Extensions of Mendelism - Essay Example

The paper 'Extensions of Mendelism' is a worthy example of genetics research.
According to Mendel’s Law, genes can exist in alternate forms called the allele. Mendel defined two alleles for each character. An allele is dominant if it has the same phenotypic (external appearance) effect in heterozygote as in homozygote, e.g. AA and Aa are phenotypically indistinguishable, while allele with aa is said to be recessive that remains suppressed in the presence of dominant allele.

Mendel was lucky as the characters he selected for his studies were not linked. Exceptions to Mendel’s Law of Dominance were perceived in incomplete dominance, Co-dominance, and multiple alleles.

Incomplete Dominance: In snapdragon, Antirrhinum majus, the heterozygote has a phenotype different from the either of its homozygotes. Red (RR) and white (rr) varieties are homozygous for different alleles of color determining gene, when they have crossed the heterozygotes (Rr) have pink flowers, indicating that allele red (R) is partially or incompletely dominant over the allele for white (r). Differences in phenotype could be due to the difference in the amount of product specified by R allele. The genotypic ration thus is 1:2:1 (RR: 2Rr: rr).

The Punnet Square:

Maleà

Female ↓

R

r

R

RR (Red flower)

Rr (Pink flower)

r

Rr (Pink flower)

Rr (White flower)

Co-dominance: in this case alleles produce independent effects when they are heterozygous, indicating that there is an independence of allele function, none of the alleles is dominant or partially dominant over the other and therefore they are written as superscripts. E.g. Karl Landsteiner’s blood group.

IAIA represents Blood group A, also IA IO also represent blood group A

IBIB represents Blood group B, also IB IO also represent blood group B

IAIB represents Blood group AB (CO-DOMINANT)

IOIO represents Blood group O

Thus, when both alleles A and B are present then both are dominant and therefore, they are said to be co-dominant.

Multiple Alleles: Genes exist in more than two forms (not in only two forms stated by Mendel). E.g. the coat color of the rabbit is controlled by many genes. Genotype cc will have the albino phenotype, genotype chch will have black hairs on the extremities and white all over, while cchcch have white and black tips on the body and c+c+ possess colored hair all over.

Most of the wild population is homozygous for the c+ allele; this allele is called wild-type whereas all the other alleles of the c gene are mutants/ altered forms of the wild-type allele that must have arisen during the process of evolution. The himalayan, chinchilla, albino or colorless are all mutant varieties.