Download Life Science I 83.101.102 Dr. Ekaterina (Kate) Vorotnikova Office

Life Science I
83.101.201
Dr. Ekaterina (Kate) Vorotnikova
Office: 413b
E-mail: [email protected]
Lecture 20
Multiple alleles, ABO blood groups.
Sex chromosomes and sex-linked genes
(page 167; 174-177.
Questions: page 179 # 2-4; 6 )
Incomplete dominance results in intermediate
phenotypes
 Incomplete dominance
– Neither allele is dominant
over the other
– Expression of both
alleles is observed as an
intermediate phenotype
in the heterozygous
individual
Copyright © 2009 Pearson Education, Inc.
Incomplete dominance in human hypercholesterolemia
Genotypes:
HH
Homozygous
for ability to make
LDL receptors
Hh
Heterozygous
hh
Homozygous
for inability to make
LDL receptors
Phenotypes:
LDL
LDL
receptor
Cell
Normal
Mild disease
Severe disease
Pleiotropy
One gene influencing
many phenotypic
characteristics.
The gene for sickle
cell disease:
•Affects the type of
hemoglobin produced;
•Affects the shape of
RBC;
•Causes anemia;
•Causes organ
damage;
•Is related to
susceptibility to
malaria.
Sickle-cell disease, multiple effect of a single human gene
Many genes have more than two alleles in the
population
 Multiple alleles
• More than two alleles are found in the
population
• A diploid individual can carry any two of these
alleles
• The ABO blood group has three alleles,
leading to four phenotypes: type A, type B,
type AB, and type O blood
• ABO blood types are inherited through genes
on chromosome 9
Copyright © 2009 Pearson Education, Inc.
Codominance
Neither allele is dominant over the
other.
Expression of both alleles is observed
as a distinct phenotype in the
heterozygous individual.
Observed for type AB blood
Genetic Inheritance Patterns
ABO blood types are inherited through genes on chromosome 9, and they do
not change as a result of environmental influences during life. An individual's
ABO type is determined by the inheritance of 1 of 3 alleles (A, B, or O) from
each parent. The possible outcomes are shown below:
Parent Alleles
IA
IB
i
IA
IB
i
IAIA
(A)
IAIB
(AB)
IAi
(A)
IAIB
(AB)
IBIB
(B)
IBi
(B)
IAi
(A)
IBi
(B)
ii
(O)
The possible ABO alleles for one parent are in the top row and the
alleles of the other are in the left column. Offspring genotypes are
shown in black. Phenotypes are red.
An international team of researchers led by Henrick Clausen of the University of
Copenhagen, Denmark have discovered a bacterial enzyme that can convert red
blood cells of types A, B, and AB into O by stripping away their identifying surface
antigens. This has the potential for dramatically improving the safety of blood
transfusions. Clinical trials of this technique are now underway. ("Bacterial
Glycosidases for the Production of Universal Red Blood Cells", published online in
Nature Biotechnology, April 1, 2007).
SEX CHROMOSOMES
AND SEX-LINKED
GENES
Copyright © 2009 Pearson Education, Inc.
Chromosomes determine sex in many species
 X-Y system in mammals, fruit flies
• XX = female; XY = male
 X-O system in grasshoppers and roaches
• XX = female; XO = male
 Z-W in system in birds, butterflies, and some
fishes
• ZW = female, ZZ = male
 Chromosome number in ants and bees
• Diploid = female; haploid = male
Copyright © 2009 Pearson Education, Inc.
X
Y
The X-Y system
(male)
44
Parents’
+
diploid
XY
cells
22
+
X
(female)
44
+
XX
22
+
Y
Sperm
22
+
X
44
+
XX
44
+
XY
Offspring
(diploid)
Egg
The X-O system
22
+
XX
Grasshoppers, roaches and some other insects have an X-O system,
in which O stands for the absence of a sex chromosome
22
+
X
The Z - W system
76
+
ZW
76
+
ZZ
Eggs determine sex in fishes, butterflies and birds.
Sex-linked traits affect female birds to a greater extent than males. A
female would need to inherit only one copy of a Z-linked recessive allele
to show a specific trait, while a male would need to inherit two copies.
Bird populations can become endangered if the numbers of females
decline due to harmful Z-linked traits.
32
Sex determination by chromosome number: bees, ants
16
Sex-linked genes exhibit a unique pattern of inheritance
Fruit fly eye color, a sex-linked characteristic
R is dominant, wild-type, red-eye allele; r is recessive, white-eye allele.
They are carried on the X chromosome.
Sex-linked genes can be
located on either of the sex
chromosomes
Reciprocal crosses show
different results
Red-eyed female  whiteeyed male = red-eyed
females and red-eyed
males
X-linked genes are passed
from mother to son and
mother to daughter
X-linked genes are passed
from father to daughter
Y-linked genes are passed
from father to son
Red-eyed female  redeyed male = red-eyed
females, red-eyed
males and white-eyed
males
Red-eyed female  red-eyed
male = red-eyed females,
white-eyed females, red-eyed
males and white-eyed males
CONNECTION: Sex-linked disorders in humans
affect mostly males
 Males express X-linked disorders such as
the following when recessive alleles are
present in one copy
• Hemophilia
• Colorblindness
• Duchenne muscular dystrophy
Copyright © 2009 Pearson Education, Inc.
DNA analysis has revealed the identity of the "cursed
blood" disorder that afflicted the British Royal Family
in the 19th and early 20th centuries.
Queen Victoria's great
grandson Crown Prince
Alexis was a hemophiliac
The mutation was transmitted from Russian Empress Alexandra to her son Crown
Prince Alexis.
The analysis of the remains of Victoria's Russian descendants helped identify the exact
form of hemophilia. Modern analytical techniques allowed the scientists to amplify the
very degraded DNA. They discovered a mutation in a gene on the X chromosome that
codes for the production of Factor IX, a substance that causes blood to clot. This
genetic mutation is the cause of hemophilia B.
Extra credit question.
A)Alexis did not die from hemophilia. At the age of fourteen he was executed
with the rest of the family. His four oldest sisters were also young and didn’t have
children, so we don’t know whether any of them was a carrier. But we can make an
estimate. a) What are the probabilities that all four of the girls were carriers of the
allele hemophilia? b) Supposing Alexis had lived and married a normal woman, what
are the chances that his daughter would be a hemophiliac? c) What are the chances
his daughters would be carriers? d) What are the chances that his sons would be
hemophiliacs? (4 points)
B)Mr. Jones has type A
blood. His wife has type AB
blood. Their first child has
type B blood. What are the
possible phenotypes for
future offspring and the
probabilities for each one?
(2 points)
For both parts of Extra credit question – 6 points
Red-green color blindness
People with intact color vision will see a 15.
Someone with color deficient vision will see a 13 or 17 or nothing.
Everyone will see a “25”.
People with normal vision
will see a “29” whereas
someone who is color
blind will only see spots.
Genes
located
on
alternative
versions called
(a) alleles
chromosomes
at specific
locations called
(b) loci
if both same,
if different,
genotype called genotype called
(c) homozygous
heterozygous
expressed
allele called
(d)
dominant
unexpressed
allele called
(e) recessive
inheritance when phenotype
In between called
(f) incomplete
dominance