MadSci Network: Genetics

Re: If my mom has blue eyes and my Dads are hazel then why does my sister have

Date: Wed Mar 19 14:55:28 2003
Posted By: Christopher Carlson, Senior Fellow, Dept. of Molecular Biotechnology
Area of science: Genetics
ID: 1047512419.Ge

Hi Brandon,

  Good question.  When you look around your class in school, you see many
different eye colors.  Eye color tends to be inherited.  For example, both
of my parents have blue eyes, and I have blue eyes, too.  It gets more
complicated when parents with different eye colors have a child.  If Dad
has brown eyes and Mom has blue eyes, what color will their child's eyes
be?  Before I explain the answer, let's talk about peas.

  In the 1800's a Bavarian monk named Gregor Mendel was growing pea plants
in his garden.  Some of the pea plants always had yellow peas, and other
plants always had green peas.  He was curious what would happen if he mated
a green pea plant with a yellow pea plant to make a "hybrid" pea plant.  To
his surprise, all of the hybrid pea plants had green peas.  He called the
green pea color "dominant" to the yellow pea color, because if you mated
pure green peas with pure yellow peas, the hybrids were always green.

  Then he mated the hybrid pea plants to each other, and in the next
generation there were some yellow pea plants.  So it was possible for a
hybrid green pea plant to have yellow pea offspring.  He realized that the
yellow trait had been transmitted through the green pea hybrid, and called
this an "allele", and figured out that each pea plant carried two alleles
for pea color.  A pure breeding green pea plant carries two green alleles,
and a pure breeding yellow plant carries two yellow alleles.  The hybrid
plant carries one green allele and one yellow allele, but because the green
allele is dominant, the hybrid always has green peas.  When a trait (like
pea color) is determined by two alleles at a single gene, it is called a
simple Mendelian trait, in honor of Gregor Mendel and his peas.

  Now let's get back to eye color.  Human eye color is kind of like pea
color, although there are more alleles.  There are blue alleles, hazel
alleles, brown alleles and green alleles.  Each person carries only two
alleles for eye color.  Just as green pea color is dominant over yellow pea
color, dark eye color is dominant over blue eye color, so a person with one
blue allele and one brown allele will have brown eyes.  Hazel is also
dominant over blue, so a person with one hazel allele and one blue allele
will have hazel eyes.

  In your family, mom has blue eyes, so she must have two blue alleles. 
Dad has hazel eyes, so he has at least one hazel allele.  If both of his
alleles were hazel, then you and all of your brothers and sisters would
have one blue allele (from mom) and one hazel allele (from Dad), so you
would all have hazel eyes.  On the other hand, if dad has one hazel allele
and one blue allele, some of your brothers and sisters would have hazel
eyes (a blue allele from mom and dad's hazel allele), while others would
have blue eyes (a blue allele from mom and dad's blue allele).  Because you
have hazel eyes and your sister has blue eyes, your Dad has to have one
blue allele and one hazel allele.


  P.S. This answer has been dramatically simplified for a K-3 audience. 
Although eye color is generally taught as a simple Mendelian trait with
dark alleles dominant over light alleles (as above), it is in fact a
quantitative trait with a continuous spectrum of phenotypes and multigenic
inheritance.  Thus, eye color does not directly reflect paternity, and dark
eyed children of light eyed parents are not necessarily adopted.

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