Beyond Mendel

Exploring inheritance patterns that extend Mendel's basic laws

Co-dominance

In co-dominance, both alleles are expressed simultaneously in the phenotype. The classic example is the ABO blood group system where individuals with AB blood type express both A and B antigens.

ABO Blood Groups

Genotype

Phenotype

Antigens

I^A I^A

Type A

A antigens

I^B I^B

Type B

B antigens

I^A I^B

Type AB

Both A & B

I^O I^O

Type O

No antigens

Key Point: In the AB blood type, both A and B alleles are expressed equally—neither dominates the other.

Sex-linked Inheritance

Genes located on sex chromosomes (X and Y) show different inheritance patterns because males (XY) and females (XX) have different chromosome compositions.

Color Blindness (X-linked)

Males (XY):

• X^C Y = Normal vision

• X^c Y = Color blind

Females (XX):

• X^C X^C = Normal vision

• X^C X^c = Normal (carrier)

• X^c X^c = Color blind

Why More Males Affected

Males need only one recessive allele to express the trait, while females need two.

Color blindness frequency:

• Males: ~8%

• Females: ~0.5%

Historical Example: Hemophilia in European royal families (the "royal disease") spread through carrier females, affecting only males.

Beyond Mendel's Foundation

From Mendel's simple pea plants to complex human inheritance patterns, genetics continues to reveal the beautiful complexity of life.

While Mendel's laws remain the foundation, modern genetics encompasses many fascinating phenomena that extend our understanding of heredity.