Transcript ch. 14 Mendelian Genetics notes

Mendelian Genetics
AP Biology
Ch. 14
Ms. Haut
Pre-Mendelian Theory of Heredity
•
Blending Theory—hereditary material from
each parent mixes in the offspring
1. Individuals of a population should reach a
uniform appearance after many
generations
2. Once traits are blended, they can no
longer be separated out to appear in later
generations
• Problems—inconsistent with observations:
1. Individuals of a population don’t reach
uniform appearance
2. Traits can skip generations
Modern Theory of Heredity
•
Based on Gregor Mendel’s
fundamental principles of heredity
1. Parents pass on discrete inheritable
factors (genes) to their offspring
2. These factors remain as separate factors
from one generation to the next
Mendel’s Discoveries
•
•
Developed pure lines—
populations that “breed
true” (always produce
offspring with the same
traits as the parents
when parents are selffertilized)
Counted his results and
kept statistical notes on
experimental crosses
Mendel’s Principles of Heredity
1. First Law of Genetics: Law of Segregation
a) alternate forms of genes are responsible for
variations in inherited traits
b) for each trait, an organism inherits 2 alleles,
one from each parent
c) If 2 alleles differ, one is fully expressed
(dominant allele); the other is completely
masked (recessive allele)
d) 2 alleles for each trait segregate during
gamete production
Useful Genetic Vocabulary
•
•
•
•
Homozygous—having 2 identical alleles
for a given trait (PP or pp)
Heterozygous—having 2 different alleles
for a trait (Pp); ½ gametes carry one allele
(P) and ½ gametes carry the other allele
(p)
Phenotype—an organism’s expressed
traits (purple or white flowers)
Genotype—an organism’s genetic makeup
(PP, Pp, or pp)
•Combinations resulting from a genetic cross may be predicted by
a Punnett square
•This law predicts a 3:1 ratio observed in the F2 generation of a
monohybrid cross
x
Ratio
3.15:1
x
3.14:1
x
3.01:1
x
2.96:1
x
2.95:1
x
2.82:1
x
2.84:1
3:1
Genotype versus Phenotype
1
PP
(homozygous)
2
Pp
(heterozygous)
Pp
(heterozygous)
1
pp
(homozygous)
Genotypic Ratio 1:2:1
Purple
3
Purple
Purple
1
White
Phenotypic Ratio 3:1
The Testcross
•
•
The cross of any
individual to a
homozygous
recessive parent
Used to determine if
the individual is
homozygous
dominant or
heterozygous
CAUTION:
Must perform many,
many crosses to be
statistically
significant
Mendel’s Principles of Heredity
2. Second Law of Genetics: Law of
Independent Assortment
a) During gamete formation, the segregation of
the alleles of one allelic pair is independent of
the segregation of another allelic pair
b) Law discovered by following segregation of 2
genes
Dihybrid Cross
Mendelian Inheritance Reflects Rules of Probability
• Rules of Multiplication: The probability that
independent events will occur simultaneously is
the product of their individual probabilities.
Question: In a Mendelian cross between pea
plants that are heterozygous for flower color (Pp),
what is the probability that the offspring will be
homozygous recessive?
Answer:
• Probability that an egg from the F1 (Pp) will
receive a p allele = ½
• Probability that a sperm from the F1 will receive a
p allele = ½
• Overall probability that 2 recessive alleles will
unite at fertilization: ½ x ½ = ¼
Mendelian Inheritance Reflects Rules of Probability
Works for Dihybrid Crosses:
Question: For a dihybrid cross, YyRr x YyRr, what
is the probability of an F2 plant having the
genotype YYRR?
Answer:
• Probability that an egg from a YyRr parent will
receive the Y and R alleles = ½ x ½ = ¼
• Probability that a sperm from a YyRr parent will
receive the Y and R alleles = ½ x ½ = ¼
• Overall probability of an F2 plant having the
genotype YYRR: ¼ x ¼ = 1/16
Mendelian Inheritance Reflects Rules of Probability
• Rules of Addition: The probability of an event
that can occur in two or more independent ways
is the sum of the separate probabilities of the
different ways.
Question: In a Mendelian cross between pea
plants that are heterozygous for flower color (Pp),
what is the probability that the offspring will being
a heterozygote?
Answer:
• There are 2 ways in which a heterozygote may
be produced: the dominant allele may be in the
egg and the recessive allele in the sperm, or the
dominant allele may be in the sperm and the
recessive allele in the egg.
Mendelian Inheritance Reflects Rules of Probability
• Probability that the dominant allele will be in
the egg with the recessive in the sperm is ½ x
½=¼
• Probability that the dominant allele will be in
the sperm with the recessive in the egg is ½ x
½=¼
• Therefore, the overall probability that a
heterozygote offspring will be produced is ¼
+¼=½
Variations to Mendel’s First Law of
Genetics
1. Incomplete dominance—pattern of
inheritance in which one allele is not
completely dominant over the other
•
Heterozygote has a phenotype that is
intermediate between the phenotypes of the 2
homozygous dominant parent and
homozygous recessive parent
Incomplete Dominance in Snapdragon Color
F2
Genotypic ratio:
1 CRCR: 2 CRCW: 1 CWCW
Phenotypic ratio:
1 red: 2 pink: 1 white
Variations to Mendel’s First Law of
Genetics
2. Codominance—pattern of inheritance in
which both alleles contribute to the
phenotype of the heterozygote
Codominance in MN Blood Groups
• MN blood group locus codes for the production
of surface glycoproteins on the red blood cell
• There are 3 blood types: M, N, and MN
Blood Type
Genotype
M
MM
N
NN
MN
MN
The MN blood type is the result of full phenotypic
expression of both alleles in the heterozygote; both
molecules, M and N, are produced on the red blood cell
Pedigree Analysis
• Analysis of existing populations
• Studies inheritance of genes in humans
• Useful when progeny data from several
generations is limited
• Useful when studying species with a
long generation time
Symbols:
= female
= male
= affected individual
= mating
I
II
= offspring in birth order
I and II are generations
= Identical twins
= Fraternal twins
Dominant Pedigree:
I
II
III
For dominant traits:
•Affected individuals have at least one affected parent
•The phenotype generally appears every generation
•2 unaffected parents only have unaffected offspring
Recessive Pedigree:
I
II
III
For recessive traits:
•Unaffected parents can have affected offspring
•Affected progeny are both male and female
Multiple Alleles
• Some genes may have more than just 2
alternate forms of a gene.
• Example: ABO blood groups
– A and B refer to 2 genetically determined
polysaccharides (A and B antigens) which are
found on the surface of red blood cells (different
from MN blood groups)
• A and B are codominant; O is recessive to A
and B
Multiple Alleles for the ABO Blood Groups
3 alleles: IA, IB, i
Pleiotropy
• The ability of a single gene to have multiple
phenotypic effects (pleiotropic gene affects
more than one phenotype)
• Example:
•In tigers and Siamese cats, the gene that
controls fur pigmentation also influences the
connections between a cat;s eyes and the
brain. A defective gene cause both abnormal
pigmentation and cross-eye condition
Epistasis
• Interaction between 2 nonallelic genes in
which one modifies the phenotypic expression
of the other.
• If epistasis occurs between 2 nonallelic genes,
the phenotypic ratio resulting from a dihybrid
cross will deviate from the 9:3:3:1 Mendelian
ratio
CC, Cc = Melanin deposition
cc = Albinism
BB, Bb = Black coat color
bb = Brown coat color
A cross between
heterozygous black
mice for the 2 genes
results in a 9:3:4
phenotypic ratio
9 Black (B_C_)
3 Brown (bbC_)
4 Albino (__cc)
Polygenic Traits
• Mode of inheritance in which the additive effect of 2 or more
genes determines a single phenotypic character
• Skin pigmentation in humans
--3 genes with the dark-skin allele
(A, B, C) contribute one “unit” of
darkness to the phenotype.
These alleles are incompletely
dominant over the other alleles
(a, b, c)
--An AABBCC person would be
very dark; an aabbcc person
would be very light
--An AaBbCc person would have
skin of an intermediate shade
Nature versus Nurture
• Environmental conditions can influence the
phenotypic expression of a gene, so that a
single genotype may produce a range of
phenotypes
• One may have a history of heart disease in
their family and thus be at risk of heart disease
themselves. If this person watches his/her
diet, exercises, doesn’t smoke, etc. his/her
risk of actually developing heart disease
decreases
Recessive Human Disorders
• Parents are generally unaffected
• Defective form of a normal trait.
Generally, more serious phenotypic
affect than dominant genes
• 2 Heterozygous normal, unaffected
parents can have affected offspring
• Probability the child of 2 carriers will be:
– affected = ¼
– Normal, but carriers = 1/2
Recessive Human Disorders
• Cystic Fibrosis; autosomal recessive
– Ineffective component of Na+/Cl-; affects
glands that produce mucus
• Tay-Sachs; autosomal recessive
– Usually fatal by 2 or 3 yrs
– Developmental retardation, followed by
paralysis, dementia, and blindness
– Lack enzyme to breakdown lipids—
accumulate in brain so cells lose function
Recessive Human Disorders
• Sickle-cell anemia; autosomal recessive
– Caused by single amino acid substitution in
hemoglobin
– Abnormal hemoglobin packs together to
form rods creating crescent-shaped cells
– Reduces amount of oxygen hemoglobin
can carry
Dominant Human Disorders
• Traits inherited in every generation
• When there is 1 affected parent; ½
progeny are affected
• 2 affected parents can have unaffected
offspring
• If prevents survival, then gene is quickly
eliminated from population
• Usually more variable in its effects. If
lethal, usually after reproductive age
Dominant Human Disorders
• Huntington’s Disease; autosomal dominant
• Average onset is 40 yrs.
• Late acting, presents itself after reproductive
age; lethal
• Affects nervous system, muscle spasms
• Destroys neurons
• Located on chromosome 4
• Children of an afflicted parent have a 50%
chance of inheriting the lethal dominant allele
Recessive Pedigree
Genetic Testing & Counseling
• Genetic counselors can help determine
probability of prospective parents
passing on deleterious genes
– Genetic screening for various known
diseases alleles (gene markers)
Genetic Testing & Counseling
• Fetal testing
Amniocentesis
– needle inserted into uterus and amniotic fluid
extracted
• Test for certain chemicals or proteins in
the fluid that are diagnostic of certain
diseases
• Karyotype-can see chromosome
abnormalities
Genetic Testing & Counseling
• Fetal testing
Chorion Villus Sampling
– Suctions off a small amount of fetal tissue
from the chorionic villus of placenta
• Karyotype-can see chromosome
abnormalities
Ultrasound at 12 weeks
--can see any physical abnormalities