sex determination

SEX DETERMINATION

Sex Chromosomes and Autosomes

Sex chromosomes are those chromosomes which determine the sex of the individual in dioecious or unisexual organisms.

The normal chromosomes, other than the sex chromosomes, of an individual are known as autosomes. Sex chromosomes may be similar in one sex and dissimilar in the other.

The two conditions are respectively called homomorphic (= similar, e.g., XX, ZZ) and heteromorphic (= dissimilar, e.g., XY, ZW).

Individuals having homomorphic sex chromosomes produce only one type of gametes.

They are, therefore, called homogametic (e.g., male birds, human female and Drosophila female).

Individuals having heteromorphic sex chromosomes produce two type of gametes (e.g., X and Y).

They are termed as heterogametic (e.g., female bird, human male and normal Drosophila male).

The factors which control the sex of an organism are under genetic control.

Various mechanism which led to sex determination can be classified into following four categories:

1. Chromosomal mechanism of sex determination.

2. Non -Allosomic genetic sex determination -Fertility factor (plasmid) in bacteria.

3. Genic balance mechanism or X/A balance.

4. Environmental mechanism of sex determination.

1. Chromosomal mechanism of sex determination.

According to this mechanism, there are certain chromosomes known as sex chromosome or heterosome or idiochromosome, which are responsible for sex determination.

This may be of following types:

(i) XX -XY type:

In most insects, plants and mammals including human beings, the females possess two homomorphic (= isomorphic) sex chromosomes, i.e., XX.

The males contain two heteromorphic sex chromosomes, i.e., XY. The Y-chromosome is often shorter and heterochromatic (made of heterochromatin).

Despite differences in morphology, the Y-chromosome pairs with X-chromosome at a certain segment during meiosis.

It, therefore, carries a segment which is homologous with a segment of X-chromosome.

The remaining segment of Y-chromosome is non-homologous and carries only Y-linked or holandric genes, e.g., Testis Determining Factor (TDF).

Human beings have 22 pairs of autosomes and one pair of sex chromosomes.

All the ova (haploid) formed by female are similar in their chromosome type (22 + X). Therefore, females are homogametic.

The male gametes or sperms (haploid) produced by human males are of two types, (22 + X) and (22 + Y).

Human males are therefore, heterogametic.

The two sexes produced in the progeny may have 50 : 50 ratio.

This type of sex-determination was reported in a plant Sphaerocarpus for the first time and is also found in plants like Melandrium and Coccinia.

Concept Builder

Barr Body and Y Spots

Barr and Bertram (1949) found a small darkly stained chromatin body (heterochromatic) adhered to the nuclear membrane of nerve cells of female cats but not in male cats.

Later on, Barr (1960) observed consistent presence of these chromatin bodies in epidermal cells of buccal and vaginal mucosa, skin cells etc. in females.

Lyon (1962) suggested that one X chromosome of maternal side becomes coiled, inert and hetero-pycnotic and forms a barr body.

Number of barr bodies is always one less than the total number of X chromosomes in female.

Normal male has no barr body.

In about 5% of polymorphonuclear leucocytes (neutrophils) in women, the heteropycnotic X chromosome occurs as a round pedunculated body attached to the nucleus.

It is called drum stick (Barr body).

A normal female has 1 drum stick per nucleus of neutrophil.

In neutrophils of males drum stick is absent.

Y spots: Males are identified by number of Y spots (Zech 1970). Number of Y spots are equal to number of Y Chromosomes.

(ii) XX -XO type.

In roundworms, Dioscorea and some insects (true bugs, grasshoppers and cockroaches), the females have two sex chromosomes, XX, while the males have only one sex chromosome, X.

There is no second sex chromosome.

Therefore, the males are designated as XO.

The females are homogametic because they produce only one type of eggs.

The males are heterogametic with half the male gametes carrying X-chromosome while the other half being devoid of it.

The sex ratio produced in the progeny is 1 : 1.

(iii) ZW -ZZ type. (WZ -WW Type).

In birds, fishes, silk worm, Fragaria elatior and some reptiles both the sexes possess two sex chromosomes.

Unlike human beings, the females contain heteromorphic sex chromosomes while the males have homomorphic sex chromosomes.

Because of having heteromorphic sex chromosomes, the females are heterogametic and produce two types of eggs, (A + Z) and (A + W).

The male gametes or sperms are of one type (A+Z). 1 : 1 sex ratio is produced in the offspring.

(iv) ZO -ZZ type.

This type of sex determination occurs in butterflies, pigeon, ducks and moths.

It is exactly opposite the condition found in cockroaches and grasshoppers.

Here the females have odd sex chromosome while the males have two homomorphic sex chromosomes.

The females are heterogametic.

They produce two types of eggs, with one sex chromosome (A + Z) and without the sex chromosome (A + 0).

The males are homogametic, forming similar types of sperms (A + Z).

The two sexes are obtained in the progeny in 1 : 1 ratio as both the types of eggs are produced in equal ratio.

Arrhenotoky/Haploid-diploid Mechanism:

This mechanism is found in honey bee. In honey bees, ants and wasps, the egg, if fertilized, gives rise to female fly.

The unfertilized egg develop parthenogenetically into male. So, the female flies are diploid while male flies are haploid.

2. Non-Allosomic genetic sex determination:

Fertility factor of plasmid in bacteria determines sex.

3. Genic Balance or XlA Balance Theory of Sex Determination:

Given by C.B. Bridges. According to him, Y chromosome plays no role in sex determination of Drosophila and it is the ratio between number of X-chromosome and set of autosomes which determines the sex of fly.

It was concluded that X/A ratio of > 1.0 expresses super femaleness, 1.0 femaleness, below 1.0 and above 0.5 intersexes, 0.5 maleness and < 0.5 supermaleness.

Gynandromorphs : Gynandromorph is a sex mosaic (an individual with one half of the body male and the other half female).
These are common in Silk moth and Drosophila. Gynandromorphism is developed due to accidental loss of X-chromosome from a 2A + XX cell during mitosis.

Gynander : A gynander may be male or female with patches of tissues of other sex on it.

4. Environmental Mechanism of Sex Determination:

This mechanism is observed by F. Baltzer in Bonnelia viridis (marine worm).

In this organism, the sex is undifferentiated in larva.

The larva which settle down in mud, grow up into mature female while those which settle down near the proboscis of female and become parasite develop into male.

It has been demonstrated that female secrete certain hormone which induces sex in larva. Crepidula and Ophryortocha also show such mechanism.

SEX LINKED INHERITANCE

Sex linkage was discovered by Morgan, while working on inheritance of eye colour in Drosophila. He made three types of crosses:

Cross -1 :

The white eyed male (w) was crossed with red eyed (w+) female.

All the flies of F₁ generation were found to be red eyed.

F₁ flies were allowed to self breed.

In F₂ generation, both the traits of red eye and white eye appeared in the ratio 3 : 1 showing that white eye trait is recessive to red eye trait.

Cross -2 :

Red eyed females of F₁ generation were crossed with white eyed male.

It is similar to test cross where hybrids are cross bred with recessive parents.

Morgan obtained red and white eyed female as well as male in equal proportions-1 red eyed female: 1 white eyed female: 1 red eyed male: 1 white eyed male.

The test cross indicated that white eye colour was not restricted to the male fly. Red eyed White eyed hybrid female male

Cross -3 :

White eyed females were crossed with red eyed males. It was a reciprocal of cross 1 and should give the similar result as obtained by Mendel. However, Morgan obtained a surprising result. All the males were white eyed while all the females were red eyed.

Taking all the crosses into consideration, Morgan came to the conclusion that eye colour gene is linked to sex and is present on the X-chromosome.

X-chromosome does not pass directly from one parent to the offspring of the same sex but follows a criss-cross inheritance, i.e., it is transferred from one sex to the offspring of the opposite sex.

In other words, in criss-cross inheritance a male transmits his traits to his grandson through daughter (Diagynic), while a female transmits the traits to her granddaughter through her son (Diandric).

Sex Linkage in Human Beings

Colour blindness and haemophilia (Bleeder's disease) are two common examples of sex-linked diseases in human beings.

(1) Colour blindness.

This is a human disease which causes the loss of ability to differentiate between red colour and green colour.

The gene for this red-green colour blindness is present on X chromosome. Colour blindness is recessive to normal vision.

If a colour blind man (X^cY) marries a girl with normal vision (XX), the daughters would have normal vision but would be carrier, while sons would also be normal (shown in cross(a)).

Cross (a)

If the carrier girl (heterozygous for colour blindness, X^CX) now marries a colour blind man X^CY, the offspring would show 50% females and 50% males.

Of the females, 50% would be carrier for colour blindness and the rest 50% would be colour blind.

Of the males, 50% would have normal vision and the 50% would be colour blind (shown in cross (b).

Cross (b)

Fig. : Sex-linked inheritance of colour blindness -cross (a) and cross (b)

(2) Haemophilia (Bleeder's disease).

The person suffering from this disease cannot synthesize a normal blood protein called antihaemophilic globulin (AHG) required for normal blood clotting (Haemophilia A - more severe).

Therefore, even a very small cut may lead to continuous bleeding for a long time.

This gene is located on X chromosome and is recessive.

It remains latent in carrier females.

Fig.: Inheritance of haemophilia when the mother is carrier and the father is normal

If a normal man marries a girl who is carrier for haemophilia, the progeny would consist of 50% females and 50% males.

Of the females, 50% would be normal and the rest 50% would be hemophilia carrier.

Of the males, 50% would be normal and the rest would be haemophiliacs.

Haemophilia-B (christmas disease) -plasma thromboplastin is absent, Inheritance is just like Haemophilia A.

Concept Builder

1. C.E. McClung (1902) concluded that some special chromosome (X-chromosome) must be responsible for determination of sex, thus providing first definite evidence for chromosomes theory.

2. Bridges (1916) correlated cytological observations of chromosomes and inheritance of characters.

3. Y-chromosome is male determiner in man but not in Drosophila. However, its accumulation is responsible for motility of sperms in Drosophila.

4. Man is hemizygous for sex chromosomes as he has XY chromosomes.

5. Henking discovered an "X-body" in reproductive cells of firefly.

6. Y-body was discovered by Stevens.

7. Wilson and Stevens -put forward chromosome theory of sex and name X and Y body as X-chromosome and Y-chromosome.

8. Tijo and Levan -Discovered exact number of chromosomes in human beings as 46.

9. F.R. Lillie -Formation of free martins (twins in cattles).