Dihybrid inheritance
Dihybrid inheritance is a genetic concept that examines how two distinct hereditary traits are passed down from parents to offspring. This pattern often provides richer insights than analyzing single traits alone, as many traits are influenced by multiple genes. The foundational work on dihybrid inheritance was conducted by Austrian botanist Gregor Mendel, who first explored the principles of inheritance through experiments with pea plants. By crossing plants with different traits, Mendel discovered that traits segregate independently, leading to characteristic inheritance ratios.
In his experiments, Mendel crossed purebred round, yellow peas with purebred wrinkled, green peas, yielding a variety of offspring. This established a dihybrid ratio of 9:3:3:1 in the second generation. The concept of independent assortment suggests that the inheritance of one trait does not affect the inheritance of another, a principle that holds true unless the genes are linked on the same chromosome. Dihybrid inheritance not only applies to plants but also extends to animals and other organisms, illustrating the complexity and variability of genetic traits. Understanding this model is essential for grasping more intricate patterns of inheritance in genetics, including those influenced by multiple genes.
Dihybrid inheritance
SIGNIFICANCE: The simultaneous analysis of two different hereditary traits may produce more information than the analysis of each trait separately. In addition, many important hereditary traits are controlled by more than one gene. Traits controlled by two genes serve as an introduction to the more complex topic of traits controlled by many genes.
Mendel’s Discovery of Dihybrid Inheritance
Austrian botanist Johann Gregor Mendel was the first person to describe both monohybrid and inheritance. When he crossed purebred round-seed garden peas with purebred wrinkled-seed plants, they produced only round seeds. He planted the monohybrid round seeds and allowed them to fertilize themselves; they subsequently produced ¾ round and ¼ wrinkled seeds. He concluded correctly that the monohybrid generation was for an allele that produces round seeds and another allele that produces wrinkled seeds. Since the monohybrid seeds were round, the round allele must be dominant to the wrinkled allele. He was able to explain the 3:1 ratio in the second generation by assuming that each parent contributes only one copy of a gene to its progeny. If W represents the round allele and w the wrinkled allele, then the original true-breeding parents are WW and ww. When eggs and pollen are produced, they each contain only one copy of the gene. Therefore the monohybrid seeds are heterozygous Ww. Since these two alleles will separate during when pollen and eggs are produced, ½ of the eggs and pollen will be W and ½ will be w. Mendel called this “segregation.” When the eggs and pollen combine randomly during fertilization, ¼ will produce WW seeds, ½ will produce Ww seeds, and ¼ will produce ww seeds. Since W is dominant to w, both the WW and Ww seeds will be round, producing ¾ round and ¼ wrinkled seeds. When Mendel crossed a purebred yellow-seed plant with a purebred green-seed plant, he observed an entirely analogous result in which the yellow allele (G) was dominant to the green allele (g).
Once Mendel was certain about the nature of monohybrid inheritance, he began to experiment with two traits at a time. He crossed purebred round, yellow pea plants with purebred wrinkled, green plants. As expected, the dihybrid seeds that were produced were all round and yellow, the dominant form of each trait. He planted the dihybrid seeds and allowed them to fertilize themselves. They produced 9/16 round, yellow seeds; 3/16 round, green seeds; 3/16 wrinkled, yellow seeds; and 1/16 wrinkled, green seeds. Mendel was able to explain this dihybrid ratio by assuming that in the dihybrid flowers, the of W and w was independent of the segregation of G and g. Mendel called this independent assortment. Thus, of the ¾ of the seeds that are round, ¾ should be yellow and ¼ should be green, so that ¾ × ¾ = 9/16 should be round and yellow, and ¾ × ¼ = 3/16 should be round and green. Of the ¼ of the seeds that are wrinkled, ¾ should be yellow and ¼ green, so that ¼ × ¾ = 3/16 should be wrinkled and yellow, and ¼ × ¼ = 1/16 should be wrinkled and green.
Sex Chromosomes
Humans and many other species have sex chromosomes. In humans, normal females have two X chromosomes and normal males have one X and one Y chromosome. Therefore, sex-linked traits, which are controlled by genes on the X or Y chromosome, are inherited in a different pattern than the genes that have already been described. Since there are few genes on the Y chromosome, most sex-linked traits are controlled by genes on the X chromosome.
Every daughter gets an X chromosome from each parent, and every son gets an X from his mother and a Y from his father. Human red-green color blindness is controlled by the recessive allele (r) of an X-linked gene. A red-green color-blind woman (rr) and a normal man (RY) will have normal daughters (all heterozygous Rr) and red-green color-blind sons (rY). Conversely, a normal woman (RR) and a red-green color-blind man (rY) will have only normal children, since their sons will get a normal X from the mother (RY) and the daughters will all be heterozygous (Rr). A heterozygous woman (Rr) and a red-green color-blind man (r Y) will have red-green color-blind sons (r Y) and daughters (rr), and normal sons (RY) and daughters (Rr) in equal numbers.
A dihybrid woman who is heterozygous for red-green and (a recessive trait that is not sex linked) can make four kinds of eggs with equal probability: R;A, R;a, r;A, and r;a. A normal, monohybrid man who is heterozygous for albinism can make four kinds of sperm with equal probability: R;A, R;a, Y;A, and Y;a. It is easy to predict the probability of each possible kind of child from this mating.
The probabilities are 6/16 normal female, 2/16 female, 3/16 normal male, 3/16 red-green color-blind male, 1/16 albino male, and 1/16 albino, red-green color-blind male. Note that the probability of normal coloring is ¼ and the probability of albinism is ¼ in both sexes. There is no change in the inheritance pattern for the gene that is not sex linked.
Other Examples of Dihybrid Inheritance
A hereditary trait may be controlled by more than one gene. To one degree or another, almost every hereditary trait is controlled by many different genes, but often one or two genes have a major effect compared with all the others, so they are called single-gene or two-gene traits. Dihybrid inheritance can produce traits in various ratios, depending on what the gene products do. A number of examples will be presented, but they do not exhaust all of the possibilities.
The comb of a chicken is the fleshy protuberance that lies on top of the head. There are four forms of the comb, each controlled by a different combination of the two genes that control this trait. The first gene exists in two forms (R and r), as does the second (P and p). In each case, the form represented by the uppercase letter is dominant to the other form. Since there are two copies of each gene (with the exception of genes on sex chromosomes), the first gene can be present in three possible combinations: RR, Rr, and rr. Since R is dominant, the first two combinations produce the same trait, so the symbols R‗ and P‗ can be used to represent either of the two combinations. Chickens with R‗;P‗ genes have what is called a walnut comb, which looks very much like the meat of a walnut. The gene combinations R‗;pp, rr;P‗, and rr;pp produce combs that are called rose, pea, and single, respectively. If two chickens that both have the gene combination Rr;Pp mate, they will produce progeny that are 9/16 walnut, 3/16 rose, 3/16 pea, and 1/16 single.
White clover synthesizes small amounts of cyanide, which gives clover a bitter taste. There are some varieties that produce very little cyanide (sweet clover). When purebred bitter clover is crossed with some varieties of purebred sweet clover, the progeny are all bitter. However, when the hybrid progeny is allowed to fertilize itself, the next generation is 9/16 bitter and 7/16 sweet. This is easy to explain if it is assumed that bitter/sweet is a dihybrid trait. The bitter parent would have the gene combination AA;BB and the sweet parent aa;bb, where A and B are dominant to a and b, respectively. The bitter dihybrid would have the gene combination Aa;Bb. When it fertilized itself, it would produce 9/16A‗;B‗, which would be bitter, and 3/16A‗;bb, 3/16aa;B‗, and 1/16aa;bb, all of which would be sweet. Clearly, both the A allele and the B allele are needed in order to synthesize cyanide. If either is missing, the clover will be sweet.
Absence of Dominance
In all of the previous examples, there was one dominant allele and one recessive allele. Not all genes have dominant and recessive alleles. If a purebred snapdragon with red flowers (RR) is crossed with a purebred snapdragon with white flowers (rr), all the monohybrid progeny plants will have pink flowers (Rr). The color depends on the number of R alleles present: two Rs will produce a red flower, one R will produce a pink flower, and no Rs will produce a white flower. This is an example of partial dominance or additive inheritance.
Consider a purebred red wheat kernel (AA;BB) and a purebred white wheat kernel (aa;bb). If the two kernels are planted and the resulting plants are crossed with each other, the progeny dihybrid kernels will be light red (Aa;Bb). If the dihybrid plants grown from the dihybrid kernels are allowed to self-fertilize, they will produce 1/16 red (AA;BB), 4/16 medium red (AA;Bb and Aa;BB), 6/16 light red (AA;bb, Aa;Bb, and aa;BB), 4/16 very light red (Aa;bb and aa;Bb), and 1/16 white (aa;bb). The amount of red pigment depends on the number of alleles (A and B) that control pigment production. Although it may appear that this is very different than the example in the first table, they are in fact very similar.
All of the inheritance patterns that have been discussed are examples of independent assortment, in which the segregation of the alleles of one gene is independent of the segregation of the alleles of the other gene. That is exactly what would be expected from meiosis if the two genes are not on the same chromosome. If two genes are on the same chromosome and sufficiently close together, they will not assort independently, and the progeny ratios will not be like any of those described. In that case, the genes are referred to as linked genes.
Key Terms
- allelesdifferent forms of the same gene; any gene may exist in several forms having very similar but not identical DNA sequences
- dihybridan organism that is heterozygous for both of two different genes
- heterozygousa condition in which the two copies of a gene in an individual (one inherited from each parent) are different alleles
- homozygousa condition in which the two copies of a gene in an individual are the same allele; synonymous with “purebred”
Bibliography
Cooper, Jessica. "What Are the Odds? Integrated STEM and Student Conceptual Understanding of Genetic Inheritance." Kennesaw State University, 2024, digitalcommons.kennesaw.edu/dissertations/39/. Accessed 10 Sept. 2024. Fletcher, Hugh, and Ivor Hickey. Genetics. London: Garland, 2013. Print.
Hartl, Daniel L., and Maryellen Ruvolo. Genetics: Analysis of Genes and Genomes. 8th ed. Burlington: Jones, 2012. Print.
Leacock, Stefanie West. "Dihybrid Crosses and Independent Assortment." University of Arkansas at Little Rock, 2021, bio.libretexts.org/Courses/University‗of‗Arkansas‗Little‗Rock/Genetics‗BIOL3300‗(Leacock)/Genetics‗Textbook/04%3A‗Inheritance/4.02%3A‗‗Mendelian‗Genetics/4.2.02%3A‗Dihybrid‗Crosses‗and‗Independent‗Assortment. Accessed 10 Sept. 2024.
Madigan, Michael M., et al., eds. Brock Biology of Microorganisms. 12th ed. San Francisco: Pearson/Benjamin Cummings, 2009. Print.
Mauseth, James D. “Genetics.” Botany: An Introduction to Plant Biology. 3rd ed. Sudbury: Jones and Bartlett, 2003. Print.
Slack, Jonathan M. W. Essential Developmental Biology. 3rd ed. Hoboken: Wiley, 2013. Print.
Thomas, Alison. “Dihybrid Inheritance.” Introducing Genetics: From Mendel to Molecule. Cheltenham: Nelson, 2003. Print.
Tortora, Gerard J., Berdell R. Funke, and Christine L. Case. Microbiology: An Introduction. 10th ed. San Francisco: Pearson, 2010. Print.
Wolf, Jason B., Edmund D. Brodie III, and Michael J. Wadel. Epistasis and the Evolutionary Process. New York: Oxford UP, 2000. Print.