VOCABULARY:•Gene map
KEY CONCEPT:•What structures actually assort independently?
Remember
• Genes are located on chromosomes
• Independent assortment—genes for different traits can segregate independently during the formation of gametes
The Big Question…
• It may be easy to see that genes located on different chromosomes assort independently, but what about genes located on the same chromosome?
Thomas Morgan’s Research
• Morgan identified more than 50 genes on Drosophila’s 4 pairs of chromosomes.
• He discovered that many seemed to be “linked” together– They are almost always inherited together &
only rarely become separated
• Grouped genes into 4 linkage groups
Morgan’s Conclusion
• Each chromosome is actually a group of linked genes
• BUT Mendel’s principle of independent assortment still holds true
• It is the chromosome that assorts independently!!– Mendel missed this because 6 of
the 7 traits he studied were on different chromosomes.
Gene Linkage
• Linked genes: genes located on the same chromosome that tend to be inherited together– Chromosomes assort independently,
NOT individual genes• BUT…If 2 genes are found on the
same chromosome are they linked forever?– NO!!
Crossing OverCrossing Over
• Crossing over is random• Crossing over during Meiosis (Prophase
I) can separate genes • Crossing over sometimes separates
linked genes and therefore there is a new combination of alleles!
• CROSSING OVER increases genetic diversity by producing new combinations of alleles
Gene Maps
• Alfred Sturtevant was a graduate student working in Morgan’s lab part-time in 1911
• He hypothesized: the farther apart 2 genes are on a chromosome the more likely they are to be separated by crossing-over
• The rate of at which linked genes are separated can be used to produce a “map” of distances between genes
Alfred Sturtevant
1891-1970
Gene Maps…in other words
• Knowing how often crossing over occurs between genes allows us to map positions of genes on chromosomes
• Yes, crossing over is random…BUT the distance between two genes determines how often crossing over occurs
• Frequency of crossing over: how often crossing over occurs between 2 genes. – If two genes are close together, they have a
LOW cross over frequency– Knowing this helps estimate the positions of
genes on chromosomes.
Gene Maps
• This map shows the relative locations of each known gene on a chromosome– We have used this
technique to help construct maps of the Human Genome
Review…look in your notes…
•If 2 genes are on the same chromosome but usually assort independently, what does that tell you about how close together they are?
And…try this
• Draw M and N as unlinked genes
• Draw M and N as linked genes
M
N
M
N
Linked Genes Unlinked Genes