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Genetics
Chapter 11
Basic Terms
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Trait – an inheritable physical characteristic
 May be internal or external
 Ex: Eye color, hair color, blood type, personality
Gene - are chemical factors (Segments of DNA) that code for physical traits
Allele - Different versions of the genes that may be inherited
 2 alleles are inherited for each trait; one from mom and one from dad
 Some alleles are Dominant and some are Recessive
 Dominant alleles are represented by a CAPITAL LETTER
 Recessive alleles are represented by a lowercase letter
 Ex: Brown hair color is dominant over blonde hair color; if one of each
allele is inherited, only the brown hair color will be seen
Phenotype – The physical appearance of the trait
Genotype – The genetic description of the trait
 Homozygous Dominant – 2 dominant alleles were inherited
 Homozygous Recessive – 2 recessive alleles were inherited
 Heterozygous – 1 dominant and 1 recessive allele were inherited
Gregor Mendel
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Austrian monk who did his work in a monastery
during the mid-1800’s
Fertilization is the process of male and female
reproductive cells joining to form a new
organism
Mendel became interested in knowing why some
pea plants had white flowers (a trait) and some
had purple…or, why some pea plants were tall
and others short
He fertilized his pea plants and examined the
traits of the offspring which later became the
basis for modern genetics
More Mendel
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Mendel started his experiments by looking at
flower color in pea plants
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Flower color is an example of a Trait
He “self-fertilized” (a form of asexual
reproduction) the purple flower plants until
ALL of the offspring continuously produced
purple flowers.
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These purple flower plants were True breeding
Their genotype was Homozygous Dominant (PP)
Mendel Cont.
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Next, Mendel self-fertilized plants that had
white flowers, until all the offspring produced
had white flowers
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These offspring were true breeding for white
flowers
The white flower plants were Homozygous
recessive (pp)
He then used a sample of the true breeding
white flowers and the true breeding purple
flowers as the “parent generation” in his
experiments
The Laws of Genetics
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Law of Dominance:
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Law of Segregation:
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The dominant allele will always show in the phenotype
unless it is not present.
In other words, the dominant allele masks the presence of
a recessive allele
Alleles separate during gamete formation (meiosis)
This provides each sex cell with one copy of a gene
instead of the usual two
Law of Independent Assortment:
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Genes are inherited separately from one another due to
“crossing-over” during meiosis and chromosome
separation during meiosis
True-Breeding Pea Plants
pp =
Homozygous
Recessive
Cross-Fertilization
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Mendel took the pollen
from a purple flower plant
and used it to fertilize a
white flower plant.
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Predict what the offspring
plants looked like
They were all Purple!
Their genotypes were
Heterozygous (Pp)
These Pp plants were the
F1 Generation
PP
PG
All Pp
pp
Cross-Fertilization Again
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Predict what happened
next when Mendel
crossed two
Heterozygous Purple
plants
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(Fertilized one plant with
the pollen of the other)
75% of the offspring were
Purple, 25% of the
offspring were White
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These offspring were the
F2 generation
Types of Dominance
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Some Dominant alleles are not completely
dominant
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These genes only partially mask the recessive allele if it
is present
Therefore, these alleles may show Incomplete
Dominance
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Ex: A pink flower results from a cross between a red flower
and white flower
Some genes may combine 2 dominant alleles
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These phenotypes show up together at the same time
Therefore, these alleles are Codominant
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Ex: A Black and white cow results when a black bull is mated
with a white cow
Examples of Incomplete Dom.
And Codom.
Incomplete Dominance:
RR
Codominance
rr
Rr
Exceptions to the Rule
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Multiple Alleles:
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Some genes have multiple alleles
EX: Blood type alleles
 IA
 IB
 i
Both IA and IB are Dominant alleles whereas “i” is a
recessive allele
Not only are there 3 possible alleles (instead of the
usual 2 alleles for most traits), but IA and IB can show
codominance
Blood Type Phenotypes
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There are six possible genotypes of blood
type
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ii, IAi, IBi, IAIA, IBIB, IAIB
These lead to 4 possible phenotypes
A, B, AB or O
Side notes:
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Type O is the Universal Donor, this means
anyone can receive this blood in transfusion,
however, Type O patients can ONLY receive
Type O blood
Type AB is the Universal Recipient, this means
they can receive any blood type in transfusion
Polygenic Traits
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Some traits are affected
by many genes at the
same time
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These are called
Polygenic Traits
Ex: Height, Skin Color, Eye
Color
Allows for huge variation in
a poplutation
Graphs of these traits for a
given population result in a
Bell Curve
Meiosis
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Meiosis is the process of making gametes
The result of meiosis are 4 sex cell with HALF the
normal amount of chromosomes for that organism
A normal number of chromosomes = 2N (Diploid)
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Half the normal amount (found in sex cells) = N
(Haploid)
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In humans, there are 23 PAIRS of chromosomes or 46 in
total
Each chromosome in a pair has the SAME genes, one
came from mom and the other from dad
These chromosomes are HOMOLOGOUS
In humans, this would be 23 individual chromosomes
This allows two gametes (sperm and egg) to combine
and form a NEW cell with the complete number of
chromosomes
Meiosis Continued
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During Prophase I of
meiosis, Homologous
Pairs join together to form
Tetrads
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Remember, one
chromosome from each
parent with the same genes
Chromosomes in each
tetrad then exchange small
segments of DNA
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This process is called
“Crossing Over”
Results in new trait
combination
Allows for genes to be
inherited separately from
one another (Law of
Independent Assortment)
Process of Meiosis
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Meiosis is broken up into two stages
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Meiosis I and Meiosis II
During Meiosis one:
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Prophase I: Same as mitosis, however, chromosomes form
“tetrads”
Metaphase I: Tetrads line up in the middle of the cell (23
pairs line up in the middle)
Anaphase I: Tetrads are ripped apart allowing one
chromosome from each homologous pair to move to
opposite sides of the cell
Telophase I, Cytokinesis I: Same as mitosis
Process of Meiosis Cont.
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Meiosis II
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THERE IS NO INTERPHASE BETWEEN MEIOSIS I and
MEIOSIS II
Prophase II: Same as mitosis prophase
Metaphase II: The 23 chromosome “x”s line up in the
middle of the cell
Anaphase II: The 23 chromosomes are ripped in half
allowing 23 chromatids to move to each side of the cell
Telophase II and Cytokinesis II: Same as usual, however,
at the end, the daughter cells have 23 individual
chromosomes
http://www.youtube.com/watch?v=D1_mQS_FZ0&feature=PlayList&p=D2A007AC2BF1878B&pla
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