Getting Organized
How genetic material is arranged
Organization Overview
http://www.accessexcellence.org/RC/VL/GG/chromosome.php
Homologous Chromosomes
These have the same
traits on them (not
always the same alleles
however)
Gene Linkage
Homologous chromosomes
During gamete formation
(meiosis) homologous
chromosomes segregate into
individual gametes
Genes on the same
chromosome are
LINKED
That means
they’re inherited
together
Like red hair and
freckles.
Image from: http://anthro.palomar.edu/biobasis/images/linked_genes.gif
Homologous Chromosomes
Looking at Chromosomes
Determined in babies through amniocentesis and
karyotyping
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 chromosomes.
The chromosomes are counted and
examined
Sex Determination
The first 22 homologous pairs are
referred to as the body
chromosomes or autosomes.
The last pair are the sex
chromosomes and
determine gender
Individuals with two X
chromosomes 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
X
Y
50% male
X
XX
XY
X
XX
XY
50% female
Sex Determination
• 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 coded on
the X and Y chromosomes.
• In dudes, the sex chromosomes are NOT
homologous
Sex-linked traits
• We’ve already discussed how alleles on
the same chromosome tend to be
inherited together showing the pattern of
inheritance known as Gene Linkage.
• Alleles on the X or Y chromosome are said
to be Sex-linked and show another
unusual pattern of inheritance.
Y-linked traits
• Since the Y chromosome is the smallest
chromosome, it actually has very few
genes on it.
• Question: can girls show Y-linked traits?
Why/whynot?
• Answer: No, they can’t. Girls don’t have a
Y chromosome
X-linked traits
• Traits on the X chromosome are X-linked.
• Boys and girls both have X chromosomes
(boys have one while girls have two).
• Question: Who would show recessive, Xlinked traits more often? Girls or boys?
• Answer: Though it would be tempting to
say girls because they have two chances
to have the alleles, it’s actually boys.
X-linked traits and Punnett
Squares
• The classic example of an X-linked trait is
color-blindness. It is recessive.
XB
The X shows it is
X-linked
Xb
The superscript letter
represents the allele
X-linked traits and Punnett
Squares
• Here’s a Punnett Square for color-blindness
B
X
Y
B
X
XBXB
XBY
b
X
XBXb
XbY
Dad = normal vision: XB Y
Mom = “carrier”:
XB
50% chance of
color-blindness in
boys.
No chance of
color-blindness in
girls. (50% chance
the girls will be
carriers though)
Xb
X-linked traits and Punnett
Squares
• Because girls have two X chromosomes
they have a chance to be heterozygous
carriers and not show the trait. Guys do
not have this chance.
• 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).