Download October 25, 2012

October 25, 2012
October 25, 2012
3.1 Section Review Questions
1. a) In Mendel’s cross for stem height, what contrasting traits did the pea plants in the P generation exhibit?
Tall and short
b) What trait or traits did the plants in the F1 generation exhibit? When you think of the traits of the parent plants, why is this result surprising?
All of the plants in the F1 generation were tall. You might expect some of the offspring to be short like one of the parent plants.
c) Contrast the offspring of the F1 generation to the offspring in the F2 generation. What did the differences in the F1 and F2 offspring show Mendel?
The F2 generation was 75% tall and 25% short, while the F1 generation was 100% tall. Individual factors, or sets of genetic information, must control the inheritance of traits in peas. These factors exist in pairs. Each parent contributes one factor. And one factor can hide another factor.
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2. a) What is a dominant allele? What is a recessive allele?
Dominant allele: an allele in which a trait always shows up when the allele is present; recessive allele: an allele in which a trait is masked whenever the dominant allele is present.
b) Explain how dominant and recessive alleles for the trait of stem height determine whether a pea plant will be tall or short.
If the plant has two dominant alleles for stem height (TT), it will be tall. If it has one dominant and one recessive allele (Tt), it will be tall. If it has two recessive alleles (tt), it will be short.
c) Can a short pea plant ever be a hybrid for the trait of stem height? Why or why not? As part of your explanation, write the letters that represent the allele’s for stem height of a short pea plant.
No, a short plant has two recessive alleles (tt); hybrids have two different alleles for a trait (Tt). A hybrid would appear tall because the tall allele is dominant.
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Choose a trait and use the symbols for alleles to
diagram the crosses that Mendel made.
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I. Mendel’s Work
A. used pea plants to understand heredity
1. traits are physical characteristics
2. heredity is the passing of traits from parent to off-spring
3. pea plant
a. petals surround pistil and stamen
b. pistil produces female sex cells (egg)
c. stamen produce male sex cell (pollen)
d. pea plant is usually self pollenating
B. Discovered foundation of genetics
C. Experiments
1. Cross pollinated pea plants using purebreds
2. F1 cross tall and short
a. all tall phenotype
b. Tt genotype
3. F2 cross hybrids
a. 3/4 tall; 1/4 short
b. genotype TT, Tt, tt
4. first generation all dominant trait
5. second generation lost or recessive trait appears.
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II. Dominant and Recessive alleles
A. genes/ alleles --factors exist in pairs
1. genes control traits
2. alleles are different forms of genes
3. offspring inherit one allele from each parent
4. dominant alleles show up
5. recessive alleles are hidden when the dominant allele is
present
6. hybrid has two different alleles
B. Symbols
1. dominant = capital letters
2. recessive = lowercase letters
3. hybrid = one capital, one lowercase (capital always first)
C. Significance
1. Traits are determined by individual separate alleles
2. Mendel presented results from experiment to scientific
society in 1866--describing principles of heredity.
3. Mendel's ideas were not recognized until after his death in
1900
4. Mendel is known as the "Father of Genetics"
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Dominant alleles → capital Letter
Recessive alleles → lowercase
Choose the letter of the dominant trait.
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3.2 Probability & Heredity
I. Principles of Probability
A. Mathematics of Probability
1. likelihood an event will occur
2. expressed as a fraction or percent
a. numerator is tested event
b. denominator is total possibilities
3. more tests conducted, closer actual results will be to
predicted probability
B. Independence of Events
1. results of one event do not affect results of next
2. ex. Tossing a coin
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II. Probability of Genetics
A. Punnett Squares
1. shows all possible combinations of alleles that can result
from genetic cross
2. each parent can pass either of its alleles to its offspring
B. Using a Punnett square
1. calculate the probability that offspring with a certain
combination of alleles will result
2. allele that each parent will pass on to its offspring based
on probability
3. dominant allele masks recessive allele
4. If there are more than one way a combination can occur,
it increases the probability
C. Predicting Probabilities
1. can use Punnett square
2. allele distribution can determine probability of inherited
traits
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III. Phenotypes & Genotypes
A. genotype-genetic makeup; allele
combinations
1. homozygous: identical alleles for a trait
2. heterozygous: different alleles for a trait (hybrid)
B. phenotype-physical appearance; visible traits
IV. Codominance
A. alleles are neither dominant nor recessive
B. both alleles are expressed in offspring
C. Written as superscripts
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3.2 Section Review
1. a) What is probability?
A number that describes how likely it is that an event will occur.
b) If you know the parents’ alleles for a trait, how can you use a Punnett
square to predict the probable genotypes of the offspring?
A Punnett square shows all the possible combinations of alleles that
can result from a genetic cross.
c) A pea plant with round seeds has the genotype Rr. You cross this plant with
a wrinkled-seed plant, genotype rr. What is the probability that the
offspring will have wrinkled seeds?
50%; Punnett square should show two Rr and two rr possibilities.
2. a) Define genotype and phenotype.
Genotype: an organism’s genetic makeup; phenotype: an organism’s
physical
appearance.
b) Explain how two organisms can have the same phenotype but two
different genotypes. Give an example.
A heterozygous organism will have the same phenotype as an organism
that is homozygous for a dominant allele. For example, tall pea plants can
be heterozygous (Tt) or homozygous (TT).
c) A pea plant has a tall stem. What are its possible genotypes?
TT or Tt
3. a) What is codominance? Give an example of codominant alleles and explain
why they are codominant.
An inheritance pattern in which alleles for a trait are neither dominant nor
recessive. For example, the alleles for black and white chicken feathers are
codominant because both colors show.
b) What is the phenotype of a chicken with the genotype FBFW?
Black and white feathers.
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3.3 Section Review
1. a) According to Sutton’s observations, how does the number of
chromosomes in a grasshoppers body cells compare to the number in its
sex cells?
Body cells have twice the number of chromosomes (24) as sex cells
(12).
b) Describe what happens to the number of chromosomes when two
grasshopper sex cells join in fertilization.
The fertilized egg gets 24 chromosomes.
c) How do Sutton’s observations about chromosome number support
the chromosome theory of inheritance?
Just as the offspring get one allele from each parent for every gene,
the offspring get half their chromosomes from one parent and half from the
other parent.
2. a) What is meiosis?
The process by which the number of chromosomes is reduced by
half to form sex cells.
b) Briefly describe meiosis I and meiosis II.
Meiosis I: The duplicated chromosomes divide into two cells, each
with half the number of chromosomes. Meiosis II: The two cells divide
once more, producing sex cells that have half as many chromosomes as the
body cells.
c) Use the events of meiosis to exlain why a sex cell normally does not
receive both chromosomes from a pair.
In meiosis I, the members of each chromosome pair separate and end
up in different cells.
3. a) How are genes arranged on a chromosome?
They are joined together like beads on a string.
b) How does the order of genes in one member of a chromosome pair
compare to the order of genes on the other chromosome?
They are lined up in the same order on both chromosomes.