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Pre-AP Genetics Notes
Basic Principles of Heredity
Mendelian Genetics:
Many of the basic concepts of heredity were worked out in the mid-1800’s
by the Austrian monk ______________ (1822-1884). Mendel
cultivated__________________, which he used to study plant inheritance. From
these experiments, Mendel concluded that each hereditary trait is controlled by
two separate _________, one from each parent, and that these factors are
passed on unchanged from generation to generation. Mendel knew nothing of
chromosomes or genes. However, what he called “factors” are now known to
be_______, which are found on the _________________.
Mendel was very careful in his studies to use plants that “____________”that is plants that showed the same traits generation after generation. Also, he
studied only _________ different traits, and there were two contrasting forms for
each trait.
Law of Dominance:
When Mendel crossed two true-breeding plants that showed one pair of
contrasting traits, only one of these traits was evident in the resulting offspring.
For example, if he crossed a tall plant and a short plant, all offspring were tall.
The trait that was expressed was described as ____________, while the trait that
did not show in the offspring was ______________.
Of the pairs of contrasting traits that Mendel studied, he found that in each
case one trait proved dominant. Tall stems were dominant to short; round seeds
were dominant, wrinkled recessive, etc. Mendel concluded that of every pair of
contrasting traits, one is dominant and the other recessive.
The pure-breeding parent generation is called the P generation. The
offspring of the crosses between members of the P generation are the
___________or F1 generation. Crosses between members of the F1 generation
produce the second filial or _____________ and so on. In Mendel’s
experiments, members of the F1 generation were __________- they were the
offspring of unlike parents.
Questions:
1. In Mendel’s experiments, he crossed pea plants with green pods with
plants with yellow pods. He found that all of the offspring had green pods.
a. The plants used in the original cross are members of the ___
generation.
b. The offspring of the original cross make up the _________ filial or
F1 generation.
2. What is a hybrid?
Law of Segregation:
In another experiment, Mendel allowed the hybrid members of the F 1
generation to undergo _____________. The offspring of this cross were the F2
generation. Of the F2 generation, ____________ showed the dominant trait and
___________ showed the recessive trait. This showed that although members
of the F1 generation all showed the dominant trait, these plants also carried the
recessive factor (gene), and that this factor was passed on to the F 2 generation.
These results led Mendel to the conclusion that each trait is controlled by a pair
of factors, one from each parent. (The F1 hybrids obviously carried the dominant
factor of one parent and the recessive factor of the other parent.) This is
Mendel’s __________________.
Since the gametes fuse in fertilization, each gamete must carry only one
of the factors, and the zygote formed by the fusion of the two gametes then
carries the two, one from each parent. In the formation of gametes, the two
factors must separate, or _____________. Each gamete receives one factor for
each trait. The dominant trait is expressed whenever the dominant factor (gene)
for that trait is present. For example, in the experiments on stem length, all
members of the F1 hybrid generation were tall. Only an organism possessing two
__________________ for a given trait will show the recessive trait.
An individual with two dominant or two recessive genes is said to be
_______________, while the hybrid, with one dominant and one recessive gene,
is ________________. The dominant trait is shown by two capital letters (TT for
tallness, for example), and the recessive by two lower case letters (tt for
shortness). The homozygous dominant is____, the homozygous recessive is
___, and the hybrid is ____.
Questions:
1. What is Mendel’s law of segregation?
2. In the pea plants studied by Mendel, how many factors controlled a given
trait?
3. Use G for green pods (dominant) and g for yellow pods (recessive).
a. A pea plant that is homozygous dominant for green pods would be
shown as:
b. A pea plant that is heterozygous for green pods would be shown
as:
c. A pea plant that is homozygous for green pods would be shown as:
Punnett Squares:
The _______________ is a convenient method for visualizing genetic
crosses. The boxes that make up the square show all possible ____________
resulting from a cross of gametes bearing a specific trait. The letter representing
each possible type of sperm are placed along the top of the square, and those for
each possible type of egg along the side. For example, in a pea plant that was
heterozygous for stem length (___), half the gametes would receive the dominant
factor for stem length (___) and half would receive the recessive (__). Thus, the
plant would produce _____ types of gametes. A plant that was pure dominant
(TT) or pure recessive (tt) would produce only one type of gamete-- T or t
respectively.
A cross between a homozygous tall plant (TT) and a homozygous short
plant (tt) is shown below.
P generation= pure short x pure tall
tt x TT
T
t
t
T
Tt
Tt
Tt
Tt
F1 offspring: 100% tall
The F1 generation is all heterozygous tall. The physical appearance of an
individual is called the ______________. Thus, the phenotype of all members of
the F1 generation is ______. The _____________, on the other hand, is the
genetic makeup of the individual. In this case, all members of the F1 generation
have a __________________, or hybrid, genotype. The genotypes of the P
generation were homozygous dominant and homozygous recessive.
Questions:
1. The physical appearance of an individual is its _________________, while
the genetic makeup of an individual is its ____________________.
2. Use a Punnett square to show the offspring of a cross between two pea
plants that are heterozygous for green color (Gg x Gg). Give the
phenotypes and genotypes of the offspring.
3. Use a Punnett square to show the offspring of a cross between a pea
plant that is homozygous wrinkled (aa) and one that is heterozygous (Aa).
Give the phenotypes and genotypes of the offspring.
Test Cross:
A _________________ is a method for determining whether an organism
that shows a dominant trait is pure dominant or whether it is heterozygous. In a
test cross, the organism in question is crossed with a _______________for the
trait. If the test organism is heterozygous, some of the offspring of the cross will
show the recessive trait. Large numbers of offspring are necessary for valid
results.
Questions:
1. In flowers, purple petal color is dominant over white. How would you find
out whether a purple flower is homozygous for petal color (PP) or
heterozygous (Pp)?
2. Using Punnett squares, show how the results of a test cross would differ
with a purple flower that was homozygous dominant for petal color (PP)
and one that was heterozygous for petal color (Pp).
Law of Independent Assortment:
Following his crosses of plants with one pair of contrasting traits, Mendel
did crosses of plants that showed two pairs of contrasting traits. Such a cross
might involve a plant with long stems and round seeds (both dominant) and one
with short stems and wrinkled seeds (both recessive). The first plant could be
represented as _______, the second as ______. Each plant would produce only
one type of _________- one would be ____and the other____. All offspring of
the cross would be hybrids with long stems and round seeds (TtRr).
To find out whether there was any relationship between the two traits- for
example, whether the dominant traits segregated together- Mendel allowed the
hybrid plans to undergo self-fertilization. This is a __________________, a
cross involving organisms that are hybrid for two traits. The results of such a
cross showed that the factors (genes) for each trait were _______________
________________of any others in the course of gamete formation. This is
Mendel’s law of_______________ _______________. It has since been found
that there are exceptions to this law.
Questions:
1. What is Mendel’s law of independent assortment?
2. Work out the following dihybrid cross. Round seeds are dominant over
wrinkled seeds, and yellow seeds are dominant over green seeds. A plant
that is hybrid for both traits (RrYy) is allowed to undergo self-fertilization
(RrYy x RrYy).
a. The four types of gametes formed are:
b. Fill in the Punnett square below. Give the phenotypes of the
offspring of this cross and the ratio of each phenotype produced.
3. In the dihybrids that Mendel worked with, the genes for the individual traits
were located on separate chromosomes. If the genes for two or more of
these traits had been located on the same chromosome, would Mendel
have developed his law of independent assortment?
4. Complete the following dihybrid crosses in the space below.
a. TTYY x TtYy
b. RrYy x Rryy
Incomplete Dominance
Incomplete Dominance means that one allele is NOT fully dominant over the
other in a _______________ individual. In Incomplete Dominance, the new
phenotype is an intermediate ____________ between the two homozygotes. An
example of incomplete dominance would be when _______ flowering plants
crossed to __________flowering plants yield __________flowering offspring.
Example1: In cattle, longhorns are incompletely dominant to shorthorns. A
heterozygous individual would have medium- horns. Cross a longhorn bull and a
shorthorn cow. Give the genotypes and phenotypes (with the ratios) of the
offspring.
LL = longhorn
SS = shorthorn
LS = medium horn
Example 2: The Panola petunia is a flower that shows incomplete dominance.
Red flowers have a genotype of RR, white flowers of WW, and pink flowers of
RW. In a flower show the following results were obtained: 48 red flowers and 52
pink flowers. What were the probable genotypes of the parent flower plants?
Explain.
Co- dominance
_____________________ means that one allele is not fully dominant over the
other in a heterozygous individual. In such cases, the heterozygous phenotype
does not look like either homozygous phenotype. Instead, the two contrasting
alleles produce a new phenotype. Both alleles are _________________ in the
heterozygous individual. An example of co- dominance would be when a redflowered plant is crossed to a white- flowered plant yielding a _____________
flowered offspring (showing both red & white colors).
Practice #1:
RR= red
RW= roan
WW= white
Cross a white bull with a red cow. Is it possible to obtain white offspring?
Practice #2:
In Tupelo chickens, the alleles for feather color are black (B) and white (W).
These two alleles are co- dominant. When both alleles are present, the resulting
birds have both black feathers and white feathers, giving it a “checked”
appearance. Cross a checked rooster with a checked hen. Give the phenotypes
and genotypes along with the ratios.
Sex Linked Traits
In 1890, it was observed that the chromosomes in cells from males and females
were identical except for ____________ pair. Scientists suspected that these
different chromosomes determined the sex of the organism. This hypothesis is
now well- proven. These unmatched chromosomes are known as the ________
________________________. All other chromosomes are called
_____________________________.
There are ________ pairs of chromosomes in human body cells: __________
pair are autosomes and ____________ pair of sex chromosomes. The sex
chromosomes are called ____________ and _______________. The cells of
human females contain two __________ chromosomes. The cells of males
contain one _________ chromosome and one ___________ chromosome. Each
egg of a female contains ___________. A sperm cell may contain either
________ or _________. When fertilization occurs, the zygote may be either
_________ (female) or __________ (male). In humans, it is the sperm of the
male that determines the sex of the offspring.
A trait that is controlled by a gene found on the sex chromosome is a
_____________________ trait. The chance of showing the trait is affected by
the sex of the individual. Most sex- linked traits are determined by genes on the
X chromosome, but not on the Y chromosome.
Many human conditions and diseases are caused by ____________
____________________ of particular genes. The normal allele enables the
body to perform some function that the abnormal allele does not. The term
__________________ gene is often used to refer to the abnormal alleles that
cause genetic diseases. Several defective genes are sex- linked. Examples of
sex- linked diseases caused are: __________________, _________________,
and ____________________. Very few women suffer from these diseases but
may be carriers. To carry a gene for the disease on one chromosome, but are
not affected as they have the normal gene on their other X chromosome.
Every male receives an X chromosome from his mother and a Y chromosome
from his father. If the mother is a carrier, there is a __________% chance that
she will transmit the defective X chromosome to any son that she has. Since the
Y chromosome has no corresponding gene for the trait, the son will suffer from
the condition. A daughter would only be a carrier. In color blindness, if both
parents have the condition, then all children will be colorblind.
Sex Linked problems
Example 1: If a normal woman marries a normal man and two of three sons are
hemophiliacs, what is her genotype? Show your work.
Example 2: Cross a hemophiliac woman with a normal man.
Can the father ever pass the gene for hemophilia on this his son? Why or why
not?
Multiple Alleles:
Many traits have more than two different alleles for the trait. This situation is
referred to as multiple alleles.
Geneticists probably know more about the inheritance of ___________ type than
any other inherited human trait. In the ABO blood group, there are _______
blood types. These blood types are ______, _______, _______, and ______.
These basic blood type are determined by the presence or absence of the ____
and _____ _________ (surface proteins) on the red blood cell membrane. There
are 3 different alleles: _____, _____, and _____ in the human population.
______ codes for the production of the _______ antigen on the RBC surface.
______ codes for the production of the _______ antigen on the RBC surface.
______ codes for neither antigen. _____ and _____ are co-dominant. If both
are present, both are expressed equally. Both _______ and _______ are
dominant to _______. Any individual only has 2 of these alleles, one from each
___________, and they may be the same or they may be different.
Phenotypes
Type A blood
Type B blood
Type AB blood
Type O blood
Genotypes
Example 1: Suppose that a woman with type O blood marries a man with Type
AB blood. What are the expected genotypes and phenotypes of their children?
Example 2: A couple preparing for marriage have their blood typed. Both are AB.
What are the possible blood types of their children and the percentage of each?
Example 3: Two parents, both with type A blood, have a son with type O blood.
What are the genotypes of the parents.
Example 4: Mr. and Mrs. Fleming, both with type B blood, claimed that a certain
JR Wilson, a billionaire technology tycoon, was the father of Mrs. Fleming’s
recent 4 month old baby. The Flemmings took the issue to court, charging that
Mr. Wilson was the father and that he should pay $300,000 per year to the
Flemmings for child support. Mr. Wilson’s blood type is AB. The baby is type O.
a. Prove that the baby is not Mr. Wilson’s.
b. Is it possible that the baby is Mr. Flemming’s? Explain.
Example 5: Would it be possible for a child to have O type blood if one parent is
type A and the other is type B?
More about blood: Transfusions and blood clotting:
As mentioned earlier, one type of protein, the antigens, are present on the
surface of red blood cells. Another type of protein is present in the plasma or
liquid component of the blood. These types are called ANTIBODIES. The two
types of protein will react with each other when they come into contact so the
antibodies are called anti- A and anti- B. If A antigen and anti- A come into
contact, the blood will clot. People with A antigen have anti- B in their plasma so
clotting does not occur. B antigen is compatible with anti- A.
Rh Factor (DD or Dd = Rh positive, dd = Rh negative) and pregnancy:
Another antigen present in the blood is the Rh factor. This factor is either labeled
positive or negative. The importance of the Rh factor, aside from causing
problems during transfusions, is during the firth of human babies. If an Rh
negative mother is pregnant with an Rh positive baby, the child could die. This is
due to antibodies crated by the mother’s blood to stop the invasion of ther system
with the detrimental Rh positive blood of her unborn child. Small amounts of fetal
blood cross the placenta and come in contact with the mother’s immune system
usually late in pregnancy or during delivery, and this stimulates the production of
antibodies to the Rh factor. Although death of a child during the first pregnancy
is rare, the possibility for death becomes greater with each succeeding
pregnancy. A vaccination is administered to an Rh negative mother who is giving
birth to an Rh positive child to prevent the problem in future pregnancies.
Example 6: Cross IBiDd x IAIBDd. Give the phenotypes in the offspring and the
ratio.
Chromosomes
In humans, the normal diploid (2n) number of chromosomes is 46 (23 pairs).
Females have 22 pairs of autosomes and 2 “X” chromosomes, while males have
22 pairs of autosomes, 1 “X” chromosome, and 1 “Y” chromosomes. The
chromosomes of an individual can be photographed and examined by a
procedure called ________________. In this procedure, cells such as blood
cells or skin cells are cultured in the growth medium for a few days and then
treated with chemicals which block them in metaphase of mitosis. These cells
have condensed chromosomes that can be stained with special stains, then
observed and photographed under a microscope. Next, the chromosomes can
be cut out and paired with one another. Finally, they are arranged according to
their size, banding pattern, and position of the centromeres. This pictorial
presentation of the chromosome is called a _________________.
Analysis of a karyotype reveals more than the sex of an individual. It may
also reveal gross ________________ _________________ (deviations from
“normal”) such as broken chromosomes or abnormal numbers of chromosomes
(______________). Gametes containing too many or too few chromosomes are
sometimes produced as a result of _______________________ (failure of
homologous chromosomes to segregate properly) during meiosis. If these
defective gametes succeed in fertilization, individuals with an abnormal number
of chromosomes will be produced. If the individual receives 3 copies of a
particular chromosome instead of the normal 2 copies, it is termed __________
(2n + 1). Alternately, if the individual receives only a single copy of a particular
chromosome instead of the normal 2 copies, it is termed _________________
(2n – 1). About 50% of all spontaneous miscarriages involve embryos with
major chromosomal defects.
Chromosomal defect disorders:
1. ____________________ (Trisomy 21): One of the most common
chromosomal defects in humans. This defect would have _________
___________________________. These individuals will have moderate
to severe mental retardation, round moon- shaped faces with thick necks
and short, stocky bodies.
2. Meta- Female or “_________________” (Trisomy X): These females
have 47 chromosomes with
______________________________________. They are relatively
normal mentally and physically, but are often sterile or have reduced
fertility. The more extra X chromosomes that are present (for example:
XXXX or XXXXX), the greater the chance of mental retardation.
3. ____________________ (Monosomy X): These females have only 45
chromosomes with only a __________________________ rather than the
usual two. These individuals are females with respect to their external
genitalia, but they lack functional ovaries and are sterile. They also fail to
mature properly at puberty.
4. _________________________: These males have 47 chromosomes with
2 copies of the X chromosome and one copy of the Y chromosome
(______). Although the external genitalia is male, the testes are small and
do not produce sperm. Also, at puberty, there is partial breast
development. This individual is mentally retarded.
5. ___________________: These males have 47 chromosomes with 1 X
chromosome and 2 Y chromosomes (______). They are usually large in
stature (over 6 feet tall) and show some over aggressive behaviors.
Compare the Karyotypes below:
Normal __________________
Normal _______________
Syndrome:
Syndrome:
Syndrome:
Pedigree Problems
Free earlobe =
О
Attached earlobe =
1.
2.
3.
4.
5.
6.
or
□ (dominant)
● ■
or
Earlobe attachment
(recessive)
How many females are represented in this pedigree?
How many females have attached earlobes?
How many females have free (normal) earlobes?
What letter would you use to represent the genes?
Which type of earlobe attachment is dominant?
Give the genotypes of each person represented.
Tongue rollers pedigree
Tongue Rolling ability:
Tongue roller =
● or ■ (dominant)
Non- tongue roller =
1.
2.
3.
4.
5.
6.
7.
8.
9.
О or □
(recessive)
How many males can roll their tongue?
Give the genotypes of each person represented.
Write the homozygous recessive genotype.
How many homozygous recessive pairs are there?
Write the homozygous dominant genotype.
What is the phenotype of the homozygous dominant genotype?
Write the heterozygous genotype.
What is the phenotype of the heterozygous genotype?
How many deceased people are represented in this pedigree?