Intro to Genetics
What genetic traits do you have?
Widow’s peak vs. straight hairline
Attached earlobes vs. free
Gapped vs. ungapped front teeth
Do any of these traits run in your family? Where do these traits come from? Who do we inherit
them from? Can you think of any other traits we could inherit?
Genes are passed from one generation to the next, but they are not all expressed in every
generation. i.e. Eye color
I. The Work of Gregor Mendel
A. What is an inheritance?
1. To most people, it is money or property left to them by a relative
who has passed away. That is an important type of inheritance but
the inheritance we care about is your genes.
2. Every living thing – plant or animal, microbe or human – has a set
of characteristics inherited from its parent or parents.
3. As a result of people wanting to know how and why they became
the person they did, genetics has been accepted as the exciting
scientific study of heredity.
B. Gregor Mendel’s Peas
1. Gregor Mendel
Austrian monk who was very important in understanding
biological inheritance.
a)
b)
He was born in 1822.
After becoming a priest, Mendel spent several years studying
science and mathematics at the University of Vienna.
c)
He then spent 14 years working in the monastery and teaching
and gardening.
d)
He began to study what we know as genetics with ordinary
peas.
e)
He first began to understand fertilization within pea plants.
During sexual reproduction, male and female reproductive cells
join which is known as fertilization.
f)
Fertilization produced a new cell, which develops into a tiny
embryo.
(1)
Pea flowers are normally self-pollinating, which means that
sperm cells in pollen fertilize the egg cells in the same flower. The
seeds that are produced by self-pollination inherit all of their
characteristics from the single plant.
(2)
When Mendel first started he had several stocks of pea plants.
These peas were true-breeding, meaning that if they were allowed
to self-pollinate, they would produce offspring identical to
themselves.
(3)
Mendel forced the plants to cross-pollinate, which allowed
seeds to have two different plants as parents. This made it possible
for Mendel to cross-breed plants with different characteristics, and
then to study the results
(4)
C. Genes and Dominance
1. Mendel studied seven different pea plant traits.
2. A trait is a specific characteristic, such as seed color or plant
height, that varies from one individual to another.
3. We call each original pair of plants the P (parental) generation.
The offspring are called the F1 generation.
4. The offspring of crosses between parents with different traits are
called hybrids.
5. Did the characters of the parent plants blend in the offspring? Not
at all. To his surprise all the offspring has the character of only one
of the parents.
6. From this set of experiments, Mendel drew two conclusions.
1st - Biological inheritance is determined by factors that are
passed from one generation to the next, which we now call genes.
a)
2nd – Principle of Dominance, which states that some alleles are
dominant and others are recessive.
b)
(1)
The different forms of a gene are called alleles
An organism with a dominant allele for a particular form of the
trait will always exhibit that form of the trait.
(2)
An organism with a recessive allele for a particular form of a
trait will exhibit that form only when the dominant allele for the
trait is not present.
(3)
D. Segregation
1. Had the recessive allele disappeared, or were they still present in
the F1 plants? To answer this question Mendel allowed all seven
kinds of F1 hybrid plants an F1 generation by self-pollination.
2. The F1 Cross
When Mendel compared the F2 plants, he discovered that the
traits controlled by the recessive alleles had reappeared!
a)
Roughly ¼ of the F2 plants showed the trait controlled by the
recessive allele.
b)
3. Explaining the F1 Cross
a)
Segregation – separation of alleles during gamete formation
b)
Gametes – specialized cell involved in sexual reproduction
When each F1 plant flowers and produces gametes, the two
alleles segregate from each other so that each gamete carries only
a single copy of each gene. Therefore, each F1 plant produces two
types of gametes – those with the allele for tallness and those with
the allele for shortness.
c)
II. Probability and Punnett Squares
A. Whenever Mendel preformed a cross with pea plants, he carefully
categorized and counted the many offspring. Every time Mendel
repeated a particular cross, he obtained similar results.
B. Mendel realized that the principles of probability could be used to
explain the results of genetic crosses.
C. Genetics and Probability
1. The likelihood that a particular event will occur is called
probability.
2. Consider an ordinary event like flipping a coin. There are two
possible outcomes:
a)
Heads up
b)
Tails up
3. The chances, or probabilities, of either outcome are equal.
Therefore, the probability that a single coin flip will come up heads is
1 chance in 2, this is 1/2, or 50%.
4. If you flip a coin 5 times in a row, what is the probability that it will
land heads up every time?
5. Because each coin flip is an independent event, the probability of
each coin’s landing heads up is ½. Therefore, the probability of
flipping 5 heads in a row is:
a)
½ x ½ x ½ x ½ x ½ = 1/32
6. The principles of probability can be used to predict the outcomes
of genetic crosses.
III. Exploring Mendelian Genetics
A. Does one pair or alleles affect the segregation of another pair of
alleles?
B. For example, does the gene that determines weather a seed is
round or wrinkled have anything to do with the gene for seed color?
C. Independent Assortment
1. The Two-Factor Cross: F1
Mendel crossed true-breeding plants that produced only round
yellow peas (genotype RRYY) with plants that produced wrinkled
green peas (genotype rryy). All of the offspring produced round
yellow peas.
a)
b)
A Punntt square for this cross is shown in Fig. 11-9 p 270.
This cross does not indicate whether genes assort, or segregate,
independently. But it produces the hybrid plants he needed for
the next generation.
c)
2. The Two-Factor Cross: F2
Mendel knew that the F1 plants had genotypes of RrYy – all
heterozygous for both the seed shape and seed color.
a)
When crossing the F2 generation would the two dominate
alleles always stay together? Or would they “segregate
independently” so that any combinations of alleles was possible?
b)
c)
In Mendel’s experiment, the F2 plants produced 556 seeds.
Mendel compared the variation in the seeds. He observed that
315 seeds were round and yellow and another 32 were wrinkled
and green, the two parental phenotypes. However 209 of the
seeds had combinations of phenotypes not found in either parent.
d)
This clearly meant that the alleles for seed shape segregated
independently of those for seed color – a principle known as
independent assortment.
e)
Mendel’s experimental results were very close to the 9:3:3:1
ratio that the Punnett square predicted.
f)
The principle of independent assortment states that genes for
different traits can segregate independently during the formation
of gametes. Independent assortment help account for the many
genetic variations observed in plants, animals, and other
organisms.
g)
D. A Summary of Mendel’s Principles
1. Mendel’s principles from the basis of the modern science of
genetics. These principles can be summarized as follows:
The inheritance of biological characteristics is determined by
individual units known as genes. Genes are passed from parents
to their offspring.
a)
In cases in which two or more forms (alleles) of the gene for a
single trait exist, some forms of the gene may be dominant and
other may be recessive.
b)
In most sexually reproducing organism, each adult has two
copies of each gene—one from each parent. These genes are
segregated from each other when gametes are formed.
c)
The alleles for different genes usually segregate independently
of one another.
d)
E. Beyond Dominant and Recessive Alleles
1. Not all genes show simple patterns of dominant and recessive
alleles. In most organisms, genetics is more complicated, because
the majority of genes have more then two alleles.
2. Some alleles are neither dominant nor recessive, and many traits
are controlled by multiple alleles or multiple genes.
3. Incomplete Dominance
A cross between two four o’clock plants shows one of these
complications. The F1 generation produced by a cross between
red-flowered (RR) and white-flowered (WW) plants consists of
pink-colored flowers (RW).
a)
b)
Which allele is dominant in this case? Neither one.
This is a case of incomplete dominance. Incomplete dominance
is a case in which one allele is not completely dominant over
another.
c)
4. Codominance
A similar situation is codominance, in which both alleles
contribute to the phenotype.
a)
For example, in certain varieties of chicken, the allele for black
feathers is codominant with the allele for white feathers.
Heterozygous chickens have speckled black and white feathers.
b)
Many human genes show codominance, also, including one for
a protein that controls cholesterol levels in the blood. People with
the heterozygose form of the gene produce two different forms of
the protein, each with a different effect on cholesterol levels.
c)
5. Multiple Alleles
Many genes have more than two alleles and are therefore said
to have multiple alleles.
a)
This means that more than two possible alleles exist in a
population.
b)
Rabbit coat color is the best example. A rabbit’s coat color is
determined by a single gene that have a least four different alleles,
each one producing a different coat color possibility.
c)
F. Applying Mendel’s Principles
1. American geneticist Thomas Hunt Morgan in the 1900s decided to
work on a tiny insect that kept showing up, uninvited, in his
laboratory, known as a common fruit fly, Drosophila melanogaster
2. Drosophila was an ideal organism for genetics because it could
produce plenty of offspring quickly. A single pair of flies could
produce as many as 100 offspring.
3. Mendel’s principles applied to Drosophila and also humans.
G. Genetics and the Environment
1. The characteristics of any organism, whether bacterium, fruit fly,
or humans, are not determined solely by the genes it inherits. Rather,
characteristics are determined by interaction between genes and the
environment.
IV. Meiosis
A. Genes are located on chromosomes in the cell nucleus.
B. Mendel’s principles of genetics
1. Organisms must inherit a single copy of every gene from each of
it’s “parents.”
2. When an organism produces its own gametes, those 2 sets of
genes must be separated from each other so that each gamete
contains just one set of genes.
C. Chromosome Numbers
1. Fruit fly, Drosophila has 8 chromosomes. Four from the male
parent, 4 from female parent.
2. These 2 sets of chromosomes are homologous, meaning that each
of the 4 chromosomes that came from the male parent have a
corresponding chromosome that came from the female parent.
3. A cell that contains both sets of homologous chromosomes is
said to be diploid, which means “two sets.” This is represented by
the symbol 2N.
Thus for Drosophila the diploid number is 8 which can be
written 2N=8.
a)
4. Gametes of sexually reproducing organisms contain only a single
set of chromosomes, and therefore only a single set of genes which
are called haploid, meaning “one set.”
D. Phases of Meiosis
1. How are haploid (N) gamete cells produced from diploid (2N)
cells?
2. Meiosis is a process of reduction division in which the number of
chromosomes per cell is cut in half through the separation of
homologous chromosomes in a diploid cell.
3. Meiosis usually involves two distinct divisions, called meiosis I
and meiosis II.
By the end of meiosis II, the diploid cell that entered meiosis
has become 4 haploid cells
a)
4. Meiosis I
Prior to meiosis I, each chromosome is replicated. The cells
then begin to divide in a way that looks similar to mitosis.
a)
In mitosis, the 4 chromosomes line up individually in the center
of the cell. The 2 chromatids that make up each chromosome
then separate from each other.
b)
In prophase of meiosis I each chromosomes pairs with its
corresponding homologous chromosome to form a structure
called a tetrad. There are 4 chromatids in a tetrad.
c)
As homologous chromosomes pair up and form tetrads in
meiosis I, they exchange portions of their chromatids in a process
called crossing-over.
d)
Crossing-over results in the exchange of alleles between
homologous chromosomes and produced new combinations of
alleles.
(1)
Then the homologous chromosomes separate, and two new
cells are formed.
e)
Because each pair of homologous chromosomes was separated,
neither of the daughter cells has the two complete sets of
chromosomes that it would have in a diploid cell.
f)
The two cells produced by meiosis I have sets of chromosomes
and alleles that are different from each other and from the diploid
cell that entered meiosis I.
g)
5. Meiosis II
The two cells produced by meiosis I now enter a second meiotic
division. Unlike the first division, neither cell goes through a
round of chromosome replication before entering meiosis II.
a)
b)
Each of the cell’s chromosomes has 2 chromatids.
During metaphase II of meiosis, chromosomes line up in the
center of the cell.
c)
d)
Anaphase II the paired chromatids separate.
Those four daughter cells now contain the haploid number (N)
– just 2 chromosomes each.
e)
E. Gamete Formation
1. In males, meiosis results in four equal-sized gametes, called
sperm.
2. In females, only one large egg cell results from meiosis. The other
three cells, called polar bodies, usually are not involved in
reproduction.
F. Comparing Mitosis and Meiosis
1. Mitosis results in the production of two genetically identical
diploid cells, whereas meiosis produces four genetically different
haploid cells.
V. Linkage and Gene Maps
A. Gene Linkage
1. After identifying more than 50 Drosophila genes, Morgan
discovered that many of them appeared to be “linked” together in
ways that, at first glance, seemed to violate the principle of
independent assortment.
2. For example, a fly with reddish-orange eyes and miniature wings
was used in a series of crosses. The results showed that the genes
for those traits were almost always inherited together and only rarely
became separated from each other.
3. It was discovered that the linkage groups sort independently, but
all of the genes in one group were inherited together.
4. It is the chromosomes, however, that sort independently, not
individual genes.
B. Gene Maps
1. If two genes are found on the same chromosome, it does not mean
that they are linked together forever.
2. Crossing-over during meiosis sometimes separates genes that
had been on the same chromosome onto homologous chromosomes.
3. The rate at which linked genes were separated and recombined
can be used to produce a “map” of distances between genes.