BIOL 311 Chapter Notes - Chapter 4: Thomas Hunt Morgan, Mendelian Inheritance, Karyotype
BIOL 311 Chapter 4: Mapping Eukaryote Chromosomes by Recombination
Linked genes: the loci of two genes are on the chromosome and the alleles on any one
homolog are physically joined by the DNA between them
• Thomas Hunt Morgan found a deviation from Medels la of idepedet assortet
when studying the genes in Drosophila
o Linkage as a hypothesis to explain the phenomenon of apparent allele
association
• The use of a test cross is important because the tester only contributes gametes with
recessive alleles so you can focus on the dihybrid
o The phenotypes of the offspring directly reveal the alleles contributed by the
gametes by the dihybrid parent
o Did not find a 1:1:1:1 ratio
o The parental gametes have the greatest number indicating they are linked
• We can calculate the percentage of recombination by adding all the recombinants and
dividing by the total multiply by 100
• Alleles were linked on the same chromosome so the parental allele combos made up
majority of the progeny
Results of a dihybrid testcross:
2 equally frequent nonrecombinant classes totaling in excess of 50 percent
2 equally frequent recombinant classes totaling less than 50 percent
• Linked genes do not assort independently but produce a RF less than 50%
o RF less than 50% is a diagnostic for linkage
• For linked genes, recombinants are produced by crossovers; chiasmata are visible
manifestations of crossovers
Nomenclature
1. Alleles on the same homolog have no punctuation between them
2. A slash separates the two homologs
3. Alleles are always written in the same order on each homolog
4. Unlinked genes are separated by a semicolon: A/a;C/c
5. Unknown linkage is shown by a dot
• Crossover results from the breakage and the reunion of DNA
o 2 parental chromosomes break in the same position and each piece joins up with
the neighboring piece of the other chromosome (rejoining in reciprocal
recombinant combination)
• CO takes place in the 4 chromatid stage; products of meiosis of a tetrad can be isolated
o Tetrad analysis in which genes are linked show tetrads that contain 4 different
allele combinations
• Several CO can occur along a chromosome pair
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