Introduction
Human beings have 46 numbers of chromosomes arranged in a human cell in 23 pairs. These carry information that is passed from the parent to the children through heredity. While the somatic cells contain 46 chromosomes in a diploid form the the sex cells contain only 23 chromosomes in haploid form. But have you ever wondered why there is a discrepancy in the chromosome number of different cells? And which chromosome decides the gender of the offspring?
Autosomes
Out of the 23 chromosomes pairs, 22 pairs are referred to as autosomes.
There are two copies of songs in both females and males. These chromosome are involved in all the functions except for sex determination. They regulate the somatic characters of the body.
These chromosomes regulate all inheritance of organisms characteristics and are responsible for transferring the genetic information from parent to offspring. Several genes in each chromosome perform a specific function.
They are numbered according to their shape and size which means that chromosome 1 with the smallest number, is the largest chromosome in size having 3000 genes on it. Thus, ranging from chromosome 1 to 22 the size of the chromosomes decrease. Except that the chromosome 21 is the smallest chromosome instead of chromosome 22.
Allosomes
23rd pair of chromosome in human beings is referred to as the sex chromosome or allosome.
In females the XX pair of chromosomes is found in males XY pair of chromosomes is found.
These chromosomss help in determining sex of offspring and also transfer sex linked traits from parents to offsprings.
Each chromosome generally has two arms named as 'p' and 'q'. In the case of an XY pair of chromosomes the X chromosome has an equal or similar p and q arm but the Y chromosome has an unequal, one long and one short arm.
Difference Between Autosomes and Allosomes
Autosomes
Somatic chromosomes.
22 pairs
Have more genes
Number of gene varies from 200 - 2000
Homologous pairs of chromosome
Control many traits
Follow Mendelian principles of inheritance
Centromeric position is identical
Allosomes
Sex chromosome
Only 1 pair represented as XO,XY,ZW or ZO
Have less number of genes
Number of gene upto 300
Non homologous pairs
Control only sex determining and sex linked traits
Do not follow Mendelian principles of inheritance
Centromeric position is non identical
Examples of Autosomal Disorders
Autosomal disorders can be identified by amniocentesis. These include-
Klinefelter's syndrome- Identified by Dr. Harry F. Klinefelter. It happens due to an additional copy of X chromosome making the overall chromosome 47 in number in individuals. Symptoms include development of mammary gland, tall stature, rudimentary testis.
Turner's syndrome- This is caused by deletion of X chromosome resulting in 45 chromosomes in individuals. Symptoms include short statured females with rudimentary ovaries.
Triple X syndrome- This is caused by the presence of 3 copies of X chromosome resulting in karyotype 47.
Supermale - In this case 2 copies of Y chromosome are present due to nondisjunction of chromosome at meiosis.
Do You Know?
Why Do Somatic Cells Have Chromosomes Double in Number?
Ans: Eukaryotic organisms have double or two copies of each chromosome which is inherited from each one of the parents. This creates diversity among the offspring and enhances adaptability to the environment. In the case of humans adaptability is in terms of good immune sy, ability to fight against diseases etc. Presence of two copies also enables the chance of passing a healthy copy to the next generation in case one gets mutated.
FAQs on Autosomes and Allosomes
1. What are autosomes and allosomes in human genetics?
In human genetics, chromosomes are categorized into two types. Autosomes are the chromosomes that determine the somatic (non-sex) characteristics of an individual. Humans have 22 pairs of autosomes. Allosomes, also known as sex chromosomes, are responsible for determining the biological sex of an individual. Humans have one pair of allosomes.
2. What is the key difference between autosomes and allosomes?
The primary difference lies in their function and structure. Here are the key distinctions:
- Function: Autosomes control the inheritance of all bodily traits except for sex-linked characteristics. Allosomes primarily determine the sex and control sex-linked traits.
- Number: In humans, there are 22 pairs of autosomes and only 1 pair of allosomes.
- Homology: Autosomal pairs are always homologous, meaning they are of the same size, shape, and carry genes for the same traits. Allosomes can be homologous (XX in females) or non-homologous (XY in males).
3. How do allosomes determine the sex of a child?
Sex determination in humans follows the XY system. The female parent has two X chromosomes (XX) and can only pass on an X chromosome to the offspring. The male parent has one X and one Y chromosome (XY) and can pass on either an X or a Y. If the offspring receives an X from the father, its genotype will be XX, resulting in a female. If it receives a Y from the father, its genotype will be XY, resulting in a male.
4. What are the main functions of autosomes in the body?
Autosomes carry the vast majority of our genetic information and are responsible for regulating most of our body's functions and traits. They contain the genes that code for characteristics like eye colour, hair colour, height, and blood type. They also control the production of proteins essential for metabolism, growth, and repair of all somatic cells.
5. What are some examples of disorders related to autosomes?
Autosomal disorders occur due to an abnormal number or structure of the 22 pairs of autosomes. Common examples include:
- Down's Syndrome (Trisomy 21): Caused by an extra copy of chromosome 21, leading to developmental and intellectual delays.
- Edward's Syndrome (Trisomy 18): Caused by an extra copy of chromosome 18, resulting in severe developmental issues.
- Patau Syndrome (Trisomy 13): Caused by an extra copy of chromosome 13, leading to severe intellectual disability and physical abnormalities.
6. Why is it biologically important for humans to have two copies of each autosome?
Having two copies of each autosome, a condition known as diploidy, offers significant biological advantages. It creates genetic diversity, as one copy is inherited from each parent. This diversity enhances a species' adaptability. Furthermore, it provides a 'backup' copy. If one gene on an autosome is mutated or damaged, the normal gene on the other homologous chromosome can often compensate, preventing a genetic disorder from being expressed.
7. Can autosomes ever play a role in sex determination?
While allosomes are the primary determinants of sex, certain genes on autosomes can influence this process. For instance, the SOX9 gene located on human chromosome 17 plays a crucial role in activating male development pathways. This gene is typically switched on by the SRY gene from the Y chromosome. However, mutations or issues with the SOX9 gene can disrupt normal sexual development, demonstrating an indirect but critical link between autosomes and sex determination.
8. How do the X and Y allosomes differ structurally from each other?
The X and Y chromosomes are non-homologous and differ significantly in structure. The X chromosome is much larger and contains over 1,000 genes that are vital for both sexes. The Y chromosome is significantly smaller and contains fewer than 100 genes, most of which, like the SRY gene, are specifically involved in male development. This size and gene-count difference is a key structural distinction between the two human allosomes.
9. What are some examples of disorders related to allosomes?
Allosomal disorders, or sex chromosome aneuploidies, are caused by an abnormal number of X or Y chromosomes. Key examples include:
- Klinefelter's Syndrome (XXY): An extra X chromosome in males, leading to underdeveloped testes and reduced fertility.
- Turner's Syndrome (XO): The absence of one X chromosome in females, resulting in short stature and ovarian failure.
- Triple X Syndrome (XXX): An extra X chromosome in females, which often has mild or no symptoms.
