Genetics II
Answered Review Questions
1.
Explain the incomplete dominance inheritance pattern.
“Alleles can show different degrees of dominance and recessiveness in relation to each
other. We refer to this range as the spectrum of dominance. One extreme on this
spectrum is seen in the F1 offspring of Mendel’s classic pea crosses. These F1 plants
always looked like one of the two parental varieties because of the complete dominance
of one allele over another. In this situation, the phenotypes of the heterozygote and the
dominant homozygote are indistinguishable.”
“The alleles for some characters fall in the middle of the spectrum of dominance. In this
case, the F1 hybrids have a phenotype somewhere in between the phenotypes of the two
parental varieties. This phenomenon, called the incomplete dominance of either allele, is
seen when red snapdragons are crossed with white snapdragons: All the F1 hybrids have
pink flowers (see figure below). This third phenotype results from flowers of the
heterozygotes having less red pigment than the red homozygotes (unlike the situation in
Mendel’s pea plants, where the Pp heterozygotes make enough pigment for the flowers
to be a purple color indistinguishable from those of PP plants).” (Text quoted from page
260 of the textbook)
With incomplete dominance, there is no dominant and recessive trait. As a result, there
are three different phenotypes. The ratio for two hybrids parents would be 50% like the
parents, 25 % with one homozygous trait expressed and 25% with the other homozygous
trait expressed. Looking at snapdragon flower color, if both parents had a pink flower, we
would expect 50% of their offspring to have the pink color flower (heterozygous pink),
25% would be expected to have a red color (homozygous red) and we would expect 25%
to have the white flower color (homozygous white). The genotype ratios for a cross
between two incompletely dominant parents would be the same as the phenotype ratio.
Going back to the snapdragon flower color, we would expect 25% of the offspring to be
homozygous red, 50% to be heterozygous pink and 25% to be homozygous white. This
is a 1:2:1 ratio the same as the phenotype ratio for incomplete dominance.
2.
Explain the codominance inheritance pattern.
With codominance, there are two not one dominant alleles and each one
dominates over the recessive allele but not each other. With incomplete
dominance there is neither a dominant nor recessive allele but two different
homozygous individuals. When a heterozygous condition exists both alleles are
expressed. This is similar to codominance in a way, but unlike codominance,
neither allele is dominant or recessive.
3.
How is ABO blood type an example of the codominance inheritance pattern?
“The ABO blood group in humans, for instance, is determined by multiple alleles
of a single gene. There are four possible phenotypes for this character: A
person’s blood group may be either A, B, AB, or O. These letters refer to two
carbohydrates—A and B—that may be found on the surface of red blood cells. A
person’s blood cells may have carbohydrate A (type A blood), carbohydrate B
(type B), both (type AB), or neither (type O), as shown schematically below.”(Text
quoted from page 262 of the textbook)
“Not only are the ABO blood groups determined by multiple alleles, the allele for
A dominates over O and the allele for B dominates over O, but neither A nor B
dominate over each other. They are considered codominant.”
4.
What are the genotypes for the following?
A.
Type A blood
Individuals with type A blood may be homozygous for type A blood (two
alleles for A) or heterozygous for type A blood (one allele for A and one
allele for O)
B.
Type B blood
Individuals with type B blood may be homozygous for type B blood (two
alleles for B) or heterozygous for type A blood (one allele for B and one
allele for O)
C.
Type AB blood
Individuals with type AB blood have one A allele and one B allele.
D.
Type O blood
Individuals with type O blood have two O alleles.
5.
If a woman has type AB blood, could she be the biological mother of a child
with type O blood? Explain your answer.
Since the woman’s blood type is AB she would have alleles for A and B. She
could produce eggs containing either an allele for A or an allele for B. Since both
the A and B alleles dominate over the allele for O she could either be the
biological mother of an individual with either type A or Type B blood. It would
not be possible for her to be the biological parent of a child with type O blood.
6.
Explain the polygenic inheritance pattern.
Many genetic traits are controlled by many genes. Polygenic traits act like
incomplete dominance. There is no dominant or recessive. The effect of each
gene and each allele is additive. For example, kernel color in wheat ranges from
white through shades of pink to dark red.
The color is dictated by three genes (ABC). Imagine that there is a light allele
(abc) and a dark allele for each gene (ABC). The more dark pigment alleles
present in the genotype the more red the kernel. Likewise, the more light pigment
alleles in the genotype makes the kernels whiter. The trait is continuous. The
more genes that are involved in a trait the less of a difference there is between
phenotypes.
Genotype
AABBCC
AaBBCC or AABbCC or AABBCc
aaBBCC or AaBbCC or AAbbCC or AABbCc or AABBcc etc…
aabBCC or AabbCC or AAbbCc or AABbcc etc..
aabbCC or AAbbcc or aaBBcc etc…
aabbCc or Aabbcc or aaBbcc etc…
aabbcc
Phenotype
Dark red
Red
Dark pink
Pink
Light pink
Pinkish-white
White
Polygenic inheritance is a common inheritance pattern for many traits like: skin color,
eye color, height, weight, temperament, personality, intelligence, heart disease, mental
illness, diabetes etc…
7.
Explain multifactorial inheritance.
Many traits, especially polygenic traits, have an environmental component. For
example, one member of a set of identical twins may have schizophrenia while
the other is perfectly normal. If schizophrenia was completely genetic, then both
twins would have the disease. Twin studies are often used to determine how
much of given trait is genetic and how much is environment. Researchers
compare the inheritance in identical twins with fraternal twins. If the chances that
identical twins share a trait are greater than fraternal twins, then the trait has a
genetic component.
8.
Who associated genes with chromosomes and what specific experimental
organism did this person use. What characteristics made this organism
convenient for genetic studies?
“For his work, Morgan selected a species of fruit fly, Drosophila melanogaster, a
common, generally innocuous insect that feeds on the fungi growing on fruit.
Fruit flies are prolific breeders; a single mating will produce hundreds of offspring,
and a new generation can be bred every two weeks. These characteristics make
the fruit fly a convenient organism for genetic studies. Morgan’s laboratory soon
became known as ‘the fly room.’ Another advantage of the fruit fly is that it has
only four pairs of chromosomes, which are easily distinguishable with a light
microscope. There are three pairs of autosomes and one pair of sex
chromosomes. Female fruit flies have a homologous pair of X chromosomes, and
males have one X chromosome and one Y chromosome.” (Text quoted from
page 276 of the textbook)
9.
How many pairs of chromosomes does the fruit fly have? Which sex
chromosomes are found in a female Drosophila? Which sex chromosomes
are found in a male Drosophila?
The fruit fly has only 4 pairs of chromosomes. A female has two X chromosomes
while a male fruit fly has an X and a Y chromosome.
10.
What does the term “wild type” phenotype refer to with the Drosophila? What
is the variant to the “wild type” called?
“The normal phenotype for a character (the phenotype most common in natural
populations), such as red eyes in Drosophila, is called the wild type. Traits that
are alternatives to the wild type, such as white eyes in Drosophila, are called
mutant phenotypes because they are due to alleles assumed to have originated
as changes, or mutations, in the wild-type allele.” (Text quoted from page 276 of
the textbook)
11.
On which chromosome did Morgan conclude that eye color was located in
the Drosophila?
“Morgan’s finding of the correlation between a particular trait and an individual’s
sex provided support for the chromosome theory of inheritance: namely, that a
specific gene is carried on a specific chromosome (in this case, the eye-color
gene on the X chromosome). In addition, Morgan’s work indicated that genes
located on a sex chromosome exhibit unique inheritance patterns, which we will
discuss later in this chapter. Recognizing the importance of Morgan’s early work,
many bright students were attracted to his fly room.” (Text quoted from page 276
of the textbook)
What is a sex-linked trait? What are some examples of X-linked traits in
humans?
“In addition to their role in determining sex, the sex chromosomes, especially X
chromosomes, have genes for many characters unrelated to sex. A gene located
on either sex chromosome is called a sex-linked gene, although in humans the
term has historically referred specifically to a gene on the X chromosome. (Note
the distinction between the terms sex-linked gene, referring to a gene on a sex
chromosome, and linked genes, referring to genes on the same chromosome
that tend to be inherited together.) Sex-linked genes in humans follow the same
pattern of inheritance that Morgan observed for the eye-color locus in Drosophila.
Fathers pass sex-linked alleles to all of their daughters but to none of their sons.
In contrast, mothers can pass sex-linked alleles to both sons and daughters (see
figure below).” (Text quoted from page 283 of the textbook)
Examples of sex-linked traits in humans are: color blindness, Duchenne
muscular dystrophy, Hemophilia.
Neither Jon nor Pam is color blind, but their son Phillip is color blind. What
is the probability that this couple could have another son who is also color
blind? From which parent did Phillip inherit the gene for colorblindness?
Since Jon has normal color vision we know that his genotype is XAY. We also
know that Pam must be heterozygous since she has a son Phillip with color
blindness. Thus Pam is XAXa. Since Phillip is color blind we know his genotype
is Xa Y. Now let’s look at the possible offspring using a Punnett square.
XA
Y
XA
XA XA
XA Y
Xa
XA Xa
Xa Y
From the Punnett square on the left we expect two of the children to be male. Of
these male children we expect one to be color blind. Thus ½ of the male children
are expected to be color blind. ½ is 50%.
Since the male child receives the Y chromosome from his father, male individuals
inherit the recessive allele for color blindness from their mother since the allele is
on the X chromosome and the male individual receives the X chromosome from
his mother.
Why are males more likely to be affected by X-linked traits?
“If a sex-linked trait is due to a recessive allele, a female will express the
phenotype only if she is a homozygote. Because males have only one locus, the
terms homozygous and heterozygous lack meaning for describing their sexlinked genes (the term hemizygous is used in such cases). Any male receiving
the recessive allele from his mother will express the trait. For this reason, far
more males than females have sex-linked recessive disorders. However, even
though the chance of a female inheriting a double dose of the mutant allele is
much less than the probability of a male inheriting a single dose, there are
females with sex-linked disorders. For instance, color blindness is a mild disorder
inherited as a sex-linked trait. A color-blind daughter may be born to a color-blind
father whose mate is a carrier. However, because the sex-linked allele for color
blindness is relatively rare, the probability that such a man and woman will mate
is low.” Text quoted from page 283 of the textbook)
What is a Barr Body? List an example of x-chromosome inactivation in
mammals.
A Barr body is only found in the nucleus of female somatic cells. The small black
dot is an inactivated X chromosome. Which X chromosome of each allelic pair
that inactivates is unpredictable.
“The tortoiseshell gene is on the X chromosome, and the tortoiseshell phenotype
requires the presence of two different alleles, one for orange fur and one for
black fur. Normally, only females can have both alleles, because only they have
two X chromosomes. If a female is heterozygous for the tortoiseshell gene, she is
tortoiseshell. Orange patches are formed by populations of cells in which the X
chromosome with the orange allele is active; black patches have cells in which
the X chromosome with the black allele is active. (“Calico” cats also have white
areas, which are determined by yet another gene.) “(Text quoted from page 284
of the textbook)
How many autosomes should a normal human somatic cell have? How
many autosomes should a normal human gamete have?
A normal human somatic cell is a diploid cell. Human diploid cells typically have
23 pair of homologous chromosomes. A normal human gamete is a haploid cell
so there should be only 22 chromosomes present in the normal human gamete.
How many sex-chromosomes should a normal human somatic cell have?
Which sex-chromosomes are present in a normal male? Which sexchromosomes are present in a normal female?
Since the somatic cell is normal, there should be one pair of sex chromosomes
and 22 pair of autosomes in this somatic cell. In a normal human sperm, there
would only be one sex chromosome. Since the human male determines the sex
of the individual, the sperm could either contain an X chromosome or it could
contain the y chromosome. For the human egg, there would only be one X
chromosome if the egg were normal.
Explain nondisjunction.
“Ideally, the meiotic spindle distributes chromosomes to daughter cells without
error. But there is an occasional mishap, called a nondisjunction, in which the
members of a pair of homologous chromosomes do not move apart properly
during meiosis I or sister chromatids fail to separate during meiosis II. In these
cases, one gamete receives two of the same type of chromosome and another
gamete receives no copy (see figure below).The other chromosomes are usually
distributed normally. If either of the aberrant gametes unites with a normal one at
fertilization, the offspring will have an abnormal number of a particular
chromosome, a condition known as aneuploidy.” (Text quoted from page 285 of
the textbook)
How is monosomy (monosomic condition) different from trisomy (trisomic
condition)? How can these conditions occur?
“If a chromosome is present in triplicate in the fertilized egg (so that the cell has a total of
2n + 1 chromosomes), the aneuploid cell is said to be trisomic for that chromosome. If a
chromosome is missing (so that the cell has 2n − 1 chromosomes), the aneuploid cell is
monosomic for that chromosome. Mitosis will subsequently transmit the anomaly to all
embryonic cells. If the organism survives, it usually has a set of symptoms caused by the
abnormal dose of the genes associated with the extra or missing chromosome.
Nondisjunction can also occur during mitosis. If such an error takes place early in
embryonic development, then the aneuploid condition is passed along by mitosis to a
large number of cells and is likely to have a substantial effect on the organism.” (Text
quoted from page 285 of the textbook)
Which chromosome is normally involved with Down’s syndrome?
“Down syndrome is usually the result of an extra chromosome 21, so that each body cell
has a total of 47 chromosomes. Because the cells are trisomic for chromosome 21, Down
syndrome is often called trisomy 21. Down syndrome includes characteristic facial
features, short stature, heart defects, susceptibility to respiratory infection, and mental
retardation. Furthermore, individuals with Down syndrome are prone to developing
leukemia and Alzheimer’s disease. Although people with Down syndrome, on average,
have a life span shorter than normal, some live to middle age or beyond. Most are
sexually underdeveloped and sterile.
Which chromosomes are involved and how many chromosomes are typically
present for the following conditions: Turner syndrome, Klinefelter syndrome,
Trisomy X, Jacob’s syndrome? What is the cause for these conditions?
“Nondisjunction of sex chromosomes produces a variety of aneuploid conditions.
Most of these conditions appear to upset genetic balance less than aneuploid
conditions involving autosomes. This may be because the Y chromosome carries
relatively few genes and because extra copies of the X chromosome become
inactivated as Barr bodies in somatic cells.” (text quoted from page 287 of the
textbook)
An extra X chromosome in a male, producing XXY, occurs approximately once in
every 2,000 live births. People with this disorder, called Klinefelter syndrome,
have male sex organs, but the testes are abnormally small and the man is sterile.
Even though the extra X is inactivated, some breast enlargement and other
female body characteristics are common. The affected individual is usually of
normal intelligence. Males with an extra Y chromosome (XYY) do not exhibit any
well-defined syndrome, but they tend to be somewhat taller than average.
Females with trisomy X (XXX) are called poly-x females. Poly-x females occur
once in approximately 1,000 live births, are healthy and cannot be distinguished
from XX females except by karyotype. Monosomy X, called Turner syndrome,
occurs about once in every 5,000 births and is the only known viable monosomy
in humans. Although these X0 individuals are phenotypically female, they are
sterile because their sex organs do not mature. When provided with estrogen
replacement therapy, girls with Turner syndrome do develop secondary sex
characteristics. Most have normal intelligence.” (Text quoted from page 287 of
the textbook)
Identify organisms which have the following chromosomal basis of sexdetermination:
A. X-Y system
“In mammals, the sex of an offspring depends on whether the sperm
contains an X chromosome or a Y. (Text quoted from page 282 of the
textbook)
B. X-O System
“In grasshoppers, roaches and some other insects, there is only one
type of sex chromosome, the X. Females are XX ; males have only
one X chromosome (XO). Sex of the offspring is determined by
whether the sperm has an x chromosome or no sex
chromosome. .“(Text quoted from page 282 of the textbook)
C. Z-W System
“In birds, some fishes and some insects, the sex chromosome present
in the ovum (not the sperm) determines the sex of offspring. The sex
chromosomes are designated Z and W. Females are ZW and males
are ZZ.“(Text quoted from page 282 of the textbook)
D. Haplo-diploid system
“There are no sex chromosomes in most species of bees and ants.
Females develop from fertilized ova and thus are thus diploid. Males
develop from unfertilized egg sand are haploid. They have no fathers”
(Text quoted from page 282 of the textbook)