CHAPTER 10.2 & 10.3 NOTES Mendelian Genetics The passing of traits to the next generation is called inheritance, or heredity. Mendel performed cross-pollination in pea plants. Mendel followed various traits in the pea plants he bred. The parent generation is also known as the P generation. The offspring of this P cross are called the first filial (F1) generation. The second filial (F2) generation is the offspring from the F1 cross. Genes in Pairs Allele An alternative form of a single gene passed from generation to generation Dominant (Purple flower color) Expressed form of a trait represented by a capital letter and first letter of the trait (P). Recessive (White flower color) Only appears when both alleles are recessive. Represented by a lower case letter of the dominant trait (p) Dominance An organism with two of the same alleles for a particular trait is homozygous. PP or pp An organism with two different alleles for a particular trait is heterozygous. Pp Genotype and Phenotype An organism’s allele pairs are called its genotype. Example would be Pp The observable characteristic or outward expression of an allele pair is called the phenotype. Example would be purple flower color Mendel’s Law of Segregation The law of segregation, states that the two alleles for a trait segregate (separate) when gametes are formed. Two alleles for each trait separate during meiosis. During fertilization, two alleles for that trait unite. Each offspring receives one allele from each parent resulting in two alleles for each trait. Mendel’s Law of Independent Assortment Mendel found that the inheritance of one trait, such as plant height, did not influence the inheritance of any other trait, such as flower color. The law of independent assortment states that the alleles of different genes separate independently of one another during gamete formation. This occurs during meiosis I when the homologous chromosomes line up along the metaphase plate Monohybrid Cross A cross that involves hybrids for a single trait is called a monohybrid cross. Dihybrid Cross The simultaneous inheritance of two or more traits in the same plant is a dihybrid cross. For example, if you crossed a yellow round pea (YYRR) with a green wrinkled pea (yyrr) What would be the predicted offspring genotypes and phenotypes? Four types of alleles from the male gametes and four types of alleles from the female gametes can be produced. The resulting phenotypic ratio is 9:3:3:1. Incomplete Dominance In some organisms, however, an individual displays a trait that is intermediate between the two parents, a condition known as incomplete dominance. For example, when a snapdragon with red flowers is crossed with a snapdragon with white flowers, a snapdragon with pink flowers is produced. Codominance For some traits, two dominant alleles are expressed at the same time. In this case, both forms of the trait are displayed, a phenomenon called codominance. Codominance is different from incomplete dominance because both traits are displayed. Instead of pink you would get a red and white flower Polygenic Inheritance When several genes influence a trait, the trait is said to be a polygenic trait. The genes for a polygenic trait may be scattered along the same chromosome or located on different chromosomes. Familiar examples of polygenic traits in humans include eye color, height, weight, and hair and skin color. Multiple Alleles Genes with three or more alleles are said to have multiple alleles. Example: Blood types A, B, AB, and O Even for traits controlled by genes with multiple alleles, an individual can have only two of the possible alleles for that gene. Pedigree Geneticists often prepare a pedigree, a family history that shows how a trait is inherited over several generations. Pedigrees are particularly helpful if the trait is a genetic disorder and the family members want to know if they are carriers or if their children might get the disorder Scientists can determine several pieces of genetic information from a pedigree: Autosomal or Sex-Linked? If a trait is autosomal, it will appear in both sexes equally. If a trait is sex-linked, it is usually seen only in males. A sex-linked trait is a trait whose allele is located on the X chromosome. Dominant or Recessive? If the trait is autosomal dominant, every individual with the trait will have a parent with the trait. If the trait is recessive, an individual with the trait can have one, two, or neither parent exhibit the trait. Sex-linked Sex-linked traits occur on the X chromosomes Females have 2 Xs so they must have both defective alleles to have the genetic disorder Males only have 1 X making sex-linked disorders much more common in males Hemophilia is a sex-linked trait a condition that impairs the blood’s ability to clot.