There are complex patterns of inheritance beyond Mendel's laws: 1. Codominance occurs when both alleles are expressed in heterozygotes. 2. Incomplete dominance results in an intermediate phenotype between homozygotes. 3. Multiple alleles exist for some traits, increasing possible genotypes and phenotypes. 4. Sex-linked traits involve alleles on the X or Y chromosome. Examples include blood types (A, B, AB, O alleles) and Rh factor (Rh+ and Rh- alleles), which have medical implications for blood transfusions and hemolytic disease of the newborn.

1. LESSON #10
MENDEL DID NOT EXPLAIN EVERYTHING

3. COMPLEX PATTERNS OF INHERITANCE
CODOMINANCE, INCOMPLETE DOMINANCE, MULTIPLE ALLELES, SEX-LINKED TRAITS

4. 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. Codominance
2. Incomplete dominance
3. Traits which have multiple alleles.
4. Sex-Linked traits

5. 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

6. 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

7. INCOMPLETE DOMINANCE: DOES NOT FOLLOW MENDEL’S RESULTS

9. 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.

10. ERYTHROCYTE PHENOTYPES: ABO BLOOD GROUPS
Human ABO blood type is another example of multiple alleles: there are 3 alleles that determine the blood type.

11. BLOOD TYPE ALLELES, GENOTYPE AND PHENOTYPE
Possible alleles
Antigen A (IA)
Antigen B (IB)
No Antigen (i)
IA = IB
IA > i
IB > 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.

12. The A blood group
plasma produces
anti-B antibodies.
The B blood group
plasma produces
anti-A antibodies.
The AB blood group
plasma does not
peither anti-A
or anti-B antibodies.
The O blood group
plasma produces both
anti-A and anti-B
antibodies.

13. 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”.

14. 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.

15. 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.