Punnet Square

Inheritance: It is the transfer of genetic characteristics from parents to offspring.

Mendelian Inheritance 

Mendel applied mathematical logic and statistical logic for the first time in biology during his investigations on inheritance. Mendel through his experiments shows the stable trait inheritance and expressions for several generations. In his experiments, he selected pea plant varieties. 

Punnett’s Square 

Punnett’s Square is a square diagram that helps us predict the offspring trait when the parents’ genes are known. In 1905, Reginald  
C. Punnett discovered Punnett Square. It is the visual representation of Mendelian Inheritance. 

Allele: It is a form of a gene variant. There will be a combination of either two dominant alleles or two recessive alleles. 

Zygosity: It is the similarity between the alleles. It can be homozygous or heterozygous. In homozygous, homo means same while in heterozygous, hetero means different. For determining height, in homozygous dominant pair’s genotype would be ‘TT’ while the homozygous recessive pair is ‘tt.’ But in heterozygous the genotype would be ‘Tt’ where the capital letter represents the dominant pair, and the small letter represents the recessive pair. 

Phenotype: While determining the characteristics of an organism, the observable characteristics are determined by phenotype. Its ratio is called the phenotypic ratio.  

Genotype: While determining the characteristics of an organism, the genetic characteristics are determined by genotype. 

Punnett’s Square Diagram 

Monohybrid cross  

When we need to determine a single trait, a monohybrid cross is used.  

For a monohybrid cross, the phenotypic ratio is 3:1, where 3 is the dominant heterozygous and 1 is the homozygous recessive. The genotypic ratio is 1:2:1, where 1 is homozygous dominant, 2 is heterozygous, and 1 is homozygous recessive. 

For example, let us consider the height of a plant, having traits tall ‘T’ and small ‘t.’  

Punnett Square for the above example will be as follows: 

The Punnett square diagram of a monohybrid cross consists of four squares.  

From the example, we can conclude that both parents have the genotype ‘Tt.’  

Here, the plants will be tall in three combinations and small in one combination hence the phenotypic ratio will be 3:1.  

The probability of getting TT and tt will be 25% and Tt will be 50%, hence the genotypic ratio will be 1:2:1. 

Dihybrid Cross 

If we have more than one gene, then we use a dihybrid cross to determine the characteristics of the offspring. They are complicated crosses.  

In a dihybrid cross, there are nine genotypes. The phenotypic ratio for a dihybrid cross is 9:3:3:1 and genotypic ratios are 1:2:2:4:1:2:1:2:1.    

For example, let us consider two characteristics of a plant such as the height and color of the plant.  

Tall will be ‘T’ and small will be ‘t.’ Similarly ‘G’ will be green and white for ‘g.’  

Here, in a dihybrid cross, there will be four possible gamete combinations: TG, Tg, tG, and tg.  

Now, if we draw a Punnett square diagram we get, 

The Punnett square diagram of a dihybrid cross consists of 16 squares.  

Here, the phenotypic combinations are nine tall green, three tall white, three small green, and one small white, hence the phenotypic ratio will be 9:3:3:1. 

The genotypic combinations are one-time TTGG, two times TTGg, two times TtGG, four times TtGg, one-time TTgg, two times Ttgg, one-time ttGG, two times ttGg, and one-time ttgg, hence the genotypic ratio will be 1:2:2:4:1:2:1:2:1.