Question

Imagine that you are performing a cross involving seed color in garden pea plants. What traits would you expect to observe in the \(F_{1}\) offspring if you cross true-breeding parents with green seeds and yellow seeds? Yellow seed color is dominant over green. a. only yellow-green seeds b. only yellow seeds c. 1: 1 yellow seeds:green seeds d. 1: 3 green seeds:yellow seeds

Short answer

The correct answer is (b) only yellow seeds, as all the F1 offspring will have the Yg genotype and display the dominant yellow seed color.

Step-by-step solution

Understand the terms true-breeding and dominant

True-breeding refers to organisms that produce offspring identical to themselves when self-fertilized or bred with another true-breeding individual with the same traits. In this case, true-breeding parents with green seeds will have a homozygous genotype (both alleles for the seed color are the same), which we can represent as "gg". Similarly, true-breeding parents with yellow seeds, the dominant trait, will have a homozygous genotype, which we can represent as "YY".

Set up a Punnett square

To determine the genotypes and phenotypes of the offspring, we will use a Punnett square. The Punnett square helps us visualize the possible combinations of alleles that the offspring can inherit from the parents. In this case, we are crossing two true-breeding parents, one with green seeds (gg) and one with yellow seeds (YY).

Fill in the Punnett square

Now, we will fill in the Punnett square with the possible combinations of alleles that the offspring could inherit from their true-breeding parents. | | Y | Y | | ------ | ------ | ------ | | **g** | Yg | Yg | | **g** | Yg | Yg | Every corner of the Punnett square now represents a possible zygote from the cross. Here, all possible combinations of alleles result in the "Yg" genotype.

Determine the phenotypes of the offspring

We established that yellow seed color is dominant over green. Therefore, any offspring with the "Yg" genotype will exhibit the dominant trait, which is yellow seeds. Thus, we can now answer the question about the expected traits in the F1 offspring: a. only yellow-green seeds b. only yellow seeds c. 1:1 yellow seeds: green seeds d. 1:3 green seeds: yellow seeds The correct answer is (b) only yellow seeds, as all the F1 offspring will have the Yg genotype and display the dominant yellow seed color.

Key concepts

Punnett Square

The Punnett square is an essential tool in genetics for predicting the possible genotypes of offspring from a particular genetic cross. It's a simple grid that helps visualize how alleles from each parent can combine to form the genotypes of the offspring.

When using a Punnett square, we'll represent the alleles of the parents along the top and the side. Each box in the grid corresponds to a potential outcome for the offspring's genotype.

• In the case of our pea plant exercise, one parent is homozygous for the dominant yellow seed trait (YY), and the other parent is homozygous for the recessive green seed trait (gg).
• In our grid of 2x2, we can see all offspring will inherit one allele from each parent, resulting in the genotype Yg in every cell.

This visualization helps us easily determine that all offspring will possess the dominant phenotype, in this case, yellow seeds.

Dominant Traits

Dominant traits are those that express themselves in the phenotype even when only one allele of a gene is present. When examining genetic crosses, it is important to identify which trait is dominant.

In our pea plant example, yellow seeds are the dominant trait over green seeds. This means that even if a pea plant inherits only one allele for yellow seeds (Y) along with one for green seeds (g), the resulting seed color will be yellow. This is due to the dominant nature of the allele for yellow seeds.

• Understanding dominant and recessive traits is crucial in predicting the phenotypes of offspring, particularly when using tools like the Punnett square.
• All offspring from our specific cross will exhibit yellow seeds because the presence of the dominant yellow "Y" masks the expression of the recessive green "g".

Therefore, recognizing dominant traits allows us to predict that any offspring with at least one dominant allele will display the associated dominant phenotype.

Homozygous Genotype

A homozygous genotype occurs when an organism has two identical alleles for a given trait. In genetics notation, this can be represented as either both dominant (YY) or both recessive (gg).

In our pea plant example, true-breeding parents have homozygous genotypes:

• The green-seeded parent is homozygous recessive (gg), meaning both alleles are the same and encode for the green seed color.
• The yellow-seeded parent is homozygous dominant (YY), indicating both alleles are identical and responsible for the yellow seed color.

When these homozygous parents are crossed, the result is offspring with heterozygous genotypes (Yg). This emphasizes the combination of genes and how dominant and recessive traits interplay.

Understanding homozygosity helps clarify why offspring consistently display a dominant trait when crossed between opposite homozygous parents, as seen in our exercise with yellow and green seeds.