Transcript Mendelian Genetics

Inheritance: Mendelian Genetics
I. Gregor Mendel (1865)
A. Before Mendel
B. Mendel’s experimental approach
II. Genetic terms
1. genes
2. gene pair
3. alleles
4. homozygous/heterozygous
5. dominant/recessive
6. homozygous dominant and recessive/heterozygous
7. genotype/phenotype
III. Genetic crosses
A. Monohybrid crosses
B. Dihybrid crosses
IV. Mendel's discoveries
A. Principle of segregation
B. Principle of independent assortment
C. Genes are particles
A. Before Mendel
Darwin and Mendel
The blending theory (paradigm)
Inheritance of acquired characteristics
• statistics
• amateur
• inductive leap
B. Mendel’s experimental approach
Hermaphrodite
Perfect flowers
pollen
Genetic barriers
egg
Pea plant traits
Pure breeding lines
Breed true
Why was Mendel’s experimental findings ignored during
his lifetime?
a. He was an amateur
b. There was a problem with inductive leaps
c. The dominant paradigm was opposed to his findings
d. He used statistics
e. All of the above
II. Genetic terms
1.
2.
3.
4.
5.
6.
7.
genes/ loci
gene pair = homologues
alleles
homozygous/heterozygous
dominant/recessive
homozygous dominant and recessive/heterozygous
genotype/phenotype
Human traits
Hand folding
III. Genetic crosses
Punnett Square
III. Genetic crosses
Red coat in foxes is a dominant trait; white is the
recessive trait. If a red fox whose mother had a
white coat is bred to a white fox, what will be the
probable percentage of red kits (baby foxes)?
a. 25%
b. 50%
c. 75%
d. 100%
A. Monohybrid cross:
genetic disorders and lethal genes
Tay-Sachs is a lethal disorder resulting in death by the
age of 4. A couple who are normal have a child with TaySachs. Which of the following is true of the parents?
a. One parent is homozygous dominant and the other is
homozygous recessive
b. both parents are heterozygous
c. both parents are homozygous dominant
d. both parents are homozygous recessive
Sickle cell trait: recessive disorder
Sickle cell trait: recessive disorder
Homozygous dominant = normal, not malaria resistant
Heterozygous = malaria resistant
Homozygous recessive = sickle cell anemia
Huntington’s disease: dominant disorder
CAGCAGCAG
Normal, 26 times
HD, 40 to >100 times
Manifests after age 40
Why is dominant lethality less common than recessive?
Dwarfism: dominant trait
Homozygous dominant = lethal
Heterozygous = dwarf
Homozygous recessive = normal height
Hardy-Weinburg rule
If two achondroplasic dwarfs have children, what
fraction of the children would be expected to be
dwarfs like their parents?
a. 1/4
b. 1/2
c. 2/3
d. 3/4
What determines how common a trait is in a population?
a. it’s frequency
b. Dominance
c. Selection
d. Chance
B. Dihybrid crosses: Human traits
Dark hair dominant to light hair
Curly hair incompletely dominant to straight hair
Brown eyes dominant to blue
Dimples dominant to no dimples
In humans, a widow's peak is dominant and a straight hairline is
recessive. Dimples are dominant and no dimples are recessive. A male
who is heterozygous for both widow's peak and dimples has a child with
a woman who has a straight hairline and no dimples.
What is the phenotype ratio of children can they produce?
a. 3: 1
b. 2:2
c. 1:1:1:1
d. 4:0
IV. Mendel’s discoveries
A. Principle of segregation
Sexually reproducing diploid
organisms have 2 alleles of
each gene.
These 2 alleles segregate from
each other to form gametes
that contain only 1 allele of
each gene.
B. Principle of independent assortment
Different genes on different chromosomes segregate into
gametes independently of each other.
Mendel’s Principle of Segregation is based on which
event in meiosis?
a. Anaphase I
b. Anaphase II
c. Metaphase I
d. Metaphase II
Mendel’s principle of Independent Assortment is based
on which event in meiosis?
a. Anaphase I
b. Anaphase II
c. Metaphase I
d. Metaphase II
C. Genes are particles
V. Eugenics
Social Darwinism
Francis Galton
Positive Eugenics
Eugenics
Eugenics
Eugenics today?
IVF/ PGD
Gene “therapy”
Non-Mendelian Inheritance
I. Dominance relations
A. Incomplete dominance
B. Codominance
II. Interactions between different gene pairs: epistasis
III.Multiple effects of single genes: pleiotropy
IV. Environmental effects on phenotype
V. Polygenic inheritance
VI. Sex chromosomes
VII.Linkage and crossing over
VIII. Genes and behavior
A. Incomplete dominance
B. Co-dominance
Bombay phenotype
ABO blood typing system
Three alleles
IA, IB, i
Rh factor is completely dominant trait (R, r)
Blood type by population
Population
O
A
B
AB
Armenians
.289
.499
.132
.080
Austrians
.427
.391
.115
.066
Bolivian Indians
.931
.053
.016
.001
Chinese
.439
.270
.233
.058
Danes
.423
.434
.101
.042
Eskimos
.472
.452
.059
.017
French
.417
.453
.091
.039
Irish
.542
.323
.106
.029
Nigerians
.515
.214
.232
.039
U.S. whites (St. Louis)
.453
.413
.099
.035
U.S. blacks (Iowa)
.491
.265
.201
.043
Source: Mourant, Kopec, and Domaniewska-Sobczak, 1976, The Distribution of the Human Blood Groups, 2nd Ed., London, UK:
Oxford University Press
Jack has B+ blood and Jill has A- blood. Their daughter,
Jenna has B- blood. What is Jack’s genotype?
a. Heterozygous for B and heterozygous for Rh
b. Homozygous dominant for B and heterozygous for Rh
c. Heterozygous for B and homozygous for Rh
d. Cannot tell from this information
II. Epistasis
One gene pair masks the expression of another gene pair
Black: B_ E_
B and E genes
Chocolate: bb E_
Yellow: _ _ ee
A black lab female whose mother was yellow and father
was chocolate has a litter of puppies sired by a chocolate
male whose father was a yellow lab. What is the
probability that one of the puppies will be a yellow lab?
a. 1/8
b. ¼
c. ½
d. ¾
One gene has multiple effects: pigmentation and eye convergence
III. Pleiotropy
IV. Environmental effects on phenotype
Height of American school-age children
Siamese cats
Cortisol and stress
V. Polygenic Inheritance
V. Polygenic Inheritance: skin color
V. Polygenic Inheritance: eye color
Eye color: two master genes
Brown/blue and Green/blue
Three modifier genes
Density of pigment
Distribution of pigment
Tone of pigment
V. Polygenic Inheritance
The result of polygenic inheritance is continuous variation.
VI. Sex chromosomes
VI. Sex chromosomes
• Y
• 95% junk
Y genes
• 78 genes
• SRY
• anti-mullerian
• sperm production
• housekeeping genes
• inversion region
Y Genes
Y evolution
• Disease genes
•X
• 1,098 genes
• Brain genes
• Gay genes? Xq28
Hemophilia: the Royal Family
X-linked traits
Male Pattern Baldness
Red-green color deficiency
Jon has male pattern baldness; his partner Michelle is
not bald but her father has male pattern baldness. What
is the probability that, if they have a son, he will be
bald?
a. 1/8
b. ¼
c. ½
d. ¾
Single Active X
At 500 to 1000 cell stage in female mammals, one of X’s in each
cell is turned off
Remaining X is called the single active X
Being a random event, approximately half of all cells will have
an active paternal X and half will have an active maternal X.
Thus, all female mammals are a patchwork of paternal and
maternal traits coded for by the X chromosome.
If the genotype is homozygous dominant of recessive, it is of no
consequence. However, if the genotype is heterozygous, … ?
Single active X and calico cats
Orange coat color dominant to black
The reason for single active X
VII. Linkage and crossing over
Genes are said to be linked if they are on the same chromosome
The closer two genes are on a chromosome, the more tightly
linked they are and the more likely they will be inherited together.
VIII. Genes and behavior
Twin studies
Genes for behavior?
Novelty seeking ‘genes’ and dopamine receptors
tryptophan
Genes for behavior?
Shyness (harm avoidance) and serotonin
The end