Transcript Mendelian Genetics - Austin Peay State University

Mendelian Genetics
An Overview
Vocabulary
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Genetics: The scientific study of heredity
Character: heritable feature
Trait: each variant for a character
True-breeding: plants that self-pollinate all
offspring are the same variety
• Allele: alternate version of a gene
• Dominate allele: An allele which is expressed
(masks the other) in the heterozygote & homozygote
• Recessive allele: An allele which is present but
remains unexpressed (masked) in the heterozygote
Vocabulary (continued)
• Homozygote – pair of identical alleles for a character
– Homozygous dominant- BB
– Homozygous recessive - bb
• Heterozygote – two different alleles for a character
(Bb)
• Genotype – genetic makeup; combination of alleles an
organism has
• Phenotype – appearance of an organism; the
characteristics determined by the genotype and
environmental influences
Vocabulary
• Monohybrid cross – a cross that tracks the
inheritance of a single character
• P generation – (parental) true-breeding
• F1- (first filial) offspring of P generation
• F2 – (second filial) offspring from F1 cross
History
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Principles of genetics were developed in the mid
19th century by Gregor Mendel an Austrian
Monk
Developed these principles without ANY
scientific equipment - only his mind.
Experimented with pea plants, by crossing
various strains and observing the characteristics
of their offspring.
Studied the following characteristics:
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Pea color (Green, yellow)
Pea shape (round, wrinkled)
Flower color (purple, white)
Plant height (tall, short)
Made the following observations (example given
is pea shape)
When he crossed a round pea and wrinkled pea,
the offspring (F1 gen.) always had round peas.
When he crossed these F1 plants, however, he
would get offspring which produced round and
wrinkled peas in a 3:1 ratio.
• Pea plants have several advantages for genetics.
– Pea plants are available in many varieties with distinct
heritable features (characters) with different variants (traits).
– Another advantage of peas is that Mendel had strict control
over which plants mated with which.
– Each pea plant has male
(stamens) and female
(carpal) sexual organs.
– In nature, pea plants typically
self-fertilize, fertilizing ova
with their own sperm.
– However, Mendel could also
move pollen from one plant
to another to cross-pollinate
plants.
Fig. 14.1
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In a typical breeding experiment, Mendel would
cross-pollinate (hybridize) two contrasting,
true-breeding pea varieties.
– The true-breeding parents are the P generation and
their hybrid offspring are the F1 generation.
• Mendel would then allow the F1 hybrids to selfpollinate to produce an F2 generation.
• It was mainly Mendel’s quantitative analysis of
F2 plants that revealed the two fundamental
principles of heredity: the law of segregation
and the law of independent assortment.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Laws of Inheretance
• Law of Segregation: When gametes
(sperm, egg, etc…) are formed each gamete
will receive one allele or the other.
• Law of Independent Assortment: Two or
more alleles will separate independently of
each other when gametes are formed
By the Law of Segregation, the two
alleles for a characters are packaged
into separate gametes
• If the blending model were correct, the F1 hybrids
from a cross between purple-flowered and whiteflowered pea plants would have pale purple
flowers.
• Instead, the F1 hybrids
all have purple flowers,
just a purple as the
purple-flowered
Fig. 14.2
parents.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Punnett Squares
• Genetic problems can be easily solved using a
tool called a Punnett square.
– Tool for calculating genetic probabilities
A Punnett square
Monohybrid cross
(cross with only 1 trait)
• Problem:
• Using this is a several step process, look at the
following example
– Tallness (T) is dominant over shortness (t) in pea plants.
A Homozygous tall plant (TT) is crossed with a short
plant (tt). What is the genotypic makeup of the
offspring? The phenotypic makeup ?
Punnet process
1. Determine alleles of
each parent, these are
given as TT, and tt
respectively.
2. Take each possible
allele of each parent,
separate them, and place
each allele either along
the top, or along the side
of the punnett square.
Punnett process continued
• Lastly, write the letter for
each allele across each
column or down each
row.
• The resultant mix is the
genotype for the
offspring.
• In this case, each
offspring has a Tt
(heterozygous tall)
genotype, and simply a
"Tall" phenotype.
Punnett process continued
• Lets take this a step further
and cross these F1
offspring (Tt) to see what
genotypes and phenotypes
we get.
• Since each parent can
contribute a T and a t to
the offspring, the punnett
square should look like
this…
Punnett process continued
• Here we have some more
interesting results: First
we now have 3 genotypes
(TT, Tt, & tt) in a 1:2:1
genotypic ratio. We now
have 2 different
phenotypes (Tall & short)
in a 3:1 Phenotypic ratio.
This is the common
outcome from such
crosses.
• When Mendel allowed the F1 plants to selffertilize, the F2 generation included both purpleflowered and white-flowered plants.
– The white trait, absent in the F1, reappeared in the
F 2.
• Based on a large
sample size, Mendel
recorded 705
purple-flowered F2
plants and 224
white-flowered F2
plants from the
original cross.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 14.2
Law of Segregation - the two alleles for each
character segregate during gamete production
Law of Independent Assortment – Each
set of alleles segregates independently
Test cross – designed to reveal the
genotype of an organism
Mendelian Inheritance and Rules
of Probability
• Rule of Multiplication – the probability
that two events will occur simultaneously is
the product of their individual probabilities
• Probability that an egg from the F1 (Pp)
will receive p = ½
• Probability that an sperm from the F1 (Pp)
will receive p = ½
• Probability that a of offspring receiving
two recessive alleles during fertilization
½x½=¼
Rule Applies to dihybrid Crosses
• For a dihybrid cross, YyRr x YyRr, what is
the probability of an F2 having the genotype
YYRR?
• Go page 267 and work #9 and #10
Dihybrid Crosses
• Dihybrid crosses are made when phenotypes and
genotypes composed of 2 independent alleles are
analyzed.
• Process is very similar to monohybrid crosses.
• Example:
– 2 traits are being analyzed
– Plant height (Tt) with tall being dominant to short,
– Flower color (Ww) with Purple flowers being dominant
to white.
Dihybrid Cross Example
• The cross with a pure-breeding (homozygous)
Tall,Purple plant with a pure-breeding Short, white
plant should look like this.
F1 generation
Dihybrid Cross Example continued
• Take the offspring and cross them since they are donating
alleles for 2 traits, each parent in the f1 generation can give 4
possible combination of alleles. TW, Tw, tW, or tw. The cross
should look like this. (The mathematical “foil” method can
often be used here)
F2 Generation
Dihybrid Cross Example continued
• Note that there is a 9:3:3:1
phenotypic ratio. 9/16
showing both dominant traits,
3/16 & 3/16 showing one of
the recessive traits, and 1/16
showing both recessive traits.
• Also note that this also
indicates that these alleles are
separating independently of
each other. This is evidence
of Mendel's Law of
independent assortment
Other Factors: Incomplete Dominance
• Some alleles for a
gene are not
completely
dominant over the
others. This
results in partially
masked
phenotypes which
are intermediate
to the two
extremes.
Incomplete Dominance
Codominance
• Two alleles affect the phenotype in separate
and distinguishable ways.
• Neither allele can mask the other and both
are expressed in the offspring and not in an
“intermediate” form.
• Example: red flowers that are crossed with
white flowers that yield red and white
flowers.
• 1) In cattle, roan coat color (mixed red and white
hairs) occurs in the heterozygous (Rr) offspring of
red (RR) and white (rr) homozygotes. When two
roan cattle are crossed, the phenotypes of the
progeny are found to be in the ratio of 1 red:2
roan:1 white. Which of the following crosses
could produce the highest percentage of roan
cattle?
• A) roan x roan
• B) red x white
• C) white x roan
• D) red x roan
• E) All of the above crosses would give the same
percentage of roan.
Multiple Alleles
Page 267 and work #6
Pleiotropy
• Most genes have
multiple
phenotypic effects.
The ability of a
gene to affect an
organism in many
ways is called
pleiotropy.
Epistasis
• Epistasis occurs
when a gene at one
locus alters or
influences the
expression of a gene
at a second loci. In
this example, C is for
color and the
dominate allele must
be present for
pigment (color) to be
expressed.
Polygenetic Inheritance
• Qualitative variation
usually indicates
polygenic inheritance.
This occurs when there
is an additive effect
from two or more
genes. Pigmentation in
humans is controlled by
at least three (3)
separately inherited
genes.
Other Factors: Continuous Variation
• Many traits may
have a wide
range of
continuous
values. Eg.
Human height
can vary
considerably.
There are not just
"tall" or "short"
humans
Chromosomes and Classical Genetics
• Walter Sutton in 1902 proposed that chromosomes
were the physical carriers of Mendel's alleles
• Problems arose however regarding the following
question:
• Why are the number of alleles which undergo
independent assortment greater than the number of
chromosomes of an organism?
• This was explained understanding of 2 additional
factors; Sex Linkage and crossing over
Sex Linkage
• All chromosomes are homologous except on sex
chromosomes.
• Sex chromosomes are either X or Y.
• If an organism is XX, it is a female, if XY it is
male.
• If a recessive allele exists on the X chromosome.
It will not have a corresponding allele on the Y
chromosome, and will therefore always be
expressed
Sex Linkage Example
• Recessive gene for white eye
color located on the Xw
chromosome of Drosophila.
• All Males which receive this gene
during fertilization (50%) will
express this.
• If a female receives the Xw
chromosome. It will usually not
be expressed since she carries an
X chromosome with the normal
gene
Human Sex Linkage
• Hemophilia:
– Disorder of the blood where
clotting does not occur
properly due to a faulty
protein.
– Occurs on the X
chromosome, and is
recessive.
• Thus a vast majority of
those affected are males.
– First known person known
to carry the disorder was
Queen Victoria of England.
Thus all those affected are
related to European royalty.
Hemophilia and Royalty
Other Factors: Multiple Alleles
• Phenotypes are controlled by more than 1 allele. Eg. Blood types are
regulated by 3 separate genes.
• ABO Blood typing
– Humans have multiple types of surface antigens on RBC's
– The nature of these surface proteins determines a person's Blood Type.
– There are 3 alleles which determine blood type IA, IB, or IO. This is
referred to as having multiple alleles
– Human blood types are designated as A, B or O.
• Type A denotes having the A surface antigen, and is denoted by IA
• Type B denotes having the B surface antigen, and is denoted by IB
• Type O denotes having neither A or B surface antigen, and is denoted by
IO
– There are several blood type combinations possible
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A
B
AB (Universal recipient)
O (Universal donor)
Blood & Immunity
• A person can receive blood only when the donor's blood type does not
contain any surface antigen the recipient does not. This is because the
recipient has antibodies which will attack any foreign surface protein.
• Thus, Type AB can accept any blood types because it will not attack
A or B surface antigens. However, a type AB person could only
donate blood to another AB person. They are known as Universal
Recipients.
• Also, Type O persons are Universal donors because they have NO
surface antigens that recipients' immune systems can attack. Type O
persons can ONLY receive blood from other type O persons.
• There is another blood type factor known as Rh.
• People are either Rh+ or Rh- based on a basic dominant/recessive
mechanism.
• Not usually a problem except with pregnancy.
• It is possible that an Rh- mother can carry an Rh+ fetus and develop
antibodies which will attack & destroy the fetal blood
• This usually occurs with 2nd or 3rd pregnancies, and is detectable and
treatable.
Other Factors
• Gene interaction:
– Many biological pathways are governed by multiple
enzymes, involving multiple steps. If any one of these
steps are altered. The end product of the pathway may
be disrupted.
• Environmental effects:
– Sometimes genes will not be fully expressed owing to
external factors. Example: Human height may not be
fully expressed if individuals experience poor nutrition.
Environmental Impact on Phenotype
pH of the soil will change the color of
hydrangea flowers from blue to pink
The
Average
American
Phenotype
Technology
And
Genetic testing
Fetal testing
Carrier
Recognition
1.
Amniocentesis
2.
Chorionic villus
sampling (CVS)
3.
Ultrasound
4.
Fetoscopy
Newborn
screening
Mendelian Genetics
End