A Pedigree is a Graphic Representation of Inheritance!
In your Ideal Mate Assignment, part II, you are required to figure
out the phenotype of your potential offspring. In order to do that, you need to know their
genotypes. To do that, you must know the genotypes (and phenotypes) of you and your ideal
mate. To do that, however, you need to know the genotypes (and phenotypes) of your parents
and your ideal mates parents, and so on . . .
In the hints section of the Ideal Mate Assignment, part II, you were
given hints to help you figure out the genotype and phenotype of your ideal mate (i.e., if
you don’t know the phenotype, make it the opposite of yourself; if the phenotype is
dominant, make it heterozygous). The reason for this is rather obvious, for such
information is not easily available. I cannot, however, be as easy on you!
In order for you to complete the Ideal Mate Assignment, part II, you
need to do the following, in the order below:
1. The Prelude -- collect phenotypic information for as many relatives as you can.
Remember the idea of sample size -- the more, the better!
2. Using this information, fill out the phenotype side of the table for your parents, and
yourself.
3. The only part of the genotype side you can now fill in is for those individuals who are
recessive, for they must be what genotype?
4. Using the hints section, fill out the phenotype and genotype for your ideal mate.
Now comes the hard part . . .
5. Construct and fill out a family pedigree as
per the given instructions.
6. As you fill out the pedigree, it becomes necessary to compare
the phenotypes of the parents and the children to the six possible punnett squares. You will need one pedigree for each of the ten
traits listed, so it is a good idea to make a master pedigree (with the
names of all the family members), and then ten copies, which you will fill out.
7. Although it may, under very specific conditions, become necessary to decide on a either
the homozygous dominant or heterozygous genotype, it is usually possible to determine the
exact genotype of most people in a pedigree (see the pedigree
instructions).
8. Using the genotypes you have determined for your parents from the pedigree (for which
you had to refer to the punnett squares, as stated above), simply copy them in the genotype
side of the table.
9. Given that you needed to refer to the punnett squares in
the construction of the pedigree for each trait, simply copy the appropriate punnett square (using the letters described, such as R’s
for tongue rolling, F’s for ear lobes) for your parents, with you and your siblings
being within the square. You will need one square for each of the ten traits listed.
10. Having determined your genotype, and your ideal mate’s (from the hints section),
fill out the appropriate punnett squares for each of the ten
traits listed, with your potential children being within the square.
11. Using the genotypes and phenotypes that are possible within each of the punnett squares in the step above, fill in the phenotypes and
genotypes for your potential children in the table.
12. Lastly, using the phenotypes for your potential children, create a drawing that
incorporates all of the ten traits.
As you will see, this assignment requires not only an understanding of the relationship between genotypes and phenotypes, but, in addition, the use of that information to build punnett squares, and interpret your unique family pedigree. Even adopted members of the class can construct a pedigree using the information from their adopted family; although this will not help the adopted student to discover her/his own specific inheritance, it will help them to learn a great deal about genetics in general, and their adopted family in particular.
Given the importance of punnett squares in this whole process, we need to start off with a bit of review:
| Fill out the Genotypic & Phenotypic Ratios: | |||||
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| Which should give you the following UNREDUCED Ratios: | |||||
| 4 : 0 :
0 4 : 0 |
0 : 0 :
4 0 : 4 |
0 : 4 :
0 4 : 0 |
2 : 2 :
0 4 : 0 |
0 : 2 :
2 2 : 2 |
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| Which should be REDUCED to the following Ratios: | |||||
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1. Females are indicated with a circle, and males are indicated with a square.
=
and
=
An individuals who is deceased should have a diagonal line passing
through her/his symbol.
2. A straight, horizontal line connecting a female and a male indicates that the two
are married, and/or produced children.
NOTE: When individuals are
deceased, we indicate them in a pedigree as follows . . .
3. A straight, vertical line connected to the vertical line above indicates the children produced by that union.
4. If more than one child is produced by that union, a straight,
horizontal line is drawn from the vertical line above, and all the children hang down from
that horizontal line.
5. All of the children from a specific union should be placed in birth order.
6. Despite placing the children from a specific union in birth order, the order of
children from multiple families need not be in order from left to right.
7. Each generation should be given a roman numeral, with the oldest in the pedigree
(traditionally at the top) being labeled “I.”
8. Each individual within a given generation should be numbered using Arabic numerals,
from left to right.
9. Using such a numbering system, each individual gets a unique identification (e.g., I-3,
II-1, III-5, etc.).
10. Start off with your grandparent’s generation, although a pedigree can start
out with the oldest generation for whom you have information.
NOTE I: If a male and female who are not immediately next to each other on the pedigree, such as II-1 and II-4, have children, then it is necessary to place them on the same horizontal level. Place the other individuals in the generation, II-2 and II-3, on a different horizontal level. It will then be necessary to indicate the two horizontal levels of generation II with a bracket. Remember, the different horizontal levels are merely a device to make it easier to indicate different marriages within one generation.
NOTE II: Given the complexity indicated in the note above, it may become
necessary to have lines cross to indicate the different children of multiple marriages.
Note that individual IV-6 in Mr. Lazaroff’s Pedigree is clearly the child of
individuals III-7 and III-8, despite the crossing of lines.
Notes I and II illustrate an interesting point. Given that life is more complex than a
simple pedigree can illustrate, it becomes necessary to extend part of a pedigree not only
up and down, but also left and right. Most pedigrees are thus drawn with the paper in
landscape view. Also, don’t forget that these pedigrees only touch upon other
families. The top generation is not truly the oldest, for thousands of years worth of
ancestry has been ignored! In addition, when we include our cousins, we need to include
the spouse of our blood-aunt or blood-uncle. Those individuals, however, have their own
pedigrees that intersect our own. With many aunts and uncles, the true situation becomes
even more complex. From our grandparents alone, we are talking about the intersection of
four other pedigrees. From Mr. Lazaroff’s pedigree, a fifth pedigree intersects in
generation II (for II-2), and six more in generation III (for III-1, III-4, III-6, III-8,
III-10, and III-12)!
NOTE III: When a person remarries, due to either death or divorce,
indicate the other marriage on the opposite side; this will make it easier to indicate the
children of each union. It is not necessary to indicate which marriage happened first, or
that a divorce took place. In the sample pedigree below,
individual II-2 was married twice; individual III-1 is the child of II-2 and II-1, and
individual III-2 is the child of II-2 and II-3. Individuals III-1 and III-2 are
half-siblings because they share a biological parent (II-2).
NOTE IV: When a person remarries, the children of
the new spouse from a previous marriage should not be included in your
pedigree. In an official pedigree, the children would be included if the
second marriage produced children (i.e., if half-siblings were produced),
but not if the only siblings are step-siblings (i.e., step-brother,
step-sister).
NOTE V: Individuals who are adopted should only be
included if they had children with a blood relative. This is not to diminish their
importance in the emotional make-up of the family, but rather to indicate that they cannot
be included in terms of the inheritance of physical traits.
The Pedigrees above are a
graphic representation of Mr. L's Family.
Look at the Pedigree below to see where Mr. L is, and, using the proper designation
(i.e., II-2? IV-5?) answer the following questions:
(1) Who is Mr. L?
(2) Who is Mr. L's Daughter?
(3) Who is Mr. L's Mother?
(4) Who is Mr. L's Paternal Grandfather?
(5) Who is Mr. L's oldest sibling? Is that person a brother or a sister?
As a graphic representation
of inheritance, it is important to have
a different pedigree for each of the ten traits being studied.
BACKGROUND
1. There are three genotypes, or combinations of
alleles for one gene (e.g., BB : Bb : bb), and only two phenotypes, or traits that can expressed from that
gene (e.g., dominant or recessive).
2. A phenotype is the physical expression of a
genotype.
3. It follows that two of the genotypes must correspond to one
phenotype. The homozygous dominant (BB) and heterozygous (Bb) genotypes are both dominant.
4. The recessive phenotype can therefore only have the homozygous recessive genotype. As
such, any individual who is recessive for a trait must be
homozygous recessive.
1. First of all, shade in the symbols for all the individuals who have the recessive trait.
2. These individuals, as explained above, must be homozygous recessive, so write their genotype (e.g., bb, rr, tt, etc.) underneath these individual’s shaded symbols.
3. Given that The homozygous dominant (BB) and heterozygous (Bb) genotypes are both
dominant, at this point it becomes necessary to look at more than one generation in order
to figure out the genotypes of all the dominant individuals.
4. The given that we don't know the genotype of the mother
above, we indicate it as B_.
5. Given that the daughter above had to have inherited a recessive allele from her father, but she is nonetheless dominant, we have to label her as heterozygous (Bb).
6. Being heterozygous, the daughter is called a carrier, as she carries the recessive allele (b), but she does not express the recessive trait. Carriers are indicated by diagonally shading the lower half of the symbol.
NOTE: When
deceased individuals are carriers, we indicate them in a pedigree as follows . . .
7. Don’t forget that working out a pedigree basically involves using the information we learned in the punnett squares, so you will need to compare the pedigree to the punnett squares in order to predict whether an individual is homozygous dominant or heterozygous.
| Click on the links
below to see a comparison of the punnett squares and specific pedigree examples! |
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| BB x BB | bb x bb | BB x bb | BB x Bb | Bb x bb | Bb x Bb |
 Don't assume this is the case if everyone is dominant! Be on the safe side and say B_ |
 You can, however, assume that everyone here is homozygous recessive (bb) |
 NOTE: Even if the mother above is Heterozygous, any dominant child will be Bb |
 Once again, you cannot be SURE of the genotype for a dominant person, so you have to put B_ |
 NOTE: Based on the recessive child, you CAN figure out the genotype of the Father above |
 Once again, based on the recessive child, you KNOW the genotypes of BOTH Parents |
8. In general, you will find that the more information you have, the
better (think about the importance of sample size from the punnett square/coin toss lab),
but occasionally we do not have enough information to be completely sure.
9. Despite this, we can usually narrow it down to one of two possibilities. For example,
an individual who is dominant has to be either BB or Bb. We know, at the very least, that
they have to have at least one dominant allele to be dominant. We can therefore indicate
what we know by indicating that the individual is B_.
10. Another example of narrowing a genotype down to one of two possibilities is slightly
more unusual. In this example, we know that a child is recessive, but we do not know the
phenotype of either parent. In such a case, the only way for a child to be recessive is
for both parents to have contributed one recessive allele. As such the parents can be
either Bb or bb, so we can therefore indicate what we know by indicating that the
individual is _b.
11. Since we know that the parents have to at least be a carrier, another way to represent such an individual is to shade them in as a carrier, but still indicate the uncertainty with a question mark.
12. It is very rare that we cannot determine anything about the genotype of an indivdual. Such an example would involve a person whose phenotype we don’t know, and whose parents’ phenotypes are also unknown. If they have had a child, we can often figure out at least if they are B_ or _b, but in the case where they have had a dominant child with a dominant adult, we still cannot be sure. We must, therefore, indicate the lack of knowledge about a genotype using _ _. (This determination, however, is not approprite when we know the phenotype!)
NOTE: A pedigree can also illustrate the principle of a
test cross.
When Mendel took two purebred (They were known to be
purebred given the extremely large sample size; our pedigrees are way too small of a
sample size to conclude that all of the dominant individuals are homozygous dominant.)
populations of peas (e.g., tall and short) and bred them, he discovered which trait was
dominant because one trait disappeared in the next generation.
This type of cross became known as a test cross. The trait that
disappeared (short) was recessive (tt), the trait that showed up in the next generation
(tall) was dominant (TT), and all the dominant children were Tt.
In the case of a homozygous recessive parent (bb), or a heterozygous
parent (Bb), and a parent for whom all we know is that they are dominant (B_), we can
sometimes determine the exact genotype of the dominant parent by looking at the children.
If there is a recessive child, who must, of course, be homozygous
recessive (bb), then the parent we had indicated as B_ must have contributed a recessive
allele to her/his child, and must, therefore, be heterozygous (Bb).
Here is a sample of a Mr. Lazaroff's pedigree, using a fictitious trait.
(1) Try to figure out the genotypes of every
individual.
(2) When you have written in the Genotypes,
and before you have done any more shading,|
click on the link below to check your work.
Click here to see the answers without any
further shading.
