Download Mendelian Genetics

Genetics
Gregor Mendel (1822-1884)
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Austrian Monk, “Father of Genetics”
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Bred Garden Peas (Pisum sativum)
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Developed a simple set of rules to
accurately predict patterns of
heredity which form the basics of
genetics
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Years later we found that traits are
determined by genes encoded in
DNA
Heredity History

Heredity – transmission of traits from parents to
offspring… before DNA was discovered it was one
of the great mysteries of science!

Modeled experiments after British farmer T.A.
Knight who bred garden peas and concluded purple
flowers show a stronger tendency to appear than
white flowers

Mendel used a mathematical approach and counted
the number of each kind of offspring
Why did Mendel choose peas?

Many easily distinguishable characteristics

2 possible traits (forms) of each characteristic
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Quantitative – he could count plants with or with out trait
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P. sativum were small, easy to grow, mature quickly, and
produce lots of offspring
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Pea plants can self-pollinate

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Male (pollen) and Female (pistil) parts are enclosed in the same
flower and it can fertilize itself
Pea plants can cross-pollinate

Transfer pollen from one plant to the pistil of another plant
Anatomy of a flowering plant
Self pollination vs. Cross Pollination
Mendel’s Experimental Design

Parental Generation (P generation):
ensure that ea/plant was true breeding –
all offspring display only one form of the
characteristics for subsequent generations

First Filial Generation (F1 generation):
Mendel cross pollinated 2 plants from P
generation w/ contrasting traits, offspring
called F1 generation

Second Filial Generation (F2
generation): Mendel allowed the F1
generation to self-pollinate, offspring
called the F2 generation
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Mendel then counted his results…
Mendel’s Results
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F1
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F2
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The recessive traits
disappears
The expressed trait is said to
be dominant
The recessive trait
reappears!!
Mendel obtained a 3:1 ratio
of dominant to recessive
for each trait of the F2
generation!
Mendel proposed a Theory of Heredity


Parents pass on “units of information” that operate in
the offspring to produce a trait (today we know these
to be genes!)
For each characteristic there are 2 factors or alleles(1
from mom and 1 from dad) at ea/locus
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Homozygous - if 2 of the same alleles are inherited (truebreeding)
Heterozygous – if 2 different alleles are inherited (hybrid)
Genotype – combination of alleles an individual has
Phenotype – physical appearance as a result of the
alleles inherited
Mendel’s Theory Became Laws of
Heredity

Law of Segregation

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Law of Independent Assortment


The members of each pair of alleles separate when
gametes are formed
Pairs of alleles separate independently of one another
during gamete formation (only applies to genes far apart
on the same chromosome or separate chromosomes)
Mendel published paper in 1866 – no interest,
rediscovered in early 1900’s
Analyzing Heredity

Use letters to represent alleles

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Capital letters represent dominant alleles
Lowercase letters represent recessive alleles
Same letter designates 2 forms of the same trait
(letter of dominant trait)

Ex. Tallness in pea plants

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T = tall dominant allele
t = short recessive allele
Genotype vs. Phenotype

2 alleles for each trait make up genotype
Genotype
Phenotype
Homozygous
dominant
Heterozygous
TT
Tall
Tt
Tall
Homozygous
recessive
tt
Short
Probability


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Probability – likelihood that a specific event
will occur
Probability = # of specific outcome
total # of all possible outcomes
Use this formula to predict the outcome of a
genetic cross
Monohybrid Cross

Monohybrid Cross - provides data about 1 pair of
contrasting traits


Ex. Homozygous tall x homozygous short
Punnett Square – diagram used to predict the probable
outcome of a cross
1.
2.
3.
4.
5.
6.
Write parental cross (genotypes)
Draw box, genotype of 1 parent goes on one side, other parents
genotype on the other side
Fill in the boxes with 1 allele from each parent to indicate possible
offspring genotypes
Determine probability of traits
Genotypic Ratio: homozygous dominant : heterozygous : homozygous recessive
Phenotypic Ratio: dominant: recessive
Test Cross

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Test cross is used to determine unknown
genotypes
Cross unknown with a homozygous
recessive individual for that trait

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If ALL offspring show dominant trait, then the
unknown is homozygous dominant
If any (about 1/2 ) offspring show recessive trait,
then the unknown is heterozygous
Do Now:
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Leslie has a long palmar muscle. Leslie has a brother, who
does not have a long palmar muscle. Leslie’s parents also
lack the muscle. Leslie is married to Lamont, who does have
the long palmar muscle. Their first two children are identical
twin boys (Larry and Lance), who both have a long palmar
muscle. Use the letters M and m to represent the alleles for
this trait.
What are the genotypes of everyone in this problem?
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Leslie, Louis, Lamont, Larry, Lance, Leslie’s Parents
What is the most probable method of inheritance (dominant
or recessive) for this trait? Explain.
Dihybrid Cross
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Dihybrid Cross involves 2 pairs of contrasting
traits

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Ex. Homozygous round yellow seeds (RRYY) x
homozygous green wrinkled seeds (rryy)
Punnett Square has 16 boxes
Determine possible allele combinations for each
parent and put on sides of Punnett square
Fill in boxes with possible allele combinations for
offspring
Dihybrid Cross (RrYy x RrYy)
Extra Credit – Trihybrid Cross
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Round is dominant to wrinkled seeds
Yellow seeds are dominant to green seeds
Purple flower color is dominant to white flower
color
Show a trihybrid cross, and use a Punnett
square to determine the phenotypic ratio for
possible offspring from parents that are each
heterozygous for all traits
Complex Patterns of Heredity

Do not follow Mendelian Genetics

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Incomplete Dominance
Codominance
Multiple Alleles
Sex linked traits
Polygenic Inheritance
Incomplete Dominance

Incomplete dominance
occurs when an
intermediate form of
the trait is displayed in
heterozygous
individuals
Ex. Snapdragons
Red x White = 100% Pink!

Codominance

Codominance – 2 dominant alleles are both
expressed at the same time
Ex. Roan horses
Red x White horse
= 100% Roan horse
(has both red and white hair)
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Do Now:
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Thomas has sickle cell but his wife, Susie,
does not have sickle cell. Their daughter,
Kelly has both regular cells and sickle cells.
What pattern of inheritance does sickle cell
follow? How do you know?
What is the probability that Kelly and her
husband Regis (who does not have sickle cell)
will have a child with all normal red blood
cells?
Multiple Alleles
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Traits with more than 2
possible alleles
Ex. Blood Type (A,B,
and O)
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3 possible alleles
IA,IB (dominant),
i (recessive)
Linked Genes
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Discovered by Thomas Hunt
Morgan
Studied Drosophila melanogastar
Crossed wildtype red-eyed female
x mutant white-eyed male
Concluded white-eye mutation
linked to sex chromosome (X)
Sex-linked Traits
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Sex-linked traits – genes are found on the X
chromosome but not on the Y chromosome
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Females have 2 X chromosomes, therefore 2 alleles for
each trait and a heterozygous female would exhibit the
dominant trait
Males have only 1 X chromosome, therefore only 1 allele
to determine traits found on the x chromosome and will
always exhibit that trait even if it is recessive
Ex. Sex-linked traits: Hemophilia, Red-Green color
blindness, Male-Pattern baldness, Duchenne Muscular
Dystrophy
Punnett Squares for Sex-linked Traits
Genetic Recombination
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Unlinked Genes - typically see 50% freq of
recombination for any 2 genes located on different
chromosomes due to independent assortment of
metaphase I
Linked Genes – freq of recombination varies
depending on distance between linked genes due to
crossing over during prophase I
Using the freq of recombination can construct a
genetic map (ordered list of loci along chromosome)
Polygenic (Multi-gene Inheritance)
Polygenic Inheritance – several genes influence 1 trait,
therefore we see a variety of phenotypes and a continuum
from one extreme to another
Linked Genes

Linked genes – genes located on the same
chromosome that tend to be inherited together

Linked genes do not always follow Mendel’s
Law of Independent Assortment (he used
genes on different chromosomes)
X Inactivation in Female Mammals
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Although female mammals inherit 2 copies of the X
chromosome, one X chromosome becomes
inactivated during embryonic development and is
called a Barr Body
The inactivation of an X chromosome occurs
randomly in each embryonic cell, therefore females
consist of a mosaic of 2 types of cells
(active x from mom or active x from dad)
Ex. Tortoise shell cats
 Some cells express black fur
and others express orange fur
Pedigree Analysis

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Pedigree - diagram of
family history of a trait
or disease used to study
heredity
By studying a pedigree,
it is possible to infer
the pattern of heredity
Analyzing a Pedigree
1.
Determine if trait is sex-linked or autosomal

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2.
Determine if trait is dominant or recessive
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3.
Sex-linked usually seen in males
Autosomal appears in both sexes equally
If every individual w/trait has a parent w/trait then it is
dominant
If individual has parents w/o trait then it is recessive
Determine if the trait is determined by a single gene
or several
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If determined by a single recessive gene, than normal
parents should produce affected children with a 3:1 ratio
If determined by several genes the proportion would be
much lower
Example Pedigree
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Ex. Pedigree 1
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Pedigree 2