

Mary Lyon hypothesis : According to the British geneticist Mary Lyon (1961), one of the two X-chromosomes of a normal female becomes heterochromatic and appears as Barr body. This inactivation of one of the two X-chromosomes of a normal female is the dosage compensation or Lyon’s hypothesis.

It is estimated that number of Barr bodies is one less from the total number of X chromosomes present in embryo. Therefore, Barr bodies are also used to decide the genic constitution of such persons who have irregular number of sex chromosomes. More than one X chromosome in such persons is transformed into Barr bodies.

Sex can also be distinguished by studies of simple blood smears. The neutrophils, the most common of the white blood corpuscles, have a nucleus divided into two or three lobes. Female neutrophils showing a small drumstick extending out from one of the nuclear lobes, is a definite indication of the female chromosome component in the cells.

Important Tips

• Goldschmidt brought forward the quantitative theory of sex..
• The term “gynandromorphism” was introduce by Goldschmidt in 1915.
• Drumstick is the sex chromatin present in the neutrophil (Polymorphonuclear leucocyte) of 3 to 5% cells in females, but not in males.
• Y chromatin (Y body) can be identified as bright spot by staining cells with acridine dyes.
• First X-linked gene was discovered by T.H. Morgan (1910) for white eye mutation.
• Pedigree of colour blindness was first described by Horner (1876).
• It is also called bleeder’s disease, first studied by John Cotto in 1803.
• Duchenne Muscular Dystrophy (DMD) is the disease which is characterized by a progressive weakness and loss of muscle.
• Inheritance of beard in a man is sex-limited.
• In melandrium (Garden flower) the sex determination type is XX-XY.