PEDIGREES
Now that you
understand different types of inheritance, we can now discuss a diagram which
is used to show the occurrence of a trait over several generations of a family.
A Pedigree is the diagram that shows
how a trait is inherited over several generations within a family. A genetic
disorder can be traced within a family using a pedigree. Sometimes parents may
want to know the likelihood of their children inheriting a disorder that may
run within their family’s past. Using a pedigree can also help parents
determine if they are carriers, because, remember, carriers do not show any
symptoms of a genetic disorder while they carry the allele for the disorder.
Symbols used in Pedigrees
Refer to Table 26-1 as you read
this section of the symbols used in pedigrees. A male is drawn as a square. If
the square is empty or not shaded he is considered normal or as not having a
trait. A square that is shaded represents a male with the particular trait. A
female is drawn as a circle. If the circle is empty or not shaded she is
considered normal or as not having a trait. A circle that is shaded represents
a female with the particular trait. A female can also be drawn as half-shaded.
If her symbol is half shaded, she is considered a carrier. A carrier symbol is
not always used. If the carrier symbol is not used, then she will be drawn as
an empty symbol because she is a carrier. Remember, carriers are considered
normal and only carry the trait with no symptoms or signs of carrying it. A
horizontal line connecting a male symbol and female symbol represents a
crossing or mating between the two parents. A vertical line drawn down from the
horizontal line of crossing or mating connects the offspring of the parents.
The offspring are normally listed in order of birth from left to right. Notice
in Table 26-1 how “P” and “F1” are used in the example next to Offspring of parents. The “P” identifies
the parents of the pedigree. The P generation may also be listed as the Roman
numeral “I” for the first generation. The “F1” (known as first
filial generation) identifies the offspring of the parents. The “F1”
may also be listed as the Roman numeral “II”. Every generation after the F1
would be given the next number in sequence whether it is listed as filial
generations (F2, F3, etc.) or roman numerals (III, IV,
etc.).
We will now examine three different
pedigrees: autosomal recessive, autosomal dominant, and x-linked (sex-linked on
the X chromosome) recessive. After completing the remainder of this unit, you
should be able to determine the genotype and phenotype of each individual in a
pedigree, explain how an individual acquired their alleles and predict possible
future generations.
Autosomal Recessive Pedigree
The pedigree below displays how the
autosomal recessive allele for attached earlobes has been passed from
generation to generation within a particular family. Free Earlobes are earlobes
that hang below the point of attachment to the head. An Attached Earlobe is one
that is directly attached to the skin of the jaw. An attached lobe is recessive
and free lobe is dominant.
Notice in
the autosomal recessive pedigree above that there are 4 generations. The
parents that began this pedigree had 4 children (2 males and 2 females). The
second female, from the parents in the first generation, had 2 children (2
females) with a male from another pedigree. The first female in the F2
generation (generation III) had one child (female) with a male from another
pedigree. Recognize that the parents that started this pedigree had 4 children
(2 males and 2 females - F1), 2 grandchildren (2 females – F2),
and 1 great grandchild (female - F3).
Some
generalities about autosomal recessive pedigrees:
·
Unaffected
parents can have affected offspring (notice
how the parents in the F2 generation did not have the trait but
their child does have the trait).
·
Affected
offspring can be male and female (notice
how two males and two females are affected in this pedigree).
·
The
recessive phenotype/genotype can skip generation (notice how the trait skipped the F2 generation then
reappeared in the F3 generation).
Now we will
attempt to determine every individual’s genotype and phenotype. Step 1, we will
determine the genotype of every individual that we are certain of by how the
symbols are drawn for the recessive trait. Therefore, every shaded symbol will
be of the genotype “ee” because it is a recessive trait and if they are shaded
that means they have the trait. Every other symbol that is not shaded will have
the genotype of either “EE” or “Ee”. For now, all we know is that each empty
symbol has an “E” in common.
Step 2, we need to determine the
second allele of every empty symbol. Remember that each individual needs two
alleles, one from the mother and one from the father. To determine the second
allele we need to examine the offspring and parents together. Follow the arrows
in the pedigree below to understand how the second allele is determined.
Again, it is very important to
understand that the two alleles each individual has are the result of one
allele from the mother and one allele from the father. Once you have determined
that a parent gives one specific allele, you can no longer look to that parent
to give another allele to the same child, for
example the first female in the F1 generation. We know she has
an “E” because her symbol is empty. Looking at her parents we know she
inherited the “E” from her father, because the mother does not have an “E” to
give. When we then look to the mother we see that all she can give to her daughter
is an “e”, so the daughter inherited “E” from her father and “e” from her
mother to make her have the genotype “Ee”.
Autosomal Dominant Pedigree
The pedigree below displays how the
autosomal dominant allele for widow’s peak has been passed from generation to
generation within a particular family. Widow’s Peak is a V-shaped point in the
hairline in the center of the forehead. Widow’s Peak is dominant and Straight
Hairline (no widow’s peak) is recessive.
Notice in
the autosomal dominant pedigree above that there are 4 generations. The parents
that began this pedigree had 4 children (2 males and 2 females). The second
female, from the parents in the first generation, had 2 children (2 females)
with a male from another pedigree. The first female in generation III had one
child (female) with a male from another pedigree. Recognize that the parents
that started this pedigree had 4 children (2 males and 2 females – Generation
II), 2 grandchildren (2 females – Generation III), and 1 great grandchild
(female – Generation IV).
Some
generalities about autosomal dominant pedigrees:
·
Affected
individuals have at least one affected parent (notice how the two males in generation II have a mother with the
trait, and the female in generation IV has a father with the trait).
·
Two
unaffected parents can only have unaffected offspring (notice how the parents in generation II had two female offspring
(generation III) neither of which have the trait).
·
The
dominant phenotype/genotype usually appears in every generation (notice how every generation has at least
one individual with the dominant trait to keep it running in the family).
Now we will
attempt to determine every individual’s genotype and phenotype. Step 1, we will
determine the genotype of every individual that we are certain of by how the
symbols are drawn for the dominant trait. The fact that this is a dominant
trait means that we need to switch our thinking from what we did in the last
recessive pedigree. Therefore, every shaded symbol will now be of the genotype
“WW” or “Ww” because it is a dominant trait and if they are shaded that means
they have the trait. Every other symbol that is not shaded will now have the
genotype “ww”. For now, all we know is that each empty symbol is “ww” and every
shaded symbol has a “W” in common.
Step 2, we need to determine the
second allele of every shaded symbol. Remember that each individual needs two
alleles, one from the mother and one from the father. To determine the second
allele we need to examine the offspring and parents together. Follow the arrows
in the pedigree below to understand how the second allele is determined.
Again, it is very important to
understand that the two alleles each individual has are the result of one
allele from the mother and one allele from the father. Once you have determined
that a parent gives one specific allele, you can no longer look to that parent
to give another allele to the same child, for
example the first male in generation II. We know he has a “W” because his
symbol is shaded and this is a dominant trait pedigree. Looking at his parents
we know he inherited the “W” from his mother, because the father does not have
a “W” to give. When we then look to the father we see that all he can give to
his son is a “w”, so the son inherited “W” from his mother and “w” from his
father to make him have the genotype “Ww”.
X-Linked (sex-linked on the X
chromosome) Recessive Pedigree
The pedigree below displays how the
sex-linked recessive allele, which is carried on the X chromosome, for
colorblindness has been passed from generation to generation within a
particular family. Colorblindness (specifically red-green colorblindness) is
the inability to distinguish red and green colors. Normal vision is dominant
and colorblindness is recessive.
Keep in mind, this is a sex-linked
trait, therefore, you must use the sex chromosomes (male XY and female XX).
Now we will
attempt to determine every individual’s genotype and phenotype. Step 1, we will
determine the genotype of every individual that we are certain of by how the
symbols are drawn for the recessive trait, remembering to use the sex
chromosomes. Therefore, we need to break down each symbol by gender.
Every shaded
square will be of the genotype “XcY” because it is a male with the
colorblind recessive trait (this trait is only carried on the X’s). Every other
square that is not shaded will have the genotype “XCY”, because the
Y chromosome carries nothing there is no other option for the male.
Every shaded
circle will be of the genotype “XcXc” because it is a
female with the colorblind recessive trait. Every other circle that is not
shaded will have the genotype of either “XCXC” or XCXc”.
Notice how this pedigree does not use the half-shaded circle for carrier
females so we need to distinguish the empty circle for either “XCXC”
or “XCXc”. For now, all we know is that each empty circle
has a “XC” in common.
Now, we can
fill in the pedigree with the genotypes we know.
Step 2, we need to determine the
second allele of every empty female circle. Remember that each individual needs
two alleles, one from the mother and one from the father. Each male, in a
sex-linked trait, always inherits the “Y” from his father. The “X” the male
inherits always comes from his mother.
In females, one “X” comes from her
father and one “X” comes from her mother. To determine the second allele we
need to examine the offspring and parents together. Follow the arrows in the
pedigree below to understand how the second allele is determined.
Again, it is very important to
understand that the two alleles each individual has are the result of one
allele from the mother and one allele from the father. Once you have determined
that a parent gives one specific allele, you can no longer look to that parent
to give another allele to the same child, for
example the first female in generation II. We know she has an “XC” for two
reasons. One reason is because her symbol is empty and she has normal vision. The
other reason is because, when looking at her father in generation I, the only
“X” he can give her is “XC”. Looking at her mother, we see the only X see can give her
daughters is “Xc”.
Pedigree Analysis Summary
You should
be able to determine and explain all individual genotypes and phenotypes of any
pedigree by analyzing parents and offspring, and knowing what type of
inheritance the pedigree uses. Remember, when analyzing a pedigree, to first
identify what the symbols tell you directly as in who has the trait. Second,
determine the second unknown allele by examining which parent gives which
allele, remembering that each parent only gives one allele at a time to each
offspring.
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