Difference Between Autosomes and Sex Chromosomes

Main Difference – Autosomes vs Sex Chromosomes

During the cell division, chromatin in the nucleus shrinks to a thread like structures named chromosomes. Two major types of chromosomes can be found in eukaryotic cells. They are autosomes and sex chromosomes. Humans have 22 homologous pairs of autosomes and one pair of sex chromosomes. The main difference between autosomes and sex chromosomes is that autosomes are involved in determining the somatic characters of an individual and sex chromosomes are involved in determining the sex and the sex-related hormonal traits.

 This article explains,

1.What are Autosomes?
     – Definition, Function, Autosomal genetic disorders
2.What are Sex Chromosomes?
     – Definition, Function, Sex-linked genetic disorders
3.What is the difference between Autosomes and Sex Chromosomes?

What are Autosomes

Non-sex chromosomes which determine the trait of an organism is identified as autosomes. They are also known as somatic chromosomes since they determine the somatic characters of an individual. A genome mainly consists of autosomes. For example, human body contains 46 chromosomes within its genome and 44 chromosomes of them are autosomes. Autosomes exist as homologous pairs and 22 autosome pairs can be identified in the human genome.

Both autosomal chromosomes contain the same genes, which are arranged in the same order. But an autosomal chromosome pair differs from other autosomal chromosome pairs within the same genome. These pairs are labeled from 1 to 22, according to the base pair sizes contained in each chromosome.

Autosomes also participate in sex determination. SOX9 gene is an autosomal gene on chromosome 17. It activates the function of TDF factor which is encoded by Y chromosome. TDF factor is critical in male sex determination. Hence, a mutation of SOX9 causes the development of Y chromosome, resulting in a female.

Autosomal genetic disorders occur due to either the non-disjunction in parent chromosomes (Aneuploidy) during gametogenesis or the Mendelian inheritance of deleterious alleles. An example for aneuploidy is Dawn’s Syndrome, which possesses three copies of chromosome 21 per cell. Disorders with Mendelian inheritance can either be dominant or recessive (Ex: Sickle cell anemia).

Figure 1: Human male karyotype

What are Sex Chromosomes

Sex chromosomes are referred to as allosomes. They determine the sex of an individual. The sex determination also happens in most animals and many plants. Humans have only 2 sex chromosomes in their genome which are labeled as X chromosome and Y chromosome. A female individual is determined by XX and a male individual is determined by XY. A female contains the same two copies of sex determining genes arranged in the same order in both X chromosomes (homomorphic). Therefore the sex chromosomes in a female are homologous to each other. In male’s, the two sex chromosomes contain different genes (heteromorphic).

During Meiosis, female gametes are made of a single X chromosome plus 22 autosomal chromosomes. Male gametes are made either of an X or Y chromosome plus 22 autosomal chromosomes. The joining of two gametes containing both X chromosomes produces a female offspring. On the contrary, the joining of two gametes, containing either an X or Y chromosomes produce a male offspring. The fertilization of two gametes, each containing a haploid set of chromosomes makes human genome diploid. Some unfertilized eggs of ants and bees develop into haploid males while fertilization makes females.

Sex-linked genetic disorders such as hemophilia and Duchenne muscular dystrophy occur due to the defective second copy of the same gene. Red/green blindness occurs due to the gene defectiveness on the X chromosome. If a male inherits the defective copy of the gene which is responsible for red/green blindness, it might cause the blindness since he contains a single X chromosome. Developmental abnormalities in infants are caused by unusual combinations of sex chromosomes (Ex: XXX, XXY).

Figure 2: X-linked recessive inheritance

Difference Between Autosomes and Sex Chromosomes

Definition

Autosomes: Autosomes determine the trait. Males and females contain the same copy of autosomes.

Sex Chromosomes: Sex chromosomes determine the gender. They are different in males and females by their size, form, and behavior.

Labeling

Autosomes: Autosomes are labeled with numbers, from 1 to 22.

Sex Chromosomes: Sex chromosomes are labeled with letters as XY, ZW, XO and ZO.

Availability

Autosomes: Most of the chromosomes within a genome are autosomes.

Sex Chromosomes: A few of the chromosomes within a genome are sex chromosomes.

Homogeneity

Autosomes: The 22 pairs of autosomes are homologous in humans.

Sex Chromosomes: Female sex chromosomes (XX) are homologous (homomorphic) while male sex chromosomes (XY) are non-homologous (heteromorphic).

Position of the Centromere

Autosomes: Since autosomes are homomorphic, the position of the centromere is identical.

Sex Chromosomes: Since the male sex chromosomes are heteromorphic, the position of the centromere is not identical. The position of the centromere in female sex chromosomes is identical. 

Number of Genes

Autosomes: Autosomes contain the number of genes varying from 200 to 2000. Chromosome 1 which is the largest, carries about 2800 genes in humans.

Sex chromosomes: X chromosome contains more than 300 genes while Y chromosome contains only a few genes since it is small in size.

Genetic Disorders

Autosomes: Autosomal disorders show Mendelian inheritance.

Sex chromosomes: Sex-linked disorders show Non-Mendelian inheritance.

Conclusion

Heteromorphic sex chromosomes inherit unequal times through offspring. Thus they do not have much implication in evolutionary processes such as mutation, selection, and genetic drift. But homomorphic chromosomes undergo evolutionary processes by homologous recombination and mutation. Thus, sex chromosomes are considered to be disproportionate in the Haldane’s rule.

Reference:

Griffiths, A. J. F., Miller, J. H., Suzuki, D. T., et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.

Johnson, N. A., and Lachance, J., The genetics of sex chromosomes: evolution and implication for hybrid incompatibility. Ann N Y Acad Sci. 2012 May; 1256: E1–22. doi:10.1111/j.1749-6632.2012.06748.x.

Image Courtesy:

“X-linked recessive” By XlinkRecessive.jpg: National Institutes of Healthderivative work: Drsrisenthil – XlinkRecessive.jpg (Public Domain) via Commons Wikimedia

“Human male karyotype” By  National Human Genome Research Institute – From w:en:Image:Human male karyotpe.gif, Uploaded by User:Duncharris. (Public Domain) via Commons Wikimedia

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