Life Science I83.101.201

Dr. Ekaterina (Kate) VorotnikovaOffice: 413b

E-mail: Ekaterina_Vorotnikova@uml.edu

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

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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