Genetic Terms Flashcards
Recombination fraction
If two loci are on different chromosones they will segregate independently. If two loci are on the same chromosome they would always segregate together were it not for the process of crossing over. The nearer two loci are on a chromosome the less likely they are to be separated by crossing over.
Blocks of alleles on a chromosome tend to be transmitted as a block through pedigree, and are known as a haplotype.
The further away two loci are on a chromosome the more likely they are to be separated by crossing over, thus the recombination fraction is a measure of the distance between loci. The recombination fraction can vary from 0% if they are extremely close and 50% if they are on different chromosomes.
Genetic mapping
Mapping the genome (i.e. establishing what every part of it does) is done in two ways.
Genetic mapping - uses techniques such as pedigree analysis.
Physical mapping - uses molecular techniques to cut the DNA into pieces (using restriction enzymes) then to look at the pattern of pieces that result.
Physical maps can be divided into three general types: chromosomal or cytogenetic maps, radiation hybrid (RH) maps, and sequence maps. The different types of maps vary in their degree of resolution.
LOD score
When genes are very near to each other on a chromosome they are unlikely to be separated during crossing over in meiosis, such genes are said to be linked.
Genes which are very far apart or on different chromosome segregate independently, and are said to follow independent assortment.
The relative distance between two genes can be calculated by using the offspring of an organism showing two strongly linked traits, and finding the percentage of offspring where the traits do not run together.
The LOD score (logarithm of the odds) is a method used to ascertain if there is evidence for linkage. By convention a LOD score of >3 is considered evidence for linkage, and a LOD score of
Epigenetic
Epigenetics involves genetic control by factors other than an individual’s DNA sequence. Epigenetic changes can switch genes on or off and determine which proteins are transcribed.
Penetrance
The probability of a gene or genetic trait being expressed. ‘Complete penetrance’ means the gene or genes for a trait are expressed in all the population who have the genes. ‘Incomplete penetrance’ means the genetic trait is expressed in only part of the population.
Heritability
Heritability is the proportion of phenotypic variance attributable to genetic variance.
Anticipation
A phenomenon whereby the symptoms of a genetic disorder become apparent at an earlier age as it is passed on to the next generation.
