Chromosomes contain many genes with each gene at a particular location or locus. Genes located on different chromosomes assort independently during meiosis because 2 chromosomes behave independently of one another as they line up on metaphase phase.
Genes located on the same chromosome may be inherited together in GENETIC CROSSES. Genetic crosses refers to the idea that genes don't assort independently because the chromosome is inherited as a single physical entity in meiosis. LINKED GENES are genes on the same chromosome and this phenomena is referred to as LINKAGE.
Groups of genes that tended to assort together in crosses were believed to be carried on the same chromosome. The first genetic map was created by Morgan who showed the relative order of genes on a chromosome, also estimated the distance separating the genes. Morgan reasoned that genes sitting relatively far apart on a chromosome would be more likely to be separated from one another during meiotic crossing-over than genes lying close together
NORMAL VS. ALLELE GENOTYPE
Geneticists work primarily with "normal" and "wild-type" genotype. Any changes in "wild-type" was mutant, that is, change in phenotype of the organism that expresses them.
MUTANTS, HOW ARE THEY WRITTEN?
- Dominant mutant = First letter uppercase
- Recessive mutant = First letter lowercase
- Notation for wild-type allele : Adding a superscripted (+) sign to the mutant allele notation
Morgan's hypothesis to explain Non-Mendelian Distribution:
- Suggested two genes are linked genetically-physically associated on the same chromosome
- Hypothesized the behaviour of these linked genes is explained by chromosome recombination during meiosis
- Proposed that the frequency of this recombination is a function of distance between linked genes
RECOMBINATION FREQUENCY
RECOMBINATION FREQUENCY is the characteristic of two particular genes involved and varied from less than 1% to a maximum of 50%. Recombinant offspring frequency can be used to make a linkage map of a chromosome, showing relative locations of genes. Note: Not all recombinant offspring frequency is equal to each other. WHY? Remember, genes farther apart on the chromosome are more likely to have more than one crossover between them.
- Single crossover between 2 genes= Recombinant Chromatids
- Double crossover between 2 genes= Parallel arrangement of alleles
The unit of a linkage map is map unit (mu) which is equivalent to the recombinant offspring frequency of 1%. MAPUNIT is relative and shows the positions of genes with respect to each other.
WIDELY SEPARATED LINKED GENES
Some genes are so widely separated that recombination is certain to happen. In this case, genes assort independently even if they are on the same chromosome. The map distance separating them is 50 mu. Linkage can still be detected by testing their linkage to one or more genes that lie between them.
No comments:
Post a Comment