Punnett square - Definition, Types and Examples | biology (2023)

Definition of Punnett-Square

A Punnett square is a graphical representation of the possible genotypes of an offspring resulting from a given mating or breeding event. Creating a Punnett square requires knowledge of the genetic makeup of the parents. The various possible combinations of your gametes are encapsulated in a tabular format. Therefore, each box in the table represents a fertilization event.

The inherent assumption is that each trait is determined by a single gene locus and that different traits are classified independently. While this applies to many useful traits, especially when choosing characters for crops or animal husbandry, there are many exceptions.

This tool was developed in the 20th century, long after Mendel's pioneering experiments in genetics. Today, however, they are widely used to explain Mendel's results, especially when combined with our current knowledge of DNA, genes and chromosomes.

Common terms in genetics

Some terms are commonly used in the study of genetics and are particularly useful for understanding how Punnett squares work. Among them is the term "allele" and is used to indicate a variant of a gene. For example, a pea plant can have red or white flowers, and the genetic variants that encode each are called alleles.

When an organism contains two copies of the same allele, its genetic makeup or genotype is said to be homozygous. These are also known as purebred specimens. For example, plants with white flowers are homozygous at the loci that code for flower color.

Individuals carrying two different alleles are considered heterozygous at this locus. Many plants with red flowers can have one allele for red and one allele for white. The externally observable characteristic of an individual is called the phenotype. The phenotype in a heterozygous individual is called the "dominant" form of the gene, and the deleted trait is considered the "recessive" allele. In the flower color example, the allele for red is dominant over the allele for white.

In a cross between a homozygous dominant and a homozygous recessive, all offspring will have a heterozygous genotype and a dominant phenotype.

Some gene loci are found on the sex chromosomes and are called sex-linked traits, while all others are called autosomal.

Punnett square functions

In large-scale experiments like those performed by Mendel, Punnett squares can accurately predict the proportions of various observable traits, as well as their underlying genetic makeup. For example, if a tall pure pea plant is pollinated with pollen from a short pure pea plant, the Punnett square can predict that all offspring will be tall and all will be heterozygous with both alleles for height and short height. It can further be predicted that if these heterozygous plants can self-pollinate, approximately seventy-five percent of the second-generation plants will be large and the remaining twenty-five percent will be small. Of the tall plants, one-third will remain purebred, while the remaining two-thirds will be heterozygous. Therefore, this tool is used by plant and animal breeders to select suitable specimens to obtain offspring that have a desired trait.

They are also used in genetic counseling to help couples make decisions about having children. For example, in cases where both parents are carriers of an autosomal recessive disease such as cystic fibrosis, there is a 25% chance that their child will have the disease and a 50% chance that their offspring will be a carrier. However, if one parent has the disease and the other is not a carrier or has the disease, the couple can be confident that their child will not develop cystic fibrosis because they only carry one copy of the abnormal gene.

Types of Punnett squares

Two types of Punnett squares are commonly used. The first is relevant when looking at a single trait determined by a genetic locus. This is called a monohybrid cross, and examples of this include some of Mendel's original experiments, in which he selected true breeders for a single trait and crossed them with members carrying a different allele. For a monohybrid cross, these are 2x2 squares with four boxes, each representing a fertilization event between the parental gametes.

The second type is used to predict the outcome of breeding experiments where two traits are tracked and the sixteen-box Punnett square is larger. The 4X4 square is necessary because each parent can produce four types of gametes depending on the allele distribution of the two genes.

When more than two features are observed, a Punnett square becomes heavy and other tools are used to predict the results of such crosses.

Examples of Punnett Squares

Most people will become familiar with Punnett squares through Mendel's experiments. Among the many characteristics of the common pea plant that he observed was the color of the peas. Other common examples used to illustrate the predictive power of this tool are the inheritance of blood types and eye color in humans.

Seed color in the common pea plantthe pea plant

Mendel created plants that were homozygous for both alleles: yellow and green seeds. When he cross-pollinated these homozygotes, he found that all the offspring had yellow seeds. When he allowed these yellow offspring to self-pollinate, he was surprised to find that nearly 25% of the second-generation peas contained green seeds. He concluded that the yellow allele was dominant over the green one. To better understand this phenomenon, he crossed some first generation plants with yellow seeds with a pure green plant. This later became known as the test cross.

In each Punnett square, an allele is represented by the initial of the dominant phenotype. In this case, the dominant yellow allele is denoted by an uppercase "Y" and the recessive allele by a lowercase "y". Each allele can divide independently into a gamete, and the gametes are shown outside the 2X2 table.

Each of the boxes shows a possible genotype of the offspring. In this test cross, half of the offspring have yellow seeds and are genotypically heterozygous. The other half is homozygous and has green seeds.

Tail and hair color in cats.

When a homozygous white-haired short-tailed cat is crossed with a brown-haired long-tailed cat, all offspring appear to inherit one trait from each parent. They all have short tails and brown hair, showing that the brown color is dominant over white and the short tail allele is more dominant than the long tail allele.

When members of this first generation mate with each other, the vast majority of their offspring will have short tails and brown hair. Also, there is a three in sixteen chance that the parental combinations will reappear: short tail with white hair or long tail with brown hair. Finally, there is a one in sixteen chance that a new combination will appear: long tail and white color.

If a breeder were looking for a specimen with white fur and a long tail, they would know that it would only appear in the second generation.

Constraints on Punnett squares

While Punnett squares are a convenient tool for understanding Mendelian genetics, they cannot be used in many situations involving complex genetic inheritance. For example, they are not effective for estimating the distribution of genotypes and phenotypes when there is an association between two genes. Genetic linkage is a phenomenon in which two genes are close together on the same chromosome. Therefore, the probability that these two traits are inherited together during gamete formation, in the same combination from the parents, is high. An example of this is the link between the location of the gene that causes nail-patella syndrome (NPS) and the one that determines blood type. Analysis of a family with members who suffered from SPN revealed that it was often inherited with a type B blood group. These links alter the random distribution of the two traits among the offspring, making the Punnett square an ineffective predictive tool.

Furthermore, when a single trait is determined by multiple genes and the effect of each of these genes is ranked, Punnett squares cannot accurately predict the distribution of phenotypes in the offspring. Human height is determined by over four hundred genes scattered throughout the genome. Furthermore, this property is also influenced by environmental factors such as diet.

Finally, genes inherited entirely from one parent, such as those found in mitochondria or the Y chromosome, and genotypes that are lethal to the fetus confound the results of a Punnett square.

• co-ownership– A situation in which two alleles are neither dominant nor recessive to each other and both are expressed as a phenotype.
• diploid– A cell containing two sets of chromosomes, one set from each parent. Diploid cells contain two copies of nearly all genes.
• gamete– Mature, haploid male and female germ cells that can fuse to form a zygote.
• haploid– A cell that contains a single set of chromosomes.

Trial

1. Which of these is inherited entirely from the mother?
A.eye color genes
B.Genes for Cystic Fibrosis
C.Y chromosome genes
D.mitochondrial gene

2. Which of these are assumptions in creating a Punnett square?
A.Alleles for each trait separate during meiosis.
B.Each property is sorted independently of the others.
C.Only one locus is involved in a given trait.
D.the whole interior

3. How many rows and columns would it take to create a Punnett square for a trihybrid cross?
A.3X3
B.6X6
C.8X8
D.9X9