Question

Figure 8.10 In pea plants, purple flowers \((P)\) are dominant to white \((p)\), and yellow peas \((Y)\) are dominant to green \((y)\). What are the possible genotypes and phenotypes for a cross between \(P p Y Y\) and ppYy pea plants? How many squares would you need to complete a Punnett square analysis of this cross?

Short answer

The possible genotypes for a cross between PpYY and ppYy pea plants are PpYY, ppYY, Ppyy, and ppyy. The possible phenotypes are Purple flowers with Yellow peas, White flowers with Yellow peas, Purple flowers with Green peas, and White flowers with Green peas. A 2x2 Punnett square with a total of four squares is needed for this analysis.

Step-by-step solution

The given parent plants have genotypes PpYY and ppYy. The first parent has one purple flower allele (P) and one white flower allele (p), and two yellow pea alleles (YY). The second parent has two white flower alleles (pp) and one yellow pea allele (Y) and one green pea allele (y). #2. Determine gametes#

For each parent, we will find the possible gamete combinations. For the first parent (PpYY), the possible gametes are PY and pY. For the second parent (ppYy), the possible gametes are pY and py. #3. Create Punnett square#

Since there are two possible gametes for each parent plant, we will use a 2x2 Punnett square to visualize all possible offspring genotypes. There are a total of four possible combinations between the parental gametes, so each square in the Punnett square will represent one unique combination. #4. Fill Punnett square with offspring genotypes#

Place the gametes from the first parent (PpYY) on the top row of the Punnett square and the gametes from the second parent (ppYy) on the left column. Then, fill in the squares by combining the parental alleles. Here are the offspring genotypes in the Punnett square: | | PY | pY | |---|----|----| | pY| PpYY | ppYY | | py| Ppyy | ppyy | #5. Determine phenotypes of offspring genotypes#

Convert the offspring genotypes to their respective phenotypes using the dominance relationships (P is dominant to p, and Y is dominant to y). Here are the phenotypes of each genotype in the Punnett square: | | PY | pY | |----|---------|---------| | pY | Purple, Yellow | White, Yellow | | py | Purple, Green | White, Green | #6. Summarize the results#

The possible genotypes for the cross between PpYY and ppYy pea plants are PpYY, ppYY, Ppyy, and ppyy. The possible phenotypes are Purple flowers with Yellow peas, White flowers with Yellow peas, Purple flowers with Green peas, and White flowers with Green peas.

Key concepts

Genotype and Phenotype Relationships

Understanding the relationship between genotype and phenotype is fundamental in genetics. Genotype refers to the genetic makeup of an organism; in other words, the set of alleles a living being carries. For example, if a pea plant has one allele for purple flowers and one for white, its genotype is expressed as Pp.

On the other hand, phenotype describes the physical characteristics or traits that are expressed, such as the color of the flowers. The phenotype is what you can see, and it is the result of the interaction between the genotype and the environment.

It is essential to note that the genotype determines the potential for certain traits, but dominant and recessive alleles can affect which traits appear. In the given pea plant example, the phenotype of the plant with the Pp genotype will be purple flowers, since the P allele (for purple) is dominant over the p allele (for white).

Monohybrid Cross

A monohybrid cross is a simple genetic cross considering only one trait. It involves individuals with different alleles for a single locus; for instance, a plant with purple flowers (heterozygous Pp) is crossed with one with white flowers (homozygous pp).

In this type of cross, we analyze the distribution of these alleles in the offspring, which can be easily visualized through a Punnett square, a tool that helps predict the offspring's probability of inheriting specific alleles.

For example, crossing a heterozygous purple-flowered plant (Pp) with a white-flowered one (pp) results in two different genotypes in the offspring: 50% will be heterozygous (Pp) and show the purple phenotype because the P allele is dominant, while the other 50% will be homozygous recessive (pp) and display white flowers, as shown in a typical 2x2 monohybrid Punnett square.

Dihybrid Cross

A dihybrid cross, on the other hand, involves two traits, which means the genetic analysis is more complex. It considers the inheritance of two different genes, like flower color and pea color in our pea plants. When creating a Punnett square for a dihybrid cross, we must consider every possible combination of alleles for each trait.

The process involves determining the gamete combinations each parent can produce, which for a heterozygous pair would include four types of gametes, and then setting up a 4x4 Punnett square to calculate the odds for each genotype. The resulting offspring can exhibit four phenotypic combinations, depending on the dominance of the alleles involved.

In our example, a cross between one parent with genotype PpYy and another with ppyy would result in sixteen different potential genotypes. The Punnett square helps to visualize and calculate the ratios of these genotypes and subsequently the phenotype ratios.

Mendelian Inheritance

Mendelian inheritance or Mendelian genetics is based on Gregor Mendel's experiments on pea plants. Mendel concluded that organisms have pairs of genetic 'factors' (now known as genes) and that these come in different forms called alleles. Traits are inherited in a predictable way following principles that include:

• Dominance and recessiveness
• Independent assortment
• Segregation of alleles

Mendelian inheritance is often displayed in monohybrid or dihybrid crosses within Punnett squares. It demonstrates how dominant alleles can mask the presence of recessive ones and how, during gamete formation, alleles for different genes segregate independently of one another.

In our pea plant scenario, the principles of Mendel's laws can be observed. The PpYy pea plant will form gametes that resemble Mendel's law of segregation, where each gamete receives only one allele of the two it carries for a trait, and the offspring's genotypes and phenotypes are determined following the law of independent assortment.