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5 Genesandtraits
If yo u e x a m i n e a family photograph of relatives from three or four genera-
tions, you may notice similar traits among the people, such as a certain eye or
hair color. When people began growing crops and breeding animals, they noticed
patterns in the traits of parent plants and animals and their offspring. These pat-
terns of traits passing from one generation to the next helped farmers decide
which animals to breed and which seeds of which plants to grow to obtain the
qualities they wanted in their farm products. More recently, scientists began
studying these patterns in human families to track and understand such traits as
those that cause genetic diseases.
In the previous activity, you modeled the possible combinations of traits passed
on to sexually reproduced corn plants. In this activity, you will learn more about
the mechanism of heredity, the passing of genetic traits from one generation to
the next.
ChallengeWhatcanweinferaboutgenesandtraitsbasedonhereditypatterns?00
Due to many generations of selective breeding, domestic carrots (Daucuscarota) (on the right) show many traits that differ from those of wild carrots (on the left).
291
GeneSAndtrAitS • Activity5
Procedure 1. The reading below involves a strategy called Stopping to Think questions.
Occasionally, in between paragraphs, there will be a question. As you read,
stop and answer these questions in your mind. They can help you determine
the main ideas of the reading. Follow your teacher’s instructions on further
exploration of these questions in discussions.
Readingearly Breeding practicesFarmers learned thousands of years ago that by selecting which parent plants and
animals to breed, those parents would produce offspring with characteristics
people wanted. Through this selective breeding, farmers’ improvements in farm
products have helped sustain human communities. The potato, for example, was
first discovered as a food source in South America more than 10,000 years ago.
Native South Americans who started farming potatoes quickly learned that they
would lose fewer potatoes to disease if they grew several kinds. Through selective
breeding, potato farmers around the world now grow thousands of varieties. A
few traits of those varieties are size, color, and how long they can be stored. Such
progress in agriculture led to the modern study of heredity.
STOPPInGTOTHInkQUeSTIOn1
Why is the study of heredity and traits important?
Gregor Mendel was a 19th century monk, teacher, and scientist, who set out to
systematically explore the relationship between traits and heredity. He worked
mainly with pea plants, which he could grow easily and which showed several
traits clearly. Among the pea traits Mendel analyzed were seed color (yellow or
green) and stem length (long or short). Over several years, he conducted carefully
controlled experiments and kept detailed records of the traits inherited by off-
spring from parent pea plants. A summary of Mendel’s findings is shown in the
table on the following page.
Science & Global iSSueS/bioloGy • GeneticS
292
Painting of Gregor Mendel working with pea plants
Mendel crossed hundreds of pea plants, and observed and counted phenotypes of
traits related to seeds, pods, and flowers in thousands of offspring. The phenotype
of an organism is its physical characteristics, which result from the organism’s genes
and their interaction with the environment. For example the color of some flowers
depends on both the genes they carry and the soil conditions in which they are
grown. He then analyzed the results and applied his knowledge of statistics to figure
out the patterns associated with individual genes and the probability of such pat-
terns occurring.
Mendel’s results for Three Generations of pea plant Crosses
floWer Color seed Color seed surfACe pod Color
original cross purple3white green3yellow wrinkled3smooth green3yellow
f1 generation allpurple allyellow allsmooth allgreen
f2 generation 705:224(purple:white)
6,022:2,001(yellow:green)
5,474:1,850(smooth:wrinkled)
428:152(green:yellow)
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GeneSAndtrAitS • Activity5
As he looked at the data, Mendel noticed an interesting relationship. With seed
color, if the original cross (parent generation) was a purebred green seed with a
purebred yellow seed, he found that all of the offspring (F1 generation) had yellow
seeds. When he bred the F1 seeds together, their offspring (F2 generation) still had
many yellow seeds, but some were green. Calculating the ratio of the two traits in
the F2 generation, he obtained a ratio very near to 3:1. This means that for every
one green-seeded plant, there were approximately three yellow-seeded plants in
the third generation. For example, for color in 8,023 pea seeds he calculated the
ratio of yellow to green seeds as
6,022 yellow 5
3.01 yellow
2,001 green 1 green
This is almost exactly a 3:1 ratio of yellow : green.
Mendel observed that the green-color trait was absent in the F1 generation, but
reappeared in the third generation, and that the probability of the seeds of an F2
generation plant having the trait was one in four—that is, about one green-seeded
plant for every four plants produced overall. Mendel found the same ratio for sev-
eral other characteristics involving the same number of generations. The 3:1 ratio
was the clue to how the parents’ genes combine in their offspring. Based on his
analysis, he proposed three principles of heredity:
Each characteristic that appears in the F• 1 generation is the dominant version of
the trait. A dominant trait, when present in an individual, will always appear in
that individual. The trait that is “hidden” in the F2 generation is called recessive.
It can be present but does not appear if there is a dominant trait masking it.
Every plant has two copies, alleles, of the gene for each trait. •
Note: The terms “allele” and “gene” were proposed long after Mendel’s research.
Every offspring receives only one allele for each trait from each parent.•
Science & Global iSSueS/bioloGy • GeneticS
294
In sexual reproduction, a gamete from a male parent carrying one allele for every trait fuses with the gamete of a female parent, also carrying one allele for every trait as shown in the ovule of a plant at left and in the human sperm and egg above. Once fused, the fertilized egg contains a complete genetic set of alleles—one from each parent.
STOPPInGTOTHInkQUeSTIOn2
Look at the information presented in the table, “Mendel’s Results,” on the previous pages. Based on these results, which allele for each trait isa) dominant? b) recessive? What evidence supports your claim?
The work of Mendel and other scientists has provided evidence that supports his
basic ideas about heredity. Today, scientists know that heredity is controlled
through genes. A gene is a segment of an organism’s genetic material, or DNA.
Each gene is present in an individual in two versions, called alleles. We now know
that when organisms reproduce sexually, each parent donates a gamete. A gamete
is a sexual reproductive cell, such as a sperm or an egg, which contains genetic
material of the organism. The gamete from each parent carries one allele for each
trait. During sexual reproduction the two gametes, one from the female and one
from the male, fuse together and create a new cell with two alleles for each trait.
This new cell eventually grows into a fully developed organism.
Consider the corn ears in the last activity. You worked with two alleles for corn
color—purple and yellow. Each kernel (offspring) received one allele for color from
each parent to make a complete set of two alleles. This genetic makeup for an
organism is its genotype.
295
In the simplest cases, there are two types of alleles, and the gene can produce only
two traits, one of which is dominant over the other. An example of this in Mendel’s
pea plants is flower color. Based on the results of breeding plants with purple
flowers with plants with white flowers, Mendel inferred that a pea plant having
two copies of the allele for the white color trait will have white flowers, while a
plant with two copies of the allele for the purple color will have purple flowers.
However, a pea plant with one allele for the purple color and one allele for the
white color will always have purple flowers. This evidence led to the conclusion
that the purple flower trait is dominant and the white trait is recessive.
For corn kernel alleles, we can designate S for smooth and s for wrinkled. The
allele pair—whether SS, Ss, or ss—is the kernel’s genotype. Genotypes that have
two identical alleles, such as SS or ss, are called homozygous. The prefix homo
means “same.” Genotypes with two different alleles, such as Ss, are referred to as
heterozygous. The prefix hetero means “different.” The kernels with homozygous
recessive alleles will express the recessive phenotype (ss 5 wrinkled kernels), those
with homozygous dominant alleles will express the dominant phenotype (SS 5
smooth kernels), and kernels with heterozygous alleles will express the dominant
phenotype (Ss 5 smooth kernels).
STOPPInGTOTHInkQUeSTIOn3
If P represents the dominant purple allele for corn color and p represents the recessive yellow color allele, what letters would represent a kernel that has the following genotypes:
Homozygous purple?•
Homozygous yellow?•
Heterozygous purple?•
GeneSAndtrAitS • Activity5
S S s s S s
Science & Global iSSueS/bioloGy • GeneticS
296
Since the time of Mendel’s work, scientists have continued to explore patterns of
heredity in organisms. For some traits, the patterns follow the rules of simple
dominance, as Mendel observed. However, some traits are not so simple.
Consider flower color in snapdragons. When red snapdragons are crossed with
white snapdragons, pink flowers result, as shown in the Punnett square below.
This is called incomplete dominance, and it occurs when neither trait is domi-
nant. The result is a blending of the two traits that produces a third trait. In
humans, the trait for curly hair shows a type of incomplete dominance. If a person
inherits one allele for straight hair and one allele for curly hair, he or she will have
the intermediate trait, wavy hair.
STOPPInGTOTHInkQUeSTIOn4
What are the possible phenotypes and genotypes of offspring from a cross between a pink snapdragon and a white snapdragon? What percentage of each phenotype and genotype would you expect to find?
A third type of dominance occurs when more than one trait is dominant, and each
is expressed instead of the two blending into one trait. This is called codominance.
Blood type in humans is a trait that exhibits codominance. Humans who have the
allele for type A red blood cell proteins and the allele for type B red blood cell pro-
teins will have red blood cells that express both proteins, type AB blood. This
means that the traits produced by the A and B alleles are equally dominant, or
codominant. However, there is a third blood trait, type O, which is recessive to
both A and B. The box on the next page explains codominance and human blood
types in more detail.
CR CW
Parent 1
Pare
nt 2
CR CW
CR CW
CR CW
CW
CW
CR CRSNAP DrAgoNS: A cASE of iNcoMPLETE DoMiNANcE
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GeneSAndtrAitS • Activity5
t h e r e a r e t h r e e possible alleles
for human blood type; each person
carries two alleles of the gene,
which may be two of the same, or
any combination of two out of the
three possible alleles.
Figure 1 shows the red blood cells of
a person who has type A blood and
has the genotype IAIA.
Figure 2 shows the red blood cells of
a person who has type A blood and
has the genotype IAi.
Figure 3 shows the red blood cells
of a person who as type B blood
and has the genotype IBIB.
Figure 4 shows the red blood cells
of a person who has type AB blood
and the genotype IAIB.
Figure 5 shows the red blood cells
of a person who has type O blood
and the genotype ii.
human Blood Type Alleles
Allele Codes for phenoType red Blood Cell surfACe proTeins
IA TypeAsurfaceproteinsonredbloodcells
IB TypeBsurfaceproteinsonredbloodcells
i nosurfaceproteinsonredbloodcells
bAckGroundinforMAtion
Codominance of Human Blood Types
1
3
2
4
5note: Cell surface proteins not drawn to scale.
Science & Global iSSueS/bioloGy • GeneticS
298
STOPPInGTOTHInkQUeSTIOn5
If a child has type O blood, what blood types might his or her parents have? Explain.
If a parent with type A blood and the genotype IAi and a parent with B blood and
the genotype IBi have children, four possible phenotypes may result, as shown in
the Punnett square below.
An understanding of genes, alleles, and recessive and dominant traits allows scien-
tists to predict the outcome of many genetic crosses. This information is the basis
for both selective breeding and modern biotechnology research. Some traits are
determined by only one gene, as illustrated in the cases above. A majority of traits
are determined by a combination of many genes. Then again, many traits are also
determined by the interaction of one or more genes with environmental condi-
tions. For example, both genes and nutrition determine size in dogs. A Chihuahua
that is poorly fed will be smaller than a well-fed Chihuahua. However, because of
their genes a Chihuahua cannot be the size of a Great Dane no matter how well it is
fed. There are also genes that control more than one phenotype. For example, if a
gene controls the production of an enzyme needed by multiple organs, one muta-
tion in that gene that changes the enzyme could affect each organ that uses the
enzyme. For most traits, an organism’s phenotype is determined by multiple genes
and a combination of environmental conditions.
Analysis 1. Explain the difference between an organism’s phenotype and its genotype.
Include an example in your answer.
2. Explain the difference between simple dominance, incomplete dominance,
and codominance.
i
IB
IA i
IA IB IB i
i iIA i
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GeneSAndtrAitS • Activity5
3. Think back to the Bt corn you considered in Activity 1, “A Genetically Modi-
fied Solution?” When an organism is genetically modified, which of the fol-
lowing is changed: genotype, phenotype, both, or neither? Explain.
4. The following is a list of a few traits in plants and animals. Determine if the
traits described are examples of simple dominance, codominance, or incom-
plete dominance. Explain your reasoning.
keyvocAbulAry
allele homozygous
codominance incomplete dominance
dominant phenotype
gamete Punnett square
gene recessive
genotype selective breeding
heredity trait
heterozygous
Trait Description Type of dominance and reasoning
Feather color in chickens
The feathers of a species of chicken can be black, white, or “erminette.” Erminette chickens have both black feathers and white feathers, but not gray feathers.
Sweet pea tendrils
When sweet pea plants with tendrils (structures that grow from the stem and help the plant attach and climb) are crossed with sweet pea plants without tendrils, all of the resulting sweet peas have tendrils.
Rabbit hair length
Longhaired rabbits crossed with short haired rabbits produce off-spring that have medium-length hair.