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11-1 Notes
The works of Gregor Mendel
Inheritance
Every living thing has a set of
characteristics inherited from its
parent or parents.
Heredity holds the key to
understanding what makes each
species unique.
Genetics is the study of heredity.
Gregor Mendel’s Peas
Mendel became a priest and spent
years studying science and math.
He taught for 14 years and worked
in a monastery garden.
This is where Mendel did his work
that changed biological inheritance
forever.
Gregor Mendel’s Peas
 Mendel carried out his work with ordinary
garden peas.
 The male and female reproductive cells
would join and fertilize to produce a new
cell.
 The pea plants are self pollinating in which
the sperm cells in pollen fertilize the egg
cells in the same flower.
 The seeds produced inherited all the
characteristics from the single plant that
bore them.
Gregor Mendel’s Peas
 The peas plant in the monastery were
true-breeding.
 They would only produce offspring
identical to themselves if allowed to selfpollinate.
 Mendel wanted to produce seeds by
joining male and female reproductive cells
from two different plants.
 He cross pollinated the plants by cutting
off the male parts and dusted the pollen
onto another plant.
Genes and Dominance
 Mendel studied seven different pea plant
traits. (color, height, shape)
 He crossed the plants with their
contrasting trait and studied the offspring.
 The original pair of plants were called the
P (parental) generation.
 The offspring were called the F1 (“first
filial”) generation.
 The offspring of crosses with the different
traits are called hybrids.
Genes and Dominance
 When the plants were cross bred the
offspring had the character of only one of
the parents.
 Mendel’s first conclusion was that
inheritance is determined by factors that
are passed from one generation to the
next known as genes.
 The different forms of a gene are called
alleles.
Genes and Dominance
 Mendel’s second conclusion was the
principle of dominance that states that
some alleles are dominant and others are
recessive.
 An organism with a dominant allele for a
specific trait will show it.
 When the dominant allele is not present
the recessive allele trait will appear.
Segregation
 When Mendel allowed the F1 plants to
reproduce by self-pollination, the traits
controlled by recessive alleles reappeared
in about ¼ of the F2 plants in each cross.
P Generation
Tall
Short
F2 Generation
F1 Generation
Tall
Tall
Tall
Tall
Tall
Short
Segregation
 Mendel assumed the dominant allele had
masked the corresponding recessive allele
in the F1 generation.
 However the recessive alleles were only
segregated because they showed up in the
F2 generation.
 Each F1 plant produces two types of sex
cells (gametes) that later paired up in the
second generation.
 One dominant, represented by a capital letter
 One recessive, represented by a lowercase letter
11-2 Notes
Probability and Punnett Squares
Genetics and Probability
 Gregor Mendel realized that the principals
of probability could be used to explain the
results of genetic crosses
 Probability is the likelihood that a specific
event will occur
Genetics and Probability
 There are two important points to
remember with probabilities
 1. Past outcomes do not affect future events
 2. Probabilities predict the average outcome of
many events.
 They do not predict what will happen in a single
event. Therefore, the more trials there are , the
closer the numbers will get to the predicted
values
Punnett Squares
 The gene combinations that might result
from a genetic cross can be determined by
drawing a punnett square
 The types of parents are shown along the
top and left sides of the punnett square
Punnett Squares
 Punnett squares help predict the chances
an offspring will be homozygous or
heterozygous for a trait
 Organisms that have two identical alleles
for a particular trait are called
homozygous
 Ex. TT or tt
Punnett Squares
 Organisms that have two different
alleles for the same trait are called
heterozygous
 Ex. Tt
 The phenotype of an organism shows
physical characteristics.
 Ex. The plant is tall
 The genotype of an organism shows
the genetic makeup of the plant
 Ex. The tall plant could be Tt or TT
Probability and Segreagation
 Mendel’s experiment showed that
 ¼ of the F2 plants have 2
(TT)
 ½ of the F2 plants have 1
and 1 allele for shortness
 ¼ of the F2 plants have 2
shortness (tt)
alleles for tallness
allele for tallness
(Tt)
alleles for
 Overall there are 3 tall plants and one
short plant resulting in a 3:1 ratio
 He concluded that segregation did
occur
11-3 Notes
Exploring Mendelian Genetics
Independent Assortment
 Does the segregation of one pair of alleles
affect the segregation of another pair of
alleles?
 To answer this question, Mendel
performed an experiment to follow two
different genes as they passed from one
generation to the next (2-factor cross).
The Two-Factor Cross:F1
 Mendel crossed true breeding plants that
produced only round yellow peas
(genotype RRYY) with plants that
produced wrinkled green peas (genotype
rryy).
 All of the F1 offspring produced round
yellow peas (genotype RrYy)
 This cross does not indicate whether
genes segregate independently.
The Two-Factor Cross:F1
 The F1 plants were all heterozygous for
both the seed shape and seed color genes.
 How would the alleles segregate when the
F1 plant were crossed to each other to
produce an F2 generation?
The Two-Factor Cross:F1
 In Mendel’s experiment the F2 plants
produced 556 seeds.
 315 seeds were round and yellow
 32 seeds were wrinkled and green
(just like the parental generation)
 209 seeds had a combo of phenotypes
and alleles, not found in either parent.
The Two-Factor Cross:F1
 This clearly meant that the alleles for seed
shape segregated independently of those
for seed color, known as independent
assortment.
 Independent assortment helps account for
the many genetic variations observed in
plants, animals, and other organisms.
Beyond Dominant and Recessive
Alleles
 Not all genes show simple patterns of
dominant and recessive alleles.
 The majority of genes have more than two
alleles.
 Many important traits are controlled by
more than one gene.
Incomplete Dominance
 Cases in which one allele is not completely
dominant over another are called
incomplete dominance.
 EX: When crossing the alleles of a red flower
and a white flower the offspring are pink, a mix
of the parents.
Codominance
 In codominance both alleles contribute to
the phenotype.
 EX: In chickens black and white feathers are
codominant and heterozygous offspring will be
speckled with both.
Multiple Alleles
 Many genes have more than two possible
alleles that exist in a population.
 EX: A rabbit’s coat color is determined by a
single gene that has at least four different
alleles. (C, cch, ch, c)
 EX: Human blood type is another.
Polygenic Traits
 Many traits are produced by the
interaction of several genes called
polygenic traits.
 Often show a wide range of phenotypes.
 EX: The wide range of skin color in humans.
Genetics and the Environment
 The characteristics of any organism are
not determined solely by the genes it
inherits.
 Characteristics are determined by
interaction between genes and the
environment.
11-4 Notes
Meiosis
Chromosome Number
 Offspring have two sets of chromosomes,
one set from each parent
 These chromosomes are homologous
 Each chromosome from the male parent has a
corresponding chromosome from the female
parent
Chromosome Number
 A cell that contains both sets of
chromosomes is considered diploid
 Represented by the symbol 2N
 A cell that contains only one set of
chromosomes is considered haploid
 Represented by the symbol N
Meiosis
 According to Mendel, living things inherit a
single copy of each gene from each of
their parents
 When gametes are formed, these two
copies are separated
 Gametes are made during a process called
meiosis
Meiosis
 Meiosis is a process in which the number
of chromosomes per cell is divided in half
through the separation of homologous
chromosomes in a diploid cell
 Meiosis involves two phases
 At the end one diploid cell will become
four haploid cells
 Meiosis only occurs in reproductive cells
Meiosis I
 Prior to meiosis cells undergo DNA
replication
 Meiosis I
 Prophase I , metaphase I, anaphase I,
Telophase I and cytokinesis occur
 Two cells have formed
 Each cell has sets of chromosomes and alleles
that are different from each other and the
original cell
Meiosis I
 During prophase I each homologous pair
forms a tetrad
 Tetrads are made of four chromatids (two from
each chromosome)
 When tetrads form crossing over occurs.
 This is where they exchange portions of their
chromatids
 This produces new combinations of alleles
Crossing Over
Meiosis II
 The cells divide again, but DNA replication
does not take place
 Four daughter cells are formed
 Each daughter cell contains half the
number of chromosomes as the original
cell
 Each cell is now haploid (N)
Gamete Formation
 In Animals
 Male gametes are sperm
 Female gametes are eggs
Comparing Mitosis and Meiosis
 Mitosis results in the production of two
genetically identical diploid (2N) cells
 Meiosis results in the production of four
genetically different haploid (N) cells