Getting Organized

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					   Getting Organized

How genetic material is arranged
Organization Overview

Homologous Chromosomes
               These have the same
               traits on them (not
               always the same alleles
                 Gene Linkage
Homologous chromosomes

                                                 During gamete formation
                                                 (meiosis) homologous
                                                 chromosomes segregate into
                                                 individual gametes

             Genes on the same
                                        That means                              Like red hair and
             chromosome are
                                        they’re inherited                       freckles.
             LINKED                     together

                                 Image from:
Homologous Chromosomes
    Looking at Chromosomes
Determined in babies through amniocentesis and
                                       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
              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                 50% female

                 X           XX                XY
      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
        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?
• 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, X-
  linked 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
• The classic example of an X-linked trait is
  color-blindness. It is recessive.

               XB         Xb
The X shows it is          The superscript letter
X-linked                   represents the allele
X-linked traits and Punnett
• Here’s a Punnett Square for color-blindness
Dad = normal vision: XB Y
                               X B    Y
Mom = “carrier”:   XB   Xb

50% chance of
color-blindness in       X B   XBXB   XBY
No chance of
color-blindness in
                           b   XBXb   XbY
girls. (50% chance
the girls will be
carriers though)
X-linked traits and Punnett
• 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).

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