Life Science I83.101.201
Dr. Ekaterina (Kate) VorotnikovaOffice: 413b
E-mail: [email protected]
Lecture 20Multiple 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
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HHHomozygous
for ability to makeLDL receptors
hhHomozygous
for inability to makeLDL receptors
HhHeterozygous
LDL receptor
LDL
CellNormal Mild disease Severe disease
Genotypes:
Phenotypes:
Incomplete dominance in human hypercholesterolemia
Sickle-cell disease, multiple effect of a single human gene
PleiotropyOne 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.
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.
CodominanceNeither 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 IAIA
(A)IAIB
(AB)IAi(A)
IB IAIB
(AB)IBIB
(B)IBi(B)
iIAi(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
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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 = maleCopyright © 2009 Pearson Education, Inc.
X
Y
(male)
Sperm
(female)
44+
XYParents’diploidcells
44+
XX
22+X
22+Y
22+X
44+
XY
44+
XX
Egg
Offspring(diploid)
The X-Y system
22+X
22+
XX
Grasshoppers, roaches and some other insects have an X-O system, in which O stands for the absence of a sex chromosome
The X-O system
76+
ZZ
76+
ZW
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.
The Z - W system
1632
Sex determination by chromosome number: bees, ants
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 exhibit a unique pattern of inheritance
Sex-linked genes can be
located on either of the sex chromosomes
Reciprocal crosses show different results
Red-eyed female white-eyed 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 red-eyed 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
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DNA analysis has revealed the identity of the "cursed blood" disorder that afflicted the British Royal Family in the 19th and early 20th centuries.
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.
Queen Victoria's great grandson Crown Prince Alexis was a hemophiliac
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
People with intact color vision will see a 15.
Someone with color deficient vision will see a 13 or 17 or nothing.
Red-green color blindness
Everyone will see a “25”. People with normal vision will see a “29” whereas someone who is color blind will only see spots.
Genes
locatedon
(b)
(a)
at specificlocations called
alternativeversions called
if both same,genotype called
expressedallele called
inheritance when phenotypeIn between called
unexpressedallele called
if different,genotype called
chromosomes
heterozygous
(d)
(c)
(f)
(e)
alleles
loci homozygous
dominant recessive
incomplete dominance
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