Question

Mendel crossed peas having round green seeds with peas having wrinkled yellow seeds. All \(\mathrm{F}_{1}\) plants had seeds that were round and yellow. Predict the results of testcrossing these \(\mathrm{F}_{1}\) plants.

Short answer

The predicted results of the testcross are that 25% of the offspring will have round yellow seeds, 25% will have round green seeds, 25% will have wrinkled yellow seeds, and 25% will have wrinkled green seeds.

Step-by-step solution

Identify traits and their alleles in the parent plants

Mendel crossed peas with round green seeds (parent 1) and peas with wrinkled yellow seeds (parent 2). The traits involved are: 1. Seed shape: round (dominant, R) vs wrinkled (recessive, r) 2. Seed color: green (dominant, G) vs yellow (recessive, g) Parent 1 has round and green seeds, so its genotype can be represented as RRGG. Parent 2 has wrinkled and yellow seeds, so its genotype can be represented as rrgg.

Determine the genotype of the F1 generation

When Parent 1 (RRGG) and Parent 2 (rrgg) are crossed, their offspring (F1 generation) will inherit one allele for shape and one allele for color from each parent. - From Parent 1, offspring will inherit allele R and allele G - From Parent 2, offspring will inherit allele r and allele g Therefore, the genotype of the F1 plants will be RrGg (round and yellow seeds).

Perform the testcross

A testcross involves crossing an organism with an unknown genotype with an organism with a homozygous recessive genotype. In this case, we cross the F1 plants (RrGg) with plants having wrinkled yellow seeds (rrgg).

Determine the offspring genotypes from the testcross

We can use a Punnett square to predict the results of the testcross between F1 plants (RrGg) and plants with wrinkled yellow seeds (rrgg). The Punnett square will involve one R or r allele and one G or g allele from the F1 plants, and one r and one g allele from the testcross plants. The possible gametes for the F1 plants (RrGg) are RG, Rg, rG, rg. The possible gametes for the testcross plants (rrgg) are rg. Now, we construct the Punnett square: | | RG | Rg | rG | rg | |----|----|----|----|----| | rg |RrGg|Rrgg|rrGg|rrgg|

Interpret the results of the testcross

From the resulting Punnett square, we can observe that the offspring will have the following genotypes: 1. RrGg (round, yellow) - 25% 2. Rrgg (round, green) - 25% 3. rrGg (wrinkled, yellow) - 25% 4. rrgg (wrinkled, green) - 25% So, the predicted results of the testcross are that 25% of the offspring will have round yellow seeds, 25% will have round green seeds, 25% will have wrinkled yellow seeds, and 25% will have wrinkled green seeds.

Key concepts

Punnett Square

A Punnett Square is a simple, yet powerful tool used in Mendelian genetics to predict the possible genotypes of offspring from a particular cross or breeding experiment. When Gregor Mendel carried out his experiments with pea plants, he needed a method for predicting genetic variations in his F1 and testcross generations. This diagram resembles a grid, where the alleles from one parent line up along the top, and the alleles from the second parent are listed along the side.

• The square then becomes populated by aligning each allele from the rows with each allele from the columns.
• This helps determine the possible allele combinations the offspring could inherit. For example, the results of the testcross between F1 plants (with genotypes RrGg) and homozygous recessive plants (with genotypes rrgg) can be effectively organized using a Punnett Square.

In the example given, we see how each of the four types of gametes from the F1 generation (RG, Rg, rG, and rg) are combined with one type of gamete (rg) from the testcross parent to form a new grid of potential genotypes for the offspring.

Dominant and Recessive Traits

In Mendelian genetics, traits are typically categorized as either dominant or recessive. Dominant traits require only one allele to be expressed, meaning if an organism inherits even one dominant allele, that trait will appear in the organism's phenotype. On the other hand, recessive traits require two copies of the recessive allele for the trait to be observed phenotypically.

In the exercise, the seed shape and seed color traits are clearly identified as being dominant or recessive.

• Round seed shape (R) is dominant over wrinkled (r).
• Green seed color (G) is dominant over yellow (g).

The F1 generation, created by crossing the two parent plants, shows dominant characteristics for both shape and color, resulting in round, yellow seeds. This occurs because one dominant allele (R or G) in the genotype is sufficient to mask the presence of the recessive allele (r or g). Understanding which traits are dominant or recessive helps predict how traits will be expressed in offspring.

Testcross

The testcross is a brilliant method to ascertain the genotype of an individual organism showing a dominant phenotype but with an unknown genotype. This technique involves breeding the "unknown" organism with a homozygous recessive individual. If the organism in question carries any recessive alleles, they may appear in the offspring.

For instance, in the given scenario, the F1 plants (RrGg), with round and yellow seeds, are testcrossed with plants having wrinkled yellow seeds (rrgg). By crossbreeding with a recessive specimen (rrgg), any dominant recessive allele combinations the parent carries will be revealed through the different phenotypic offspring.

• The predicted results from this test include all possible combinations of the dominant and recessive traits, namely round yellow (RrGg), round green (Rrgg), wrinkled yellow (rrGg), and wrinkled green seeds (rrgg), each with a probability of 25%.

Testcrosses are widely used in genetics to help scientists understand which alleles an organism carries, especially in the case where the dominant trait is present, making it unclear what the other allele could be.