Introduction to DNA Marker
DNA marker as the name suggests is a sequence of DNA which is present at a specific site on DNA. In other words, DNA marker is that DNA sequence which has marked DNA and present in repetitive manner. For example, suppose there is a 15-20 nucleotide long sequence present on DNA that is repeating several times in tandem. These markers are not genes (genes are nucleotide sequences that code for a protein). These marker DNAs never form mRNA and proteins.
In other words, these DNA sequences are non-coding in nature and stay silent on DNA. These sequences are transferred from one generation to the next generation. DNA markers are used in forensic science and also used in identifying genetic diseases. The article discusses the types of DNA markers in a simple and brief manner.
Types of DNA markers
Examples of DNA markers include SSR (simple sequence repeats), STR (short tandem repeats), RFLP (Restriction fragment length polymorphism), ISSR (inter simple sequence repeats), RAPD (random amplification of polymorphic dna), and AFLP (amplified fragment length polymorphism).
On the basis of how these DNA markers are inherited to the next generation – DNA markers have been divided into two types:
Co-dominant markers: Those markers which are inherited from parents to offspring in equal proportion. For instance, if Parent 1 and Parent 2 inherit a marker equally in the next generation, it is called co-dominant marker.
In the picture above, it is shown that P1 is having a DNA marker and P2 is having a DNA marker. When parental gametes fuse, new offspring is produced as F1. Now, in F1 generation, both markers from P1 and P2 are seen. It means that both markers are expressed and this phenomenon is called co-dominance. In terms of alleles, if P1 is having AA allele and P2 is having aa allele, the F1 generation shows Aa allele. Both dominant and recessive alleles are expressed in F1 generation. Examples of co-dominant markers include SSR, STR, and RFLP.
Dominant markers: In this case, marker of only one parent is expressed in F1 generation. The P1 marker i.e., the dominant marker allele form DNA band in the gel, but the recessive marker DNA does not form any band. However, in F1 generation, a single DNA band is seen. However, it is not possible to conclude whether this DNA marker allele is dominant or recessive in nature.
In the above picture, P1 is having a DNA band with dominant allele AA and P2 is not having any DNA band formation, however, its allele is represented as aa. In the F1 generation, a single DNA band is formed with genotype Aa, however, only A is expressed. And that is the reason why it is called dominant marker. Examples of dominant DNA marker include ISSR, RAPD, and AFLP.
Q. Given below is a marker profile of two parental lines (P1 and P2) and their derived F1 progeny:
The marker that is represented in the above figure is most likely to be:
- RFLP and SSR
- SSR only
- SSR or RAPD
- RAPD only
Answer: The above picture shows the DNA markers bands for P1 and P2. F1 generation shows two bands which are also found in P1. It means, only one parent’s DNA band is visible in the F1 generation and that is the case of dominant marker. RAPD is the only dominant marker given in the option. So, the correct answer would be option 4.