Apples have many different genetic traits, including skin color, size, sweetness, and crunchiness. Each variety (type) of apple has a unique combination of phenotypes for these traits. The first trait most people notice about an apple is the color of its skin. A single gene that comes in two forms (alleles) controls this trait. The allele for red color (C) is dominant, and the allele for yellow color (c) is recessive. The seeds in apples form by sexual reproduction that involves cross-pollination. All the apples on a tree that grows from a seed will be the same color — either red or yellow. That's because the color of their skin depends on the pair of alleles the tree inherited from its parents. An apple orchard usually has trees of many varieties. Because of cross-pollination, each apple flower can be pollinated by pollen from several different trees. Thus, each seed in an apple from an orchard probably has a unique combination of genes. This can be a problem for an apple tree farmer. If the farmer grows new apple trees from seed, the apples on the new trees will be unlikely to look or taste like either parent tree's apples. That's because they will not have the same mix of traits.
So, how does a farmer make sure that all the apples on a tree look and taste a certain way? When farmers find an apple they like, they use methods of asexual reproduction to make new trees. Grafting is one way to asexually reproduce apple trees that are identical to the parent tree. A farmer cuts off a branch of the apple tree that produces the best apples. Then the branch is grafted onto, or joined with, the trunk of another tree. The apples that grow on the new tree will be genetically identical to those of the parent tree that the branch came from.
In this activity, you will develop two models for growing new trees that make either red or yellow apples. The two models should compare sexual reproduction and asexual reproduction (grafting). Include the genotypes of the parents and the offspring in your models.
To model sexual reproduction, use Punnett squares to show the results of crosses between trees with different colors of apples. Start by showing a cross between one red parent that has the genotype CC and one yellow parent that has the genotype cc. Then choose two trees from the first generation of offspring to make a second generation. Cross each of them with another tree that has the genotype Cc.
To model asexual reproduction, use a branching diagram (concept map or tree diagram) to show the results of grafting. Start with a parent that has the genotype Cc. Show four offspring for the first generation of grafting. Then choose two of these offspring as parents for a second generation of grafted trees.
Finally, write two short paragraphs: one under the Punnett squares and another under the branching diagram. Each paragraph should summarize what the diagram shows about genetic variation.