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Getting Organized
How genetic material is arranged
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![](3D"slide0007_image002.gif")
Organizati=
on
Overview
http://www.accessexcellence.org/RC/VL/GG/chromosom=
e.php
Homologous Chromosomes
![3Dallele](3D"slide0004_image004.jpg")
These have the same tr=
aits
on them (not always the same alleles ho=
wever)
Gene Linkage
Genes on the same =
chromosome
are LINKED<=
/div>
That means =
they’re
inherited together
![](3D"slide0005_image006.gif")
![](3D"slide0005_image007.gif")
Homologous chromosomes
![](3D"slide0005_image008.gif")
During gamete formation <=
span
style=3D'position:absolute;top:25.0%;left:0%;width:85.09%;height:25.0%'>(m=
eiosis)
homologous chromosomes segregate into individual gametes
![](3D"slide0005_image009.gif")
![](3D"slide0005_image010.gif")
![](3D"slide0005_image011.gif")
![](3D"slide0005_image012.gif")
Like red hair and fr=
eckles.
Image from:
http://anthro.palomar.edu/biobasis/images/linked_genes.gif
![](3D"slide0006_image014.jpg")
Homologous Chromosomes
Determined in babies through amniocentesis and karyotyping
Looking at Chromosomes
Withdrawing some amniotic fluid =
from
around the fetus for analysis
Taking a look at the cell going
through mitosis and taking a picture of the ch=
romosomes. The chromosomes are co=
unted
and examined
![](3D"slide0003_image015.gif")
![](3D"slide0003_image016.gif")
![](3D"slide0003_image018.jpg")
![](3D"slide0003_image019.gif")
Sex Determination=
The first 22 homologous pairs a=
re =
referred
to as the body chromosomes or autosomes.
![](3D"slide0008_image020.jpg")
The last pair are the sex <=
i>chromosomes
and determine gender
![](3D"slide0008_image021.gif")
![](3D"slide0008_image022.gif")
Individuals with two X ch=
romosomes
are female and those with an X and a Y are male
This is a ….?
Sex Determination=
Since=
we
now know that it isn’t individual alleles that segregate but whole
chromosomes, we can use a Punnett square to show the chances=
of
a child being male or female
![](3D"slide0009_image023.gif")
X
X
X
Y
X X
X X
X Y
X Y
50% male
50% fem=
ale
•This chromosomal difference between boys
and girls, boys being XY and girls XX means that boys’
chromosomes do NOT all come in pairs.
•There are actually different traits co=
ded
on the X and Y chromosomes.
Sex Determination=
•We’ve already discussed how alle=
les
on the same chromosome tend to be inherited toget=
her
showing the pattern of inheritance known as Gene
Linkage.
•Alleles on the X or Y chromosome are sa=
id to
be Sex-linked and show another unusual pattern of
inheritance.
Sex-linked traits=
•Since the Y chromoso=
me is
the smallest chromosome, it actually has very few genes
on it.
•Question:
can girls show Y-linked traits? Why/whynot?
•Answer: N=
o, they
can’t. Girls don’=
;t
have a Y chromosome
Y-linked traits
<=
span
class=3DBB style=3D'position:absolute;left:-3.8%'>•Traits on =
the X
chromosome are X-linked.
•Boys and g=
irls
both have X chromosomes (boys have one while girls have
two).
•Question:
Who would show recessive, X-linked traits more often? Girls or boys?
•Answer: Th=
ough it
would be tempting to say girls because they have two chanc=
es to
have the alleles, it’s actually boys.X-linked traits
•The classic example of an X-linked tra=
it
is color-blindness. It
is recessive.
X-linked traits and Punn=
ett Squares
XB
Xb
The X shows it is X-=
linked
![](3D"slide0014_image024.gif")
The superscript letter re=
presents
the allele
![](3D"slide0014_image025.gif")
![](3D"slide0014_image026.gif")
•Here’s a Punnett Square for color-blindne=
ss
Dad =3D normal vision: XB Y
Mom =3D “carrier”: XB =
Xb
X-linked traits and Punn=
ett Squares
![](3D"slide0015_image027.gif")
XbY
XBXb
XBY
XBXB
XB
Xb
Y
=
;
XB
50% chance of =
color-blindness
in boys.
No chance of =
color-blindness
in girls. (50% chance the girls will be =
carriers
though)
•Because girls have two X chromosomes <=
/span>they
have a chance to be heterozygous carriers and not show the
trait. Guys do not have this chance.
<=
span
class=3DBB style=3D'position:absolute;left:-3.88%'>•Dad’=
;s
cannot pass X-linked traits to sons because they pass Y
chromosomes to sons.
If they pass their X chromosome to their offspring, it will=
be
a girl (not a son).
X-linked traits and Punn=
ett Squares
' + ENDSHOW_MESG + '
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No=
tes![](3D"buttons.gif")
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