genes and traits i - polytech high school · genes and traits i f you examine a family photograph...

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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).

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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

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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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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.•


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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.


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.

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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).


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?•


S S s s S s


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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.


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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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.


of a person who has type AB blood

and the genotype IAIB.


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.


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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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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.

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