4 - Molecular basis of dominance

Allelic forms of a gene

Mendel

• He chose to breed pairs of antagonistic phenotypic traits 

• He studied phenotypic traits that are controlled by a single gene, whose alleles showed complete dominance or recessiveness towards each other 

Three main phenotypic trait pattern categories 

1. Incomplete / partial dominance or codominance 

• Trait can controlled by a single gene but alleles do not show complete dominance or recessiveness towards each other

2. Pleiotropy 

• One gene controls several phenotypic traits 

• Eg. vestigial wings in Drosophila

3. Polygenic 

• Phenotypic traits controlled by more than one gene 

• Eg. most human traits 

Dominant and recessive relationships between alleles of the same gene 

Incomplete or partial dominance 

• When a dominant allele does not completely mask the effects of a recessive allele 

• Offspring shows a blend of both alleles 

• Different from codominance

Dominance vs incomplete dominance 

Molecular explanation:

• Flower colour “A” gene encodes pigment producing enzyme 

• “A” allele encodes a functional enzyme 

• “A” allele encodes a non-functioning enzyme 

• AA - 2 functioning alleles - plenty of pigment (darker pink) 

• Aa - 1 functioning allele - less pigment (light pink) 

• Aa - 0 functioning alleles - no pigment (white) 

Codominance 

• Both parental phenotypes are seen in the F1 hybrid 

• Phenotype of F1 heterozygote is distinct from the homozygous and the F2 segregation of phenotypes is also 1:2:1

Eg. Human blood types 

• Alternative alleles of the I gene (I^A and I^B) encode enzymes that add slightly different sugars to the cell membrane of blood cells 

Genes with more than 2 alleles 

• Many genes show this

• May show different dominant and recessive relationships between each other

Eg. blood types 

• 3 alleles of the “I” gene control human blood type 

• Ia produces surface sugar A

• Ib produces surface sugar B 

• I produces no surface sugar 

• Each individual carries 2 copies of the I gene but there are 3 possible alleles instead of 2 

• Because there are 3 possible alleles, there are 6 possible genotypes

I^A is dominant to I 

I^A and I^B are codominant 

Examples 

Dominance: 

Incomplete dominance 

Codominance:

Dominance series 

• Pair-wise crosses - used to determine dominance relationships when there are numerous alleles for a gene 

• See which allele is dominant for each pair to establish a dominance series 

Pleiotropy 

• Where one gene can control multiple phenotypes 

• Apparently unconnected phenotypes can be inherited 

• Eg. coat colour and recessive-lethal allele in mice

• Wild type coat is grey - hairs have black and yellow bands - adaptive camouflage 

• Agouti gene (A) controls this 

• This gene is also involved in an aspect of metabolism 

• Homozygous wt AA = grey coat

• Heterozygous A^YA = yellow coat and obesity 

BUT 

• Never get A^YA^Y homozygotes 

• Heterozygote cross gives phenotypic ratio of 2:1 

• Does this violate Mendel's principle of segregation?

• Like incomplete dominance - each genotype has its own phenotype 

• In this case -  A^YA^Y homozygotes have a phenotype that's impossible to read - they're dead 

• 2:1 segregation is the signature of a recessive lethal allele - modification of 3:1 segregation

Eg. sickle cell anaemia 

• Haemoglobin composed of 2 types of polypeptides - alpha and beta globin

• Abnormal allele of beta globin gene (B^S) causes an abnormal polypeptide to form 

• Homozygous B^SB^S causes sickling of RBCs at low oxygen levels 

Sickled cells cause other abnormal phenotypes:

• More fragile and break easily giving rise to anaemia (sickle cell) 

• Cells are less flexible and flow less easily in the bloodstream, causing clogging 

• Poor circulation and tissues deprived of oxygen 

• Heart and kidney failure, paralysis, rheumatism in the joints 

• Co-inheritance of different phenotypic traits from one mutant allele 

• B^S allele persistent in humans because it confers some resistance to malaria - heterozygote advantage 

• Heterozygotes are less badly affected by above conditions