Incomplete Dominance and Co-Dominance
It is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. Incomplete dominance is a deviation from Mendel’s law of segregation or monohybrid ratio.
Generally, there are two phenotypic expressions of a particular character. For example, flower colour in four o’clock plant (Mirabilis jalapa) has two phenotypic expressions- red (dominant) and white (recessive). Only one of these is expressed-i.e., dominant phenotype in homozygous (AA) and heterozygous (Aa) conditions and recessive phenotype in only homozygous (aa) condition. However, in this case F1 (heterozygous) shows pink flowers which is intermediate between the dominant (red colour of flower) and the recessive (white colour of flower). This type of inheritance where heterozygote shows an intermediate phenotype, is termed as incomplete dominance. Further, in F2 generation both phenotypic and genotypic ratios are also similar, i.e.,1 red (RR): 2 pink (Rr): 1 white (rr).
In snapdragon, when a true breeding (homozygous) red coloured plant is crossed with true breeding (homozygous) white coloured plant, all the F1 offspring (heterozygous) are pink. When pink F1 hybrid (heterozygous) were selfed, there were three types of offsprings in F2 -1 red : 2 pink : 1 white.
Andalusian fowls also show incomplete dominance. A cross between pure white splashed fowl and a pure black fowl resulted into a blue F1 hybrid. F2 generation produces the same phenotypic and genotypic ratios of 1 white (WW): 2 blue (Ww) : 1 black (ww).
There are certain gene combinations which affect viability of an organism instead of phenotypic expressions. Such a gene combination causes death of an organism and, therefore, it is termed as lethal gene combination. The most commonly known example is the coat colour in mice.
In mice, yellow coat colour (Y) is dominant over non- yellow coat colour (y). Dominant homozygous (YY) organism dies because this is a lethal combination. Therefore, homozygous organism with yellow coat colour will never be obtained. Thus instead of 3 : 1 ratio, 2 : 1 ratio is obtained. This is a modified monohybrid ratio.
Another expression to Mendel’s result is the phenomenon of co-dominance. In this case alleles express themselves independently even when present together. In heterozygous condition both alleles would be present and each would express independent of the other. While considering co-dominance different symbols are used. Capital letters are used for both the alleles with different subscripts for example, IA, IB, or HbA HbS or M and N (for MN blood system). Following are some of the examples:
MN blood group system
There are three genotypes- MM, NN and MN. MM genotype produces a specific kind of protein produced by M protein. N allele produces slightly different protein, so that individual with genotype NN produces protein N. A heterozygote, MN, produces both these proteins.
AB blood group
There are three genotypes-IAIA, IBIB and IA IB. IAIA produces antigen A, IBIB produces antigen B while genotype IA IB produces both antigens A (from IA) and B (from IB).
Coat colour in short horn cattles
There are two types of true breeding (homozygous) shorthorn cattles -red and white coat coloured. When these are crossed F1 hybrids are roan coat coloured. This is produced due to the presence of small patches of red and white colours. Roan hybrids (F1) on breeding produce three types of cattle 1 : red : 2 roan :1 white.
Sickle Cell Disease
In the presence of two recessive alleles for sickle cell haemoglobin (HbS HbS), the shape of red blood cells is abnormal. This is due to synthesis of haemoglobin molecules with wrong amino acid sequence. These abnormal molecules tend to attach to one another in long, rod- like chains when oxygen is in short supply i.e., during exercise, pneumonia, emphysema, etc. These rod-like chains distort the shape of red blood cells into sickle shape. It causes blocking of small blood vessels. It is often lethal in homozygous recessive condition.
Normal red blood cells occur in homozygous dominant condition (HbA HbA). In heterozygous condition ( HbA HbS) both types of red blood cells occur i.e.,normal as well as sickle shaped.
Mendelian concept of allele was that there are only two alternative forms of a gene. This post-Mendelian inheritance is termed as multiple allelism. In a diploid individual, only two alleles are present but multiple alleles are the entire group of alleles present in the population.
ABO blood group system
The ABO blood group system in humans is an example of multiple allelism. At this locus, there are two copies of gene I. The locus is polymorphic i.e., there are several alleles competing for this position. Specifically, there are three alleles IA, IB and IO.
These alleles determine the final structure of the type of protein that coats red blood cells. Besides being an example of multiple allele system, this example is a combination of dominance and co-dominance at the same time. The IOallele produces a ‘bogus’ gene product that leaves the red blood cell with an incomplete coat (i.e., it lacks a coat). Homozygous individuals with this allele IO IO possess red blood cells with this particular protein.
The IA allele produces protein A which coats the surface of red blood cells. The IB allele produces protein B beyond the surface of red blood cell. An individual heterozygous with alleles IA, IB is able to produce red blood cells covered with both proteins A and B (thus showing co-dominance).
An individual that has IO allele will not produce coating protein, but pairing with either IA or IB as the other allele, produces a coating of protein A or protein B on the red blood cells. Thus allele IO is recessive to both the alleles IA or IB.