Can independent assortment be demonstrated in a monohybrid cross

Therefore, the F 1 generation of offspring all are YyRr. For the F2 generation, the law of segregation requires that each gamete receive either an R allele or an r allele along with either a Y allele or a y allele. The law of independent assortment states that a gamete into which an r allele sorted would be equally likely to contain either a Y allele or a y allele. Thus, there are four equally likely gametes that can be formed when the YyRr heterozygote is self-crossed as follows: YR, Yr, yR, and yr.

These are the offspring ratios we would expect, assuming we performed the crosses with a large enough sample size. Because of independent assortment and dominance, the dihybrid phenotypic ratio can be collapsed into two ratios, characteristic of any monohybrid cross that follows a dominant and recessive pattern.

Ignoring seed color and considering only seed texture in the above dihybrid cross, we would expect that three-quarters of the F 2 generation offspring would be round and one-quarter would be wrinkled. Similarly, isolating only seed color, we would assume that three-quarters of the F 2 offspring would be yellow and one-quarter would be green.

The sorting of alleles for texture and color are independent events, so we can apply the product rule. These proportions are identical to those obtained using a Punnett square. When more than two genes are being considered, the Punnett-square method becomes unwieldy.

For the F2 generation, the law of segregation requires that each gamete receive either an R allele or an r allele along with either a Y allele or a y allele. The law of independent assortment states that a gamete into which an r allele sorted would be equally likely to contain either a Y allele or a y allele. Thus, there are four equally likely gametes that can be formed when the YyRr heterozygote is self-crossed as follows: YR, Yr, yR, and yr. These are the offspring ratios we would expect, assuming we performed the crosses with a large enough sample size.

Independent assortment of 2 genes : This dihybrid cross of pea plants involves the genes for seed color and texture. Because of independent assortment and dominance, the dihybrid phenotypic ratio can be collapsed into two ratios, characteristic of any monohybrid cross that follows a dominant and recessive pattern. Ignoring seed color and considering only seed texture in the above dihybrid cross, we would expect that three-quarters of the F 2 generation offspring would be round and one-quarter would be wrinkled.

Similarly, isolating only seed color, we would assume that three-quarters of the F 2 offspring would be yellow and one-quarter would be green. The sorting of alleles for texture and color are independent events, so we can apply the product rule.

These proportions are identical to those obtained using a Punnett square. When more than two genes are being considered, the Punnett-square method becomes unwieldy. It would be extremely cumbersome to manually enter each genotype. For more complex crosses, the forked-line and probability methods are preferred. To prepare a forked-line diagram for a cross between F 1 heterozygotes resulting from a cross between AABBCC and aabbcc parents, we first create rows equal to the number of genes being considered and then segregate the alleles in each row on forked lines according to the probabilities for individual monohybrid crosses.

We then multiply the values along each forked path to obtain the F 2 offspring probabilities. Note that this process is a diagrammatic version of the product rule. The values along each forked pathway can be multiplied because each gene assorts independently. For a trihybrid cross, the F 2 phenotypic ratio is Independent assortment of 3 genes : The forked-line method can be used to analyze a trihybrid cross. Here, the probability for color in the F2 generation occupies the top row 3 yellow:1 green.

The probability for shape occupies the second row 3 round:1 wrinked , and the probability for height occupies the third row 3 tall:1 dwarf. The probability for each possible combination of traits is calculated by multiplying the probability for each individual trait.

While the forked-line method is a diagrammatic approach to keeping track of probabilities in a cross, the probability method gives the proportions of offspring expected to exhibit each phenotype or genotype without the added visual assistance.

To fully demonstrate the power of the probability method, however, we can consider specific genetic calculations. For instance, for a tetrahybrid cross between individuals that are heterozygotes for all four genes, and in which all four genes are sorting independently in a dominant and recessive pattern, what proportion of the offspring will be expected to be homozygous recessive for all four alleles?

Rather than writing out every possible genotype, we can use the probability method. Genes that are located on separate non-homologous chromosomes will always sort independently. However, each chromosome contains hundreds or thousands of genes organized linearly on chromosomes like beads on a string.

The segregation of alleles into gametes can be influenced by linkage, in which genes that are located physically close to each other on the same chromosome are more likely to be inherited as a pair. Homologous chromosomes possess the same genes in the same linear order.

The alleles may differ on homologous chromosome pairs, but the genes to which they correspond do not. In preparation for the first division of meiosis, homologous chromosomes replicate and synapse. Like genes on the homologs align with each other.

At this stage, segments of homologous chromosomes exchange linear segments of genetic material. This process is called recombination, or crossover, and it is a common genetic process. Because the genes are aligned during recombination, the gene order is not altered. Instead, the result of recombination is that maternal and paternal alleles are combined onto the same chromosome.

Across a given chromosome, several recombination events may occur, causing extensive shuffling of alleles. Linked genes can be separated by recombination : The process of crossover, or recombination, occurs when two homologous chromosomes align during meiosis and exchange a segment of genetic material. Here, the alleles for gene C were exchanged. The result is two recombinant and two non-recombinant chromosomes. When two genes are located in close proximity on the same chromosome, they are considered linked, and their alleles tend to be transmitted through meiosis together.

To exemplify this, imagine a dihybrid cross involving flower color and plant height in which the genes are next to each other on the chromosome. If one homologous chromosome has alleles for tall plants and red flowers, and the other chromosome has genes for short plants and yellow flowers, then when the gametes are formed, the tall and red alleles will go together into a gamete and the short and yellow alleles will go into other gametes.

These are called the parental genotypes because they have been inherited intact from the parents of the individual producing gametes. But unlike if the genes were on different chromosomes, there will be no gametes with tall and yellow alleles and no gametes with short and red alleles. If you create the Punnett square with these gametes, you will see that the classical Mendelian prediction of a outcome of a dihybrid cross would not apply.

As the distance between two genes increases, the probability of one or more crossovers between them increases, and the genes behave more like they are on separate chromosomes.

Geneticists have used the proportion of recombinant gametes the ones not like the parents as a measure of how far apart genes are on a chromosome. Using this information, they have constructed elaborate maps of genes on chromosomes for well-studied organisms, including humans. The garden pea has seven chromosomes and some have suggested that his choice of seven characteristics was not a coincidence. However, even if the genes he examined were not located on separate chromosomes, it is possible that he simply did not observe linkage because of the extensive shuffling effects of recombination.

In fact, single observable characteristics are almost always under the influence of multiple genes each with two or more alleles acting in unison.

For example, at least eight genes contribute to eye color in humans. In some cases, several genes can contribute to aspects of a common phenotype without their gene products ever directly interacting.

In the case of organ development, for instance, genes may be expressed sequentially, with each gene adding to the complexity and specificity of the organ. Genes may function in complementary or synergistic fashions: two or more genes need to be expressed simultaneously to affect a phenotype. Genes may also oppose each other with one gene modifying the expression of another.

In epistasis, the interaction between genes is antagonistic: one gene masks or interferes with the expression of another. Using this information, they have constructed elaborate maps of genes on chromosomes for well-studied organisms, including humans.

The garden pea has seven chromosomes, and some have suggested that his choice of seven characteristics was not a coincidence. However, even if the genes he examined were not located on separate chromosomes, it is possible that he simply did not observe linkage because of the extensive shuffling effects of recombination.

Question : What will be the offspring of a dihybrid cross? Background : Consider that pea plants mature in one growing season, and you have access to a large garden in which you can cultivate thousands of pea plants.

There are several true-breeding plants with the following pairs of traits: tall plants with inflated pods, and dwarf plants with constricted pods. Hypothesis : Both trait pairs will sort independently according to Mendelian laws. When the true-breeding parents are crossed, all of the F 1 offspring are tall and have inflated pods, which indicates that the tall and inflated traits are dominant over the dwarf and constricted traits, respectively.

A self-cross of the F 1 heterozygotes results in 2, F 2 progeny. Test the hypothesis : Because each trait pair sorts independently, the ratios of tall:dwarf and inflated:constricted are each expected to be Each member of the F 1 generation therefore has a genotype of TtIi. Construct a grid analogous to Figure Each individual can donate four combinations of two traits: TI , Ti , tI , or ti , meaning that there are 16 possibilities of offspring genotypes.

Because the T and I alleles are dominant, any individual having one or two of those alleles will express the tall or inflated phenotypes, respectively, regardless if they also have a t or i allele. Only individuals that are tt or ii will express the dwarf and constricted alleles, respectively. As shown in Figure Test the hypothesis : You cross the dwarf and tall plants and then self-cross the offspring. For best results, this is repeated with hundreds or even thousands of pea plants.

What special precautions should be taken in the crosses and in growing the plants? Reduce these findings to a ratio and determine if they are consistent with Mendelian laws.

Form a conclusion : Were the results close to the expected phenotypic ratio? Do the results support the prediction? What might be observed if far fewer plants were used, given that alleles segregate randomly into gametes? Try to imagine growing that many pea plants, and consider the potential for experimental error.

For instance, what would happen if it was extremely windy one day? When both the A and B loci are homozygous recessive aabb , the seeds are ovoid. However, if the dominant allele for either or both of these genes is present, the seeds are triangular. Based on this information, what are the expected phenotypic ratios for a cross between plants that are heterozygous for both traits?

What is the expected ratio of phenotypes from a dihybrid cross? In fact, single observable characteristics are almost always under the influence of multiple genes—each with two or more alleles—acting in unison. For example, at least eight genes contribute to eye color in humans. Eye color in humans is determined by multiple genes. Use the Eye Color Calculator to predict the eye color of children from parental eye color.

In some cases, several genes can contribute to aspects of a common phenotype without their gene products ever directly interacting. In the case of organ development, for instance, genes may be expressed sequentially, with each gene adding to the complexity and specificity of the organ. Genes may function in complementary or synergistic fashions, such that two or more genes need to be expressed simultaneously to affect a phenotype.

Genes may also oppose each other, with one gene modifying the expression of another. In epistasis , the interaction between genes is antagonistic, such that one gene masks or interferes with the expression of another. Epistasis is a word composed of Greek roots that mean standing upon.

The alleles that are being masked or silenced are said to be hypostatic to the epistatic alleles that are doing the masking. Often the biochemical basis of epistasis is a gene pathway in which the expression of one gene is dependent on the function of a gene that precedes or follows it in the pathway. An example of epistasis is pigmentation in mice. The wild-type coat color, agouti AA , is dominant to solid-colored fur aa. However, a separate gene C is necessary for pigment production.

A mouse with a recessive c allele at this locus is unable to produce pigment and is albino regardless of the allele present at locus A Figure Therefore, the genotypes AAcc , Aacc , and aacc all produce the same albino phenotype. In this case, the C gene is epistatic to the A gene. Epistasis can also occur when a dominant allele masks expression at a separate gene.

Fruit color in summer squash is expressed in this way. Homozygous recessive expression of the W gene ww coupled with homozygous dominant or heterozygous expression of the Y gene YY or Yy generates yellow fruit, and the wwyy genotype produces green fruit.

However, if a dominant copy of the W gene is present in the homozygous or heterozygous form, the summer squash will produce white fruit regardless of the Y alleles. Finally, epistasis can be reciprocal such that either gene, when present in the dominant or recessive form, expresses the same phenotype.

When the genes A and B are both homozygous recessive aabb , the seeds are ovoid. If the dominant allele for either of these genes is present, the result is triangular seeds. As you work through genetics problems, keep in mind that any single characteristic that results in a phenotypic ratio that totals 16 is typical of a two-gene interaction. Similarly, we would expect interacting gene pairs to also exhibit ratios expressed as 16 parts.

Note that we are assuming the interacting genes are not linked; they are still assorting independently into gametes. This section may include links to websites that contain links to articles on unrelated topics. See the preface for more information. Skip to main content. How can the forked-lined method and probability rules be used to calculate the probability of genotypes and phenotypes from multiple gene crosses?