Tag: classical genetics

A total number of types of gamete produced by an organism is 2$^n$, where n is the number of heterozygous genes present. As we know that trihybrid plant is heterozygous for three genes, these total possible gametes by it = 2$^3$ = 8. During fertilization, any of the 8 types of sperms can fuse with any of the 8 types of egg to produce 64 possible combinations of zygotes. This makes option A correct but C and D wrong. Four different types of gametes are produced by an organism who is heterozygous for two genes; 2$^2$ = 4 and resultant four gametes produce total 4 x 4 = 16 zygotic combinations. Thus, option B is wrong.

AaBbCC will produce 4 types of gametes which are as follows- ABC, AbC, aBC, abC. The number of gametes formed is decided by the number of heterozygous alleles present in the given genotype. 2^n is the formula used to find it out, where n=number of heterozygous alleles present in the genotype. Say for example, in the above genotype Aa & Bb are the 2 heterozygous alleles, so here n=2. Putting the values in the formula , we get 2^2=4. Hence 4 types of gametes are formed.

For quantitative inheritance by four genes, we have a total of 8 alleles that will determine the phenotypes of the individual. Those offspring with all 8 dominant alleles ( Let's assume it as AABBCCDD ) will show the highest additive effect. Gradually as we keep decreasing the number of dominant alleles, additive effect will become less prominent. Hence, offspring with 7,6,5,4,3,2 and 1 dominant alleles will show different phenotypes. Finally, those individuals with no dominant alleles i.e. all recessive ( aabbccdd ) will show the least additive effect of characters. If we count all of them we have a total of 09 types of phenotypes possible. 

The tetrahybrid cross of AaBbCcDd can be easily calculated by assuming their independent assortment.

In complete dominance, the dominant allele fully masks the recessive allele. Therefore, the phenotype of the heterozygote is identical to the homozygous dominant, making the phenotypic and genotypic ratios identical in a monohybrid cross (3:1 vs 1:2:1 is not the case here, but rather the expression of traits).

According to Mendel"s law of segregation, alleles segregate from one another so that the gametes will have only one copy of the particular gene. In a diploid organism, there are two alleles for a particular gene, which segregate and only one of each allele can be present in a gamete, therefore, since there are two alleles for a gene, chances of a gamete containing either of the allele is 50%. 

Incomplete dominance: It is a type of intragenic (or interallellic) interaction where both the alleles of a given trait express as a blend (mixture) as against a normal Mendelian pattern where one allele is dominant over the other. As a resulting of this blending, an intermediate character is expressed. This situation occurs due to the fact that the dominant gene is not in a position to completely suppress the expression of recessive gene. With the result, the heterozygous offspring will be phenotypically and genotypically different from either of the homozygous parent.

Hybrid - Rr pink flower and white flower - rr
Genotypes: Rr            x      rr
Gametes:     R, r                 r
Offspring:      Rr, rr
The ratio of pink flower: white flower = 1:1

Incomplete dominance refers to a genetic situation in which one allele does not completely dominate another allele and, therefore, results in a new phenotype. It shows that the dominant allele is incompletely dominant.
Examples of incomplete dominance are:

Incomplete dominance is the only option which does not obey Mendel's principles. Mendel postulated law of segregation, independent assortment, dominance, purity of gametes, etc. According to Mendel's theories, the alleles constituting the genes will separate and segregate during meiosis for the formation of gametes. It produces the expression of either dominant or recessive - dominant expresses in both homo and heterozygous conditions, recessive expresses only in homozygous condition. But incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a third phenotype in which the expresses physical trait is a combination of the dominant and recessive phenotypes.