Life
Science: Why are My Eyes Brown?
Introduction:
In 1868, Gregor Mendel published some experiments he performed
with pea plants. In this work, he discovered the principles of
heredity. He spent 8 years following what happened to various
traits of pea plants such as pea shape as he crossed different
varieties of plants. He meticulously followed each generation
and recorded the crosses he made and the traits of the offspring.
From this he was able to conclude: 2 genes control each trait
in a hybrid (Law of Segregation), and in a Mendelian hybrid, one
of the two genes controlling the given trait will be dominant.
Only the dominant trait will be expressed. This second observation
is now called the Law of Dominance.
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In humans, one allele is
inherited from ones biological father and one allele from ones biological
mother for each genetic trait. This pair of alleles can be dominant,
recessive, or hybrid. The dominant and recessive conditions are
called pure or homozygous since both genes present are identical.
The hybrid condition possesses one of each gene type, and is called
heterozygous. The genetic structure is called the genotype.
Although the genotype of an individual may differ from another's,
their outward appearance for a particular trait may be identical.
The outward appearance is called the phenotype. Take for example
someone who inherits R (can roll tongue) from their mother and r
(cannot roll tongue) from their father. Their genotype is Rr. Another
individual inherits an R from both their mother and father. This
individuals genotype is RR. Since the ability to roll your tongue
is dominant, both the Rr and RR individuals will be able to roll
their tongues. Hence, their phenotype is R.
The laws of inheritance are inherently the laws of chance. In a
hybrid x cross, (the case where the mother and father are both heterozygous
for a given trait), every offspring has a 50:50 chance of inheriting
the dominant or recessive gene. A common way of illustrating this
is with a Punnett Square:
The offspring in this example have a 50% change of being heterozygous
for the X trait and 25% chance of being homozygous for the recessive
x and 25% chance of 25% of being homozygous for the dominant X.
Notice that the offspring have a 75% change of having the X phenotype.
Note all traits are simple Mendelian types. Many traits demonstrate
incomplete dominance, where no gene is expressed over the others.
Traits will be blended together to form an intermediate trait. An
example of this is hair color. The combination of a brown-haired
gene and a blonde gene will most likely produce light brown hair.
Other traits, such as skin color, are determined by multiple genes.
One special form of linkage affects inheritance in a very visible
fashion. Certain genes are located on the chromosomes that determine
sex. Traits that do, such as color-blindness, appear primarily in
males. Hemophilia is another example. In each of these sex-linked
traits, the recessive gene is carried on the X chromosome. A female
is XX and a male is genetically XY. Thus, a male only has one X
chromosome and if it carries a gene for color blindness or hemophilia,
then he will exhibit that trait. However, for a female, there is
a second X chromosome. For her to exhibit such a trait there must
be the much rarer occurrence of the recessive gene present on both
chromosomes. Demonstration: Inheritance
of dominant and recessive traits.
We will study human heredity using the family tree (FT) board.
Scientists have developed a standard notation that is used on
this board for discussing heredity. The squares on this board
represent males and the circles represent females. A horizontal
line between male and female indicates marriage. A vertical line
indicates their descendants. When there are more than one descendent
it is shown as given in line F2 on the board.
As you look at the FT board, the top row makred P1 represents
two couples whom we call parents. Their offspring are labelled
F1. The four descendants of these two are labelled F2. These are
the grandchildren of the P1 generation. The F2 generation shows
four offspring. It represents a 25% probability that the F1 parents
can have a child with the traits being studied. (These correspond
to the four entries in the Punnett square discussed above.)
Genes, in this kit, are represented by plastic parts illustrating
various facial traits. Each part shows the trait which is the
effect of the particular gene. (The phenotype is shown on the
board and not the genotype.)
Lesson 1:
On a small pallet, place 10 "brown eyes", 10
"blues" and 4 "H" labels. Have the students
do these activities along with you using their worksheets and
colored pencils. Line P1 on the Family Tree board represents the
parents. Place two brown-eye traits into each male face along
the P1 line. These male parents are both PURE brown in eye color.
(Both genes in the pair are for brown eyes.)
Next, place two blue-eye traits onto each female face along P1.
These female parents are both PURE blue in eye color. (Both genes
in the pair are for blue eyes.) Now each couple has one child.
Each parent contributes one gene for eye color to his or her F1
child. Remove a gene from each parent (P1) and place it onto the
face of its offspring. (Place another gene similar to the one
just removed onto the parent since each gene occurs in pairs.)
Place one additional brown and one blue onto each F1 parent. (This
will give them enough genes to pass on to the next generation.)
Now make the four possible offspring. Discuss the results. What
are their eye colors? (3 brown, 1 blue) How many brown-eyed? (3)
How many blue eyed? (1) How many hybrids? (2) Is their an equal
chance to receive either gene? (yes) If these parents had four
children, would they necessarily produce one blue-eyed child?
(NO). (Lesson #2 will help the girls understand this.)
Lesson #2: Laws of chance
Tie the girls observations to the F2 offspring in the first lesson.
Lesson #3: Dominant Traits
The P1 line should show the first set of parents to be
father, pure brown (BB), mother, pure blue (bb). The mother and
father for the second set of parents should both be pure blue.
This is the same as saying BB x bb and bb x bb. In this notation,
males are always given to the left of the 'x' and females to the
right. The uppercase letters (B) indicate a dominant trait. The
lower case letters (b) indicate a recessive trait.
Have the students guess what the possibilities are for F1 and
F2. Put the appropriate eyes on the board. The students should
put the results on their worksheets.
There are 3 cases (2,3, & 4) in this lesson. Do as many as
time permits.
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