Sex determination is a system that decides the sex of a particular offspring. It determines whether an organism will be female or male. The organism of different species has a different number of chromosomes, out of which particular chromosome or a pair of chromosomes called allosomes or sex chromosomes are associated with the sex determination. A sex chromosome is a type of chromosome that participates in sex determination. Similar to humans, plants, insects, birds, etc., also have separate sexes.

Humans and most other mammals have two sex chromosomes, the X and the Y. Females have two X chromosomes in their cells, while males have both X and Y chromosomes in their cells. Egg cells all contain an X chromosome, while sperm cells contain an X or Y chromosome. This also states that it is the male that determines the sex of the offspring when fertilization occurs. y

Definition of Sex Determination

Sex determination deals with the study of factors that are responsible for deciding the sex of an individual that can either be male or female or hermaphrodite.

Theories of Sex Determination

Different theories of sex determination have been proposed that are classified into the following two main categories: 

1. Classical Theories of Sex Determination
2. Modern Theories of Sex Determination
3. Non-genetic Theories of Sex Determination

Learn About Female Reproductive System Here

Classical Theories of Sex Determination

The following two classical theories have been proposed to determine the sex of an individual:

1. Metabolic Theory of Sex Determination: This theory was proposed by Riddle. 

A. According to this theory, sex is determined by the results of metabolic activities in the body cells of an individual.  

B. The rate of oxidation of food, the quantity of water and protein content are the facts that determine the sex of an individual. 

C. A male individual arises when the rate of oxidation of food is maximum and the protein content is less, while the reverse conditions lead to a female individual.

2. Enzymatic Theory of Sex Determination: This theory was proposed by Goldschmidt.

A. According to this theory, the specific enzyme in the individual determines the sex of an individual. 

B. The enzyme andrase and gynase are responsible for the development of male and female individuals, respectively. 

C. When the quantity of andrase is more than the gynase, it leads to a male individual and vice versa. 

D. On the other hand, at equilibrium, the individual will have different sex or intersex.

Modern Theories of Sex Determination

A number of modern theories have been proposed to determine the sex of an individual. These can be described as follows:

1. Chromosomal Theories of Sex Determination: In the early years of the 20^th century, Nettie Stevens and E.B. Wilson discovered the factors or bodies that are responsible for the male and female sex in humans. These are called sex chromosomes or allosomes. Different biologists contribute to the determination of sex in different organisms based on their sex chromosomes. A few can be discussed as follows:

i. XX-XY Type of Sex Determination: This type of sex determination is found in certain insects like Drosophila melanogaster and mammals, including humans.

a. The human male has a pair of heterosomes called sex chromosomes, and the female has a homologous pair of sex chromosomes. It is represented as XY and XX respectively in males and females.

b. The Y chromosome carries a gene SRY (sex-determining region), also called testis-determining factor (TDF).

c. TDF is required for the development of the male sex, and its absence leads to the development of the female sex.

d. This observation shows that Y is responsible for maleness. The evidence collected from the study of variation in sex chromosomes and the molecular genetic analysis proves that Y does determine maleness in humans and other mammals but not in Drosophila.

e. Although XX individuals in Drosophila are female and XY are males, the presence of Y is not necessary for the male phenotype.

ii. XX-XO Type of Sex Determination: Roundworms and some insects (true bugs, grasshopper, cockroaches etc.) exhibit XX-XO type of sex determination.

a. Males have only one X chromosome and produce two types of sperms, i.e. 50% with an X chromosome and 50% without the X chromosome.

b. Females are homogametic that have a pair of XX chromosomes and produce all the gametes having a single X chromosome.

c. Eggs fertilized by a sperm having an X chromosome leads to femininity, and those fertilized with sperm without an X chromosome leads to masculinity. The cross can be described as follows:

XX-XY and XX-XO types of sex determination are examples of male heterogamety as the males produce two types of gametes during meiosis. One with the X chromosome and another with the Y chromosome (In case of XX-XY) or devoid of chromosome (XX-XO type).

iii. ZZ-ZW Type of Sex Determination: This method of sex determination is seen in birds, fowls, fishes.

a. Females produce two types of gametes, one containing autosomes with Z chromosomes and the other containing autosomes with W chromosomes.

b. Males produce all similar types of gametes, each having a single Z chromosome. A cross between the males and females can be described as follows:

iv. ZZ-ZO Type of Sex Determination: This type of sex determination has been observed in insects and butterflies.

a. The females are heterogametic, i.e. they produce two types of gametes, one with the Z chromosome and another without the Z chromosome.

b. The males are homogametic, producing all the gametes with Z chromosomes. The following cross determines the gender in certain insects:

ZZ-ZW and ZZ-ZO type of sex determination are examples of female heterogamety as the females produce two types of gametes during meiosis. One with the Z chromosome and another with the W chromosome (In the case of ZZ-ZW) or devoid of chromosomes (ZZ-ZO type).

v. Haploid-Diploid Type of Sex Determination: The biologist Whitings conducted this type of sex determination method in one of the wasp species, namely, Harbobracon. There are three types of honey bees found in the colony, namely the queen (female), worker and drone (male).  The males arise parthenogenetically and therefore have a haploid number of chromosomes. While the queen bees and workers have a diploid number of chromosomes.

II. Genetic Balance Theory of Sex Determination:

This theory of sex determination is being proposed by C.B. Bridge in Drosophila after the discovery of XXY normal females. 

a. In Drosophila, four pairs of chromosomes are found, including one pair as a sex chromosome.

b. Though the Y chromosome is paired with the X chromosome, the presence of the Y chromosome in a pair does not determine the maleness in the individual.

c. As a whole, it can be said that the sex-determining factor is the ratio of the X chromosome and the set of autosomes. If the ratio is 1, female Drosophila arises, and if the ratio is 0.5, the gender of Drosophila will be male. The ratio value more than 1 and less than 0.5 leads to super female and super male, respectively.

A general formula for sex-determining ratio (sex index) is as follows:

The possible combinations of chromosomes in males and females can be tabulated as follows:

What are Gynandromorphs?

In Drosophila, a zygote with AA+XX has to develop into a female. After the first division, two cells are formed, each with AA+XX. If from one cell, either of the X chromosomes is lost due to any reason, the ratio of the X chromosome and set of autosomes will be 0.5, hence out of the two cells, one will develop into a male, and the other will develop into a female. The male Drosophila will have half the body of the male and half the body of the female. Such organisms are called gynandromorphs.

iii. Single Gene Sex Determination:

There are several instances where the single gene was found to be responsible for sex expression. In microorganisms such as Chlamydomonas, Neurospora, Saccharomyces, and Asparagus species, sex is determined by a single gene.

a. An asparagus is a dioecious form. However, the female flower bears rudimentary stamens, and the male flower bears rudimentary pistils. When the seeds from the male are raised into a plant, both male and female flower-bearing plants are obtained in a ratio of 3:1. 

b. When the male plants obtained from the cross were used to pollinate female flowers, only two-third of them showed segregation, indicating that sex is controlled by a single gene.

c. This represents that maleness is dominant over femaleness, and therefore the heterozygotes will be male.

d. Similarly, in some plants and animals, sex is determined by a single gene. For example, single-gene sex determination is observed in maize.

iv. Cytoplasmic or Non-Allosomic Theory of Sex Determination:

This theory of sex determination has been observed in the bacterium E. coli.  

a. The bacterium contains an extrachromosomal DNA called F-plasmid that determines the sexuality in bacteria.

b. The bacteria-containing F-plasmid is known as the donor (male) and denoted as F⁺ (positive), while the bacteria lacking F-plasmid is known as the recipient (female) and denoted as F^– (negative).

c. During bacterial conjugation, the donor bacterial cell transfers its F-factor into the recipient cell, and it becomes a male due to the presence of F-plasmid.

However, besides plasmids, other cytoplasmic factors may also be responsible for sexuality in some microorganisms.

Fig: Cytoplasmic theory of sex determination

Non-genetic Theory of Sex Determination

There are some other factors that also determine the sex of some organisms. Based on the different factors, the following theories have been proposed for the determination of sex:

i. Nutritional Theory of Sex Determination

This theory is proposed by Sharp. According to this theory, the nutritional supply also determines the sex of some organisms. 

a. The spores of Equisetum grown under favourable growth conditions (such as nutrients and water supply, optimum temperature, etc.) develop into females. Unfavourable conditions lead to the development of males from the spores.

b. In a marine worm (Dinophilus), the size of the egg determines the nature of sex. The small egg, due to the lack of nutrients, develops into males, whereas the large egg develops into females.

ii. Environmental Theory of Sex Determination

A number of environmental factors are also responsible for determining the sex of an individual. The theory stated that every zygote contains the genes for both sexes. But only one of the genes is able to express itself and determine the sex of an individual. The expression of sex-determining genes depends on several environmental conditions. A few examples are as follows:

a. The temperature determines the sex of certain reptilians. The high temperature of about 30-35 degrees Celsius induces the expression of the female determining genes. At the same time, the lower temperature of about 23-28 degrees Celsius induces the expression of the male determining gene.

b. Photoperiod determines the sex in cucumber and melon.

iii. Hormonal Theory of Sex Determination

In some animals, the hormones also determine the sex of an individual. A few examples can be discussed as follows:

a. The sex of Bonellia larvae is being determined by the hormonal substance secreted from the proboscis of the mature female worm.

b. The larvae that come in contact with the female worm and get attached to its proboscis develops into a male as the hormone-like substance secreted by the female worm inhibits the expression of the female-determining gene of the larvae.

c. Amartin is also called hormonal intersex cattle. If cattle produce dizygotic twins of the opposite sex, the probability of producing sterile female and normal male is higher because of the cross hormonal circulation in between the two embryos.

d. It was found that during cross circulation, some of the hormone released by the male embryos crosses the placenta of the normal female embryo inhibits the development of secondary sexual characters in the female embryo and simultaneously induce male characters. As a result, the female embryo turns more likely into a male after birth, but it is sexually female (sterile female).

Summary

The sexes are more often separate in advanced invertebrates and all the vertebrates. The sex of an individual is determined during the fusion of gametes. The chromosomal theory of sex determination is widely accepted for sex determination in different types of organisms. According to this theory, the constitution of a pair of sex chromosomes in males is different from females. In humans, the XX and XY set of sex chromosomes respectively determines a female and a male. Similarly, in other organisms, XX-XO, ZZ-ZW, ZZ-ZO are the different pairs of sex chromosomes. Beyond the chromosomal theory, several other sex determination theories such as metabolic theory, enzymatic theory, genic balance theory, single-gene theory, hormonal theory, and non-genetic theory have been proposed for sex determination in different species of organisms.

Frequently Asked Questions (FAQs) on Sex Determination

Q.1. How is sex in humans determined?
Ans: The sex in humans is determined by the type of male gamete that fuses with the female gamete during fertilization. 
1. If the male gamete with the X chromosome fuses with the female gamete, the child will be a girl with a homologous pair of sex chromosomes (XX).
2. If the male gamete with the Y chromosome fuses with the female gamete, the child will be a boy with a heterologous pair of sex chromosomes (XY).

Q.2. How many types of sexes are there?
Ans: There are mainly two types of sexes: the male and the female.

Q.3. Who is responsible for the gender of a baby in humans?
Ans: The gender of a baby is a matter of chance during the fertilization of gametes. However, the haploid gametes of the father determine the sex of the baby.

Q.4. Which animal has 32 chromosomes in honey bees?
Ans: Queen and worker bees have 32 chromosomes.

Q.5. What is the ZZ-ZW type?
Ans:  ZZ and ZW are the types of sex chromosomes birds, fishes, butterflies, and moths. ZZ determines the male sex, and ZW determines the female sex.

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