Table of Contents
Sex Determination
Sex determination has long been a part of life on this planet. The well-known X and Y chromosomes in humans and other model species provide the idea that sex determination mechanisms are ancient and well-preserved. In many taxa, men and females are determined by a variety of factors that evolve fast. Despite the diversity of fundamental sex-determining cues, male and female development is triggered by similar molecular pathways. Many of these transitions may be driven by competing selection on various sections of the genome and between the sexes. Despite this, only a few systems with rapidly changing sex determination mechanisms have been thoroughly investigated. In this paper, we review our current knowledge of how and why sex determination evolves in animals and plants, identifying critical gaps in our understanding that present exciting research opportunities to characterize the evolutionary forces and molecular pathways underlying sex determination evolution.
Overview
Determination of sex is the process that defines the biological sex of an offspring and, as a result, the sexual characteristics that they’re going to develop. Humans typically develop as either male or female, counting on the mixture of sex chromosomes that they inherit from their parents. The human sex chromosomes, named X and Y, form cells made up of tightly bound deoxyribonucleic acid, DNA, and proteins. Those are molecules containing the instructions for the event and functioning of all life forms, including the possibility of physical traits and body parts corresponding to each biological sex. Humans who inherit two X chromosomes generally develop as females, while humans with one X and one Y chromosome typically develop as males. Sex determination is the beginning of the event of the many characteristics that influence how a person looks and functions because of other humans’ societal expectations.
Sex determination – In humans, Birds, Honey bee
Humans
In humans, the male-specific Y chromosome plays a pivotal role in sex determination and bears genes required for spermatogenesis. However, not all the genes needed to make a testis or male germ cells need to be on the Y chromosome, and many are known to be located on the X chromosome. Human females carry two X chromosomes (XX), and males hold a single X and a single Y chromosome (XY).
50% of the total sperm produced to possess the X-chromosome, and the rest 50% have Y-chromosome besides the autosome. There is an equal probability of fertilisation of the ovum (22+X) with the sperm carrying either the X or y chromosome. If the ovum fertilizes with (22+ X) type sperm, the zygote develops into a female (44+ XX).
Therefore, the genetic makeup of the sperm determines the sex of the child. That is always a 50% or 1/2 probability of either a male or a female child in each pregnancy.
Birds
The sex chromosomes in birds are identified as Z and W, and the male is the homomorphic sex (ZZ) while the female is heteromorphic (ZW). The Z chromosome is apparent in most avian species, usually the fourth or fifth largest, and contains almost all the known sex-linked genes. The W chromosome is generally a much smaller microchromosome, having a high proportion of repeat sequence DNA.
Honey Bee
The number of sets of chromosomes an individual receives is the basis of the sex determination in the honey bee. An offspring composed of a sperm and an egg coalition develops as a female (queen or worker), and an unfertilised egg grows as a male (drone) utilising parthenogenesis. It means that males have half the number of chromosomes than that of a female. The females are diploid, enclosing 32 chromosomes and males are haploid, i.e., having 16 chromosomes. It is called the haplodiploid sex-determination technique. It has special characteristic features. The males produce sperms by mitosis shown in the figure below; they do not have a father and thus cannot have sons, but have a grandfather and can have grandsons.
Also Check: Sex Determination – Type of Sex Chromosomes in Males and Females
Sex determination
Sex determination establishes sex through differential development in an organism or offspring. It defines whether it will be male or female. Various species use very distinct strategies for this purpose. Some microorganisms like turtles rely entirely on environmental factors such as temperature for sex determination. The sex of human beings and insects like grasshoppers, Firefly, Drosophila, etc., is determined genetically.
Types Of Chromosomal Cause Of Sex Determination
Chromosomal basis of sex-determination is of the following types:
- Male heterogamety: A male individual produces two different gametes in this type. Thus, the sex of the offspring is defined by the sperm. It involves two types of sex-determining mechanisms; XO type and XY type.
- XO type (XX-XO type): It is observed in many insects, for example, grasshoppers. The number of chromosomes is distinct in male and female individuals. All eggs (ova) bear an additional X-chromosome besides the autosomes, while only 50% of the sperms take X-chromosomes. In grasshopper, eggs fertilized by (A+X) type sperm become females while those fertilized by (A+O) type sperm become males. Therefore, sperm defines the sex of the offspring. Due to the involvement of the X-chromosome in sex determination, it was designated to be the sex chromosome.
- XY type (XX-XY) type: In several other insects like Drosophila and mammals, including human beings, the males contain two kinds of sex chromosomes (X and Y). In comparison, females possess two similar types of sex chromosomes (XX). Both males and females have an equal number of chromosomes. In males, Y-chromosome is usually shorter than the X-chromosome.
