BIOL205:week 4, lecture 10
Genes on the same chromosome: Linkage
Mapping by recombinant frequency
• independent assortment of genes on different chromosomes at meiosis is one way to “shuffle”
• How do we detect recombination?
• compare the “inputs” into meiosis with the “outputs”
• in humans the “inputs” are sperm and egg from mother and father and the “outputs” are the
person’s eggs or sperm
• any meiotic product that has a new combination of the alleles provided by the two input
genotypes is by definition a recombinant
In diploids, recombinants are best detected in a testcross Independent assortment produces 50 percent recombinants
But wait a second…
• chromosomal theory of inheritance suggested that genes are on chromosomes
• remember Mendel did not know what genes were
• for him, chromosomes and genes were the same thing: “factors”.
• but what if Mendel decided to study more than 7 traits?
• the pea plant has 7 pairs of chromosomes.
• eventually he would have a situation where 2 genes would be on the same chromosome.
Who was correct? Mendel? Chromosomal Theory?
• although all of Mendel’s 7 genes showed independent assortment, several were on similar
chromosomes (three on chromosome 4 and two on chromosome 1).
• however, the distance separating the genes were far apart so it behaved as if they were on
Dihybrid (double heterozygous) test cross:
If the two genes were on the same chromosome Was Mendel correct?
• early 1900s: William Bateson + R.C. Punnett studying inheritance of two
factors (genes) in peas
• they carried out standard F1 Dihybrid self crosses but it did not show the
9::3:3:1 ratio as predicted by the principle of independent assortment
• and suggested that the factors were coupled
• confirmation of Bateson and Punnett’s hypothesis had to await the
development of Drosophila as a genetic tool
• Thomas Hunt Morgan found a similar deviation from Mendel's Second Law
while studying two autosomal recessive genes in Drosophila
• Morgan proposed linkage as a hypothesis to explain phenomenon of apparent
Arecombination-based map of one of the chromosomes of Drosophila →
• recombination maps of chromosomes are usually assembled two or three genes at a time
• when geneticists say 2 genes linked, they mean the loci of those genes are on same chromosome
• hence, the alleles on any one homolog are physically joined (linked) by DNAbetween them
• the way in which early geneticists deduced linkage is a useful means of introducing most of the
key ideas and procedures in the analysis Why make genetic maps?
• gene position is crucial information needed to build complex genotypes required for
experimental purposes or for commercial applications
• knowing position occupied by a gene provides a way of zeroing in on its structure and function
• a gene's position can be used to define it at the DNAlevel
• in turn, the DNAsequence of a wild-type gene or its mutant allele is a necessary part of
deducing its underlying function
• the gene arrangement on chromosomes are often slightly different in related species
• for example, the rather long human chromosome number 2 is split into two shorter
chromosomes in the great apes
• hence, chromosome maps are useful in interpreting mechanisms of evolution
• chimpanzees (closest living relative to humans) have 48 chromosomes, humans have 46
• either lost one, or they fused together to form one
• more likely fused, don't lost → important
Evolution and Human Chromosome 2
Iffused, would predict find telomere sequences