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Instructions for Calculating the Percent Chances in Genetics
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Step 1 - Choose a letter.
Choose a letter for the characteristic you are calculating. Be
careful, because characteristic means "hair
color" or "blood type"
and not "brown
hair" or "type A blood".
Example
Color of flowers: |
R
= red |
r
= white |
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The capital (or big) letter represents the dominant gene and the
lower case (or small) letter represents the recessive gene. It is
important that you use the same letter for one
characteristic. In other words, use R and r, not R and w. A
and a, not A and b.
Be careful to chose a letter which looks very different when it is
written as a big or small letter. Do not use letter such as Cc, Oo, Pp, Ss,
Uu, Vv or Ww.
Step 2 - Write the genotypes.
In other words, write the genetic information about the parents concerning
their alleles for this particular characteristic. Remember that an
allele is one form of a gene (examples are given in the book). A
red flower could be RR or Rr.
For this first example, we will choose RR. If white is recessive
than a white flower must be
rr. If you do not know why, you need to go back,
re-read and re-learn what "dominant" and
"recessive" mean.
This flower could be RR or Rr.
In our example, let's suppose that the red flower is the one which
will give the male reproductive cells in the pollen. The white
flower will hold the ovule which will be fertilized. The genotype
of the flower which made the pollen is RR. The genotype of the
flower with the ovule to be fertilized is rr.
This flower can only be rr.
(If it was Rr, it would be red because red is dominant in this species
of flower.)
When talking about genotypes, we use certain technical terms:
heterozygous or homozygous. For example, RR is
"homozygous dominant" and rr is "homozygous
recessive". The term 'homo' means "same". We have
to specify if the genotype is dominant or recessive since just saying
"homozygous" only says that they are the same, not what they
are. Since 'hetero' means "different", we use the term
"heterozygous" to describe the genotype Rr. When
a genotype is heterozygous, the terms dominant and recessive have no
meaning since each allele is represented.
Step 3 - Prepare the gametes.
We need to separate the genes in each pair to prepare for the next
generation. In other words, we need to make the gametes - the
sperm and the egg cells. To remember what happens in meiosis to
make the reproductive cells, here is a table and a diagram:
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Parent Cell |
Gametes |
Male |
RR |
4 sperms with only
one "R" in each |
Female |
rr |
4 eggs with only one
"r" in each. |
Male:
Female:
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Remember, this only shows one gene. In fact what happens
is that each gene pair (and there are 10's of thousands of them) are
split up. Remember that each chromosome carries thousands of
genes.
Step 4 - Prepare a cross diagram.
Draw a table as shown below with three rows and three columns and
place the letters from the parent's gametes on the top and on the side:
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r |
r |
R |
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R |
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The rest is as easy as 1, 2, 3. Simply copy each letter
vertically and horizontally to fill in each box.
1...
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2...
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3...
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Note that the big letter is always written first.
Here is a little movie of another possible cross: Rr
x Rr:
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Step 5 - Calculate the percent chance.
What do the boxes tell us? Look back at the
results obtained in step 3 above. (not the one in the movie just
above) Each box represents a 25% chance of
the offspring having the genotype listed inside the box. Since
there are 4 boxes, and since all of them contain Rr, then this couple of
flowers has a 100% chance of having offspring with a genotype of Rr. Since R (for red
flowers) is dominant, all of the offspring
from this cross will be red. It is impossible to have any white
flowers.
Chance of having red flowers in the offspring |
4 out of 4 squares = 100% |
Chance of having white flowers |
0 out of 4 squares = 0% |
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Another example:
If the red flower was Rr instead of RR, what would the chances be for
having white flowers? |
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Here, the cross is Rr x rr: |
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Chance of having red flowers in the offspring |
2 out of 4 squares = 50% |
Chance of having white flowers |
2 out of 4 squares = 50% |
Before you try the questions for homework, make sure you
understand how this works. If you have any questions, contact
me.
Questions for Homework
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