24 - 10. Heritability & concordance

10. Heritability & concordance

© SPMM Course known to carry out PCR. It is not error free as laboratory contaminants can have DNA which gets amplified erroneously. The technique has three steps. (1) Double-stranded genomic DNA is denatured by heat into singlestranded DNA. The reaction is then cooled to favour DNA annealing, and the primers bind to their target DNA. (2) DNA polymerase is used to extend the primers in opposite directions using the target DNA as a template. After one cycle there are two copies of double-stranded DNA, after two cycles there are four copies, and this number rises exponentially with the number of cycles. (3) The cycling is set to produce necessary number of amplifications.  FISH – Fluorescent in situ hybridisation FISH is a cytogenetic technique to detect and localize specific DNA sequences on chromosomes. It uses fluorescent probes that bind to only those parts of the chromosome with which the probes have high degree of sequence similarity. Fluorescence microscopy is employed to detect the location where the fluorescent probe binds to the chromosome. FISH is often employed to detect specific features in DNA.  DNA Cloning Plasmid is a bacterial DNA, which is extra chromosomal, and independently replicating (similar to mitochondrial DNA in humans). Any particular DNA fragment of interest can be isolated and inserted (using a DNA ligase enzyme) into the genome of such self-replicating plasmids. When used for such a purpose the plasmids are called vectors (vehicles for DNA replication). Bacteriophages and other viruses can also be used as vectors. Replication by the millions of the vectors results in multiple copies or clones of the inserted sequence. Removal of inserted gene sequences from the host vector results in large quantities of the required genes. 10. Heritability & concordance

Concordance: A twin pair is said to be concordant when both co twins have the same disease expression (or both are disease free). The pair can be discordant if one of them harbours a disease while the other does not. Due to higher degree of genetic sharing among homozygous individuals, one would expect higher concordance among monozygotes compared to dizygotes if the disease being studied has a significant genetic component. In contrast, a trait that has no genetic basis should have equivalent concordance rates for MZ and DZ twins. Heritability is the main measure of genetic variation in polygenic (quantitative) traits. The total variation of a trait in a population can depend on genetic variation or environmental variation, so heritability is the proportion that is genetic, not environmental, out of that total. The relative influence of genetic factors in defining the variance in a trait is expressed as heritability. If this is defined as the proportion of the total phenotypic variance attributable to additive genetic variance, then it is known as narrow-sense heritability. Heritability is also sometimes used to describe the proportion of variance explained by the total genetic variance (additive and non-additive genetic variance). Here it is called broad-sense