Tag: classical genetics

A.Codominance – Phenotypic and genotypic ratio is 1:2:1

B.Dihybrid cross – Phenotypic ratio is 9:3:3:1 and genotypic ratio is 1:2:2:4:1:2:1:2:1

C.Monohybrid cross with complete dominance – phenotypic ratio is 3:1 and genotypic ratio is 1:2:1

D.Monohybrid cross with incomplete dominance - Phenotypic and genotypic ratio is 1:2:1

So, the correct option is ‘Codominance & Monohybrid cross with incomplete dominance’.

The inheritance of flower colour in the Antirrhinum majus (snapdragon or dog flower) is an example of incomplete or partial dominance. Incompleted dominance is the phenomenon in which neither of the two alleles of a gene is completely dominant over the other. In a cross between true-breeding red-flowered (RR) and true-breeding white-flowered plants (rr), the $F _1$ plants obtained were pink (Rr) coloured. When the $F _1$ plants were self-pollinated, the $F _2$ generation resulted in the ratio, 1 (RR) Red :2 (Rr) Pink: 1 (rr) White. The phenotypic ratios had changed from the normal 3 : 1 dominant : recessive ratio to 1: 2: 1. R was not completely dominant over rand  this made it possible to distinguish Rr (pink) from RR (red) and rr (white). 

So, the correct answer is '1:2:1'.

Incomplete dominance is the phenomenon of neither of the two alleles being dominant  so that expression in the hybrid is intermediate between the expressions of the two alleles in homozygous state. $F _2$ phenotypic ratio is : 2 : 1, similar to genotypic ratio.

• Incomplete dominance is the condition where a dominant trait fails to show dominance completely in the heterozygous condition. For example, in snapdragon homozygous dominant results in the red color of the flower while homozygous recessive results in white flower. While heterozygous condition results in the pink color of the flower.
• Hence, in each case of incomplete domination, the heterozygous condition will result in a new phenotype.
• Following Punnet's square is representing dihybrid cross for genes A and B showing incomplete domination - 
  |   |  AB |  Ab |  aB |  ab | | --- | --- | --- | --- | --- | | AB |  AABB |  AABb |  AaBB |  AaBb | |  Ab |  AABb |  AAbb |  AaBb |  Aabb | |  aB |  AaBB |  AaBb |  aaBB |  aaBb | |  ab |  AaBb |  Aabb |  aaBb |  aabb |
• As we can see by counting phenotype ratio is same as genotype ratio here i.e. 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1.
• So, the correct option is '1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1'. 

Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles. In this case, the F2 generation of a cross between dihybrids involving both the genes with incomplete dominance would result in developing eight different classes of phenotypes as evident from the calculation and phenotype variation in the Punnett square. 

• The starch synthesis in pea plants is controlled by a single gene which has two alleles B and b.
  
• BB (homozygote) seeds were round whereas, bb was wrinkled. 
• The ratio obtained is 1 (long grain): 2 (intermediate grain): 1 (small grain), therefore, out of 2000 progenies obtained 1000 progenies have intermediate size starch grain.

Exception to Mendel's laws can be explained in Mirabilis jalapa with incomplete dominance. Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the dominant and recessive phenotypes. Incomplete genetic dominance is similar to, but different from co-dominance. In co-dominance, an additional phenotype is produced, however both alleles are expressed completely. Co-dominance is exemplified in AB blood type inheritance. Example: Incomplete dominance is seen in cross-pollination experiments between red and white snapdragon plants. The allele that produces the red color (R) is not completely expressed over the recessive allele that produces the white color (r). The resulting offspring are pink. The genotypes are: (RR) Red, (rr) White, and (Rr) Pink.