Genetic linkage

• When it was first relised that chromosomes contain genetic material, they began ti suspect that discrepancies might occur between the law of independent assortment of genes and the behaviour of chromosomes during meiosis. 
• Genetictis assumed that each speciea of organism must contain thousands of diffrent genes, yet they have a few dozen chromosomes.
• Each chromosome carries loads of genes 
• Transmission of genes located close to each other on the same chromosome violates law of independent assortment. 

Overview of linkage 

• Chromosome carries many individual functional units- called genes - influences an organism's traits. 
• Typical chromosome is expected to contain many hundreds or throusand diffrent genes. 
• Term syntemy means that two or more genes are located on the same chromosome. Genes that are syntetic are physically linked to each other, because each eukaryotic chromsome contains a single, continous, linera moleccule of DNA. 
• Chromosomes ar ecalled linkage groups, chromosomes contain a group of genes that are physically linked together. 
• The number of linkage groups equal the number of chromosome types. 
• Human somatic cells have 46 chromosomes, compose 22 types of autosomes, that come in pairs plus one pair of sex chromosomes X and Y
• Humans have 22 autosomal linkage groups and an X chromosome linkage groupm human males have a Y chromosome linkage group. 
• Human mitocondrial geneome is another linkage group. 
• Genetictys are oftern intrested in the transmission of two or more characters in a genetic cross. 
• Genticists follows the variants of two diffrent characters in a ceoss, this is called a two-factor crpss
• Outcome of a two factor or three factor cross depends on whether or not the genes are linked to each otger on the same chromosome. 

Bateson and punnet 

• Mendel's law of independent assortmet, two factor cross between two individuals that are heterozygous for two genes should yield a 9:3::3:1 phentotypic ratio among offspring. 
• An unexpected result came from the F2 generation, even theough F2 generation has four diffrent phentopic catergories, observed number of offspring with various phenotype did not conform to the ratio. 
• They found that the F2 generation had a much greater proprtion of the two phenotypes found in the P generation 
• They suggested that the transmission of these two characters from the P generation to the F2 generation was somehow coupled, allels were not assorted in a independent manner. 
• They did not relize that this coupking was due to the linkage of the flower gene and the pollen shape gene on the same chromsome. 

Relationship betwen linkage and crossing over 

• Even thougfh the allels for diffrent genes may be…