Sex Determination

Table of Contents

The History of Sex Determination
Exploration of Sex Determination
Social Implications of Sex Determination
Potential Futures of Sex Determination

1. Introduction

Sex Determination is defined as a biological system that determines the sexual characteristics of an organism. Typically sexual characteristics are defined as being either male or female, and the characteristics are usually described by the XX-XY system of sex determination. Except when mutations occur, and the result is different syndromes and disease for example: Klinefelter and Turner Syndromes. This is what interested me about sex determination, that there could be so many possibilities for the XX-XY system to have mutations that alter the model X and/or Y-chromosomes.

2. The History of Sex Determination

In 1891 a German biologist by the name of Herman Henking discovered the sex chromosomes while studying sperm formation in wasps. He called the structure he identified the "X body". About ten years later a zoologist Clarence E. McClung studied the "X body" and found that it was a chromosome, but since that particular chromosome functioned differently than the rest of the chromosomes McClung called it the "accessory chromosome". Eventually the "accessory chromosome" became known as the X chromosome. Then McClung studied the cells of female grasshoppers relative to those of male grasshoppers, and found that the females had one more chromosome than males did in their cells. From this McClung concluded that these "accessory" chromosomes played a key part in sex determination.
external image McClung.gif
Clarence E.McClung

In 1905 Nettie Stevens and Edmund Wilson also experimented on grasshoppers and found that females have two X chromosomes while males have just one X chromosome. They experimented in different insects and found that they could count the same amount of chromosomes in male and female with the exception of one pair, the last pair. In females the last pair would be double X while in males it would be single X along with a smaller chromosome that is called the Y. Stevens and Wilson also concluded that during sperm formation half of the sperm took an X chromosome while the other half took a Y chromosome, and all eggs that are produced by females receive a X chromosome. In the reproduction process a sperm with the Y chromosome combines with an egg with the X chromosome to produce a XY male, also a sperm with the X chromosome combines with an egg carrying the X chromosome to form a XX female. Through all of their experimentation, Stevens and Wilson concluded that the last pair of chromosomes, the sex chromosomes, determine sex.
external image SK220_1_011i.jpg
Karyotypes for male and female organisms.

3. Exploration of Sex Determination

Sex Determination is defined as a biological system that determines an organism's sexual characteristics. There are three different systems of sex determination, they are: the XX-XO system, the ZZ-ZW system, and the most common XX-XY system. There are also three other ways of determining sex that aren't seen as often: Genic Sex Determination, Haplodiploid Sex Determination, and Environmental Factors.
  • The XX-XY system
    • The cells of female and male organisms have the same amount of chromosomes except when it comes to the sex chromosomes, XX or XY. In most organisms the Y chromosome is acrocentric, meaning that the centromere is located closely to one end of the chromosome.

external image chromo.jpg

Acrocentric Y chromosome
    • In the XX-XY sex determination system the male is what is considered to be heterogametic sex, because half of the gametes have the X chromosomes while the other half have the Y-chromosomes. Also in this system the female is considered to be homogametic sex because all of the gametes have the X chromosomes.
    • X and Y chromosomes pair because of a small homologous region called the psedoautosomal regions.
    • Many organisms have the XX-XY sex determination system such as some plants, insects, reptiles, and all mammals including humans. Other organisms not included in the list have a variation of the XX-XY system, variations include: the XX-XO system and the ZZ-ZW system.
  • The XX-XO system
    • In this system females have two X chromosomes and the males have only one X chromosome. An O to show the absence replaces the second sex chromosome in males.
    • In meiosis the female X chromosomes both X chromosomes enter into an egg. The same happens during the male meiosis except the X chromosome enters into half the sperm and the other half of the sperm do not get any of the sex chromosomes.
    • Females are said to be homogametic sex because they produce gametes that all have the same chromosomes. Males are said to be heterogametic sex because they produce gametes that differ in their sex chromosomes.
    • In this system the gender of the individual is determined by which of the male gametes fertilizes the female gametes. If a sperm carrying an X chromosome fertilizes an egg carrying the X chromosome the result is female (XX), but if a sperm carrying no sex chromosome fertilizes an egg carrying the X chromosome the result will be male (XO).
  • The ZZ-ZW system
    • In this system the roles are reversed. The females are heterogametic sex (ZW) and the males are homogametic sex (ZZ).
    • After meiosis the females in this system are ZW, half the eggs have a Z chromosome and the other half has a W chromosome. After meiosis in males all the sperms have Z chromosomes.
    • If a male sperm containing the Z chromosome fertilizes an egg carrying the Z chromosome produces a male (ZZ), but if a sperm containing the Z chromosome fertilizes an egg carrying the W chromosome produces a female (ZW).
    • This system is found in many birds, snakes, butterflies, amphibians, and fish.

  • Another system that is found in nature is called Haplodiploid sex determination this system is found in some insects such as bees, wasps, and ants.
    • In this system there are no sex chromosomes, the sex of an organism is based on how many sets of chromosomes there are, or how many sets are inherited from the parents.
    • Males evolve from unfertilized eggs and females from fertilized eggs, those that come from fertilized eggs are usually female but can be sterile males. Males have only one set of chromosomes meaning that they are haploid, whereas females have two chromosomes sets meaning that they are diploid
  • Genic Sex Determining Systems
    • In some plants and protozoans sex is determined genetically, but there is no distinction between the chromosomes of males and females, and there are no sex chromosomes.
    • Genic sex determination is defined as "genotypes at one or more loci determine the sex of an individual plant or protozoan".
    • In the genic sex determination system, independent genes control gender, but don't look different in male or female organisms.
  • Environmental Factors
    • Environmental factors are always a key part of deciding the gender of an organism.
    • In environmental sex determination gender is determined in part or wholly by environmental factors.
    • Environmental factors are apart of deciding the gender of reptiles. The temperature decides the gender of turtles, crocodiles, and alligators during development in the egg.
      • During turtle development if the temperature is warmer the egg will develop into a female during certain times in the year, while in cooler temperatures the egg will develop to be a male.
      • During alligator development if the temperature is cooler the egg will be a female, but when the temperatures are warmer during development the egg will be male.

external image 2_turtles.jpg
Two turtles whose gender is decided by environmental factors during development.

Social Implications

In normal day-to-day life there are people who live with mutations of the typical sex chromosomes. There are diseases like Turner and Klinefelter syndrome that alter your appearance greatly, where as women who have Poly-X syndrome look like a person with typical sex chromosomes. Most people do not understand where the mutations arise from like Turner syndrome comes from the presence of only one X chromosome, so the women with this disease may look different from most people. Like in any system there will be complications, and in the sex determination system there are some diseases that arise from mutations.
  • Turner Syndrome
    • This syndrome occurs in 1 in every 3000 females
    • Characteristics:
      • Short, receding hairline, a broad chest, and folds of skin on their necks
    • In 1959 Charles Ford used advanced technology to study human chromosomes, and when studying a 14-year-old girl with Turner syndrome found that in the disease there is only one X chromosome.
  • Klinefelter Syndrome
    • This syndrome occurs in 1 in 1000 males
    • The male cells have one or more Y-chromosomes and many X chromosomes.
    • Most cells of this syndrome are XXY, but in some cases they are XXXY, XXXXY, or XXYY.
    • Characteristics:
      • `Male, have smaller syndrome, reduced pubic and facial hair, and taller than normal
  • Poly-X Syndrome
    • This syndrome occurs in about 1 in 1000 females
    • Characteristics:
      • Taller than normal, and thin
    • The chromosomes in these cells have three X chromosomes; this is usually called Triplo-X Syndrome.

Potential Futures for Sexual Determination

At this point there is no cure for gender mutations however in the future scientists might be able to fix this problem. I think that in the future scientists will create a way to remove the extra X chromosome in Poly-X syndrome, the extra X or Y-chromosomes in Klinefelter syndrome, and add an extra chromosome to the single X chromosome in Turner syndrome. Scientists might also be able to find some way to eliminate a disease during child development in the womb. This would mean that scientists would revise the sex chromosomes in children with sex mutations, and replaced the sex mutations with normal sex chromosomes.


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