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LESSON #10MENDEL DID NOT EXPLAIN EVERYTHING
How do we interpret
inheritance here!?
COMPLEX PATTERNS OF INHERITANCECODOMINANCE, INCOMPLETE DOMINANCE, MULTIPLE ALLELES, SEX-LINKED TRAITS
THERE ARE COMPLEX PATTERNS OF INHERITANCE
Many traits do not follow Mendel’s laws of inheritance exactly. Not necessarily is there a dominant and a recessive allele. These other patterns of inheritance are more complex. They include: 1. Codominance2. Incomplete dominance 3. Traits which have multiple alleles.4. Sex-Linked traits
CODOMINANCE
Both alleles are expressed in the heterozygous condition.
Genotype
RR RW WW
Phenotype
Red petals
Red and white petals
White petals
Red (R)
White (W)Possible alleles:
Gene: Flower petal color in Camellias
Red = White
INCOMPLETE DOMINANCE
The heterozygous phenotype is an intermediate phenotype between the two homozygous phenotypes.
Red (CR )
White (C W )
Genotype CRCR CRC W C WC W
Phenotype Red flowers Pink flowers White flowers
Possible alleles:
Gene: Flower color in roses
INCOMPLETE DOMINANCE: DOES NOT FOLLOW MENDEL’S RESULTS
What happened here!?
If curious, research “Heterochromatic chimera”
MULTIPLE ALLELES
When more than two alleles can code for a gene, that is, more than two alleles exist for a trait.
The presence of multiple alleles increases the possible number of genotypes and phenotypes.
An example is the coat color of rabbits or the eye color of Drosophila melanogaster.
ERYTHROCYTE PHENOTYPES: ABO BLOOD GROUPS
Human ABO blood type is another example of multiple alleles: there are 3 alleles that determine the blood type.
BLOOD TYPE ALLELES, GENOTYPE AND PHENOTYPE
Possible allelesAntigen A (IA)Antigen B (IB)No Antigen (i)
IA = IBIA > iIB > i
Genotype
IAIA IAi IAIB IBIB IBi ii
Phenotype
Blood type A
Blood type A
Blood type AB
Blood type B
Blood type B
Blood type O
The first two are co-dominant and the third is recessive. There are molecules in the membrane of red blood cells that act as antigens when they come in contact
with the blood of another individual. The A blood group has the A antigen and it is produced by the first allele. The B blood group has the B antigen and it is produced by the second allele. The AB blood group has the A and B antigens and it is produced by the codominance of the first and
second allele. The O blood group doesn´t have either of these antigens and it is produced by the third allele.
The A blood groupplasma produces anti-B antibodies.
The B blood group plasma produces anti-A antibodies.
The AB blood groupplasma does not peither anti-Aor anti-B antibodies.
The O blood groupplasma produces bothanti-A and anti-B antibodies.
RHESUS FACTOR – OTHER SYSTEM OF BLOOD GROUPS
There are two blood groups, categorized according to their Rhesus factor: Rh+ (Rhesus positive) and Rh- (Rhesus negative)
People with Rh+ have an antigen on their red blood cells. People with Rh- don´t have this antigen and its plasma produces anti-Rh antibodies. For further knowledge on Rh read the article on “Learning Extra”.
BLOOD TRANSFUSIONS When a blood transfusión is needed,
the blood groups of the donor and the recipient must be compatible.
If the donor´s blood is incompatible with the recipient´s blood, then the antibodies in the recipient´s blood react agains the antigens in the donor´s blood. This leads to the antibodies destroying the invading cells, which can have serious consequences for the recipient.
People with type O blood are known as universal donors because type O blood doesn´t have A or B antigens.
Similarly, Rh- donors can give blood to Rh+ and Rh- recipients.
HEMOLYTIC DISEASE OF THE NEWBORN
Women who are negative for a Rhesus factor antigen and have had a child that is positive for that antigen, may develop antibodies anti-Rh when part of the baby’s blood passes to the mother during birth.
If this happens (development of antibodies anti-Rh), in a following pregnancy the newborn could suffer the hemolytic disease if he or she is Rh+. The mother’s antibodies anti-Rh will attack and kill the
fetus’ red blood cells. To avoid this, the woman will be treated with
artificial antibodies during the delivery of every Rh+ baby.
The woman will hence not develop these antibodies.
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