Chapter 3 Heredity and Evolu4on Chapter Outline The Cell DNA Structure and Function Cell Division: Mitosis and Meiosis The Genetic Principles Discovered by Mendel Mendelian Inheritance in Humans Misconceptions Regarding Dominance and Recessiveness Polygenic Inheritance Chapter Outline Genetic and Environmental Factors Mitochondrial Inheritance New Frontiers Modern Evolutionary Theory Factors That Produce and Redistribute Variation Natural Selection Acts on Variation 1
Gene4cs The study of gene structure and action and of the patterns of inheritance of traits from parent to offspring. Genetic mechanisms are the underlying foundation for evolutionary change. The Cell Cells are the basic units of life in all living things. In some forms, such as bacteria, a single cell constitutes the entire organism. More complex multi- cellular forms, such as plants, insects, birds, and mammals, are composed of billions of cells. An adult human is made up of as many as 1,000 billion cells, all functioning to promote survival. Structure of a Generalized Eukaryo4c Cell 2
Parts of a Eukaryo4c Cell Nucleus - A structure found in all eukaryotic cells; contains chromosomes (nuclear DNA). Molecules - Structures made up of two or more atoms. They can combine with other molecules to form more complex structures. Deoxyribonucleic acid (DNA) - The double- stranded molecule that contains the genetic code. Parts of a Eukaryo4c Cell" Ribonucleic acid (RNA) - A molecule, similar in structure to DNA. Three single- stranded forms of RNA are essential to protein synthesis. Cytoplasm - The portion of the cell contained within the cell membrane, excluding the nucleus Consists of a semifluid material and contains numerous structures involved with cell function. Proteins Three- dimensional molecules that serve a wide variety of functions through their ability to bind to other molecules. Protein Synthesis - The assembly of chains of amino acids into functional protein molecules. The process is directed by DNA. Occurs in ribosomes 3
Two Types of Cells Somatic cells All the cells in the body except those involved with reproduction. Gametes (sex cells) Reproductive cells (eggs and sperm in animals) developed from precursor cells in ovaries and testes. Gametes (Sex Cells) Their sole function is to unite with a gamete from another individual to form a zygote. Zygotes contain the full complement of chromosomes (in humans, 46) and has the potential of developing into an entire organism. Nucleo4des Basic units of the DNA molecule, composed of a sugar, a phosphate unit, and one of four DNA bases. 4
Part of a DNA Molecule DNA Replica4on A cell can t function without the appropriate amount of DNA, and for new cells to receive DNA, the DNA must replicate. Prior to cell division, enzymes break the bonds between bases in the DNA molecule, leaving the two previously joined strands of nucleotides with their bases exposed. The exposed bases attract unattached nucleotides, which are free- floating in the nucleus. 5
DNA Replica4on Because one base can be joined to only one other, the attraction between bases occurs in a complementary fashion. Each of the two previously joined parental nucleotide chains serve as models for the formation of a new strand of nucleotides. As each new strand is formed, its bases are joined to the bases of an original strand. When the process is complete, there are two double- stranded DNA molecules exactly like the original, and each consists of one original nucleotide chain joined to a newly formed one. Enzymes Specialized proteins that initiate and direct chemical reactions in the body. One function is to break bonds of DNA molecules to permit replication. Hemoglobin and other Proteins A protein molecule that occurs in red blood cells and binds to oxygen molecules. Example of a very important protein that is essential to life. Lactose is another example. 6
Hormones A class of proteins. Specialized cells produce and release hormones into the bloodstream to circulate to other areas of the body, where they produce specific effects in tissues and organs. Insulin is one example. Regulatory hormones, switching genes on and off, are another example. Proteins and Amino Acids Proteins are made up of chains of smaller molecules called amino acids. There are 20 amino acids, which are combined to produce millions of proteins. What makes proteins different from one another is the number of amino acids involved and the sequence in which they are arranged. Proteins and Amino Acids The sequence of DNA bases determine the order of amino acids in a protein molecule. In the DNA instructions, groups of three bases specifies a particular amino acid. E.g. Alanine: CGA, CGG, CGT, CGC 7
Protein Synthesis Protein synthesis involves an additional molecule similar to DNA called RNA (ribonucleic acid). DNA provides the instructions for protein synthesis and RNA reads the instructions and assembles amino acids to form proteins. RNA acts as intermediary in producing amino acids of specified type. Genes A gene is a segment of DNA that dictates the sequence of amino acids in a particular protein. A gene may consist of only a few hundred bases, or it may be composed of thousands. Muta4on A change in DNA. The term can refer to changes in DNA bases as well as changes in chromosome number or structure. If the sequence of DNA bases is altered through mutation, some proteins may not be manufactured, and the cell (or the organism) may not function. 8
Homeobox (Hox) Genes An evolutionary ancient family of regulatory genes. Hox genes direct the segmentation and patterning of the overall body plan during embryonic development. Cell Division Cell division results in the production of new cells. During this process, the DNA becomes tightly coiled and is visible under a light microscope as chromosomes. A chromosome is composed of a DNA molecule and associated proteins. Cell Division During the early stages of cell division, chromosomes are made up of two strands joined at a constricted area called the centromere. Every species is characterized by a specific number of chromosomes in somatic cells. In humans there are 46 chromosomes, 23 pairs. One member of each pair is inherited from the father and the other member is inherited from the mother. 9
Two Types of Chromosomes Autosomes carry genetic information that governs all characteristics except primary sex determination. Sex chromosomes are the X and Y chromosomes All genetically normal females have two X chromosomes (XX). All genetically normal males have one X and one Y chromosome (XY). Normal human somatic cells have 22 pairs of autosomes and one pair of sex chromosomes. Model of Human Chromosome Standard Chromosomal Complement in Various Organisms 10
Mitosis Cell division in somatic cells. Mitosis occurs during growth of the individual. It also permits healing of injured tissues and replaces older cells with newer ones. Mitosis is referred to as simple cell division because a somatic cell divides one time to produce two daughter cells that are genetically identical to each other and to the original cell. Mitosis Mitosis 11
Meiosis Cell division in specialized cells in ovaries and testes. Meiosis involves two divisions and results in four daughter cells, each containing only half the original number of chromosomes. These cells can develop into gametes. Meiosis Meiosis 12
Mitosis and Meiosis Compared Recombina4on The exchange of DNA between paired chromosomes during meiosis; also called crossing over. Crucially important because increases genetic combination potential Means that genes are shuffled in infinite combinations, so offspring not same as either parent. Gregor Mendel (1822 1884) Mendel was a monk living in an abbey in what is now the Czech Republic. When he began his research, he had already studied botany, physics, and mathematics and performed experiments in the monastery gardens. These experiments led him to explore how physical traits, such as color or height, could be expressed in plant hybrids. 13
Principle of Segrega4on Genes (alleles) occur in pairs (because chromosomes occur in pairs). During gamete production, the members of each gene pair separate, so that each gamete contains one member of each pair. During fertilization, the full number of chromosomes is restored, and members of gene or allele pairs are reunited. Traits Mendel Studied in Peas Alleles Alternate forms of a gene. Alleles occur at the same locus on paired chromosomes and thus govern the same trait. However, because they are different, their action may result in different expressions of that trait. The term allele is often used synonymously with gene. 14
Locus The position on a chromosome where a given gene occurs. The term is sometimes used interchangeably with gene. If the same allele occurs in the pair, the individual is homozygous for that allele. If different alleles occur in the pair, the individual is heterozygous for that allele. Alleles Dominant A trait governed by an allele that can be expressed in the presence of another, different allele (that is, in heterozygotes). Dominant alleles prevent the expression of recessive alleles in heterozygotes. (This is the definition of complete dominance.) 15
Recessive A trait that is not expressed in heterozygotes; also refers to the allele that governs the trait. For a recessive allele to be expressed, there must be two copies of the allele (that is, the individual must be homozygous). Results of Crosses When Only One Trait at a Time Is Considered PunneT Square 16
Genotype The genetic makeup of an individual. Genotype can refer to an organism s entire genetic makeup or to the alleles at a particular locus. Phenotypes The observable or detectable physical characteristics of an organism; the detectable expressions of genotypes. Principle of Independent Assortment The distribution of one pair of alleles into gametes does not influence the distribution of another pair. The genes controlling different traits are inherited independently of one another. 17
Mendelian Traits Characteristics that are influenced by alleles at only one genetic locus. Examples include many blood types, such as ABO. Many genetic disorders, including sickle- cell anemia and Tay- Sachs disease, are Mendelian traits. Some Mendelian Traits in Humans: Dominant Condi4on Achondroplasia Brachydactyly Hun4ngton disease Manifesta4ons Dwarfism due to growth defects involving the long bones of the arms and legs; trunk and head size usually normal. Shortened fingers and toes. Progressive degenera4on of the disease nervous system accompanied by demen4a and seizures; age of onset is commonly between 30 and 40 years. Some Mendelian Traits in Humans: Recessive Condi4on Tay- Sachs disease Albinism Sickle- cell anemia Manifesta4ons Most common among Ashkenazi Jews; degenera4on of the nervous system begins at 6 months of age; lethal by age 2 or 3. Inability to produce normal amounts of melanin; results in untannable skin, light blond hair, and light eyes. Caused by an abnormal form of hemoglobin that results in collapsed red blood cells, blockage of capillaries and reduced blood flow to organs. 18
An4gens Large molecules found on the surface of cells. Several different loci governing antigens on red and white blood cells are known. Foreign antigens provoke an immune response in individuals. Codominance The expression of two alleles in heterozygotes. In this situation, neither is dominant or recessive, so that both influence the phenotype. ABO Genotypes and Associated Phenotypes 19
Discon4nuous Distribu4on of a Mendelian Trait Bar chart of the frequency of the ABO blood type in a hypothetical population. 60% of the popula4on has blood type O. AB is the least common blood type. Polygenic Traits Traits governed by two or more loci, with each locus making a contribution to the phenotype. Human height is one example. An example in humans is skin color, and the single most important factor influencing skin color is the amount of melanin. Melanin production is believed to be influenced by 3 to 6 genetic loci, with each locus having at least two alleles, neither of which is dominant. Con4nuous Distribu4on of a Mendelian Trait Histogram of a polygenic trait (height) in a large group of people. From this chart, we can see that the average height is 66 inches and that the tallest individuals measure 80 inches. 20
A Comparison of Mendelian and Polygenic Traits Mitochondria Structures contained within the cytoplasm of eukaryotic cells that convert energy, derived from nutrients, into a form that is used by the cell. Mitochondrial DNA (mtdna) is DNA found in the mitochondria; it is inherited only through the maternal line. Polymerase Chain Reac4on (PCR) A technique scientists can use to make thousands of copies of a DNA sample as small as one molecule. This technique, developed in 1986, is important since DNA samples obtained from crime scenes or fossils are frequently too small to permit a reliable analysis of nucleotide sequences. Using PCR, scientists have been able to examine nucleotide sequences in Neandertal fossils and Egyptian mummies. 21
Clone An organism that is genetically identical to another organism. The term may also be used to refer to genetically identical DNA segments and molecules. Recombinant DNA Technology A process in which genes from the cell of one species are transferred to somatic cells or gametes of another species. Human Genome Project An international effort aimed at sequencing and mapping the entire human genome now completed. Genome The entire genetic makeup of an individual or species. In humans, it is estimated that each person possesses approximately 3 billion DNA nucleotides. 22
The Modern Synthesis Define evolution as a two- stage process: The production and redistribution of variation. Natural selection acting on this variation, whereby inherited differences, or variation, among individuals differentially affect their ability to successfully reproduce. Varia4on In genetics, inherited differences among individuals; the basis of all evolutionary change genetic differences. Evolu4on (Modern Gene4c Defini4on) A change in the frequency of alleles from one generation to the next. In a population, the percentage of all the alleles at a locus accounted for by one specific allele is the allele frequency. A population is a community of individuals within a species where mates are usually found. 23
Microevolu4on & Macroevolu4on Microevolution Small changes occurring within species, such as a change in allele frequencies. Macroevolution Changes produced only after many generations, such as the appearance of a new species. Gene Flow Exchange of genes between populations. Term migration also sometimes used. Few population are genetically isolated today. African Americans: between 4% (deep South) and 25% (large urban centers) of genetic migration has occurred through intermarriage. Gene4c Drih Evolutionary changes; changes in allele frequencies produced by random factors. Genetic drift is a result of small population size. When individuals move to isolated area, founder effect can occur. 24
Founder Effect A type of genetic drift in which allele frequencies are altered in small populations that are taken from, or are remnants of, larger populations. Succeeding generations are more genetically uniform than the original population. Genetic bottleneck of limited selection causes reduced diversity. Natural Selec4on and Varia4on Occurrence of sickle- cell anemia example of trait affected by natural selection. Individuals homozygous for trait have serious illness usually have shortened life expectancy (45 years). But individuals who are heterozygous for the sickle trait have reduced susceptibility to malaria. Advantage in tropical areas- environmental influence on allele frequencies. Frequency Map of Sickle- cell Distribu4on in the Old World 25
Distribu4on of Malaria in the Old World 26