Download Untitled

Chromosome Numbers
Humans have 46 chromosomes:
2 are the sex chromosomes
XX or XY
The other 44 are autosomes
•Egg cells carry one X chromosome.
•Sperm cells can carry either an X or a Y
chromosome.
With that in mind, which parent determines the
sex of the child?
Is this individual male or female??
Some genes are carried only on the X chromosome
“Sex-Linked” Traits
Examples: Red/Green Colorblindness, Hemophilia
**Since males have only one copy of the X
chromosome, any traits found on the X (even
recessive ones) are automatically expressed.**
Wait.
Why’s
that??
Think about it…
How many alleles would a female have for
the gene that determines color vision?
2- one on each X chromosome
The normal gene is dominant and the colorblind
gene is recessive. The only way a female could be
colorblind is if she receives 2 copies of the
defective gene.
NN
Nn
nn
However…
How many alleles for the same gene does a
male receive?? (Remember that this gene is
carried ONLY on the X chromosome…)
1
If his X chromosome carries the gene for
colorblindness, he is automatically
colorblind.
• Pairs of genes that tend
to be inherited together
• Why?
• Found on the same
chromosome
Linked Genes
Fruit Fly
Black Body
Purple Eyes
Shriveled Wings
http://ws4.niai.affrc.go.jp/dbsearch2/pmap/chrom13.gif
Crossing
Over
• Exchange of pieces of
DNA between pairs of
chromosomes
• The further apart genes
are on a chromosome, the
more likely they are to
recombine during
crossing over.
Chromosome Map
Shows the order of genes
on a chromosome
1 map unit = frequency
of recombining during
crossing over 1 percent
of the time
Black Body (48.5)
Purple Eyes (54.5)
Shriveled Wings (67.0)
http://ws4.niai.affrc.go.jp/dbsearch2/pmap/chrom13.gif
Example Chromosome Map
(Human X Chromosome)
http://embryology.med.unsw.edu.au/Notes/images/urogen/Xchr.gif