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.