How many phenotypes are expected in a trihybrid cross

Chapter Cell Division. Chapter Meiosis. Chapter Cancer. Full Table of Contents. This is a sample clip. Sign in or start your free trial. Previous Video Next Video. Next Video Embed Share. Mendel further extended his research on pea plants to trihybrid crosses, where the organisms vary in three different traits, for example, plant height - designated here by uppercase or lower case T-, seed shape shown here by the allele R, and seed color - delineated by the letter Y.

Now the values along each forked path are multiplied for each of the eight different outcomes. Snustad, D. Peter, and Michael J. Principles of genetics. Pages 46 - Please enter your institutional email to check if you have access to this content.

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Please enjoy a free hour trial. In order to begin, please login. Please click here to activate your free hour trial. If you do not wish to begin your trial now, you can log back into JoVE at any time to begin. Save to playlist. Both methods make use of the product rule and consider the alleles for each gene separately. Earlier, we examined the phenotypic proportions for a trihybrid cross using the forked-line method; now we will use the probability method to examine the genotypic proportions for a cross with even more genes.

For a trihybrid cross, writing out the forked-line method is tedious, albeit not as tedious as using the Punnett-square method. 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 and 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. For the same tetrahybrid cross, what is the expected proportion of offspring that have the dominant phenotype at all four loci? The question asks for the proportion of offspring that are 1 homozygous dominant at A or heterozygous at A, and 2 homozygous at B or heterozygous at B , and so on.

If you are ever unsure about how to combine probabilities, returning to the forked-line method should make it clear. Predicting the genotypes and phenotypes of offspring from given crosses is the best way to test your knowledge of Mendelian genetics. Given a multihybrid cross that obeys independent assortment and follows a dominant and recessive pattern, several generalized rules exist; you can use these rules to check your results as you work through genetics calculations Table 1.

To apply these rules, first you must determine n , the number of heterozygous gene pairs the number of genes segregating two alleles each. For example, a cross between AaBb and AaBb heterozygotes has an n of 2. 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. Figure 4. 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. 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 Figure 4.

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. 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.

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 5, in which you cross two TtIi individuals. 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.

Figure 5. 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? Repeat the process for traits B and C. Follow the same pattern for father's traits. Enjoy your results! Second, you find the possible alleles combinations of a given parent. FAQ What is a trihybrid cross Punnett square?

What is the trihybrid cross Punnett square used for? How many boxes are there in the trihybrid cross Punnett square? How to calculate genotype probability? It's easier than it seems! Do the Punnet square. Count how many times a given genotype is present in your table e. Find out the total number of combinations in a table e. Try out the regular Punnett square calculator , or the dihybrid cross version!

Mother's trait 1. Mother's trait 2. Mother's trait 3. Father's trait 1. Father's trait 2. Father's trait 3. Offspring genotype frequency. Punnett square. Phenotype and Genotype.