Linkage and Recombination

• linkage is the linear arrangement of non allelic genes in the same chromosome.
• linkage do NOT assort independently
• Linkage Group is the physical association of genes on a chromosome
• number of linkage group is equal to haploid number
• if the number of genes exceed the number of chromosomes then there is linkage
• type of linkage: complete linkage is when genes are found at the same chromosome and are closely associated
• in complete linkage there is 100% parental type
• there is no recombinant in complete linkage
• Bateson, Saunders, and Punnett (1906) proposed partial linkage.
• Types of Linkage: Incomplete Linkage wherein genes found at the same chromosome however they are NOT closely associated with each other
• in complete linkage, recombinant is < 50% while parental type is > 50%
• recombinant in incomplete linkage occurs due to crossing over of non sister chromatids
• Thomas Hunt Morgan (1910) proposes that the the closer the genes are located in the chromosome, the greater the probability that they would be inherited together.
• in cis form or coupling phase (AB/ab) linked genes enter the zygote together because they are in the same chromosome.
• trans form or repulsion phase (Ab/aB) the linked genes located in separate chromosome enters the zygote separately.
• Type of linkage: Sex Linkage is where sex chromosomes carry genes other than those needed for sex determination.
• Sex chromosome are chromosome that determine the organism's sex.
• Autosomes are chromosomes that do not influence an organism's sex.
• X linked genes is a type of sex linkage where genes found on X chromosome of humans.
• X linked genes expression may differ between sexes.
• X linked dominant is a father to daughter transmission.
• X linked recessive is mother to son transmission.
• Females can be heterozygous or homozygous.
• Heterozygous female are carrier of recessive allele.
• Male can ONLY be hemizygous because they only have one X chromosome.
• Y linked genes is father to son transmission.
• Y linked genes is when genes are on the Y chromosome and only males can be affected.
• Linkage Mapping or Genetic Mapping is linear arrangement of non allelic genes on the chromosome
• Linkage Mapping establishes the correct gene sequence and their distances relative to each other.
• Strength of linkage is expressed by the linkage value.
• % recombinant (or linkage value) = 1 cM (centimorgan)
• Genes that are far apart from each other tend to assort independently.
• If two genes are very close together, then there is little chance that recombinant gametes will form.