What is the Difference Between Normal and Abnormal Karyotype

The main difference between normal and abnormal karyotype is that in a normal karyotype, the number and the appearance of chromosomes in the genome are similar to the normal genome of the species whereas, in an abnormal karyotype, the number and the appearance of chromosomes in the genome is dissimilar to the normal genome of the species. 

Normal and abnormal karyotype are the two possible karyotypes in individuals of a particular group of organisms. Normal karyotype does not develop genetic diseases while abnormal karyotype typically develops genetic diseases, which can even be lethal. 

Key Areas Covered 

1. What is a Normal Karyotype
     – Definition, Chromosome Number, Appearance
2. What is an Abnormal Karyotype
     – Definition, Chromosome Number Abnormalities, Chromosome Appearance Abnormalities
3. What are the Similarities Between Normal and Abnormal Karyotype 
     – Outline of Common Features
4. What is the Difference Between Normal and Abnormal Karyotype
     – Comparison of Key Differences

Key Terms 

Aneuploidy, Chromosome Appearance, Chromosome Number, Abnormal Karyotype, Normal Karyotype

What is a Normal Karyotype 

Normal karyotype refers to a karyotype of a normal individual in a particular population. It is the most common karyotype among the individuals of that species as well. The number of chromosomes in a somatic cell of a particular species is called the somatic number, which is designated as 2n. In humans, this 2n is equal to 46. This means each somatic cell in the human body has to contain 46 chromosomes. These 46 chromosomes include 44 autosomal chromosomes and 2 sex chromosomes. Autosomal chromosomes come with two homologous sets. One set shows a maternal origin while the second shows a paternal origin.  

Figure 1: Normal Human Karyotype

When considering the appearance, the attention mainly goes to the length of each chromosome, the position of the centromere, and the banding pattern. These can be observed under the microscope. The number and the appearance of chromosomes in the genome play a key role in the determination of the phenotype of each individual. 

What is an Abnormal Karyotype 

Abnormal karyotype is the karyotype with the alterations in the number of chromosomes or their appearance. This type of karyotype is less common among individuals of a particular species and it often causes genetic disorders. Three mechanisms introduce abnormalities into the genome.  

Alterations of Chromosome Number 

It occurs due to the nondisjunction of homologous chromosomes during anaphase I of meiosis I or the nondisjunction of sister chromatids during the anaphase of meiosis II. This results in the unequal distribution of chromosomes among gametes. Upon fertilization, these gametes can cause aneuploidy. Missing one chromosome in the genome is called monosomy (2n-1). Most cases of monosomy are lethal in humans except Turner’s syndrome (monosomy XO). Trisomy (2n+1) is another form of aneuploidy determined by the presence of an extra chromosome. Trisomy 21 causes Down syndrome, trisomy 13 causes Patau syndrome, and trisomy 18 causes Edward’s syndrome in humans. The nondisjunction of sex chromosomes causes Klinefelter syndrome (47, XXY), Trisomy Y: 47, XYY in males, and trisomy X: 47, XXX in females.  

Figure 2: Possible Abnormal Gametes

Alterations of Chromosome Structure 

This occurs due to the inversion, deletion, duplication or translocation of chromosome parts. A deletion of a small part of chromosome 5 causes Cri du chat. Fragile X is a situation of duplication. Acute myelogenous leukemia is an example of a chromosome translocation disorder.  

Similarities Between Normal and Abnormal Karyotype 

• Normal and abnormal karyotype are the two possible karyotypes in individuals of a particular species. 
• The two components of a karyotype are the number of chromosomes and the appearance of chromosomes 

Difference Between Normal and Abnormal Karyotype

Definition 

Normal karyotype is a karyotype in which the number and the appearance of chromosomes in the genome are similar to the normal genome of the species while abnormal karyotype is a karyotype in which the number and the appearance of chromosomes in the genome is dissimilar to the normal genome of the species. This explains the basic difference between normal and abnormal karyotype.

Occurrence 

Normal karyotype is the most common karyotype among the individuals of a particular species while abnormal karyotype is less common among the individuals. 

Chromosome Number 

Number of chromosomes in a normal karyotype is equal to the chromosome number of the species while number of chromosomes in an abnormal karyotype is altered from the number of chromosomes in the species. This is a major difference between normal and abnormal karyotype.

Chromosome Appearance 

The appearance of chromosome creates another difference between normal and abnormal karyotype. That is, the appearance of the chromosomes in a normal karyotype is similar to the appearance of the chromosomes of the species while the appearance of the chromosomes in an abnormal karyotype is altered from the appearance of chromosomes of the species. 

Effect 

Furthermore, normal karyotype is important for the determination of the phenotype of organisms while abnormal karyotype can cause genetic disorders.  

Conclusion 

Normal karyotype is a karyotype in which the number and the appearance of chromosomes in the genome are similar to the normal genome of the species while abnormal karyotype is a karyotype in which the number and the appearance of chromosomes in the genome is dissimilar to the normal genome of the species. Abnormal chromosome number always leads to genetic disorders. The main difference between normal and abnormal karyotype is the number of chromosomes and their appearance with respect to that of the species.   

Reference: 

1. “Human Chromosomal Disorders.” IUPUI DEVELOPMENT OF BIOLOGY, 30 Apr. 2003, Available Here

Image Courtesy:

1. “Karyotype (normal)” By National Cancer Institute – This image was released by the National Cancer Institute, an agency part of the National Institutes of Health, with the ID 2721 (image) (next). (Public Domain) via Commons Wikimedia  
2. “Translocation Down syndrome” (CC BY-SA 3.0) via Commons Wikimedia