Sex Determination: Genotypic and Environmental Sex Determination, Genic Balance Theory of Sex Determination, Practice Problems and FAQs

You have seen lots of movies. Certain movies portray a social stigma related to sex determination in humans. In our society women are blamed for giving birth to female children not the men. Movies show that women are ill-treated because of this false notion.

Fig: Sex determination

But the fact is that man are responsible for the sex of an offspring. This is because human male possess different types of sex chromosomes, whereas the human female possess same type of sex chromosomes. The 50% of male sex chromosomes can result in the male offspring and the rest 50% of male sex chromosomes can result in female offspring.

Fig: Sex determination

You know sex determination in organisms differ. Now let’s take a dive deep into the details of sex determination of various organisms.

Table of contents:

Sex determination
Genotypic sex determination
Genic balance theory of sex determination
Environmental sex determination
Practice Problems
FAQs

Sex determination

The genetic determination of sex of the unborn offspring is known as sex determination. It was first studied by Henking in the year of 1891 in insects.

Discovery of X body

In a few insects, Henking was able to track a specific nuclear structure during spermatogenesis. He also observed that 50% of the sperm population receive this structure, whereas the rest 50% of the sperm population did not receive it. This structure was named as X body by Henking but he was unable to explain its significance. Other scientists' investigations led them to the conclusion that Henking's 'X body' was actually a chromosome, which is why it was given the name X chromosome.

Discovery of Y body

Y body was discovered by Stevens.

Chromosome theory of sex

It was put forward by Wilson and Stevens. According to this theory X body and Y body are nothing but the sex chromosomes X and Y chromosomes respectively. The determination of sex is decided based on the presence of absence of these chromosomes in most organisms.

Types of sex determination

Sex determination can be done in two ways based on the factors involved. These are as follows:

Genotypic sex determination
Environmental sex determination

Fig: Type of sex determination

Genotypic sex determination

This type of sex determination is based on the genetic makeup of an offspring. In this, genetic elements specify whether the offspring is male or female. Genotypic sex determination is further of two types:

Based on sex chromosomes
Based on haplodiploid system

Sex determination based on sex chromosomes

In this type, sex chromosomes are used to determine the sex of an offspring. It is classified into two types:

Male heterogamety
Female heterogamety

Male heterogamety

In this type of sex determination, the male contains different sex chromosomes. Therefore, it is known as male heterogamety. It involves two conditions as follows:

XX-XY type
XX-X0 type

XX-XY type

It is a lygaeus type of sex determination because it was first studied in the milkweed bug, Lygaeus turcicus. In this type, male possess two heteromorphic sex chromosomes i.e. X-chromosome and Y-chromosome. The female possess two homomorphic or isomorphic sex chromosomes, i.e. X-chromosome and X-chromosome. This type of condition can be seen in most mammals including humans.

Sex chromosomes in Humans

Humans contains 46 or 23 pairs of chromosomes, in which 22 pairs are autosomes and 1 pair is sex chromosome. There are 22 pairs of autosomes that are identical to one another and contain genes for traits other than sex. The one pair is XX in females and XY in males.

Fig: Human chromosomes

Due to differences in one pair of chromosomes, two types of male gametes are produced. These are androsperms and gynosperms.

Androsperms

The androsperms contain 22 autosomes and one Y chromosome. If an egg is fertilised with androsperm, it produces male child that contains 44 autosomes and XY-chromosomes.

Gynosperms

On the other hand, the gynosperms contain 22 autosomes and one X-chromosome. If the egg is fertilised with gynosperm, it produces a female child with 44 autosomes and XX-chromosomes.

Fig: Male gametes in humans

The male sperm is responsible for deciding the sex of the offspring. That means male sperm decides whether the offspring is male or female. But, the probability of a girl or boy is 50%.

Fig: Sex determination in humans

Y chromosome

For a long time, it was considered that the human Y chromosome is blank genetically. According to the data of the human genome project, it indicates that the Y chromosome possesses 75 genes as compared to X chromosomes that possess 900 - 1400 genes.

Regions of Y chromosome

The Y chromosome contains two regions as follows:

Homologous regions
Differential regions

Differential regions of Y chromosome

The differential regions of Y-chromosomes have Y-linked genes, which are also known as holandric genes. Sry (Sex determining region) is a gene that codes for TDF (Testis determining factor). Sry is considered as the smallest gene that only contains 14 base pairs. TDF is essential for the development of male sex. Female sex develops in absence of TDF.

Fig: Y-chromosome

Homologous regions of Y chromosome

Pseudoautosomal regions (PARs) can be found on both ends of the Y chromosome. These regions have homology with X chromosome and synapse and recombine with X chromosome during the process of meiosis. The remaining part of the Y chromosome which is about 95% of it does not synapse with the X chromosome. As a result, it was previously known as the non-recombining region of Y chromosome (NRY). This region of the Y chromosome has lately been classified as the male-specific region of the Y chromosome (MSY). The MSY is split evenly between euchromatic and heterochromatic sections, with euchromatic regions carrying functional genes and heterochromatic regions containing no genes.

Fig: Male-specific region of Y-chromosome

Sex determination in humans

The absence of a Y chromosome almost usually results in female development in humans. During six to eight weeks of human embryo development, the SRY gene becomes active in XY embryos. It codes for a TDF protein (testis-determining factor) which causes the undifferentiated tissue to form testes. Males also express genes found in the differential region of the X chromosome (whether dominant or recessive). These genes are hemizygous in males.

Pseudodominance

It is a condition in which a recessive allele expresses itself in phenotype when only one copy of the allele is present. For example, red-green colour blindness and haemophilia.

Fig: Sex determination in humans

XX-X0 type

XX-X0 type of sex is seen in roundworms and some insects, such as grasshoppers and cockroaches. In these organisms, the female possess two identical sex chromosomes i.e. XX. The male of these organisms only have one sex chromosome i.e. X (designated as X0). In this case, females are homogametic (A+X) and males are heterogametic, in which half the population carry X-chromosome and the rest half are devoid of X-chromosome.

Fig: Sex determination in some insects

Female heterogamety

In this type of sex determination, the female carry different sex chromosomes and therefore, it is known as female heterogamety. It involves two conditions as follows:

ZZ-ZW type
ZZ-Z0 type

ZZ-ZW type

Both the sexes contain two sex chromosomes here. ZZ-ZW type of sex determination is seen in birds and some reptiles. Males have homomorphic sex chromosomes (AA+ ZZ), whereas females have heteromorphic sex chromosomes (AA + ZW). Females are heterogametic here. That means they lay two types of eggs like (A + Z) and (A + W). There is only one type of male gametes or sperm (A + Z). In this case, the female determines the sex of an offspring. Similar to humans, the chances of producing male or female offspring is 50% here.

Fig: Sex determination in birds

ZZ-Z0 type

This type of sex determination is seen in some butterflies and moths. Males have two homomorphic sex chromosomes (AA + ZZ), while females have an odd sex chromosome (AA + Z). In this case, the male are homogametic and form only one type of sperms (A+Z), whereas the female produces two types of eggs like (A+Z) and (A+0).

Fig: Sex determination in butterflies and moths

Sex determination in Drosophila

Drosophila has four pairs of chromosomes in which three pairs are autosomes and one pair is sex chromosome. The female Drosophila has identical chromosomes i.e XX and the male Drosophila has different chromosomes i.e. XY. The male produces two heterogametes: (3+X) and (3+Y), whereas the female produces homogametes with (3+X). The Y-chromosome does not control expression of maleness here. Instead, autosomes are involved in this process. Female defining genes, such as Sxl, are found on the X-chromosomes.

Fig: Drosophila

Fig: Sex determination in Drosophila

Genic balance theory of sex determination

It says that sex is determined by the genic balance or ratio between X chromosomes and autosome genomes, rather than XY chromosomes. This theory is primarily applicable to the Drosophila melanogaster on which Bridges worked. According to the observations:

If the Genic ratio (X /A) of 1.0 produces fertile females.
If the Genic ratio (X /A) of 0.5 forms a male fruit fly.

Further, Bridges proposed that:

Metamales with a genic ratio of less than 0.5 (XY + 3A or X/3A or 0.33) are infertile (super males)
Intersexes are produced if a genic ratio is between 0.5 and 1.0
Sterile metafemales or super females are produced if the genic ratio is 1.5 (3X/2A).

Dodson refers to the sterile metamales and metafemales as the "glamour boys and girls of the fly world."

Fig: Sex determination in fruit fly

Sex determination based on haplodiploid system

Honey bees, ants, and wasps are examples of highly social community insects with this type of sex determination system.

Fig: Examples of haplodiploid system

Sex determination in honeybee

Honeybees and ants follow a hierarchy system in the colony.

Members of honeybee colony

There are queen, drones and workers in their colony.

Drones

They are males. They are haploid. They help in fertilising the unfertilised eggs which are ready to fertilise in a receptive queen. Male insects develop parthenogenetically from unfertilised eggs. This phenomenon is called arrhenotokous parthenogenesis or arrhenotoky. In this system, males generate sperm through the process of mitosis. This process is the same for ants and wasps.

Workers

They are females who are not capable of reproducing. Housekeeping, feeding the individual of the colony (queen, drones, and larvae), pollen and nectar collection, and wax production are their main responsibilities. So, we can say that everything in the hive is done by worker bees. The females are developed from the fertilised eggs and hence they are diploid.

Queen

The queen honeybee is the only reproductive female in the colony. She is specialised in egg laying. The male ‘drones’ only mate with a virgin queen. When a queen mates with a drone, she saves the drone's sperm.

The sex of honeybee is determined on the basis of number of sets of chromosomes an individual receives

Fig: Members of the honeybee colony

Environmental sex determination

This type of sex determination is based on the environmental factors, such as temperature and location. It is considered as two types as follows:

Sex determination based on temperature
Sex determination based on location

Sex determination based on temperature

This type of sex determination system is seen in crocodiles, alligators and turtles.

Sex determination in crocodile and alligator

Both for crocodiles and alligators, the sex determination method is similar. They require a critical temperature which is a minimum incubation temperature for egg development. A female will be formed, if the eggs are exposed to higher temperatures. A male will be formed, if the eggs are exposed to lower temperatures.

Fig: Sex determination in crocodile and alligator

Sex determination in turtle

For turtles, the critical temperature is 29℃. If the eggs are exposed to temperatures above 29°C, they develop into females. A male will be formed, if the eggs are exposed to temperatures below 29°C.

Fig: Sex determination in turtle

Sex determination based on location

This type of sex determination system is seen in green spoonworm. Sex is determined on the basis of location of larvae landing. If the larva lands on the bare ocean floor, it develops into a female. On the other hand, if a larva lands on a female's body, it develops into a male.

Fig: Sex determination in green spoonworm

Practice Problems

1. The individuals possess homomorphic sex chromosomes produce __________.

a. two types of gametes
b. no gametes
c. one type of gamete
d. only one gamete in the complete life span

Solution: Individuals with homomorphic chromosomes create only one type of gamete. Human females, for example, generate only gametes with 22+X chromosomes. Individuals with heteromorphic sex chromosomes have the ability to produce two different types of gametes. Human men, for instance, can create gametes with 22+X or 22+Y chromosomes. Hence, the correct option is c.

2. XX-X0 sex determination and XX-XY sex determination are two types of sex determination and are examples of __________.

a. male heterogamety
b. female heterogamety
c. male homogamety
d. haplo-diploid system

Solution: XX-X0 type of sex determination is seen in roundworms and some insects, such as grasshoppers and cockroaches. In these organisms, the female possess two identical sex chromosomes i.e. XX. The male of these organisms only have one sex chromosome i.e. X (designated as X0). In this case, females are homogametic (A+X) and males are heterogametic, in which half the population carry X-chromosome and the rest half are devoid of X-chromosome. In XX-XY type of sex determination, male possess two heteromorphic sex chromosomes i.e. X-chromosome and Y-chromosome. The female possess two homomorphic or isomorphic sex chromosomes, i.e. X-chromosome and X-chromosome. This type of condition can be seen in most mammals including humans. Hence, the correct option is a.

3. In which organism, XX-X0 type of sex determination is found ______________.

a. Hens
b. Drosophila
c. Parrots
d. Grasshoppers

4. Temperatures below 28 degrees celsius during egg incubation in turtles cause _________.

a. Maleness
b. Femaleness
c. Both (a) and (b)
d. No effect on sex determination

Solution: For turtles, the critical temperature is 29℃ which is the minimum incubation temperature required for egg development. If the eggs are exposed to temperatures above 29°C, they develop into females. A male is formed when the eggs are exposed to temperatures below 29°C. Hence, the correct option is a.

5. What is a sex chromosome?

Solution: Sex chromosome is a type of chromosome that determines the sex of an organism. For example, in humans, two types of sex chromosomes are present, X-chromosome and Y-chromosome.

6. Which type of sex determination is seen in honeybee?

Solution: In honeybees, haplodiploid sex determination method is seen. The males are haploid here and are formed parthenogenetically from unfertilised eggs. Females are diploid and formed from the fertilised eggs.

7. Which type of sex determination method do birds have?

Solution: In birds ZZ-ZW type of sex determination method is seen. In this method, males are homomorphic, whereas females are heteromorphic.

FAQs

Q 1.What happens if a human male is missing the SRY gene?
Answer: If a human male whose genotype is XY is missing the SRY gene, then without functional sex-determining region Y protein, this fetus will not form testes but will normally develop fallopian tubes and uterus.

Q 2. Does every embryo start off as a girl?
Answer: All human and mammalian embryos begin their lives as females normally. The foetal testes produce enough androgens to balance out the maternal estrogens about the second month, and maleness emerges.

Q 3. In which organism, sex is not determined genetically?
Answer: In snails, the process of determining sex is not genetically defined. This indicates that the offspring's sex is not determined by the genes passed down from parents. Individuals in organisms like snails can alter their sex. For example, in Crepidula (slipper snails), if the larva is present with the female, it develops into a male, however if it is present alone, it develops into a female.

Q 4. Can a human male have two Y chromosomes?
Answer: The presence of an additional Y chromosome causes XYY syndrome, an uncommon chromosomal abnormality. Males usually have 46 chromosomes, including one X and one Y chromosome. XYY syndrome affects men who have 47 chromosomes, two of which are Y chromosomes. They are normally taller than average height with a low muscle tone.