Chapter 11 Mendelian Genetics Copyright © 2010 Pearson Education Inc.

 Genetics is the study of the structure and function of genes. ◦ a.Early genetics focused on how traits are passed from parents to offspring (transmission genetics). ◦ b.Advances in biochemistry and molecular methods allow the study of the structure and function of genes at the molecular level (molecular genetics).  Gregor Mendel (1822–1884) laid the foundation for our current understanding of heredity. Mendel did not know about chromosomes or genes, which were discovered after his lifetime

 Hereditary traits are under the control of genes (Mendel called them factors).  Genotype is the genetic makeup of an organism, a description of the genes it contains.  Phenotype is the characteristics that can be observed in an organism.  Phenotype is determined by interaction of genes and environment. Genes provide potential, but environment determines whether that potential is realized

 Mendel began his work in 1854 with the garden pea, Pisum sativum, by crossbreeding plants with different characteristics. ◦ He reported his in 1865, but its significance was not realized until several decades later.  He focused on well-defined traits one at a time, quantifying the offspring and analyzing the results mathematically.  Garden peas are excellent for this type of research: ◦ they grow easily, ◦ produce large numbers of seeds quickly, ◦ routinely self-fertilize. ◦ experimental cross-fertilization is possible.

 First strains of peas were allowed to self-fertilize to be certain that the traits are unchanged in subsequent generations (true- breeding or pure-breeding strains).  Inheritance of traits with only two distinct possibilities for phenotype  The traits are: ◦ a.Flower/seed coat color (one gene controls both): *grey/purple vs. white/white. ◦ b.Seed color: *yellow vs. green. ◦ c.Seed shape: *green vs. yellow. ◦ d.Pod color: *green vs. yellow. ◦ e.Pod shape: *inflated vs. pinched. ◦ f.Stem height: *tall vs. short. ◦ g.Flower position: *axial vs. terminal.  Dominant trait is with *

 a.Parental generation is the P generation.  b.Progeny of P generation is the first filial generation, designated F 1.  c.When F 1 interbreed, the second filial generation, F 2, is produced.  d.Subsequent interbreeding produces F 3, F 4, and F 5 generations.

 A monohybrid cross involves true-breeding strains that differ in a single trait.  To determine whether both parents contribute equally to the phenotype of a particular trait in offspring, a set of reciprocal crosses is performed. By convention, the female parent is given first.  In Mendelian genetics, offspring of a monohybrid cross will exactly resemble only one of the parents. This is the principle of uniformity in F 1.

 Traits that disappear in the F 1 reappear in the F 2.  The F 2 has a ratio of about one individual with the “reappearing” phenotype to three individuals with the phenotype of the F 1.  Mendel reasoned that information to create the trait was present in the F 1 in the form of “factors,” now called genes.

 Each gene exists in alternative forms (alleles) that control a specific trait. ◦ True-breeding strains contain identical genes. The F 1 contain one of each, but since the trait is just like one of the parents rather than a mix, one (dominant) allele has masked expression of the other (recessive) one.  By convention, letters may be used to designate alleles, with the dominant a capital letter (S) and the recessive in lowercase (s). ◦ Individuals with identical alleles (e.g., genotypes SS and ss) are homozygous for that gene. ◦ Individuals with different alleles (e.g., Ss) are heterozygous, because 1 ⁄ 2 of their gametes will contain one allele, and 1 ⁄ 2 the other.

 Diagrams of a smooth 3 wrinkled cross.  The Punnett square is a diagram showing all possible gamete combinations of each parent.  3:1 ratio in the F 2 generation.

 After Mendel’s experiments for the seven different traits in garden peas he made these conclusions: ◦ a. Results of reciprocal crosses are always the same. ◦ b.The F 1 resembled only one of the parents. ◦ c.The trait missing in the F 1 reappeared in about 1 ⁄ 4 of the F 2 individuals.

 The first Mendelian law, the principle of segregation, states: ◦ “Recessive characters, which are masked in the F 1 from a cross between two true-breeding strains, reappear in a specific proportion in the F 2.”  This is because alleles segregate during anaphase I of meiosis, and progeny are then produced by random combination of the gametes.

 Mendel observed that plants with the recessive phenotype are all true-breeding. When plants with the dominant phenotype are selfed, 1 ⁄ 3 are true-breeding, and 2 ⁄ 3 produce progeny with both phenotypes.

 Better approach to homozygous or heterozygous determination is testcross by crossing the individual with one that is homozygous recessive.

 Analyses of dihybrid cross (two pairs of traits) resulted in Mendel’s second law, the principle of independent assortment: ◦ factors for different traits assort independently of one another. This allows for new combinations of the traits in the offspring.

 If alleles assort independently, all possible phenotypes will be represented in the F 2, in a ratio of 9:3:3:1. ◦ If the F 1 are testcrossed, all types of offspring in a ratio of 1:1:1:1 will be produced.  In the F 2 of a dihybrid cross there will be four phenotypic classes and nine genotypic classes.

 Mendel’s work was published in 1866 with little attention from the scientific community until about 1900, when Correns, deVries, and von Tschermark independently conducted experiments with similar results.  In 1902 William Bateson, experimenting with fowl, showed that Mendelian principles apply in animals. He coined the terms genetics, zygote, F 1, F 2, and allelomorph (which was shortened to allele).  W. L. Johannsen named Mendelian factors genes in 1909, from the Greek genos, meaning “birth.”

 W. Farabee in 1905 was the first to demonstrate Mendelian principles in humans, showing that brachydactyly is inherited as a simple dominant trait.

 The study of the phenotypic records of a family over several generations is pedigree analysis. The individual upon whom the study focuses is the propositus (male) or proposita (female).  The symbols of pedigree analysis are summarized in,

 Recessive traits are usually the result of a mutation causing loss or modification of a gene product. ◦ Albinism is an example.  Deleterious recessive alleles persist in the population because heterozygous individuals carry the allele without developing the phenotype and so are not at a selective disadvantage.

 Characteristics of recessive inheritance of a relatively rare trait: ◦ a.Parents of most affected individuals have normal phenotypes but are heterozygous. ◦ b.Mating of heterozygotes will produce 3 ⁄ 4 normal progeny and 1 ⁄ 4 with the recessive phenotype. ◦ c.If both parents have the recessive trait, all their progeny will usually also have the trait.

 Dominant trait is mutation causing a function to be gained because of an altered gene product capable of a new activity. ◦ Achondroplasia is an example.  Dominant alleles produce a distinct phenotype when in a heterozygote with wild type allele. ◦ Due to the rarity of dominant mutant alleles causing recognizable traits, homozygous dominant individuals are very unusual.

 Characteristics of dominant inheritance of a relatively rare trait: ◦ a.Affected individuals have at least one affected parent. ◦ b.The trait is present in every generation. ◦ c.Offspring of an affected heterozygote will be 1 ⁄ 2 affected and 1 ⁄ 2 wild type.  Other examples include: ◦ a.Autosomal dominant polycystic kidney disease (ADPKD). ◦ b.Brachydactyly. ◦ c.Marfan syndrome.