CONCEPTUAL LIFE SCIENCE
Genetics
INTRODUCTION
Living organisms resemble their parents. This is due to the transmission of traits
or genetic characters from one generation to the next. We know from our study of
reproduction that sexual reproduction involves receiving genetic material from each
parent. In humans, half of your genetic material comes from your mother and half from
your father.
We know that the genetic material is DNA. The DNA is organized into genes that
are located on the chromosomes in the nucleus of the cell. Historically, the understanding
of the transmission of the traits came before the understanding of the mechanism of the
transmission. 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
19 century. One of his duties in the monastery involved tending the garden where he
became interested in the traits of the pea plants. Table 19-1 lists some of the characters
of his peas.
th
Table 19-1. Characters in Mendel’s garden peas.
Character
Appearance
Flowers – color
Red or white
Flowers – location
Axial or terminal
Pods – color
Green or yellow
Pods – structure
Inflated or constricted
Seeds – appearance
Round or wrinkled
Seeds – color
Green or yellow
Plants – height
Tall or short
Some notes about Mendel’s peas. In Mendel’s day there was much more variation in
plants that are used for food than you find today. Go to a store where they sell garden
seeds and see what they have to offer. You will probably be able to buy seeds for peas
that give round, green peas; in inflated, green pods; with white flowers that are terminal
(on the ends of the branches). There are yellow string beans, the so-called “wax beans.”
People generally expect that peas are green and are found in green pods. Mendel also
had yellow peas and yellow pods. Just about the only variation found in modern days is
the height of the plants.
19-1
19-2
Mendelian inheritance
We begin studying Mendelian inheritance with red and white flower color.
Mendel noted that the peas in his garden from year to year had red and white flowers.
Some plants with red flowers came from plants with red flowers and made plants next
year with red flowers. These were pure-breeding red-flowered plants. Similarly he
noticed that he had pure-breeding white-flowered plants. Mendel kept very detailed,
meticulous notes on his work with his peas. However, he had no clue as to the
mechanism by which the characters were transmitted. He might have thought that it was
like mixing cans of paint together.
The flower is the reproductive structure of the plant. Figure 19-1 shows the parts
of the flower. The female component of the flower is called the pistil. The male parts are
called the stamens.
Figure 19-1. Reproductive structures of the flower.
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. 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 will become a seed while the ovary becomes a fruit.
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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. 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 involves transfer of pollen. 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 might 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 redflowered plant. In either case, this constitutes a cross between a red-flowered plant and a
white-flowered plant. Mendel did hundreds of these crosses over a period of many years.
Once the cross has been performed, you need to wait for the peas to develop in
their pods. Then you must harvest the peas and put them away in storage over the
Winter. Then, next Spring, you plant the seeds and see what grows. So, as we discuss
the types of crosses that Mendel performed with his peas, keep in mind that each stage in
the cross takes one year.
Sample mating to illustrate simple Mendelian inheritance
Mendel took red-flowered peas that were pure-breeding and crossed them with
pure-breeding white-flowered plants. These plants were the parental generation,
represented by P. 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 is
the generation of offspring and is called the F1 generation. The letter “F” comes from the
adjective “filial” derived from a Latin word referring to children (offspring) and their
relation to each other (the “sons” of the parental generation). This cross and its result is
illustrated in Figure 19-2.
Figure 19-2. Parental cross in simple Mendelian inheritance.
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Crossing of the F1 generation
The following year, Mendel crossed the F1 red-flowered plants from the previous
year. Recall that all of the F1 plants had red flowers even though one of the original
parents had white flowers. The result of crossing the red-flowered F1 plants the
production of an F2 generation. In the F2 generation, the white trait returned. Mendel
also noticed that there were about three times as many plants with red flowers as there
were with white flowers. See Figure 19-3.
Figure 19-3. F1 cross and F2 results. After not being expressed in the F1 generation,
white was expressed in the F2 generation.
Pod color exhibits simple Mendelian inheritance
For another example we can consider pod color. Mendel crossed pure-breeding
plants having green pods with pure-breeding plants having yellow pods. The F1
generation all had green pods as shown in Figure 19-4.
Figure 19-4. Parental cross involving green and yellow pods.
19-5
Once again, one of the traits was not expressed in the F1 generation. This time the trait
was for yellow pods.
Mendel then crossed two F1 plants having green pods. The results of this F1 cross
are shown in Figure 19-5.
Figure 19-5. Cross of F1 plants with green pods.
The cross of F1 plants with green pods yielded yellow pods. The ratio of plants
with green pods to plants with yellow pods was again approximately 3:1. Clearly the
traits are not expressed, as one would expect from mixing paints together. One trait was
not expressed in the F1 generation. Mendel thought that these traits were hiding in a
“recess” somewhere in the plant. He termed them recessive. The traits that were
expressed in the F1 generation were termed dominant.
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In studying genetics there are some terms that need to be defined.
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.
The genotype is the combination of alleles that the organism has in its cells. In the
case of flower color, there are three genotypes. The plant can have two genes for red
color, two genes for white color, or one for each color.
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.
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
the dominant alleles and lower case letters for the recessive alleles. For flower color:
 C – dominant color gene (red) – gives red flowers.
 c – recessive color gene (white) – gives white flowers
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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, of course, would be the sex cells; they are haploid. So, there are three
possible combinations of flower alleles. These are the genotypes.
 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 parent.
 cc – homozygous recessive – gives white flower color. The plant received a
recessive gene from each parent.
Homozygous means that the cell has both alleles of the same type (CC or cc).
Heterozygous means that the alleles are different (Cc).
Genetic symbols can be used to represent genetic crosses. We can repeat the first
example of red and white flowers using genetic symbols. See Figure 19-6.
Figure 19-6. Cross of pure-breeding red and white-flowered plants indicating genotypes.
Figure 19-6 depicts the cross illustrated in the first example. The pure-breeding
red-flowered plant is homozygous dominant. It has two genes for red flower color. The
white-flowered plant is homozygous recessive. It has two genes for white flower color.
Each parent contributes one gene in a cross. When pure-breeding plants are
crossed with each other, each one contributes one gene. The red-flowered plant
contributes a gene for red color. This is all it can do. Both genes are for red color. The
white-flowered plant contributes a gene for white color. It, too, can only contribute one
type of gene. Thus, the offspring in the F1 generation are all heterozygous. The genotype
of each plant is that it has one gene for red color and another for white color. As the red
is dominant and white is recessive, all the plants display the red phenotype.
19-7
When the F1 plants are crossed a 3:1 phenotypic ratio results. Mendel’s cross of
red-flowered F1 plants is illustrated in Figure 19-7.
Figure 19-7. F1 flower cross using genetic symbols.
In the F1 cross each parent can contribute either gene. A heterozygous plant has
two different genes, C and c. When these plants are crossed, the genes separate and can
produce any of four possible combinations. This separation of genes is known as the Law
of Segregation. Mendel’s Law of Segregation (also called Mendel’s First Law) indicates
that the genes in the heterozygous parents segregate from each other and that each gamete
has an equal probability of possessing either allele.
One way to determine the possible combinations of alleles is the FOIL method
that is used in 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). This explains the phenotypic ratio of 3 red:1 white. Note that one
combination is written as cC. When the genes are in the cell, the sequence does not
matter so we generally write it as Cc.
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.
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Figure 19-7 illustrates that the F2 offspring are of three genotypes.
Homozygous dominant for red flowers (CC). This genotype accounts for one-quarter
of the offspring in the F2 generation.
Heterozygous for red flowers (Cc). This genotype accounts for half of the offspring.
Homozygous recessive with white flowers (cc). This genotype accounts for onequarter of the offspring.
19-8
Mendel’s results were not quite 3:1. When he did crosses, they were a little higher or a
little lower than 3:1. This is due to random fluctuations in the way the genes combined
with each other. The 3:1 ratio is a theoretical prediction that is based on probability.
The test cross determines whether a dominant plant is homozygous or
heterozygous. When a plant has red flowers you do not know if it is homozygous for red
flowers or heterozygous. The test cross can determine this information. In a test cross,
the plant with red flowers is crossed with a homozygous recessive plant—one with white
flowers. There are two possible outcomes.
 A) the red parent is homozygous. All of the F1 progeny will be red.
 B) the red parent is heterozygous. Half of the F1 progeny will be red and the other
half will be white.
This is demonstrated below using genetic symbols.
Figure 19-8. Test cross.
As shown in Figure 19-8, Possibility A is a cross between a homozygous, redflowered plant and a homozygous, white-flowered plant. The red parent contributes the
red allele [C] to the offspring while the white parent contributes the white allele [c]. The
result is that all of the offspring will be heterozygous for red flowers.
In Possibility B, the red parent is heterozygous. It has two alleles for flower
color, one red and one white. Each allele has a 50:50 chance of being transmitted to the
progeny. The white parent can only contribute the white allele. The result is that there
are both red and white offspring, in a 1:1 ratio.
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Crosses can be diagrammed using the Punnett square, named after Reginald
Punnett (1875-1967), an early English geneticist. A Punnett square consists of rows and
columns. We use the Punnett square when the parents are heterozygous. The alleles
(gametes) of one parent are written across the top and there is one column for each allele.
Similarly the alleles of the other parent are written down the side with one row per allele.
See Figure 19-9. While the alleles can be put into the diagram in either order, we
generally place the female gametes across the top and the male gametes down the side.
Figure 19-9. Punnett square. Use the Punnett square for heterozygous parents. Write the
female gametes across the top with a column for each. Write the male gametes down the
side with a row for each.
In the case of Mendel’s heterozygous F1 red-flowered plants, both F1 parents were
heterozygous red with genotype Cc. The Punnett square predicts all possible outcomes
from this cross as shown in Figure 19-10.
Figure 19-10. Cross of F1 red-flowered plants. Whatever is above each column is
written below it. Whatever is on the side of the row is written across.
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From Figure 19-10 we can 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 Punnett square predicts a similar outcome for the results of
a cross between F1 plants that are heterozygous for green and yellow pods. See Figure
19-11.
Figure 19-11. Punnett square of F1 cross of plants heterozygous for green and yellow
pods.
As shown in Figure 19-11, the outcome of a cross between F1 plants
heterozygous for green and yellow pods gives a 3:1 phenotypic ratio and a 1:2:1
genotypic ratio.
The Punnett square predicts possible outcomes. The genotype of a given
individual is determined by random chance. With the Punnett square, we know that the
outcome will be in one of the four squares. You will not get a different outcome that is
not in the diagram.
Some human diseases follow simple Mendelian inheritance. There are some
human diseases that are inherited according to Mendelian inheritance patterns. They are
listed in alphabetical order as follows:
 Cystic Fibrosis
 Gout
 Sickle-Cell Anemia
 Tay-Sachs Disease
Dihybrid inheritance
Dihybrid inheritance is where two different genes are inherited simultaneously.
We can demonstrate this with the inheritance of flower color and pod color together.
19-11
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 of
which is homozygous recessive for both genes. See Figure 19-12.
Figure 19-12. Diagram of dihybrid P cross.
In Figure 19-12, one parent is homozygous dominant for red flowers and green
pods. The other parent is homozygous recessive, having white flowers and yellow pods.
When these two are crossed, all of the F1 progeny are dihybrids; they are heterozygous
for flower color and pod color.
The Punnett square is used to find the F2 generation. When F1 dihybrids are
crossed, each makes four different types of gametes, each with a unique combination of
alleles. The combinations are: CG, Cg, cG, and cg. (You can find these results by using
FOIL on the letters of the F1 genotype.) These combinations of alleles are used at the
tops of the columns and on the sides of the rows. See Figure 19-13.
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Figure 19-13. Results of dihybrid cross.
We see that the most predominant phenotype is red flowers and green pods.
There are several genotypes that can give this result. A plant with red flowers and green
pods may be CCGG, CcGG, CCGg, or CcGg. We can shorten this notation to C-Gwhere the dash indicates that the second allele does not matter. This permits us to
summarize the results of the dihybrid cross as shown in Figure 19-14.
Figure 19-14. Summary of results of dihybrid cross.
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Figure 19-14 indicates that there are four phenotypes possible in a dihybrid cross.
They always appear in a 9:3:3:1 ratio. This ratio is a theoretical prediction of the results.
In actuality, Mendel’s results came very close to this except that he had the usual
variation that is associated with events that occur at random. The alleles for flower color
and pod color separate independently. This is called the Law of Independent Assortment
(or Mendel’s Second Law). The reason for the independent assortment is that the
different genes are on different chromosomes. So, we can conclude that when different
genes are on different chromosomes, they will assort independently.
A cross involving three alleles
It is often necessary to determine the outcomes of crosses involving more than
two alleles. In the example below, the parents have different genotypes for three
different genes (Figure 19-15).
Figure 19-15. Genotypes of plants in a cross involving three genes.
Use a series of steps to diagram this type of cross. The steps involved in
diagramming this type of cross are as follows:
 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.
We will now diagram this cross following the steps.
Step 1—write the genotypes. As Figure 19-15 indicates, the genotypes of the
parents are as follows:
 Female parent—CCGgTt
 Male parent—CCGgtt
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Step 2—determine the possible gametes. For each parent, the possible gametes depend
on the alleles present for each gene.
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 is the product of these numbers. So we multiply 1 x 2
x 2 to give 4 possible combinations. Then we determine the possible combinations,
which are CGT, CGt, CgT, Cgt.
Similarly, 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 for this parent is 2 (1 x 2 x 1). These combinations are
CGt and Cgt.
Step 3—Make a diagram for the cross. The diagram for this cross will be a rectangle
with 4 columns and 2 rows. It is not a square because the female parent has 4 gametes
while the male parent only has 2 gametes. The completed diagram is shown in Figure
19-16.
Figure 19-16. Diagram of a cross involving three genes.
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Sex Linkage
All genes on a single chromosome are said to be linked. Sex linkage refers to
genes that are found on the X chromosome. An example is the gene for hemophilia.
Hemophilia results from a sex-linked recessive allele that results in lack of
production of clotting factor VII. The gene is carried on the X chromosome. Females
have two X chromosomes, so they generally have a normal gene on one of them. They
do not express hemophilia but they are 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 to track than the individual alleles, especially since the male does not have
these types of alleles on the y chromosome. So, in a sex-linkage diagram, the sex
chromosomes of the parents are shown, with the alleles in question represented as
superscripts. Figure 19-17 shows a diagram for the distribution of the hemophilia (h)
allele and the normal (H) allele.
Figure 19-17. Inheritance of hemophilia. In this case, the mother is a carrier of
hemophilia but has normal blood clotting. The father is normal.
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.
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In an average of 1000 people, the distribution of the major blood types will be as
shown in Table 19-2.
Table 19-2. Distribution of major human blood groups.
Number
Group
390
Rh positive, group O
350
Rh positive, group A
90
Rh positive, group B
40
Rh positive, group AB
60
Rh negative, group O
50
Rh negative, group A
15
Rh negative, group B
5
Rh negative, group AB
Percentage
39%
35%
9%
4%
6%
5%
1.5%
0.5%