Punnett Square
Purpose
The objective of the experiment was to use a Punnett square to determine the genotypes and phenotypes of the offspring produced from a self-pollination cross. Flower color was the main trait that was observed.
Introduction
The foundation of Mendel’s law is that units donated by two parents separate independently in the germ cells of the offspring within the sperm and egg (The mechanism of Mendelian heredity, n.d). The progeny subsequently inherits each allele for a specified attribute from the mother and father. An offspring with two identical alleles for a given trait is “homozygous for that trait” (Audesirk, Audesirk, & Byers, 2008).
Only one of the inherited alleles is visualized physically in the offspring. Therefore, the outward physical appearance of a trait is the phenotype, whereas the genotype is the precise allelic blend for a particular gene (Mendel’s first law of genetics, n.d.). The allele that is physically seen in the offspring is the dominant allele, whereas the feature whose appearance is covered up when the dominant allele is present is the recessive allele. A recessive allele is only expressed when it is homozygous (Mendelian genetics, n.d.).
Hypothesis/Predicted Outcome
It was hypothesized that crossing homozygous purple sperms with heterozygous purple eggs produced progenies with more purple flowers than white flowers.
Methods
The M.U.S.E link was used to perform the virtual experiment. A Punnett square was used to predict the genotypes and phenotypes of a self-pollination cross using flower color as the key trait. Capital letter ‘P’ was allotted to the dominant allele (color purple), whereas the lowercase letter ‘p’ was consigned to the allele for the white color, which was a recessive gene. Labels of the genotypes of the egg and sperm were put on every row and column of the Punnett square. The genotypes of the progenies were filled in the boxes by merging the sperm genotypes in the rows with the egg genotypes in the matching columns. This procedure was repeated for all the boxes. The number of progenies possessing each genotype was counted and changed to fractions of the total progenies.
Results
It was observed that the genotypic ratio was 1homozygous purple: 2 heterozygous purple: 1 homozygous white. It was also seen that three progenies had purple flowers, whereas one offspring had white flowers. The following table summarizes the obtained results.
Discussion/Analysis
The experiment yielded the expected results. It was realized that most of the progenies had purple flowers (3 out of 4). This was because of the presence of the dominant allele (P) in the sperm and eggs. It was possible to produce one white flower because both parent plants had the recessive allele (p).
The gene for purple-colored flowers was dominant, whereas the gene for white-colored flowers was recessive. The dominant gene was identified by looking at the phenotype expressed whenever a particular gene was present. For example, the color purple was expressed whenever the gene ‘P’ was present homozygously or heterozygously. The recessive gene, on the other hand, was only expressed phenotypically when it was homozygous. Only a quarter of the progenies had white flowers, whereas three quarters had purple flowers. Therefore, the ratio with the highest chances of being observed in all the traits was 3 purple-colored flowers to 1 white-colored flower.
References
Audesirk, T., Audesirk, G., & Byers, B. E. (2008). Biology: Life on earth with physiology. Upper Saddle River, NJ: Prentice Hall.
Mendelian genetics. (n.d.). Web.
Mendel’s first law of genetics (n.d.). Web.
The mechanism of Mendelian heredity. (n.d.). Web.