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