Biology Today Third Edition Chapter 2 Genes, Chromosomes, and DNA Copyright © 2004 by Garland Science Eli Minkoff Pam Baker

Know that Gregor Mendel is the father of modern genetics. Define “true-breeding,” “P generation,” “F 1 generation,” F1 generation.” Define dominant and recessive, homozygous and heterozygous, genotype and phenotype, homologous chromosomes, diploid, haploid. Be able to use a Punnett square to visualize the outcome of a genetic cross. Be able to state Mendel's two laws of heredity. Realize that genes are located along chromosomes. Understand that sexual reproduction and meiosis increases the diversity in a species Know how many chromosomes are in normal human cells and that nondisjunction leads to trisomy and aneuploidy. LEARNING OBJECTIVES

Figure 2.1

Mendel and the Garden Pea Heredity is the passing along of traits from one generation to the next. Genetics is the study of heredity. Mendel contributed to the understanding of genetics in the 1800s by counting numbers of offspring in crosses in pea plants. Mendel’s Experimental System: The Garden Pea Mendel used the garden pea because of the varieties available. peas are small, grow and reproduce quickly, and the flowers self-pollinate. Mendel’s Experimental Design Mendel began his crosses with pure-breeding varieties that contained distinct phenotypes. The first generation of crosses is the P or parental generation; their offspring are the F1 generation or first filial generation. Offspring of the F1 generation comprise the F2 generation.

Figure 2.2

Monohybrid cross-2 purebreeding phenotypes RR X rr were cross fertilized. F1 heterozygotes exhibited dominant traitand the trait not expressed was recessive. The F2 Generation F1 plants self-fertilized, F2 a 3:1 dominant to recessive phenotype A testcross of F2 generation. found a 1:2:1 genotypic ratio. The F1 Generation-

Figure 2.3

Mendel’s Results Monohybrid crosses between 2 purebreeding parents the dominant trait only appeared in the F1 generation. The F2 generation the recessive trait reappeared.

Testcross the genotype of an individual expressing a dominant trait, he did a testcross by crossing the individual with a homozygous recessive for the trait. Testcrosses are used to determine the genotype of an individual who expresses the dominant trait but may be heterozygous or homozygous.

Mendel’s First Law Law of segregation-monohybrid cross; phenotypic ratio 3:1, genotypic ratio 1:2:1 Mendel’s Second Law Law of Independent Assortment- dihybrid cross; phenotypic ratio 9:3:3:1

Punnett Square The shorthand method for determining the ratio among offspring from a particular cross is called a Punnett Square. The allele present in the male and female haploid gametes are represented and diploid offspring from fertilization appear in squares,

Figure 2.4

How Genes Influence Traits From DNA to Protein 1. DNA is transcribed into RNA in the nucleus. 2.RNA is translated into protein on ribosomes in the cytoplasm. How Proteins Determine the Phenotype 1.The specific sequence of amino acids determines a protein’s function, specifying phenotype

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Chromosomes Structures visible during cell division where the genetic material-DNA-resides Homologous chromosomes – alike in size, structure, and genes they carry One member of each pair of homologs is received from each haploid gamete at fertilization.

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Germ-Line Tissues Cells that produce gametes are called germ-line tissues. Germ-line cells will undergo meiosis to produce haploid gametes.

The Stages of Meiosis The formation of haploid gametes Replication of the genetic material occurs prior to meiosis Meiosis has two divisions: meiosis I and meiosis II Crossing over occurs during prophase I of meiosis I when pieces of nonsister chromatids exchange places to promote new genetic combinations in the offspring. Nonsister chromatids are replicated chromosomes, held together and derived from a different member of the homologous pair

Meiosis I and II Formation of haploid gametes Further genetic diversity is achieved by the independent orientation of replicated homologous chromosomes at the metaphase I plate Meiosis II results in four haploid (1N) gametes. Genetic diversity is also achieved at fertilization-which of the diverse gametes fertilize

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How Meiosis Differs From Mitosis Meiosis has three unique features: 1.Synapsis and cross-over The process of pairing throughout the length of the homologous chromosomes and exchanging genetic fragments is called crossing-over 2. Reduction Division Since DNA only replicates once, before meiosis I, the two divisions result in halving the chromosome number in the daughter cells (gametes) which are then haploid. 3. Fertilization The diploid number is restored at fertilization.

Human Chromosomes Humans have 23 pairs, or 46, chromosomes that vary by size, shape, and appearance. Photographing the chromosomes produces a karyotype. Nondisjunction is the unequal separation of chromosomes Trisomy is having an extra copy of a chromosome, the lack of one is monosomy and usually lethal.

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Sex Determination The 23 rd pair of chromosomes are sex chromosomes; the others are somatic chromosomes Females are XX Males are Xy The Y must have active genes to determine maleness

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The Griffith Experiment By experimenting with Streptococcus pneumoniae, he found that the virulent strain’s polysaccharide coat was necessary for infection. He experimented further and found that the information specifying the polysaccharide coat could be passed from dead, virulent bacteria to coatless,nonvirulent strains. Hereditary information could thus be passed from dead cells to live ones, transforming them. The hereditary information was later determined to be DNA

Figure 2.18

From Genotype to Phenotype How Genes Influence Traits From DNA to Protein 1. The genetic code in DNA is transcribed into RNA in the nucleus. 2. RNA is translated into protein on ribosomes in the cytoplasm. 3. A protein is a specific sequence of amino acids determined the genetic code How Proteins Determine the Phenotype The specific sequence of amino acids determines a protein’s function, thus specifying phenotype

Structure of DNA Nucleotides: consist of a phosphate, sugar and one of 4 possible bases. The bases are Adenine, Thymine, Guanine and Cytosine

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How the DNA Molecule Copies Itself The Double Helix DNA molecule consists of two strands Each individual strand of a DNA molecule is complementary to its opposite strand Base Pairing Rule: The base A always bonds to T and G to C. If one chain has the bases ATTGCAT, its partner will have the complementary sequence of TAACGTA. ATTGCAT TAACGTA The 2 complementary Strands of DNA separate and serve as a template for the positioning of nucleotides in the new strand according to base pairing

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