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Ch 11: Introduction to Genetics
11-1 The work of Gregor Mendel
A. Gregor Mendel’s Peas
1. __________________- the study of heredity
2. Gregor Mendel was an Austrian monk. His work was important to the
understanding of heredity.
3. Mendel carried out his work with ordinary garden peas.
4. Mendel knew that
a. the male part of each flower produces pollen, (containing sperm).
b. the female part of the flower produces egg cells.
5. During sexual reproduction, sperm and egg cells join in a process called
_________________.
a. Fertilization produces a new cell.
6. Pea flowers are self-pollinating meaning that sperm cells in pollen fertilize the
egg cells in the same flower.
a. The seeds that are produced by self-pollination inherit all of their
characteristics from the single plant that bore them.
7. Mendel had true-breeding pea plants meaning if they were allowed to selfpollinate they would produce offspring identical to themselves.
8. Mendel wanted to produce seeds by joining male and female reproductive cells
from two different plants.
9. Cross-pollination- cutting away the pollen-bearing male parts of the plant and
dusting the plant’s flower with pollen from another plant.
10.Mendel was able to produce seeds that had two different parents.
B. Genes and Dominance
1. _______- a specific characteristic that varies from one individual to another.
2. Mendel studied seven pea plant traits, each with two contrasting characters.
a. Seed shape, seed color, seed coat color, pod shape, pod color, flower
position, and plant height.
3. He crossed plants with each of the seven contrasting characters and studied their
offspring.
4. Each original pair of plants is the P (parental) generation.
5. The offspring are called the F1, or “first filial,” generation.
a. ________- the offspring of crosses between parents with different traits
6. The F1 hybrid plants all had the character of only one of the parents.
7. Mendel's first conclusion: biological inheritance is determined by factors that are
passed from one generation to the next.
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8. Today, scientists call the factors that determine traits genes.
9. Each of the traits Mendel studied was controlled by one gene that occurred in two
contrasting forms that produced different characters for each trait.
10._______________- the different forms of a gene
11.Mendel’s second conclusion: called the principle of dominance
12.Principle of dominance- some alleles are dominant and others are recessive.
13.An organism with a dominant allele for a trait will always exhibit that form of the
trait.
14.An organism with the recessive allele for a trait will exhibit that form only when
the dominant allele for that trait is not present.
Activity: Dominance 
On the line provided, fill in the genotype of the offspring. The first one is done for you.
~ What is the dominant shape of a pea pod?
~ What is the recessive color of a pea plant’s coat?
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C. Segregation
1. Mendel crossed the F1 generation with itself to produce the F2 (second filial)
generation.
2. The traits controlled by recessive alleles reappeared in one fourth of the F2 plants.
3. Mendel assumed that a dominant allele had masked the corresponding recessive
allele in the F1 generation.
4. The trait controlled by the recessive allele showed up in some of the F2 plants.
5. The reappearance of the trait controlled by the recessive allele indicated that at
some point the allele for shortness had been separated, or segregated, from the
allele for tallness.
6. Mendel suggested
Complete the diagram to show how alleles segregate…
that the alleles for
tallness and
shortness in the F1
plants segregated
from each other
during the formation
of the sex cells, or
gametes
7. When each F1 plant
flowers and
produces gametes,
the two alleles
segregate from each
other so that each
gamete carries only
a single copy of
each gene.
8. Therefore, each F1
plant produces two
types of gametes—
those with the allele
for tallness, and those with the allele for shortness.
9. Alleles separate during gamete formation.
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11-2 Probability and Punnett Squares
A. Genetics and Probability
1. Probability- likelihood that a particular event will occur
2. The principles of probability can be used to predict the outcomes of genetic
crosses.
B. Punnett Squares
1. The gene combinations that might result from a genetic cross can be determined
by drawing a diagram known as a Punnett square.
2. Punnett squares can be used to predict and compare the genetic variations that
will result from a cross.
3. Capital letter represents the dominant allele
a. Ex: tall = _____
4. Lowercase letter represents the recessive allele
a. Ex: short = ____
5. Homozygous- organisms that have two identical alleles for a particular trait
a. Also called “true-breeding” or “purebred”
i. Ex: Homozygous dominant = ____ (tall)
ii. Ex: Homozygous recessive= ____ (short)
6. Heterozygous- organisms that have two different alleles for the same trait
a. Also called “hybrid”
i. Ex: Heterozygous= ____ (tall)
7. All of the tall plants have the same phenotype, or physical characteristics.
a. A phenotype is what you see with your eyes… the outward appearance
b. Example of phenotypes  ________or_________.
8. The tall plants do not have the same genotype, or genetic makeup.
a. Example of genotypes  ______,_______,________.
Activity: Homozygous or Heterozygous? 
- color the homozygous recessive plant yellow
- color the homozygous
dominant plant red
- color the heterozygous plant
orange
- in the space provided, fill in
the missing genotype
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C. Probability and Segregation
1. Cross TT x TT
a. Probability 4:0 = 4 tall: 0 short
2. Cross tt x tt
a. Probability  0:4 = 0 tall: 4 short
3. Cross Tt x Tt
a. Probability  3:1 = 3 tall: 1 short
D. Probabilities Predict Averages
1. Probabilities predict the average outcome of a large number of events.
2. Probability cannot predict the precise outcome of an individual event.
3. In genetics, the larger the number of offspring, the closer the resulting numbers
will get to expected values.
Activity: F2 Generation Punnett Square 
~ Write the phenotype of the
three genotypes shown
TT _________
Tt ___________ Tt
___________
~ If two heterozygous
plants create four offspring,
how many do you predict
would be tall? And short?
- Tall =
- Short=
11–3 Exploring Mendelian
Genetics
A. Independent Assortment
1. To determine if the
segregation of one
pair of alleles affects
the segregation of
another pair of alleles,
Mendel performed a two-factor cross.
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2. The Two-Factor Cross: F1
a. Mendel crossed true-breeding plants that produced round yellow peas
(genotype RRYY) with true-breeding plants that produced wrinkled green
peas (genotype rryy).
b. All of the F1 offspring produced round yellow peas (RrYy).
c. The alleles for round (R) and yellow (Y) are dominant over the alleles for
wrinkled (r) and green (y).
3. The Two-Factor Cross: F2
a. Mendel crossed the heterozygous F1 plants (RrYy) with each other to
determine if the alleles would segregate from each other in the F2
generation.
b. RrYy × RrYy
c. The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation
4. In Mendel’s experiment, the F2 generation produced the following:
a. some seeds that were round and yellow
b. some seeds that were wrinkled and green
c. some seeds that were round and green
d. some seeds that were wrinkled and yellow
5. The alleles for seed shape segregated independently of those for seed color. This
principle is known as independent assortment.
6. Genes that segregate independently do not influence each other's inheritance.
7. Mendel's experimental results were very close to the 9 : 3 : 3 : 1 ratio predicted by
the Punnett square.
8. Mendel had discovered the principle of independent assortment.
9. The principle of independent assortment states that genes for different traits can
segregate independently during the formation of gametes.
10.Independent assortment helps account for the many genetic variations observed in
plants, animals, and other organisms
B. A Summary of Mendel's Principles
1. Genes are passed from parents to their offspring.
2. If two or more forms (alleles) of the gene for a single trait exist, some forms of
the gene may be dominant and others may be recessive.
3. In most sexually reproducing organisms, each adult has two copies of each gene.
These genes are segregated from each other when gametes are formed. The
alleles for different genes usually segregate independently of one another.
4. Some alleles are neither dominant nor recessive, and many traits are controlled by
multiple alleles or multiple genes.
C. Incomplete Dominance
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D.
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1. ________________- when one allele is not completely dominant over another
2. In incomplete dominance, the heterozygous phenotype is between the two
homozygous phenotypes
a. Ex: A cross between red (RR) and white (WW) flowered plants produces
pink-colored flowers (RW).
Codominance
1. In codominance, both alleles contribute to the phenotype.
2. In certain varieties of chicken, the allele for black feathers is codominant with the
allele for white feathers.
3. Heterozygous chickens are speckled with both black and white feathers.
a. The black and white colors do not blend to form a new color, but appear
separately.
Multiple Alleles
1. ______________- genes that are controlled by more than two alleles
2. An individual can’t have more than two alleles. However, more than two possible
alleles can exist in a population.
3. A rabbit's coat color is determined by a single gene that has at least four different
alleles.
Polygenic Traits
1. ______________- traits controlled by two or more genes
2. Skin color in humans is a polygenic trait controlled by more than four different
genes.
Applying Mendel's Principles
1. Thomas Hunt Morgan used fruit flies (drosophila) to advance the study of
genetics.
2. Morgan and others tested Mendel’s principles and learned that they applied to
other organisms as well as plants.
3. Mendel’s principles can be used to study inheritance of human traits and to
calculate the probability of certain traits appearing in the next generation.
Genetics and the Environment
1. Characteristics of any organism are determined by the interaction between genes
and the environment
a. Ex: genes affect a sunflowers height and color. But the environment (soil,
climate, amount of water, pH, etc.) also influence the characteristics of the
plant.
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11-4 Meiosis
A. Introduction
1. Each organism must inherit a single copy of every gene from each of its
“parents.”
2. Gametes are formed by a process that separates the two sets of genes so that each
gamete ends up with just one set.
3. _____________ = sex cell (sperm & egg)
4. _____________ = body cell (skin cell, liver cell, etc.)
B. Chromosome number
1. All organisms have different numbers of chromosomes.
a. A body cell in an adult fruit fly has 8 chromosomes:
i. 4 from the fruit fly's male parent
ii. 4 from its female parent.
b. These two sets of chromosomes are homologous.
c. Each of the 4 chromosomes that came from the male parent has a
corresponding chromosome from the female parent.
2. Diploid- a cell that contains both sets of homologous chromosomes
d. The number of chromosomes in a diploid cell is sometimes represented by
the symbol 2N.
i. For Drosophila, the diploid number is 8, which can be written as
2N=8.
3. The gametes of sexually reproducing organisms contain only a single set of
chromosomes, and therefore only a single set of genes.
a. These cells are haploid.
b. Haploid cells are represented by the symbol N.
i. For Drosophila, the haploid number is 4, which can be written as
N=4.
C. Phases of Meiosis
1. Meiosis is a process of reduction division in which the number of chromosomes
per cell is cut in half through the separation of homologous chromosomes in a
diploid cell.
2. Meiosis involves two divisions, meiosis I and meiosis II.
3. Prior to meiosis I, each chromosome is replicated
4. ___________- each chromosome pairs with its corresponding homologous
chromosome to form a tetrad.
a. There are 4 chromatids in a tetrad
b. The tetrads exchange portions of their chromatids in a process called
crossing-over
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c. Crossing-over produces new combinations of alleles.
5. After this Metaphase I, Anaphase I, Telophase I and cytokinesis follow
a. The fibers pull the homologous chromosomes toward opposite ends of the
cell.
b. Nuclear membranes form.
c. The cell separates into two cells.
6. The two cells produced by meiosis I have chromosomes and alleles that are
different from each other and from the diploid cell that entered meiosis I.
7. Meiosis II
a. The two cells produced by meiosis I now enter a second meiotic division.
b. Unlike meiosis I, neither cell goes through chromosome replication.
c. Each of the cell’s chromosomes has 2 chromatids.
d. Meiosis I results in two haploid (N) daughter cells, each with half the
number of chromosomes as the original cell.
e. The chromosomes line up in the center of cell.
f. The sister chromatids separate and move toward opposite ends of the cell.
g. Meiosis II results in four haploid (N) daughter cells
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Activity: Meiosis 
Look at the labeled pair of homologous chromosomes in the original cell.
- color one chromosome red
- Color one chromosome blue
Follow these two chromosomes through meiosis. Show this by coloring the correct
structures in the cells that result from meiosis I and II
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D. Gamete Formation
1. In male animals, meiosis results in four equal-sized gametes called sperm.
2. In many female animals, only one egg results from meiosis.
a. The other three cells, called polar bodies, are usually not involved in
reproduction.
E. Comparing Mitosis and Meiosis
1. Mitosis results in the production of two genetically identical diploid cells.
2. Meiosis produces four genetically different haploid cells.
3. Mitosis
a. Cells produced by mitosis have the same number of chromosomes and
alleles as the original cell.
b. Mitosis allows an organism to grow and replace cells.
c. Some organisms reproduce asexually by mitosis.
4. Meiosis
a. Cells produced by meiosis have half the number of chromosomes as the
parent cell.
b. These cells are genetically different from the diploid cell and from each
other.
c. Meiosis is how sexually-reproducing organisms produce gametes
11-5 Linkage and Gene Maps
A. Gene Linkage
1. Thomas Hunt Morgan’s research on fruit flies led him to the principle of linkage.
2. Morgan discovered that many of the more than 50 Drosophila genes he had
identified appeared to be “linked” together.
3. They seemed to violate the principle of independent assortment.
4. Morgan and his associates grouped the linked genes into four linkage groups.
5. Each linkage group assorted independently but all the genes in one group were
inherited together.
6. Each chromosome is actually a group of linked genes.
7. Morgan concluded that Mendel’s principle of independent assortment still holds
true.
8. Chromosomes assort independently, not individual genes.
B. Gene Maps
1. Crossing-over during meiosis sometimes separates genes that had been on the
same chromosomes onto homologous chromosomes.
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2. Crossover events occasionally separate and exchange linked genes and
produce new combinations of alleles.
3. Alfred Sturtevant, a student of Morgan, reasoned that the farther apart two
genes were, the more likely they were to be separated by a crossover in
meiosis.
4. Recombination frequencies can be used to determine the distance between
genes.
5. Sturtevant created a gene map showing the relative locations of each known
gene on one of the Drosophila chromosomes.
6. If two genes are close together, the recombination frequency between them
should be low since crossovers are rare.
7. If they are far apart, recombination rates between them should be high.
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