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 Experimental, Quantitative
Approach
• Advantages of pea plants for genetic
study:
– There are many varieties with distinct heritable
features, or characters (such as flower color);
character variants (such as purple or white
flowers) are called traits
– Mating of plants can be controlled
– Each pea plant has sperm-producing organs
(stamens) and egg-producing organs (carpels)
– Cross-pollination (fertilization between different
plants) can be achieved by dusting one plant
with pollen from another
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
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
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
•
the gene for flower color in pea plants has two
alleles, one for purple flowers and the other for
white flowers
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)
Mendel’s Principles of Heredity
1.
First Law of Genetics:
Law of Segregation
d)
Each gene resides at a
specific locus on a
specific chromosome
•
2 alleles for each trait
segregate during gamete
production
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers
The Testcross
• How can we tell the genotype of an individual with
the dominant phenotype?
• Such an individual must have one dominant allele,
but the individual could be either homozygous
dominant or heterozygous
• The answer is to carry out a testcross: breeding the
mystery individual with a homozygous recessive
individual
• If any offspring display the recessive phenotype, the
mystery parent must be heterozygous
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin
Cummings
The Testcross
•
•
The cross of an
individual with
dominant
phenotype 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
Rr
Rr

Segregation of
alleles into sperm
Segregation of
alleles into eggs
Sperm
1/
R
2
R
1/
2
R
R
Eggs
4
r
2
r
2
R
1/
1/
1/
r
1/
4
r
r
R
r
1/
4
1/
4
Rules of Multiplication:
• The probability that
independent events
will occur
simultaneously is the
product of their
individual
probabilities.
Mendelian Inheritance Reflects Rules of Probability
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
homozygous dominant parent and
homozygous recessive parent
http://www.rivergardens-indiana.com/images/snapdragons400.jpg
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
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
http://academic.kellogg.cc.mi.us/herbrandsonc/bio201_McKinley/f21-7a_abo_blood_types_c.jpg
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)
• Examples:
•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
•Marfan’s syndrome—one gene causes the slender
physique, hypermobility of the joints, elongation of the
limbs, dislocation of the lens, and susceptibility to
heart disease
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
C = Melanin deposition
c = No Deposition (Albinism)
B = Brown coat color
b = Tan coat color
A cross between
heterozygous brown
horses for the 2 genes
results in a 9:3:4
phenotypic ratio
9 Black (B_C_)
4 Albino (__cc)
3 Brown (bbC_)
http://courses.bio.psu.edu/fall2005/biol110/tutorials/tutorial5_files/figure_14_11.gif
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
Pedigree Analysis
• A pedigree is a family tree that describes
the interrelationships of parents and
children across generations
• Inheritance patterns of particular traits
can be traced and described using
pedigrees
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
Pedigree Analysis
• Is widow’s peak a
dominant or
recessive trait?
Dominant
Key
Male
Female
1st generation
(grandparents)
Affected
male
Affected
female
Ww
Mating
Offspring, in
birth order
(first-born on left)
ww
2nd generation
(parents, aunts,
Ww ww ww Ww
and uncles)
ww
Ww
Ww
ww
3rd generation
(two sisters)
WW
or
Ww
ww
No widow’s peak
Widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
1st generation
(parents, aunts,
Ww ww ww Ww
and uncles)
ww
Ww
3rd generation
(two sisters)
WW
or
Ww
Pedigree Analysis
• Is attached earlobe
a dominant or
recessive trait?
Recessive
ww
No widow’s peak
Widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
1st generation
(grandparents)
Ff
Ff
ff
2nd generation
(parents, aunts,
FF or Ff ff ff Ff
and uncles)
Ff
Ff
ff
3rd generation
(two sisters)
ff
Attached earlobe
FF
or
Ff
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?
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-; causes
mucus buildup in some internal organs and
abnormal absorption of nutrients in the
small intestine
• 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
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
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
Amniotic fluid
withdrawn
Centrifugation
Fetus
Placenta
Uterus
Cervix
Fluid
Fetal
cells
BioSeveral chemical
hours
tests
Several
weeks
Several
weeks Karyotyping
(a) Amniocentesis
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
Fetus
Placenta
Biochemical
tests
Karyotyping
Chorionic
villi
Several
hours
Suction tube
inserted
through
cervix
Fetal
cells
Several
hours
(b) Chorionic villus sampling (CVS)
Ultrasound at 12 weeks
--can see any physical abnormalities