Our discussion of homozygous and heterozygous organisms brings us to why the F 1 heterozygous offspring were identical to one of the parents, rather than expressing both alleles. The genotypic ratios are the ratios of gene combinations in the offspring, and these are not always distinguishable in the phenotypes. The phenotypic ratios are the ratios of visible characteristics.
Capital letters represent dominant alleles and lowercase letters represent recessive alleles. Figure 8.5 Phenotypes are physical expressions of traits that are transmitted by alleles. For example, the F 1 yellow plants that received a Y allele from their yellow parent and a y allele from their green parent had the genotype Yy. When P plants with contrasting traits were cross-fertilized, all of the offspring were heterozygous for the contrasting trait, meaning their genotype had different alleles for the gene being examined. Mendel’s parental pea plants always bred true because both produced gametes carried the same allele. The letter used for the gene (seed color in this case) is usually related to the dominant trait (yellow allele, in this case, or “ Y”).
The dominant allele is capitalized and the recessive allele is lower case. The genotype is often written as YY or yy, for which each letter represents one of the two alleles in the genotype. Diploid organisms that are homozygous for a gene have two identical alleles, one on each of their homologous chromosomes. The P plants that Mendel used in his experiments were each homozygous for the trait he was studying. Therefore, the F 1 plants must have been genotypically different from the parent with yellow seeds. However, we know that the allele donated by the parent with green seeds was not simply lost because it reappeared in some of the F 2 offspring ( Figure 8.5). That is, the hybrid offspring were phenotypically identical to the true-breeding parent with yellow seeds. When true-breeding plants were cross-fertilized, in which one parent had yellow seeds and one had green seeds, all of the F 1 hybrid offspring had yellow seeds. The yellow-seed allele is dominant and the green-seed allele is recessive. Seed color is governed by a single gene with two alleles. We will use a second trait that Mendel investigated, seed color, as an example. For example, the phenotypes that Mendel observed in his crosses between pea plants with differing traits are connected to the diploid genotypes of the plants in the P, F 1, and F 2 generations. Mendel’s hybridization experiments demonstrate the difference between phenotype and genotype. An organism’s underlying genetic makeup, consisting of both the physically visible and the non-expressed alleles, is called its genotype. The observable traits expressed by an organism are referred to as its phenotype. Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics. Mendel examined the inheritance of genes with just two allele forms, but it is common to encounter more than two alleles for any given gene in a natural population. Gene variants that arise by mutation and exist at the same relative locations on homologous chromosomes are called alleles. For example, one individual may carry a gene that determines white flower color and a gene that determines violet flower color.
The offspring are formed when that gamete unites with one from another parent and the two copies of each gene (and chromosome) are restored.įor cases in which a single gene controls a single characteristic, a diploid organism has two genetic copies that may or may not encode the same version of that characteristic. This separation, or segregation, of the homologous chromosomes means also that only one of the copies of the gene gets moved into a gamete. Recall that in meiosis these chromosomes are separated out into haploid gametes. In fact, it is the chromosomes we inherit and the two copies of each gene are located on paired chromosomes. The reason we have two copies of each gene is that we inherit one from each parent. We now call those two copies genes, which are carried on chromosomes. Mendel deduced from his results that each individual had two discrete copies of the characteristic that are passed individually to offspring. The seven characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits. Explain the purpose and methods of a test cross.Explain Mendel’s law of segregation and independent assortment in terms of genetics and the events of meiosis.Use a Punnett square to calculate the expected proportions of genotypes and phenotypes in a monohybrid cross.Explain the relationship between genotypes and phenotypes in dominant and recessive gene systems.By the end of this section, you will be able to:
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