A Punnett square is a graphical representation of the genotypes that an offspring could have as a result of a specific cross or breeding event. Creating a Punnett square necessitates knowledge about the parents’ genetic profile. In a tabular arrangement, the many conceivable combinations of their gametes are contained. As a result, each box represents one fertilisation event in the table.
The underlying premise is that each feature is determined by a separate gene locus and that different traits assort independently. There are many exceptions to this rule, especially when selecting features for plant or animal breeding.
This tool was developed in the twentieth century, many years after Mendel’s key genetic experiments. However, they are now frequently utilised to explain Mendel’s results, particularly when combined with our present understanding of DNA, genes, and chromosomes.
Common Terms in Genetics
Some phrases are frequently used in genetics, and they are especially useful in comprehending the operation of Punnett squares. The term ‘allele’ is one of them, and it refers to a gene variety. A pea plant, for example, can have red or white blossoms, and the gene variants that code for each of these are referred to as alleles.
The genetic composition or genotype of an organism is said to be homozygous when it contains two copies of the same allele. True-breeding specimens are another name for these. Plants with white flowers, for example, are homozygous at the genetic loci that code for blossom colour.
Individuals that have two distinct alleles at the same locus are considered to be heterozygous. Many plants with red flowers might have two alleles, one for red and the other for white. The phenotype is the externally observed characteristic of an individual. A heterozygous individual’s phenotype is referred to as the ‘dominant’ version of the gene, while the suppressed feature is referred to as the recessive allele.
The allele coding for red colour is dominant over the one for the white colour in the case of flower colour.
All of the offspring of a dominant homozygote and a recessive homozygote will have a heterozygous genotype and a dominant phenotype.
Some gene loci are found on sex chromosomes and are referred to as sex-linked traits, while the rest are referred to as autosomal traits.
Types of Punnett Squares
Punnett squares can be divided into two sorts. When a single trait determined by a single genetic locus is observed, the first is relevant. A monohybrid cross occurs when true-breeders for a single characteristic are crossed with members carrying a different allele.
Examples include some of Mendel’s first experiments, in which he chose true-breeders for a single trait and crossed them with members carrying a different allele. These are 2X2 squares with four boxes, each representing one fertilisation event between the parent gametes for a monohybrid cross.
The Punnett square is larger, having sixteen boxes, and is used to forecast the outcome of breeding trials where two traits are being followed. The 4X4 square is required because, based on the distribution of alleles in the two genes, each parent can create four different types of gametes.
A Punnett square gets unmanageable when more than two features are detected, hence different approaches are used to forecast the results of such crosses.
Limitation of Punnett Squares
Punnett squares are a useful tool for learning Mendelian genetics, however, they can’t be used in many cases when complex genetic inheritance is involved. When there is a linkage between two genes, for example, they are ineffective in estimating the distribution of genotypes and phenotypes. Genetic linkage occurs when two genes on the same chromosome are near to each other. As a result, the chances of these two qualities being inherited together, in the same combination as the parent, are high during gamete development.
One example is the relationship between the gene that causes Nail-patella Syndrome (NPS) and the one that determines the blood group. NPS was typically inherited along with a B-type blood group, according to a study of one family whose members suffer from it. As a result of these links, the random distribution of the two qualities among offspring will shift, rendering the Punnett square unreliable as a predictor.
Punnett squares cannot accurately predict the distribution of phenotypes in children when a single trait is determined by numerous genes and the effect of each of these genes is graded. Over 400 genes are spread across the genome that determines the human height. Furthermore, environmental factors such as nutrition have an impact on this feature.
Finally, genotypes that are fatal to the foetus, as well as genes acquired entirely from one parent, such as those in the mitochondria or on the Y-chromosome, complicate the results of a Punnett square.
Conclusion
A Punnett square is a graphical representation of the genotypes that an offspring could have as a result of a specific cross or breeding event. Creating a Punnett square necessitates knowledge about the parents’ genetic profile. The Punnett square is larger, having sixteen boxes, and is used to forecast the outcome of breeding trials where two traits are being followed. The 4X4 square is required because, based on the distribution of alleles in the two genes, each parent can create four different types of gametes.
Punnett squares cannot accurately predict the distribution of phenotypes in children when a single trait is determined by numerous genes and the effect of each of these genes is graded.