Download Inheritance

Biology
Patterns of Inheritance
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Key Terms & Scientists
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Genetics
Traits
Blending hypothesis
Mendel
Self-fertilization (true breeding)
Hybrid
Cross pollination
Monohybrid/Dihybrid
Parental generation
First/second filial (generation)
Genes/alleles
Dominant/recessive
Homozygous & heterozygous
Law of Segregation & Law of
Independent Assortment
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Probability
Punnett Square
Testcross
Complete & incomplete
dominance
Codominance
Intermediate inheritance
Polygenic traits
Multiple alleles
Pleiotropy
Chromosomal Theory of
Inheritance
Linked & sex-linked genes
Sex Chromosomes
Autosomes
Inheritance

Genetics is the scientific study of
heredity.

A trait is a characteristic that is
passed from parent to offspring
(ex. Eye color).
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The blending hypothesis was once
believed to be the way traits were
inherited from generation to
generation. Think mixing paints. This
is the idea that each generation
is a mix (or blend) of both
parents genes (traits). This does
not account for the appearance
of unexpected traits.

Traits are passed to offspring
through chromosomes.
Genes
DON’T
Mix!
http://www.google.com/imgres
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Gregor Mendel, an
Austrian Monk, (1860’s)
studied the pea plant.

He knew nothing of
molecular biology (or
chromosomes).
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He did NOT support the
blending hypothesis, and
in fact, disproved it
through his studies.

He is the father of
genetics.
http://mendel.imp.ac.at/mendeljsp/images/mendel3.jpg
Mendel used the pea plant for 3 reasons:
1. The structure of the pea flowers allowed:
 self fertilization (which means the plant can breed with itself, a
process called pure breeding)
 OR he could cross pollinate the flowers and produce a hybrid (this
is an organism that receives different forms of a genetic trait
from each parent, or 2 sets of DNA: 1 from each parent).
2. The rapid reproduction cycle: the pea plant reproduces about
every 90 days.
http://www.google.com/imgres
3. The presence of distinctive traits allowed Mendel to
observe his results easily. He studied 7 traits (we will
look at 5). Traits in the pea plant have only 2 forms
(there is NO intermediate or in between form; it is
either/or):
Purple (P) vs. white (p)= flower color
Yellow (Y) vs. green (y)= pea color
Round (R ) vs. wrinkled (r )= pea shape
Green (G) vs. yellow (g)= pod color
Tall (T) vs. short (t)= height
http://www.google.com/imgres
Mendel’s Observations:
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When Mendel worked
with the pea plants he
used 2 different
groups of purebred
plants, looking at 1
trait at a time.
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For example, he used 1
group of purebred
purple flower pea
plants & 1 group of
purebred white flower
pea plants.
http://www.google.com/imgres
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He crossed these 2
groups with each other
(cross pollinated them)
and called them the
parental generation, or
P.
◦ This is a monohybrid cross
(crossing 1 trait).
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All of the offspring had
purple flowers.
◦ This generation did not
show up as a blend of
parents (no mix b/c they
are not less purple). But,
where did the white
flower trait go?
http://www.google.com/imgres
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He called this generation of offspring the
First Filial or F1 generation (filial refers to
offspring).
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The offspring is a hybrid of the parents.
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He allowed the F1 generation to selffertilize. He called this generation the
second filial, or F2 generation.
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The F2 offspring revealed 3 out of 4 had
purple flowers and 1 out of 4 had white
flowers. Again, no blending resulted.
Also, the white flower trait had NOT
disappeared.
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Mendel performed this experiment with
all 7 traits and received the same results:
the offspring is not a mixture of the
parents; the original traits do not
disappear.
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In his work, all F1 revealed 1 characteristic:
this characteristic is dominant. All F2
generations were in a 3:1 ratio (3
dominant: 1 recessive).
MONOHYBRID CROSS, etc.
https://www.youtube.com/watch?v=i0rSv6oxSY
F1 generation
F2
generation
http://wps.prenhall.com/wps/media/objects/487/498795/CDA10_1.jpg
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Genes are sections of a chromosome that code for a
trait.
◦ Most organisms have 2 copies for every gene and chromosome
(1 from each parent).
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An allele is a distinct form of a gene.
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◦ If an organism has 2 different alleles for 1 trait, only 1 allele is
expressed or visible (usually).
http://www.google.com/imgres
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The dominant allele is a form of a gene
that is fully expressed when 2 different
alleles are present.
◦ This is represented with a capital letter
(and is written 1st).
◦ Ex. Purple= P
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The recessive allele is a form of a gene
that is not expressed when paired with
a dominant allele (it takes 2 recessives
to be expressed).
◦ This is represented by a lower case letter &
is written 2nd.
◦ Ex. White= p
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The Chromosome Theory of Heredity (developed by
Walter Sutton) states that the material of
inheritance is carried by the genes in the
chromosomes.
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A genotype is the genetic makeup of an organism.
Ex: GG, Gg, gg or BB, Bb, or bb
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A phenotype is the physical expression of the
genotype or the outward expression of that trait.
Ex: yellow peas.
http://www.google.com/imgres
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Homozygous is having 2
of the same alleles (2
identical alleles). Ex: GG
or gg
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Heterozygous is having 2
different alleles. Ex: Gg
Mendel’s Laws:
These are the Rules of inheritance:
1. The Law of Segregation:
Gene pairs separate when gametes form. This
means: genes (alleles) are on chromosomes;
chromosomes separate during meiosis; gametes form
during meiosis; therefore, genes separate when
gametes form.
2. The Law of Independent Assortment:
When looking at 2 traits at the same time, it is
seen that traits are inherited independently from
each other. Gene pairs segregate into gametes
randomly and independently of each other.
Genetics & Predictions:

In genetics we use mathematical
probability (P). If you flipped a coin what are
the chances of it landing on heads?
◦ P= ½ or 50%
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If you flipped a coin 10X what would you
expect the chances of it landing on heads?
◦ About 5 times or 50% or ½ or 1:1 (ratio)
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In science, we generally use the ratio.
A punnett square is used to organize
& predict genetic information.
 Let’s use Mendel’s purebred purple
flowers & purebred white flowers:
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PP X pp
Always show the cross
Set up square
Genotype= 4Pp
Phenotype= All Purple
Always use ratios!
Use WHOLE #s (no
fractions)!
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Let’s cross the F1 generation.
Pp X Pp
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Genotype= 1PP: 2Pp: 1pp
Phenotype= 3 purple: 1 white
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Now you have some practice problems!
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What happens if we have a purple flower but
we don’t know if it is heterozygous or
homozygous? How would we figure out what
it is?
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We would perform a testcross. This is a cross
between a recessive organism (in this case a
white flower because we know the genotype) with
an organism that has an unknown genotype
(the organism that is showing the dominant
phenotype) in an attempt to discover the
genotype of the unknown.
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If the offspring result in a recessive organism
then the unknown parent must be
heterozygous.
Variations in Inheritance:
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Complete dominance is what Mendel saw. One trait is
completely dominant (expressed) over another.
Either/or; dominant or recessive. Purple flowers or
white flowers.
Intermediate Inheritance:
 Not all genes are cut and dry; one allele is not always
clearly dominant over another & there are not always
just 2 distinct forms in nature.
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Intermediate inheritance is when the heterozygous
offspring has its own trait (different than either
parent). This is not seen in pea plants. This includes
codominance & incomplete dominance.
Incomplete dominance, etc.
https://www.youtube.com/watch?v=YJHGfb
W55l0
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Incomplete dominance is
when there is a
heterozygote BUT neither
the dominant or recessive
allele is completely
expressed. Look at
snapdragons.
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A red snapdragon (RR) is
crossed with a white
snapdragon (rr).
◦ As you would expect, the F1
generation is Rr BUT they are
not Red, they are PINK!
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This almost looks like the
blending hypothesis, right?
But it is not. Why??
http://www.nkellogg.com/codominance.gif
Allow the F1 generation to selffertilize.
Rr
X
Rr
The genotypic results are 1RR:
2Rr: 1rr
The phenotypic results are
1 red: 2 pink: 1 white
http://fig.cox.miami.edu/~cmallery/150/m
endel/c14x9incomplete-dominance2.jpg
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The original traits are NOT
lost; therefore this is NOT
the blending hypothesis.
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An example of incomplete
dominance in humans is
hypercholesterolemia (having
too much cholesterol in the
blood).
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Codominance is seen when there are more than 2
alleles for 1 trait and 2 different dominant alleles are
together but neither dominant alleles overpower the
other.
◦ This is seen in human blood types.
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There are 4 blood types in humans: type A, type B,
type AB, and type O. These are phenotypes!
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Alleles for blood types in humans are represented
with the letter I.
◦ IA represents A, IB represents B, and i represents O.
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Codominance is human blood types is phenotypically
represented by type AB and genotypically
represented by IAIB.
http://science.uniserve.edu.au/mirror/biolproject/mende
lian_genetics/problem_sets/monohybrid_Cross/graphic
s/12T.gif
http://www.biologycorner.com/resources/bloodtype_chart.gif
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Polygenic traits are when traits are affected by
more than 1 gene.
◦ Eye color, hair color & skin color are
examples of polygenic traits.
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Multiple alleles are when there are more than
2 alleles per trait.
◦ Again human blood types are examples.
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Pleiotropy is when 1 gene
affects more than 1 trait. An
example of this is sickle cell
anemia or sickle cell disease.
This affects the shape of red
blood cells (RBCs).

RBCs are normally round.
◦ In sickle cell anemia, they are
crescent-moon shaped (sickle
shaped).
◦ This blocks normal blood flow
through blood vessels causing
circulatory system damage,
weakness, anemia, brain
damage & other organ damage.
Chromosomal Theory of Inheritance
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Specific genes are located on specific
chromosomes, or have loci
http://www.anselm.edu/homepage/jpitocch/genbio/locus.JPG
Genetic Linkage

Genetic linkage (or linked genes)
genes that are located on the
same chromosome.
◦ Generally, these genes will be inherited
together.
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The closer these genes are on a
chromosome, the higher the
chances are that they will be
inherited together.
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Thomas Morgan worked with
fruit flies (Drosophila
melanogaster) and discovered
linked genes.
Sex-Linked Traits

Sex chromosomes determine the sex of the organism. In
humans, XX is female; XY is male.
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Autosomes are non-sex chromosomes.
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Sex-linked traits are genes that are located on the X or Y
chromosomes. There are more genes on the X than the
Y.
Sex-linked Traits in Humans:
 Colorblindness is recessive and
found on the X chromosome.
◦ This is when someone cannot see red
or green.
◦ More males suffer from this than
females.
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Hemophilia is recessive and Xlinked also.
◦ This causes excessive bleeding and no
normal blood clotting.
◦ More males suffer from this than
females.
http://healthresources.caremark.com/Imagebank/Articles_images/Hemophilia_02.gif
FYI: Environmental Effects:
External & internal environmental conditions
can affect genetic expression.
Some examples:
 Environmental temperature affects the
Himalayan rabbit’s fur coat & the western
white butterfly’s wing coloration for flight.
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Soil acidity affects the color of hydrangeas
(acidic=blue; neutral=pink)
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Japanese Goby fish changes sex in response to
social environment
Nature vs Nuture:
 Study of identical twins that were separated at
birth & brought up differently revealed that
there are genetic links between individuals.
The results of the studies revealed that these
twins had similar likes, dislikes, opinions, etc.