Chapter 11: Introduction to Genetics

Chapter 11:Introduction to Genetics By: Riley Thomas

11-1 The Work of Gregor Mendel Gregor Mendel, the “Father of Genetics” Geneticsis the scientific study of heredity. Genetics, explain every living thing and how it has a set of characteristics inherited from its parent or parents.

*Pea plants use part of their flowers to reproduce. The male part produces pollen; pollen is the male sex cell. The female part produces eggs, which are the female sex cell* Gregor Mendel was very important, because he understood biological inheritance. Mendel was a so own as the "Father of Genetics " He was born in 1822 in what is now the Czech Republic. Mendel was a teacher and also took charge of the garden duties at the monastery. Mendel worked with garden peas.

When Mendel took the part as monastery gardener, he had several stocks of pea plants. These peas were true breeding. True breeding is a term used to describe organisms that produce offspring identical to themselves if allowed to self-pollinate. The true-breeding plants were the basis of Mendel's experiment.

However, pea plants can also cross-pollinate. In cross-pollination, male sex cells in pollen from the flower on one plant fertilize the egg cells of a flower on another plant. The seeds produced from cross-pollination have two plants as parents.

To perform his experiment, Mendel had to select the pea plants that he would mate with each other. He had to prevent the pea flowers from self-pollinating and control their cross-pollination. Mendel accomplished this task by cutting away the male parts of a flower and then dusting the flower with pollen from a second flower.

Mendel studied seven different pea plants traits. A trait is a specific characteristic. Mendel studied crossed plants with each of the seven contrasting characters and studied their offspring. Mendel called each original pair of plants the P(parental) generation. He called the offspring the F1, or “first filial'“ generation. The offspring of crosses between parents with different traits are called hybrids.

Below is a chart of the different crosses Mendel did.

From his first experiments Mendel drew two conclusions: 1. Biological inheritance is determined by factors that are passed from one generation to the next. 2. The principle of dominance, which states that some alleles are dominant and others are recessive.

Alleles are the different forms of a gene . Mendel also had a question he wanted to, answer: Had the recessive alleles disappeared, or were they still present in the F1 plant? To answer this question, he allowed all seven kinds of F1 hybrid plants to produce an F2 (second filial) generation by self-pollination. The traits controlled by the recessive alleles appeared.

Magnified Image of flower pollen. Segregation means separation. Gametes are sex cells Ex: Pollen is a male sex cell Eggs are a feminine sex cell

11-2 Probability and Punnett Squares The likelihood that a particular event will occur is called probability. The principles of probability can predict the outcomes of genetic crosses. Probabilities predict the average outcome of a large number of events. But cannot predict the precise outcome of an individual event.

A Punnett square is a diagram that shows the different gene combinations that might result from a genetic cross. Below is an example: Tt*Tt

The letters in the Punnett square represent alleles: Capital letters for dominant alleles and lowercase letters for recessive alleles. In this example, T represents tallness and t represent the recessive allele for shortness.

Organisms that have two. identical alleles for a particular trait TT or tt (for this example) are said to be homozygous Organisms that have 2 different alleles for the same trait are heterozygous. Homozygous organisms are true breeding for a particular trait. Heterozygous organisms are hybrid for a particular trait.

A human’s genetic makeup. Phenotype means physical characteristics. Genotype means genetic makeup.

11-3 Exploring Mendelian The principle of independent assortment, states that genes for different traits can segregate independent during the formation of gametes.

Mendel's principles form the base on which the modern science of genetics has been built. These principles can be summarized as follows: Individual units known as genes determine the inheritance of biological characteristics. In organisms that reproduce sexually, genes are passed from parents to their offspring

In cases in which two or more forms of the gene for a single trait exists, some forms of the gene may be dominant and others may be recessive. In most sexually reproducing organisms each adult has two copies of each gene-one from each parent. These genes are segregated from each other when gametes are formed. The alleles from different genes usually segregate independently of one another.

*Some alleles are neither dominant nor recessive, and multiple alleles or multiple genes control many traits. * Incomplete Dominance Cases in which one allele is not completely dominant over another are called incomplete phenotype is incomplete dominance. In incomplete dominance, the heterozygous phenotype is somewhere in between the two homozygous phenotypes.

An example of Incomplete Dominance is when: RED Flower x WHITE Flower PINK Flower Codominanceis a similar situation (as Incomplete Dominance) in which both alleles contribute to the phenotype of the organisms.

Example: red x white ---> red & white spotted Multiple Alleles Many genes have more than two alleles and are therefore said to have multiple alleles. This does not mean that an individual can have more than two alleles. It onlymeans that more than two possible alleles exist in a population.

Polygenic Traits Many traits are produced by the interaction of several genes. Traits controlled by two or more genes are said to be polygenic traits which means, "having many genes".

11-4 Meiosis Homologous is a term used to refer to chromosomes that each have a corresponding chromosome from the opposite sex parent. A cell that contains both sets of homologous chromosomes is said to be diploid, which means, "two sets". Haploid is a term used to refer to a cell that contains only a single set of chromosomes and therefore only a single set of genes.

Meiosis is a process of reduction division, in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Meiosis usually involves two distinct stages: the first meiotic division, called meiosis I, and the second meiotic division, meiosis II.

Below are the various phases of meiosis and what happens during them: MEIOSIS 1 Interphase 1: Cells undergo a round of DNA replication, forming duplicate chromosomes. Prophase 1: Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Metaphase 1: Spindle fibers attach to the chromosomes. Anaphase 1: The fibers pull the homologous chromosomes toward opposite ends of the cell.

MEIOSIS II Prophase ll: Meiosis I result in two haploid daughter cells, each with half the number of chromosomes, as the original cell. Metaphase ll: The chromosomes line up in a similar way to the metaphase stage of mitosis. Anaphase ll: The sister chromatids separate and move toward opposite ends of the cell. Telophase II: Meiosis II results in four haploid daughter cells.

*Mitosis results in the production of two genetically identical diploid cells. whereas meiosis produces four genetically different haploid cells. *

11-5 Linkage and Gene Maps Thomas Hunt Morgan did research on fruit flies, which led him to the principle of linkage. Morgan and his associates observed so many genes that were inherited together that before long they could group all of the fly's genes into four linkage groups. The linkage groups assorted independently, but all of the genes in one group were inherited together.

Drosophila has four chromosomes, which led to two remarkable conclusions: 1. Each chromosome is actually a group of linked genes 2. Mendel's principle of independent assortment still hold true. (It is the. chromosomes, however, that assort independently, not individual genes.)

This has been an R-Thom Production.

Chapter 11: Introduction to Genetics