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 (wwww). 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

The F1 generation has a phenotypic ratio of 1 green : 0 white seeds since all individuals have the heterozygous genotype (WwWw) and at least one dominant allele is sufficient to produce green seeds. For the F2 generation, after performing a self-cross of the F1 individuals, the phenotypic ratio is 7 greens: 1 white. This result is obtained by counting the different genotypes and phenotypes of the F2 offspring (4 WWWW, 3 WwWw, and 1 wwww).

Step-by-step solution

Determine the gametes produced by each parent

First, we need to find the possible gametes produced by each parent. For the parent with green seeds (WWWW), there is only one type of gamete, which is WW. For the parent with white seeds (wwww), there is only one type of gamete as well, which is ww.

Predict the genotype and phenotype of the F1 generation

Since there is only one type of gamete for each parent, we cross these gametes to get the genotype for the F1 generation: WW x ww = WwWw (which is the heterozygous genotype). Since we are given that only one dominant allele is sufficient to produce green seeds, all individuals of the F1 generation will have green seeds.

Determine the gametes produced by the F1 generation

Now, we need to find the possible gametes produced by the F1 generation. Since chromosome pairs synapse randomly during meiosis, the F1 generation of autotetraploids can produce multiple types of gametes. In this case, the F1 individual can produce gametes: WW, Ww, and ww.

Predict the genotype and phenotype of the F2 generation

To predict the genotype and phenotype of the F2 generation, we perform a self-cross between the gametes from the F1 individuals. We get the following set of outcomes: - WW x WW = WWWW (green seeded plants) - WW x Ww = WWWW, WwWw (green seeded plants) - WW x ww = WwWw (green seeded plants) - Ww x Ww = WWWW, WwWw, wwww (green and white seeded plants) - Ww x ww = WwWw, wwww (green and white seeded plants) - ww x ww = wwww (white seeded plants) Counting the F2 offspring genotypes, we get: - 4 WWWW - 3 WwWw - 1 wwww

Determine the phenotypic ratio in the F2 generation

Now let's count the phenotypes in the F2 generation: - 7 green seeded plants (WWWW and WwWw) - 1 white seeded plant (wwww) So, the phenotypic ratio for the F2 generation is 7 green: 1 white.

Key concepts

Autotetraploids

Autotetraploids are unique because they have four sets of chromosomes. This is twice what is typical for organisms like humans, who have two sets. In plants, like the ones with seed traits described, having four chromosome sets can greatly impact inheritance.
These extra chromosome sets come from doubling the genome of a single species. Therefore, autotetraploids are genetically uniform compared to the mix of species found in allopolyploids.
This has significant implications for genetic traits, such as seed color. The presence of more chromosomes means there are more chances for genetic combinations, which is why Mendelian ratios might not appear in expected patterns.

Genotype and Phenotype Prediction

When predicting the genotype and phenotype of plants, it's essential to understand what each term means.
**Genotype** refers to the genetic makeup, the alleles present (like Ww or ww). In our example, it defines whether a plant is homozygous or heterozygous.
**Phenotype** is the physical expression of these genes, such as green or white seeds.
By predicting these, we can see the likely outcomes of a genetic cross.

• Green seeds arise if at least one dominant allele (W) is present.
• White seeds occur when there are no dominant alleles (ww).

In autotetraploids, because you have more alleles interacting, it creates more complexity in both genotype and phenotype predictions.

Synapsis During Meiosis

Synapsis is crucial during meiosis as it ensures proper pairing of chromosomes.
It involves the crossing over and exchange of genetic material.
In autotetraploids, because there are more chromosomes, synapsis can be more complex, occurring randomly.
This random pairing leads to various combinations of gametes, significantly affecting inheritance outcomes. During meiosis, each chromosome pair lines up and exchanges genetic information. This randomness can breed diversity in offspring by generating different gamete combinations.

• **Random pairing:** Results in unpredictable gamete production (WW, Ww, ww).
• **Chromosome segregation:** Without proper synapsis, genetic inheritance patterns may fail.

F1 and F2 Generations

The F1 and F2 generations are terms used to describe the offspring in genetic crosses.
The F1 generation is the first filial generation, or the direct offspring from a parental cross.
In the context of autotetraploids, the F1 generation results from mixing gametes from two homozygous parents (WWWW and wwww), leading to heterozygous genotypes like WwWw. In this exercise:

• **F1 Generation:** All green seeds due to the presence of at least one dominant allele.
• **F2 Generation:** More diverse outcomes from a self-cross involving F1 plants, showing both green (WWWW, WwWw) and white seeds (wwww).

This cross results in a modified Mendelian ratio, demonstrating the genetic richness and variability when dealing with autotetraploids.