MR. SURRETTE VAN NUYS HIGH SCHOOL CHAPTER 19: GENETICS CLASS NOTES GREGOR MENDEL Gregor Mendel published his work on the genetics of peas in 1866. Mendel chose traits that were clear and distinct: seed shape – round or wrinkled; seed color – yellow or green; seed coat color – grey or white; pod shape – inflated or constricted; pod color – green or yellow; flower position – axial or terminal; stem length – tall or dwarf. MONOHYBRID CROSS Mendel tested the cross between a pure yellow-seeded plant, one whose family showed only the yellow seed trait, and a pure green-seeded plant. In his notes, the letter P represented the parent plants. The resulting offspring, the F1 generation (F stands for filial), were all yellow-seeded. MONOHYBRID CROSS From this, Mendel concluded that yellow seeds were controlled by a dominant factor, and that the factor that caused the seeds to be green was recessive. MONOHYBRID CROSS Next, he took two of the F1 plants and crossed them. The F2 offspring showed a ratio of three yellowseeded plants for each green-seeded plant. His notes show 6,022 yellow seeds and 2,001 green seeds. FAMILY TREE FOR PEA PLANTS LAW OF SEGREGATION Mendel came to the conclusion that each genetic trait was controlled by a distinct factor within each plant. Parents had two of these factors that were separated during reproduction so that a set of chromosomes from each parent was given to each offspring. LAW OF SEGREGATION Mendel’s Law of Segregation explained this separation. He called these factors genes. When the genes come in two or more forms for a trait, they are called alleles for that trait. 1|Page INTEGRATED SCIENCE MR. SURRETTE VAN NUYS HIGH SCHOOL HOMOZYGOUS VERSUS HETEROZYGOUS Mendel discovered that pure yellow-seeded parent plants (P) had two dominant alleles for yellow (Y/Y). They were homozygous for the dominant trait. The green-seeded parent plant was homozygous for the recessive green gene (y/y). F1 was always heterozygous (Y/y). MONOHYBRID CROSS The terms pure (homozygous) and hybrid (heterozygous) are also used by breeders. The F1 cross between two parents each hybrid for a given trait is called a monohybrid cross. PUNNETT SQUARE A Punnett square is divided vertically and horizontally in half. If parental plants hybrid for seed color are the subject, then the letters Y and y represent their possible genetic contribution. PUNNETT SQUARE The letters Y and y from one parent are placed above each column, and the letters Y and y from the other parent are placed beside each row. The possible offspring in the four squares take a gene from each parent. PHENOTYPE VERSUS GENOTYPE Of the four possible kinds of offspring, one is homozygous yellow (Y/Y), two are heterozygous yellow (Y/y), and one is homozygous green (y/y). In a monohybrid cross, the dominant to recessive phenotypic ratio is three to one. Phenotype refers to the appearance and genotype to whether it is homozygous or heterozygous. MONOHYBRID PEA CROSS TEST CROSSING To determine if a yellow-seeded plant was pure or hybrid, Mendel used a test cross. He crossed the unknown plant, either pure (Y/Y) or hybrid (Y/y), with a plant possessing green seeds. Green seeds indicated the pure recessive genotype. If the offspring were all yellow, the unknown parent must be homozygous. DYHYBRID CROSS Consider the inheritance of two traits: seed shape and plant height. Round is dominant, so use R for round and r for wrinkled. Tall is dominant, so use T and t. Cross a homozygous tall-round parent (T/T R/R) with a homozygous short-wrinkled parent (t/t r/r). 2|Page INTEGRATED SCIENCE MR. SURRETTE VAN NUYS HIGH SCHOOL DYHYBRID CROSS The gametes (sex cells) of the first parent would carry T/R. The gametes of the second parent would carry t/r. The F1 offspring can have only one genotype (T/t R/r). This is called a dihybrid. It would have the phenotype tall and round. LAW OF INDEPENDENT ASSORTMENT Crossing two of these dihybrid F1s gets interesting. Each F1 can produce the gametes: T/R, T/r, t/R, and t/r. Mendel’s Law of Independent Assortment describes the results. For this dihybrid cross, the Punnett Square is divided into quarters each way. LAW OF INDEPENDENT ASSORTMENT Across one side put T/R, T/r, t/R, and t/r. The same goes on the other side; there are sixteen possible offspring types. LAW OF INDEPENDENT ASSORTMENT Filling in the sixteen squares shows that the phenotypic ratio is nine F2 with the dominant phenotype (T/- R/-), three with a dominant-recessive phenotype (T/- r/r), three with a recessive-dominant phenotype (t/t R/-), and one with the recessive phenotype (t/t r/r). DIHYBRID PEA CROSS 3|Page INTEGRATED SCIENCE MR. SURRETTE VAN NUYS HIGH SCHOOL PARTIAL DOMINANCE Partial dominance, also called incomplete dominance, is when a heterozygous individual shows a third phenotype, often a blended phenotype. For example, a red snapdragon flower (R/R) crossed with a white (W/W) will produce pink offspring R/W. Partial dominance is easier to work with because there are no hidden traits. HAPLOID VERSUS DIPLOID Most organisms are diploid, which means they have two sets of chromosomes in their nuclei. One set comes from one parent, and the other comes from the other parent. Having one set of chromosomes is called haploid. Bacteria, some primitive plants and gametes are haploid. HUMAN SEX CHROMOSOMES In humans, the sex chromosomes are called “X” and “Y.” Females have a pair of X chromosomes, and males have an X and a Y chromosome. The X chromosome is larger than the Y with more genes on it. Human males are therefore haploid for some traits. MEIOSIS Meiosis is the cell division process that produces gametes. STAGES OF MEIOSIS 4|Page INTEGRATED SCIENCE