1. Introduction to Genetics
Part 3
Location of genes
Meiosis
Linkage and mapping
A map of the human Y chromosome

2. Location of genes
One of the things Mendel didn’t solve was exactly where genes are located in cells. His precise description of how genes behaved, though, helped other scientists determine that genes are on chromosomes.
Normal body cells have two copies of each chromosome. These copies are NOT identical, since they may carry different alleles for the genes they have. We say that these two chromosomes are homologous (meaning “similar in appearance, structure, and function”), and each comes from a different parent.
When a cell has both sets of homologous chromosomes, we say it is diploid (“two sets”) and give it a symbol for chromosome number: 2N
Current estimates are that humans have around 25,000-30,000 genes. Since we have 23 homologous pairs of chromosomes, this is a little over 1000 genes per chromosome (One allele of a gene on each copy of a particular chromosome).
For humans, 2N = 46. For fruit flies (four chromosomes) 2N = 8. Peas have seven pairs, so 2N = 14.

3. Human female karyotype

4. Gamete Formation: Meiosis
Gametes (sex cells like eggs and sperm), though, only carry one copy of each chromosome, so we call them haploid (“one set”), with the symbol N. For us, the haploid number is N=23.
What is the haploid number for fruit flies or peas?
How does that happen, since body cells have both sets?
In making gametes, there is an extra round of cell division WITHOUT duplication of chromosomes. This is sometimes called a reduction division. 2N gets split to N, making diploid cells into haploid cells.
This specialized cell division process that results in gametes is called meiosis.

5. Meiosis I
Most of what happens is like mitosis, and uses the same machinery.
The main differences:
Pairs of homologous chromosomes (called tetrads) line up together during Metaphase I. These were already duplicated during interphase, so each tetrad consists of four copies of the chromosome. At Anaphase I the homologous chromosomes move apart (centromeres do not split).
The daughter cells produced by Meiosis I are now haploid.
This is followed by a meiotic interphase. No DNA/Chromosome duplication occurs.

6. Meiosis II
Meiosis II is essentially just mitosis and the sister chromatids that remained together during Meiosis I now separate.
At the end of meiosis, from one original diploid cell, we now have four haploid cells. Since they contain single copies of homologous chromosomes, they are NOT identical.
Remember: sister chromatids are identical since they are exact copies of each other, but homologous chromosomes are not identical, since one comes from each parent.

7. Male gametes in animals are sperm. In some plants they are pollen.
Female gametes in animals are eggs, while in some plants they are egg cells.
At right:
Products of meiosis are different in male and female animals.

8. Another look at Meiosis showing segregation of alleles.

9. Mitosis vs. Meiosis

10. Quickcheck What do haploid and diploid mean?
What are the haploid and diploid numbers for humans?
What are the products of meiosis?
Why do we call meiosis a reduction division?
What do we call the two main parts of meiosis?
What is a tetrad and when does it exist?

11. Quickcheck What do haploid and diploid mean?
One and two sets of chromosomes.
What are the haploid and diploid numbers for humans?
n=23 2n=46
What are the products of meiosis?
Gametes (sex cells)
Why do we call meiosis a reduction division?
Because the final products are haploid cells (the number of copies of each chromosome has been reduced)
What do we call the two main parts of meiosis?
Meiosis I and Meiosis II
What is a tetrad and when does it exist?
A homologous pair of duplicated chromosomes (four chromatids; tetra = four)

12. Crossing Over 1 ABCDE (parental type) 1 ABcde (recombinant)
Recombination
One interesting and important event can happen during Meiosis I at the tetrad stage: Chromatids can sometimes exchange parts (called “crossing over”). This can result in chromatids (and gametes) with different combinations of genes or alleles than they originally had. This is one way new genetic combinations can occur. This process is also called recombination, and the chromatids that are now different from the parental types are called recombinants.

15. Gene Linkage
Mendel said genes independently assort, but it is really chromosomes that do this. Genes on different chromosomes are independent. If two or more genes are on the same chromosome, though, they are physically linked, and tend to stay together in crosses (genetically linked).
This was first discovered by Thomas Hunt Morgan and his students, using fruit flies (Drosophila melanogaster) as a test subject. They noticed that there were four different groups of genes that tended to stay together (=linkage groups), and they already knew that Drosophila have four chromosomes.
From this they concluded that chromosomes are essentially a group of linked genes.
It is only when a cross-over occurs between them that linked genes don’t stay together.

16. Linkage mapping
One of Morgan’s students, Alfred Sturtevant, realized that there was a pattern to when crossing over occurred.
The farther apart two genes are on a chromosome, the higher the chance that a crossover will occur between them.
He further realized that this information could be used to make a map of where genes are on a chromosome.
1% recombination between two genes = 1 “map unit” of distance between them.
For instance: there is 13% recombination between prune eye (pr) and vestigial wing (vg) in Drosophila, so they are 13 m.u. (map units) apart on the same chromosome. We can draw the map: vg pr
13 m.u.

17. Linkage mapping
In this way, we can make maps of where genes are on the chromosomes.
One note: Genes on different chromosomes do assort independently, so they seem to have a “recombination” rate of 50%.
For this reason, genes that really are on the same chromosome, but 50 or more map units apart, will act like they are on separate chromosomes. We would say that they are physically linked (on same chromosome) but not genetically linked (because they do independently assort).

18. Linkage mapping
A linkage map of selected genes (written as descriptions, not genotypes) on Drosophila melanogaster’s Chromosome 2.
Notice that the total length of the chromosome is 110 map units.
What percent recombination would you expect between the Purple eye and Curved wing genes?

19. Quickcheck What is crossing over?
What did Morgan conclude about genes and chromosomes?
What can we conclude if there is 23% recombination between the long nose and floppy ear genes of imaginary aardvarks?
What if the recombination rate was 50%?

20. Quickcheck What is crossing over?
During the tetrad stage, parts of chromosomes can be exchanged. This may change the combination of alleles on the chromosome.
What did Morgan conclude about genes and chromosomes?
Chromosomes are groups of linked genes.
What can we conclude if there is 23% recombination between the long nose and floppy ear genes of imaginary aardvarks?
They are on the same chromosome and 23 map units apart.
What if the recombination rate was 50%?
Either more than 50 m.u. apart OR on different chromosomes.

21. Meiosis Animations
hill.com/sites/ /student_view0/chapt er28/animation__how_meiosis_works.html
mples/majorsbiology/meiosis.html
/meiosis/main.html