Principles of Biology
By
Frank H. Osborne, Ph. D.
Genetics
Introduction
Living organisms resemble their parents.
•This is due to the transmission of traits or
genetic characters from one generation to the
next.
•With sexual reproduction, the offspring receives
genetic material from each parent.
•In humans, half of your genes come from your
mother and half from your father.
Introduction
We know that the genetic material is DNA.
•DNA is organized into genes that are located on
chromosomes in the nucleus of the cell.
•Historically, the understanding of the
transmission of traits came before the
understanding of the mechanism of
transmission.
Introduction
Gregor Mendel worked on traits with his
peas in the mid-19th century.
•The chromosome was not discovered until
the 1890s. Once biologists put the traits and
the chromosomes together, we began to
understand the science of genetics.
Classical Mendelian Inheritance
Gregor Mendel was the
abbot of a Catholic
monastery in central Europe
in the mid-19th century.
•One of his duties in the
monastery involved tending
the garden where he became
interested in pea plants.
Characters of Mendel's Peas
Character
Flowers-color
Flowers-location
Pods-color
Pods-structure
Seeds-appearance
Seeds-color
Plants-height
Appearance
Red or white
Axial or terminal
Green or yellow
Inflated or constricted
Round or wrinkled
Green or yellow
Tall or short
Mendelian Inheritance
Red and white flower color.
Mendel noted that some peas always had red
flowers while others always had white flowers.
Those with red flowers came from plants with
red flowers and produced plants with red
flowers. These were pure-breeding red-flowered
plants. Similarly, there were pure-breeding
white-flowered plants.
Reproduction in Flowering Plants
The flower is the reproductive structure of the
plant.
The female component of the flower is called
the pistil.
The male parts are called the stamens.
Reproduction in Flowering Plants
•The male stamens produce the pollen. The
pollen grains are the male sexual units of the
plant. They are produced in the anther of the
flower that is supported by a filament.
Reproduction in Flowering Plants
•In the female pistil is an ovary which
contains ovules. The ovules are the female
sexual units of the plant. Each ovule contains
an egg that will become fertilized by the
pollen. After fertilization, each ovule
becomes a seed while the ovary becomes a
fruit.
Reproduction in Flowering Plants
•When plants reproduce, pollen from the
anther of one flower is transferred to the
stigma of another flower. The pollen grains
digest their way through the style to the
ovary.
Reproduction in Flowering Plants
•In the ovary, chromosomes from one of the
pollen grains fertilize each ovule. Sometimes,
flowers can self-pollinate by transferring
pollen from the anthers to the stigma in the
same flower.
Crossing Plants
•A cross involves transfer of pollen from the
stamens of one flower to the pistil of another.
•For example, pollen from a plant producing red
flowers could be placed on the stigma of a plant
with white flowers.
•Or, pollen from a white-flowered plant could be
used to inoculate the stigma of a red-flowered
plant.
•In either case the cross is the same.
Observing Results
•Once the cross has been performed you must
wait for the peas to develop in their pods.
•Then you must harvest the peas and put them
away in storage to plant next Spring.
•Next Spring, you plant the seeds and see what
comes up.
•So, in the case of Mendel's experiments, each
cross took a year to complete.
Sample Mating
•Mendel took pure-breeding red-flowered
plants and crossed them with pure-breeding
white-flowered plants. These plants were the
parental generation, represented by P.
Sample Mating
•The following year, the plants that came up
all had red flowers. None of the plants had
white flowers even though one of the parents
had white flowers. This generation is known
as the F1 generation of offspring.
Sample Mating
•P generation plants are pure breeding .
•F1 generation has all red flowers. Mendel
called the red color dominant.
Sample Mating
•The following year, Mendel crossed F1 redflowered plants from the previous year.
•The result was the F2 generation. In the F2
generation, the white trait returned. Mendel
noticed that there were about three times as
many red-flowered F2 plants as there were
white-flowered F2 plants. Mendel called the
hidden trait recessive.
Sample Mating
•F1 cross and F2 results. After not being
expressed in the F1 generation, white was
expressed in the F2 generation.
Pod Color
•Mendel crossed pure-breeding plants with green
pods and pure-breeding plants with yellow pods.
•The F1 generation had all green pods.
Pod Color
•The following year, Mendel continued by
crossing F1 plants having green pods. In the F2
generation yellow returned.
Phenotypic Ratio
•Mendel noted in each case that there were
about three times as many dominant plants as
there were recessive plants.
•The ratio each time was about 3:1.
Phenotypic Ratio
•Each time, one trait was not expressed in the F1
generation. Mendel explained that the color was
hiding in a recess somewhere in the plant. He
termed them recessive. The traits expressed in
the F1 generation he termed dominant.
Definitions
•An allele is a contrasting form of a gene. It is
found on a chromosome. In the case of flower
color, the alleles are red and white. An organism
receives two alleles, one from the female parent
and one from the male parent.
Definitions
•The genotype is the combination of alleles that
the organism has in its cells. In the case of
flower color, there are three genotypes. A plant
may have two genes for red color, two genes for
white color, or one gene for each color.
Definitions
•The phenotype is the appearance of
the organism when the genotype is
expressed. A plant with red flowers is
displaying the red phenotype, while a plant with
white flowers is displaying the white phenotype.
•Human characters following simple
Mendelian inheritance include: rightleft handedness, curly- straight hair,
light-dark eyes, widow's peak.
Genetic Symbols
•Genetics problems are expressed using symbols.
We use letters to represent the various genes and
alleles. To reduce confusion, we generally use
upper case letters for dominant alleles and lower
case letters for the recessive alleles. For flower
color:
•C - dominant color gene (red) - red flowers
•c - recessive color gene (white) - white flowers
Each Organism is Diploid
•Because the plant receives one gene from the
female parent and one gene from the male
parent, every cell in the plant has two of each
gene. The exception is the sex cells because they
are haploid.
•Therefore, there are three possible
combinations of flower alleles. Homozygous
means both alleles are the same; heterozygous
means that the alleles are different.
Genotypes of Flower Alleles
•CC - homozygous dominant - gives red flower
color. The plant received a dominant gene from
each parent.
•Cc - heterozygous - gives red flower color. The
plant received a dominant gene from one parent
and a recessive gene from the other.
•cc - homozygous recessive - gives white flower
color. The plant received a recessive gene from
each parent.
Using Genetic Symbols
•The first example of red and white flowers is
repeated using genetic symbols.
Using Genetic Symbols
•Each parent in a cross contributes one gene.
•With a pure-breeding red plant, it can contribute
only a red gene. A white plant contributes a white
gene.
•The F1 plant is heterozygous. It got a red gene
from its red parent and a white gene from its
white parent. The red is dominant so all F1 plants
are red.
Using Genetic Symbols
•When the F1 plants are crossed, a 3:1
phenotypic ratio results.
Using Genetic Symbols
•In the F1 cross each parent can contribute either
gene. When these plants are crossed, the genes
separate and can produce any of four
combinations.
•This separation of genes is known as Mendel's
Law of Segregation (also called Mendel's First
Law.
Using Genetic Symbols
•One way to determine the possible combinations
of alleles is the FOIL method that is used for
multiplying binomials in algebra.
•From the cross Cc X Cc you take the first allele
from each (CC), the outer alleles (Cc), the inner
alleles (cC), and the last alleles (cc).
•The third (cC) can also be written as (Cc) because
in the cell the sequence does not matter.
Phenotypic and Genotypic Ratios
•We have already seen that the phenotypic ratio
of plants with red flowers to plants with white
flowers is 3:1.
•These two phenotypes account for all of the F2
offspring. But there are three genotypes.
•Homozygous dominant (CC) - ¼ of offspring
•Heterozygous (Cc) - ½ of offspring
•Homozygous recessive (cc) - ¼ of offspring
Phenotypic and Genotypic Ratios
•Mendel's results were not exactly 3:1. When he
did crosses, the results were a little higher or a
little lower than 3:1.
•This is due to random fluctuations in the way
that that the genes combined with each other.
The 3:1 ratio is a theoretical prediction that is
based on probability.
Test Cross
•The test cross determines whether a dominant
plant is homozygous or heterozygous.
•With red flowers you cannot tell just by looking
at the plant. The test cross can determine this
information.
•In a test cross, the plant with the dominant
character is crossed with a homozygous
recessive. In this case one with white flowers.
Test Cross
•In a test cross of a red-flowered plant with a
plant having white flowers, two outcomes are
possible.
•A) the red parent is homozygous.
–Result: all F1 progeny will be red.
•B) the red parent is heterozygous.
–Result: half of the F1 progeny will be red and the
other half will be white.
Test Cross
•In Possibility A, the red flowered plant
contributes its red allele [C] while the white
parent contributes the white allele [c]. The
result is that all offspring will be heterozygous
and display the red color.
Test Cross
•In Possibility B, the red parent is
heterozygous. It has two different alleles.
Each allele has a 50% change of being
transmitted to the progeny. The result is red
and white offspring in a 1:1 ratio.
Punnett Square
•Crosses can be diagrammed using the Punnett
square, named after Reginald Punnett (18751967), an early English geneticist.
•A Punnett square consists of rows and columns.
The alleles (gametes) of one parent are written
across the top, and the alleles of the other parent
are written down the side.
•Then the letters are placed down or across to fill
the square.
Punnett Square
•In the case of Mendel's heterozygous F1 red
plants, both parents were heterozygous with
genotype Cc. The Punnett Square gives the
possible outcomes.
Punnett Square
•We predict that there will be a 3:1 ratio of
phenotypes (red to white) with a 1:2:1 genotypic
ratio (1 homozygous dominant, 2 heterozygous, 1
homozygous recessive). The same holds true for
green and yellow pods.
Punnett Square
•The Punnett square predicts possible outcomes.
•The genotype of the individual is determined by
random chance.
•With the Punnett square, we know that the
outcome for an individual will be found in one of
the four cells. You will not get a different
outcome that is not found in the diagram.
Human Genetic Diseases
•Some human diseases follow simple Mendelian
inheritance. These are as follows.
•Cystic Fibrosis
•Gout
•Sickle-Cell Anemia (caused by
inheritance of a specific change in
the DNA molecule)
•Tay-Sachs Disease.
Codominance
•When both genes are dominant, the flowers
display a blended appearance when they are
heterozygous.
•As both genes are dominant, we would use C for
red and C' for white. The results would be
•CC - red
•CC' - pink
•C'C' - white
Dihybrid Inheritance
•Dihybrid inheritance is where two different
genes are inherited simultaneously.
•We demonstrate this using flower color and
pod color when they are inherited
simultaneously.
Dihybrid Inheritance
•A dihybrid is heterozygous for two genes. To
produce a dihybrid we would begin with
parents, one of which is homozygous dominant
for both genes and the other which is
homozygous recessive for both genes.
Dihybrid Inheritance
Dihybrid Inheritance
•The Punnett square is used to find the F2
generation.
•When F1 dihybrids are crossed, each makes
four types of gametes, each with a unique
combination of allelles.
•In the case of CcGg, the combinations are CG,
Cg, cG, and cg. (You can find this by doing
FOIL on CcGg.)
Dihybrid Inheritance
•We see that the most predominant phenotype is
red flowers with green pods.
•A plant with red flowers and green pods may be
CCGG, CcGG, CCGg or CcGg. We can shorten
this to C-G- where the dash indicates that the
second allele does not matter.
•This permits us to summarize the results of the
dihybrid cross as follows.
Dihybrid Inheritance
•The phenotypic ratio in a dihybrid cross is
always 9:3:3:1. This ratio is a theoretical
prediction of the results. Mendel's results
came very close to this except that he had the
usual variation associated with randomness.
Dihybrid Inheritance
•The alleles for flower color and pod color sort
independently. This called the Law of
Independent Assortment (or Mendel's Second
Law). This is because the genes are on
different chromosomes.
Polygenic Inheritance
•Polygenic inheritance involves the effects of
multiple genes.
•An example is the inheritance of skin color in
humans.
A Cross Involving Three Alleles
•Sometimes it is
necessary to determine
outcome of crosses with
more than two alleles.
•Consider this cross.
A Cross Involving Three Alleles
Solving a problem like this involves a series of
steps.
•Write down the genotypes of the parents.
•Determine all of the possible gametes each
parent can produce.
•Use the Punnett technique to determine all of
the possible combinations.
A Cross Involving Three Alleles
Step 1. Write the genotypes.
•Female parent - CCGgTt
•Male parent - CCGgtt
A Cross Involving Three Alleles
Step 2. Determine the possible gametes.
With the female parent we have the following:
•One allele for flower color (C)
•Two alleles for pod color (G and g)
•Two alleles for height (T and t)
•The total number of combinations in the
product of these, 1 x 2 x 2 = 4. The combinations
are CGT, CGt, CgT, and Cgt.
A Cross Involving Three Alleles
Step 2. Determine the possible gametes.
With the male parent we have the following:
•One allele for flower color (C)
•Two alleles for pod color (G and g)
•One allele for height (t)
•The total number of combinations in the
product of these, 1 x 2 x 1 = 2. The combinations
are CGt, and Cgt.
Step 3. Make a diagram for the cross.
Sex Linkage
•All genes on a single chromosome are said to be
linked. Sex linkage refers to the genes that are
found on the X chromosome. An example is the
gene for hemophilia.
•Hemophilia results from a sex-linked recessive
gene that results in a lack of clotting factor VII.
The gene is carried on the X chromosome.
Sex Linkage
•Females have two X chromosomes, so they
generally have a normal gene on one of them.
•They do not express hemophilia but can be
carrying the recessive allele.
•In males, there is only one X chromosome. If a
male carries the defective X chromosome, he will
express the hemophilia trait. The y chromosome
is considered to be genetically inert.
Diagram of Sex Linkage
•When making diagrams of crosses involving sex
linkage, it is important to keep the sex of the
individuals in mind. The distribution of the sex
chromosomes is more important than the
distribution of the individual alleles, especially
since the male does not have these types of alleles
on the y chromosome.
Diagram of Sex Linkage
•In a diagram, the sex chromosomes of the
parents are shown with the alleles represented as
superscripts.
•Inheritance of the hemophilia gene (h) and
the normal gene (H).
Human Blood Groups
•There are four main groups of human blood.
These are known as blood types. The major blood
types are O, A, B, and AB. Within each blood
type a person can be Rh positive or Rh negative.
•In an average of 1000 people, the distribution of
major blood types will be as shown in the table.
Number
Group
Percentage
390
Rh positive, group O
39.0%
350
Rh positive, group A
35.0%
90
Rh positive, group B
9.0%
40
Rh positive, group AB
4.0%
60
Rh negative, group O
6.0%
50
Rh negative, group A
5.0%
15
Rh negative, group B
1.5%
5
Rh negative, group AB
0.5%
The End
Principles of Biology
Genetics