Question

Mendelian ratios are modified in crosses involving autotetraploids. Assume that one plant expresses the dominant trait green seeds and is homozygous (WWWW). This plant is crossed to one with white seeds that is also homozygous \((w w w w) .\) If only one dominant allele is sufficient to produce green seeds, predict the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) results of such a cross. Assume that synapsis between chromosome pairs is random during meiosis.

Short answer

Answer: The predicted F2 generation phenotypic ratio is 7:1 for green seeds to white seeds.

Step-by-step solution

Determine parental genotypes and phenotypes

The parental plants have the following genotypes and phenotypes: - Parent 1: Genotype - WWWW, Phenotype - Green seeds - Parent 2: Genotype - wwww, Phenotype - White seeds

Determine possible gametes from parental genotypes

In the parental generation, each plant produces the following gametes: - Parent 1 (WWWW): Gametes - WW, WW, WW, WW - Parent 2 (wwww): Gametes - ww, ww, ww, ww

Predict F1 generation genotypes and phenotypes

The F1 generation is produced by the cross between the gametes of Parent 1 and Parent 2: - F1 Genotype: (WW)(ww) or WWWW As only one dominant allele is sufficient to produce green seeds, all individuals in the F1 generation will have green seeds.

Determine possible gametes from F1 genotypes

In the F1 generation, a plant has the genotype WWWW. There are different possibilities for the formation of gametes due to random synapsis during meiosis: - Gametes: Ww, Ww, Ww, Ww

Predict F2 generation genotypes and phenotypes

The F2 generation is produced by the cross between the gametes of the F1 individuals (self-fertilization): - F2 Genotypes: * 1/8 WWWW * 4/8 WwWW * 1/8 (ww)(WW) or WWww * 2/8 (Ww)(Ww) or WwWw The F2 Phenotypes: - Green seeds (having at least one dominant W allele): 7/8 - White seeds (having all four recessive w alleles): 1/8 The predicted F2 generation phenotypic ratio is 7:1 for green seeds to white seeds.

Key concepts

Autotetraploids

Autotetraploids refer to plants or organisms with four copies of each chromosome, unlike the usual diploids that have two sets. This genetic characteristic is formed due to a process called autopolyploidy, where additional chromosome sets are derived from a single species. Autotetraploidy can occur naturally through events like chromosome doubling or artificially through certain treatments.

In genetic terms, being an autotetraploid means individuals have more alleles to potentially combine during reproduction. For instance, rather than having two alleles (like W and w) for a gene as in diploids, autotetraploids might have four alleles (WWWW or wwww). This results in a wide range of possible genetic combinations, which can affect both the phenotype and genotype ratios in offspring.

One significant aspect of autotetraploids is how they affect Mendelian ratios. While Mendel's laws predict specific ratios in simpler organisms, these ratios can change when dealing with organisms like autotetraploids because of the increased number of allele combinations possible during gamete formation and random synapsis in meiosis.

Dominant and Recessive Traits

In genetics, traits are determined by alleles, where each organism carries two or more alleles for a given trait. These alleles can either be dominant or recessive. A dominant trait is one that will always be expressed if at least one dominant allele is present. In contrast, recessive traits are only expressed when an individual has two recessive alleles.

For example, in the original exercise, the dominant trait is green seeds represented by the allele 'W'. Whenever present, a single 'W' is enough to dominate over recessive 'w', which represents the white seed trait. Thus, if the plant has at least one 'W' allele (e.g., Ww, WW, WWWW), it will show the phenotype of green seeds. However, a recessive trait, like white seeds, only appears if all alleles are recessive (e.g., wwww).

This notion of dominance and recessiveness is central to understanding how organisms inherit traits and why certain conditions or features are expressed in offspring, especially in autotetraploids where the complexity can increase due to multiple allele interactions.

Genotypes and Phenotypes

Genotype and phenotype are fundamental concepts tied to the genetic makeup and resulting traits of organisms. A genotype comprises the genetic constitution of an organism, specifically the alleles it possesses for a particular trait. This can include combinations like WWWW or wwww in autotetraploids.

The phenotype, on the other hand, is the observable physical or physiological trait that results from the genotype. An organism with a genotype of WWWW will have a phenotype of green seeds, due to the presence of the dominant 'W' allele. Similarly, plants with wwww genotype will express white seeds, since no dominant allele is there to mask the effect of the recessive alleles.

In genetics, especially with autotetraploids, predicting the relationship between genotypes and phenotypes can become complex. This is because multiple alleles participate in determining the phenotype, and the presence of even one dominant allele can alter the outward appearance, as seen in the dominant green seed trait. Understanding this allows scientists to track trait inheritance patterns and predict outcomes of genetic crosses.