P 1 gg genotype in offspring 2 05 05 025 p 1 g from

This preview shows 17 out of 19 pages.

P 1 gg genotype in offspring 2 = 0.5 * 0.5 = 0.25 P 1 g from mother and g from father 2 = P 1 g from mother 2 * P 1 g from father 2 P 1 G or g 2 = 0.5 + 0.5 = 1.0 P 1 g 2 = 1 2 , P 1 G 2 = 1 2 , 1 1 , 1 6 , 1 2 ,
Image of page 17

Subscribe to view the full document.

78 C HAPTER 3 Mendelian Genetics: How Are Traits Inherited? When Mendel crossed pea plants, he counted thousands of offspring. Because there were so many, the ratios of the different phenotypes were very close to the theoretical values that his laws of segregation and independent assortment predicted. They were close, but never exactly right on. Half Is Enough The examples we have considered thus far apply to organisms, like humans, that spend most of their life history carrying two alleles for each gene. Only the gametes carry single alleles. Some organisms, however, such as algae and mosses, spend much of their lives carrying only a single copy of each gene. How do Mendel’s laws apply to these creatures? Piecing It Together Mendel’s principles, derived from his experiments with peas, are true of all sexually reproducing organisms—those that produce gametes that join at fertilization. Here, we summarize Mendel’s conclusions about the inheritance of traits, as well as some of the insights that have emerged from his work: 1. Hereditary characteristics are passed from parent to offspring as units or particles. We refer to the basic unit of inheritance for a given trait as a gene. The different forms of a gene are called alleles. Mendel’s factors correspond to alleles. 2. Individuals carry two alleles for every gene.The two alleles for a given trait may be identical, in which case the individual is said to be homozygous for that trait.Alter- natively, the two alleles may differ, and hence, the individual is said to be heterozy- gous for that trait. 3. Prior to reproduction, pairs of alleles are separated so that specialized reproductive cells called gametes contain only one allele from each pair.At fertilization, gametes fuse, each contributing one allele for each trait to the new offspring. 4. Some genes show dominance; that is, heterozygous individuals may express only one allele—the dominant allele—while the other, the recessive allele, is masked. Thus, the phenotype—the traits that are expressed in an individual—may not always reveal the genotype, or the full complement of alleles that the individual carries. Recessive alleles remain hidden in the genes. 5. Mendel’s law of segregation states that heterozygous parents are equally likely to pass either of their two alleles on to their offspring. In other words, gametes combine at fertilization without regard to which alleles they carry. Because the alleles that an individual inherits are purely a matter of chance, the rules of probability can be used to determine the likelihood that any given allele is passed on.The Punnett square is a tool that illustrates the law of segregation.
Image of page 18
Image of page 19
You've reached the end of this preview.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern