12.4:
Trihybrid Crosses
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.
A homozygous dominant plant in this cross will be a tall plant with round, yellow seeds and the genotype uppercase TTRRYY. A homozygous recessive plant will be a short plant with wrinkled, green seeds and the genotype lowercase ttrryy.
When these plants are crossed, all of the F1 generation plants are trihybrids, meaning they are heterozygous for all three traits with the genotype shown here.
The F1 generation plants display the dominant phenotype where all of the plants are tall… with round…, yellow seeds.
When there are three pairs of contrasting characteristics, a Punnet square quickly becomes impractical because there are 64 potential genotypes in the F2 generation.
In such cases, the forked line method is often used instead. Here, the F1 heterozygotes with three pairs of traits are arranged in three rows, where each gene occupies one row.
The alleles for plant height are placed in the first row and are segregated into the ratio expected from the monohybrid crosses, where three plants are tall and one plant is short.
The alleles for seed shape are placed in the second row and are segregated on a forked line in a similar manner, in a 3 to 1 ratio. The process is repeated again in the third row with the alleles for seed color.
Now the values along each forked path are multiplied for each of the eight different outcomes.
For example, following the fork furthest on the left, three times three times three is 27. Therefore, among the 64 potential genotypes for this generation, there are 27 tall plants with round, yellow seeds.
Each path can be subsequently multiplied to find the phenotypic ratios for the entire F2 generation.
12.4:
Trihybrid Crosses
Trihybrid Crosses
Some of Mendel’s crosses examined three pairs of contrasting characteristics. Such a cross is called a trihybrid cross. A trihybrid cross is a combination of three individual monohybrid crosses. For example, plant height (tall vs. short), seed shape (round vs. wrinkled), and seed color (yellow vs. green).
The F1 generation plants of a trihybrid cross are heterozygous for all three traits and produce eight gametes. Upon self-fertilization, these gametes have an equal chance to give rise to 64 different combinations of genotypes in the F2 generation. In cases like this, when there are more than two pairs of contrasting characteristics to be studied, a Punnett square is unwieldy and impractical. The forked line method can be used instead of a Punnett square to simplify predicting genotype and phenotype ratios.
While it is impossible to predict the actual number of individuals per genotype in the F2 generation, this method can predict the phenotypic ratio, 27:9:9:9:3:3:3:1. In a cross involving tall plants with round, yellow seed and dwarf plants with wrinkled, green seeds, one can expect to find 27 tall plants with round and yellow seeds, 9 short plants with yellow, round seeds, 9 tall plants with yellow, wrinkled seeds, 9 tall plants with green, round seeds, 3 short plants with yellow, wrinkled seeds, 3 short plants with green, round seeds, 3 tall plants with green, wrinkled seeds, and 1 short plant with green, wrinkled seeds.
Rules of Multi Hybrid Fertilization
There are rules to identify the gametes and genotypes of the offspring of the F1 and F2 generations, respectively. These rules apply to all the multi-hybrid crosses that obey the law of independent assortment and follow the dominant-recessive pattern. The number of gametes formed in F1 generation can be identified by using the 2n formula, where n is the number of heterozygous gene pairs. For example, breeding between XxYy and XxYy heterozygotes has n of 2. Thus, the number of gametes formed by the F1 heterozygotes will be 22, which is four.
Similarly, breeding between XXYy and XXyY heterozygotes has n of 1 because X is not heterozygous. Hence, the number of gametes formed by the F1 heterozygotes will be 21, which is 2. Similarly, the genotype of the F2 generation can be identified using the 3n formula.
Suggested Reading
- Snustad, D. Peter, and Michael J. Simmons. Principles of genetics. John Wiley & Sons, 2015. Pages 46 – 47.
- “Laws of Inheritance”, accessed 10 May 2021 https://www.oercommons.org/courseware/module/14996/student/?task=5