Question

The blending-inheritance hypothesis proposed that the genetic material from parents is mixed in the offspring. As a result, traits of offspring and later descendants should lie between the phenotypes of parents. Mendel, in contrast, proposed that genes are discrete and that their integrity is maintained in the offspring and in subsequent generations. Suppose the year is \(1890 .\) You are a horse breeder and have just read Mendel's paper. You don't believe his results, however, because you often work with cremello (very light-colored) and chestnut (reddish-brown) horscs. You know that when you breed a cremcllo individual from a pure-breeding line with a chestnut individual from a pure- breeding line, the offspring are palomino-meaning they have an intermediate (golden-yellow) body color. What additional cross would you do to test whether Mendel's model is valid in the case of genes for horse color? According to his model, what offspring phenotype frequencies would you get from your experimental cross? Explain why your cross would provide a test of Mendel's model versus blending inheritance.

Short answer

Cross two palomino horses (heterozygous LD) and observe their offspring. Mendel's model predicts the offspring phenotype frequencies will be 25% Cremello (LL), 50% Palomino (LD), and 25% Chestnut (DD). If Mendel's model is correct, all three phenotypes will appear. If blending inheritance is correct, only intermediate colors (palomino or others) will be observed, and no distinct cremello or chestnut phenotypes.

Step-by-step solution

Design the experimental cross

We will perform an experimental cross between two palomino individuals (heterozygous LD).

Predict offspring phenotypes according to Mendel's model

According to Mendel's model, the Punnett square for crossing two palomino (LD) individuals would result in the following genotype frequencies: - 25% LL (Cremello) - 50% LD (Palomino) - 25% DD (Chestnut)

Interpret the results to test Mendel's model

Mendel's model predicts that if we cross two heterozygous palomino individuals, we will obtain offspring with all three different phenotypes (cremello, palomino, and chestnut) with frequencies of 25%, 50%, and 25% respectively. On the other hand, the blending inheritance hypothesis predicts that offspring will have continuous and intermediate phenotypes, so the offspring would only have palomino or other intermediate colors. Performing the suggested cross experiment would provide a test between Mendel's model and blending inheritance. If Mendel's model is correct, we will observe offspring with the three distinct phenotypes (cremello, palomino, and chestnut). If the blending inheritance hypothesis is correct, we will only observe offspring with intermediate (palomino) or other intermediate colors, and will not observe the distinct cremello and chestnut phenotypes among the offspring from a palomino cross.

Key concepts

Genotype Frequencies

Genotype frequencies are essential to understanding genetic variation within a population. They refer to the proportion of different genotypes present in a given group. For example, if a certain trait in a population is determined by two alleles, A and B, we could have three possible genotypes: AA, AB, and BB. The frequency of each genotype is calculated by counting the number of individuals with that genotype and dividing by the total number of individuals.

In the context of Mendelian genetics, genotype frequencies help predict the outcomes of crosses between individuals. Mendel’s laws, notably the law of segregation and the law of independent assortment, dictate that alleles separate during gamete formation and combine randomly during fertilization. This means that the genotype frequencies of the offspring can be calculated using a mathematical approach like the Punnett square. If we know that the genotype frequencies of the parental generation are each 50% for A and B alleles, we can predict the expected frequencies of the genotypes in the offspring after mating.

Understanding how these frequencies work helps in determining if a population is evolving or if non-Mendelian factors influence the inheritance patterns seen in actual breeding experiments, as opposed to the expected Mendelian ratios.

Punnett Square

The Punnett square is a powerful tool used in genetics to predict the genotype and phenotype frequencies of the offspring from a cross between two parents. Named after its inventor, Reginald Punnett, this grid-based diagram is a visual representation of how different alleles from each parent combine, based on the principles of Mendelian inheritance.

To use a Punnett square, one parent's alleles for a particular gene are written along the top, while the other parent's alleles are listed along the side. By filling in the squares, you determine all possible combinations of alleles that could occur in their offspring. For each square, there is an equal probability of that combination occurring, assuming that segregation and independent assortment occur without any deviations. This is particularly helpful for simple traits that follow Mendelian patterns of inheritance such as the horse color example provided in the exercise.

Using the Punnett square helps to reinforce the concept that genotype and phenotype frequencies in offspring can be calculated and predicted, providing a clear test for Mendelian versus non-Mendelian inheritance patterns like blending inheritance.

Blending Inheritance Hypothesis

The blending inheritance hypothesis, which predates Mendelian genetics, suggested that the genetic material contributed by each parent mixes in their offspring, creating an intermediate phenotype. According to this model, like two colors of paint mixed together, the individual traits of parents would blend in their children, and these blended traits would pass on to subsequent generations.

However, this model could not account for how certain traits seemed to skip a generation only to reappear in the next, which is a classic sign of Mendelian inheritance. Moreover, blending inheritance would predict a population to eventually reach uniformity since all varied traits would blend into a single, intermediate trait after many generations.

Gregor Mendel’s research on pea plants demonstrated that inheritance patterns were actually particulate, not blending. Mendel showed that traits are passed from generation to generation through discrete units, now known as genes. The horse breeding example would directly test these competing theories, as Mendel's model expects the appearance of all possible parental phenotypes in the offspring, while the blending hypothesis would result in a uniform intermediate phenotype. The results of such a breeding test could decisively support or refute the blending inheritance hypothesis in favor of Mendelian genetics.