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 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 crossingover
• 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
Linked Genes
M
N
Unlinked Genes
M
N