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Analyzing Inheritance

Offspring resemble their parents. Offspring inherit genes for characteristics from their parents. To learn about inheritance, scientists have experimented with breeding various plants and animals.

In each experiment shown in the table on the next slide, two pea plants with different characteristics were bred. Then, the offspring produced were bred to produce a second generation of offspring. Consider the data and answer the questions that follow.

Section 11-1

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1. In the first generation of each experiment, how do the characteristics of the offspring compare to the parents’ characteristics?

2. How do the characteristics of the second generation compare to the characteristics of the first generation?

Section 11-1

Parents

Long stems short stems

Red flowers white flowers

Green pods yellow pods

Round seeds wrinkled seeds

Yellow seeds green seeds

First Generation

All long

All red

All green

All round

All yellow

Second Generation

787 long: 277 short

705 red: 224 white

428 green: 152 yellow

5474 round: 1850 wrinkled

6022 yellow: 2001 green

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11–1 The Work of MendelA. Gregor Mendel’s Peas

B. Genes and Dominance

C. Segregation

1. The F1 Cross

2. Explaining the F1 Cross

Section 11-1

Section Outline

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11–1 The Work of Mendel• Gregor Mendel (1822-1884) was a monk at a

monastery in Brunn, Austria • He taught school and did other chores, but in

his spare time he set out to study plant breeding

• He started breeding pea plants about 1855 • He wanted to learn the rules of heredity so that

better varieties could be produced

Section 11-1

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11–1 cont.• Mendel’s original pea plants were true-breeding.

– When allowed to self-pollinate they produced offspring identical to themselves

• Mendel wanted to cross-pollinate the plants and study the results

• Mendel studied seven different pea plant traits (specific characteristic that varies from individual to individual) with 2 contrasting characteristics

Section 11-1

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Seed Shape

Flower Position

Seed CoatColor

Seed Color

Pod Color

Plant Height

PodShape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

Section 11-1

Figure 11-3 Mendel’s Seven F1 Crosses on Pea Plants

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11–1 cont.• Mendel called each original pair of plants

the

P Generation (parental) and the offspring of these the F1 generation (first filial)

• Mendel produced hybrids when he crossed parents with different traits; and to his surprise the offspring only resembled one of the parents

Section 11-1

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P Generation F1 Generation F2 Generation

Tall Short Tall TallTall Tall Tall Short

Section 11-1

Principles of Dominance

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P Generation F1 Generation F2 Generation

Tall Short Tall TallTall Tall Tall Short

Section 11-1

Principles of Dominance

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11–1 cont.• Mendel drew 2 conclusions from this

experiment:

1.Biological inheritance is determined by factors that are passed from one generation to the next; today known as genes

2.Principle of Dominance which states that some alleles (different forms of a gene) are dominant and others are recessive

Section 11-1

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11–1 cont.• Dominant alleles will always exhibit that form

of the trait and is represented with a capital letter

ex. (T) - Tall• Recessive alleles will only exhibit that form of

the trait if the dominant allele is not present and is represented with a lower case letter

ex. (t) - short

Section 11-1

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11–1 cont.• Mendel then allowed his F1 hybrid plants to

self-pollinate to produce an F2 generation (second filial) – Traits controlled by the recessive alleles

reappeared in ¼ of the F2 generation– Mendel explained that the dominant allele

masked the recessive allele in the F1 generation and due to the segregation of alleles during gamete formation reappeared in the F2 generation

Section 11-1

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P Generation F1 Generation F2 Generation

Tall Short Tall TallTall Tall Tall Short

Section 11-1

Principles of Dominance

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Section 11-1

T t

T t

F1 Generatio

n

T t T tGametes

T T T t T t t tF2

Generation

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Tossing Coins

If you toss a coin, what is the probability of getting heads? Tails? If you toss a coin 10 times, how many heads and how many tails would you expect to get? Working with a partner, have one person toss a coin ten times while the other person tallies the results on a sheet of paper. Then, switch tasks to produce a separate tally of the second set of 10 tosses.

Section 11-2

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1. Assuming that you expect 5 heads and 5 tails in 10 tosses, how do the results of your tosses compare? How about the results of your partner’s tosses? How close was each set of results to what was expected?

2. Add your results to those of your partner to produce a total of 20 tosses. Assuming that you expect 10 heads and 10 tails in 20 tosses, how close are these results to what was expected?

3. If you compiled the results for the whole class, what results would you expect?

4. How do the expected results differ from the observed results?

Section 11-2

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11–2 Probability and Punnett SquaresA. Genetics and Probability

B. Punnett Squares

C. Probability and Segregation

D. Probabilities Predict Averages

Section 11-2

Section Outline

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11–2 Probability and Punnett Squares• Probability is the likelihood that a particular

event will occur and Mendel realized that the principles of probability could be used in predicting the outcomes of genetic crosses

Section 11-2

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11–2 cont.• Punnett Squares can be used to predict and

compare the genetic variations that will result from a cross– The gametes are shown along the top and

left side– Possible gene combinations of the offsping

appear in the boxes

Section 11-2

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Section 11-2

Tt X Tt Cross

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Section 11-2

Tt X Tt Cross

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11–2 cont.• Homozygous organisms are true-breeding

which means they have two identical alleles for a particular trait

Ex. TT• Heterozygous organisms are hybrids that

have two different alleles for the same trait

Ex. Tt

Section 11-2

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11–2 cont.Genotype vs. Phenotype

The genotype is the actual genetic makeup of the traits, while the phenotype is the actual physical characteristics of an organism

Section 11-2

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11–3 Exploring Mendelian GeneticsA. Independent Assortment

1. The Two-Factor Cross: F1 2. The Two-Factor Cross: F2

B. A Summary of Mendel’s Principles

C.Beyond Dominant and Recessive Alleles

1. Incomplete Dominance 2. Codominance

3. Multiple Alleles 4. Polygenic Traits

D.Applying Mendel’s Principles

E. Genetics and the Environment

Section 11-3

Section Outline

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11-3 Exploring Mendelian GeneticsAfter proving that alleles segregate in the formation of gametes, Mendel wanted to know if they did so independently – did the segregation of one pair of alleles affect the segregation of other pairs?

Two-Factor Cross – F1

- Mendel crossed homozygous round yellow peas (RRYY) with wrinkled green peas (rryy).

- F1 were all round yellow (RrYy)

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Two-Factor – F2

- Mendel knew F1’s were heterozygous for both traits

- when allowed to self-fertilize the F1 gave rise to F2’s w/ a

definite ratio

- this led Mendel to his Principle of Independent Assortment

- alleles for different characteristics segregate independently of

each other – so as long as genes for two traits are on

different chromosomes every combination of alleles is possible.

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Section 11-3

Figure 11-10 Independent Assortment in Peas

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concluded that

which is called the

which is called the

GregorMendel

Law ofDominance

Law ofSegregation

Peaplants

“Factors”determine

traits

Some alleles are dominant,

and some alleles are recessive

Alleles are separated during gamete formation

Section 11-3

Concept Map

experimented with

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Incomplete Dominance* when neither allele is dominant over the other

- heterozygotes differ phenotypically from either

homozygote

Ex.: Four O’clock plants

- RR = red

- WW = white

- RW = pink

** this is not blending because red and white

flowers will reappear.

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Section 11-3

Figure 11-11 Incomplete Dominance in Four O’Clock Flowers

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Section 11-3

Figure 11-11 Incomplete Dominance in Four O’Clock Flowers

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Codominance* situation in which both alleles contribute to the phenotype

- both are expressed if present

Ex.: chickens – white and black feathers are codominant

- heterozygotes produce both white and black feathers

** A and B alleles in human blood types

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Multiple Alleles* when a gene has more than two alleles

- each individual still only gets two of the possible alleles that exist in the population

Ex.: Human blood groups

- three alleles = A, B, O

- A and B are codominant

- O is recessive

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Polygenic Traits* traits which are influenced by several genes

- results in a wide range of phenotypes

Ex.: Human height, skin color, color in eyes of fruitflies

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Height in Humans

Height in pea plants is controlled by one of two alleles; the allele for a tall plant is the dominant allele, while the allele for a short plant is the recessive one. What about people? Are the factors that determine height more complicated in humans?

Section 11-3

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1. Make a list of 10 adults whom you know. Next to the name of each adult, write his or her approximate height in feet and inches.

2. What can you observe about the heights of the ten people?

3. Do you think height in humans is controlled by 2 alleles, as it is in pea plants? Explain your answer.

Section 11-3

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Applying Mendel’s Principles* Mendel’s principles apply to all organisms

- Thomas Hunt Morgan – began use of fruitflies

- ideal organism

- fast breeder

- lots of young

- many observable traits

- small size

- Environment also affects development of organisms

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Sex Linkage

- genes located on autosomes behave as we have studied

- genes located on the sex chromosomes often exhibit a unique

pattern of inheritance.

- in mammals females have two X-chromosomes (XX)

- in mammals males have an X and a Y chromosome (XY)

- the Y chromosome has a gene (SRY) that makes the

embryo male

- but Y has fewer genes than X and so males only get one

copy of some genes on the Y

- genes that males only get one copy of are sex-linked and are

expressed more often in males than females

- color-blindness, muscular dystrophy

The latest discoveries in genetics

Interactive test

Articles on genetics

For links on Punnett squares, go to www.SciLinks.org and enter the Web Code as follows: cbn-4112.

For links on Mendelian genetics, go to www.SciLinks.org and enter the Web Code as follows: cbn-4113.

For links on meiosis, go to www.SciLinks.org and enter the Web Code as follows: cbn-4114.

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Interest Grabber Answers

1. In the first generation of each experiment, how do the characteristics of the offspring compare to the parents’ characteristics?

All offspring had the same characteristic, which was like one of the parents’. The other characteristic seemed to have disappeared.

2. How do the characteristics of the second generation compare to the characteristics of the first generation?

Both characteristics appeared in this generation. The characteristic that had “disappeared” in the first generation did not appear as often as the other characteristic. (It appears about 25 percent of the time.)

Interest Grabber Answers

1. Assuming that you expect 5 heads and 5 tails in 10 tosses, how do the results of your tosses compare? How about the results of your partner’s tosses? How close was each set of results to what was expected?Results will vary, but should be close to 5 heads and 5 tails.

2. Add your results to those of your partner to produce a total of 20 tosses. Assuming that you expect 10 heads and 10 tails in 20 tosses, how close are these results to what was expected?The results for 20 tosses may be closer to the predicted 10 heads and 10 tails.

3. If you compiled the results for the whole class, what results would you expect?The results for the entire class should be even closer to the number predicted by the rules of probability.

4. How do the expected results differ from the observed results?The observed results are usually slightly different from the expected results.

Interest Grabber Answers

1. Make a list of 10 adults whom you know. Next to the name of each adult, write his or her approximate height in feet and inches.

Check students’ answers to make sure they are realistic.

2. What can you observe about the heights of the ten people?

Students should notice that there is a range of heights in humans.

3. Do you think height in humans is controlled by 2 alleles, as it is in pea plants? Explain your answer.

No, height does not seem to be controlled by two alleles, as it is in pea plants. Height in humans can vary greatly and is not just found in tall and short phenotypes.

Interest Grabber Answers

1. How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis?

46 chromosomes

2. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Do you think this would create any problems in the developing embryo?

46 + 46 = 92; a developing embryo would not survive if it contained 92 chromosomes.

3. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?

Sperm and egg should each have 23 chromosomes.

Interest Grabber Answers

1. In how many places can crossing over result in genes A and b being on the same chromosome?

One (between A and B)

2. In how many places can crossing over result in genes A and c being on the same chromosome? Genes A and e?

Two (between A and B and A and C); Four (between A and B, A and C, A and D, and A and E)

3. How does the distance between two genes on a chromosome affect the chances that crossing over will recombine those genes?

The farther apart the genes are, the more likely they are to be recombined through crossing over.

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