Genetic Terms Gene - a unit of inheritance that usually is directly responsible for a trait or character. Allele - an alternate molecular form of a gene. Homozygous - when the two alleles are the same. Heterozygous - when the two alleles are different, in such cases the dominant allele is expressed. Dominant - a term applied to the trait (allele) that is expressed irregardless of the second allele. Recessive - a term applied to a trait that is only expressed when the second allele is the same (e.g. short plants are homozygous for the recessive allele). Phenotype - the physical expression of the allelic composition for the trait under study. Genotype - the allelic composition of an organism. Punnett squares - probability diagram illustrating the possible offspring of a mating. Gregor Mendel Mendel's work showed: 1. Each parent contributes one factor of each trait shown in offspring. 2. The two members of each pair of factors segregate from each other during gamete formation. 3. The blending theory of inheritance was discounted. 4. Males and females contribute equally to the traits in their offspring. 5. Acquired traits are not inherited. Summary of Mendel's Results: 1. The F1 offspring showed only one of the two parental traits, and always the same trait. 2. Results were always the same regardless of which parent donated the pollen (was male). 3. The trait not shown in the F1 reappeared in the F2 in about 25% of the offspring. 4. Traits remained unchanged when passed to offspring: they did not blend in any offspring but behaved as separate units. 5. Reciprocal crosses showed each parent made an equal contribution to the offspring. Mendel's Conclusions: 1. Evidence indicated factors could be hidden or unexpressed, these are the recessive traits. 2. The term phenotype refers to the outward appearance of a trait, while the term genotype is used for the genetic makeup of an organism. 3. Male and female contributed equally to the offsprings' genetic makeup: therefore the number of traits was probably two (the simplest solution). 4. Upper case letters are traditionally used to denote dominant traits, lower case letters for recessives. Characteristics of X-linked Traits 1. Phenotypic expression more common in males 2. Sons cannot inherit the trait from their fathers, but daughters can. 3. Sons inherit their Y chromosome from their father. Only a few genes have been identified on the Y chromosome, among them the testisdetermining factor (TDF) that promotes development of the male phenotype. Codominant alleles occur when rather than expressing an intermediate phenotype, the heterozygotes express both homozygous phenotypes. Incomplete dominance is a condition when neither allele is dominant over the other. Epistasis is the term applied when one gene interferes with the expression of another (as in the baldness/widow's peak mentioned earlier). Polygenic inheritance is a pattern responsible for many features that seem simple on the surface. Many traits such as height, shape, weight, color, and metabolic rate are governed by the cumulative effects of many genes. Usually polygenic traits are distinguished by 1. Traits are usually quantified by measurement rather than counting. 2. Two or more gene pairs contribute to the phenotype. 3. Phenotypic expression of polygenic traits varies over a wide range. Pleiotropy is the effect of a single gene on more than one characteristic. -Sickle Cell Anemia Linkage occurs when genes are on the same chromosome. Remember that sex-linked genes are on the X chromosome, one of the sex chromosomes. Linkage groups are invariably the same number as the pairs of homologous chromosomes an organism possesses. Recombination occurs when crossing-over has broken linkage groups, as in the case of the genes for wing size and body color that Morgan studied. Chromosome mapping was originally based on the frequencies of recombination between alleles. Chromosome Abnormalities Chromosome abnormalities include inversion, insertion, duplication, and deletion. These are types of mutations. Since DNA is information, and information typically has a beginning point, an inversion would produce an inactive or altered protein. Likewise deletion or duplication will alter the gene product. Prokaryotes Ancient lineages. Ubiquitous: they are found everywhere! Abundant: 4 ? 6 X 1030 cells globally, perhaps 40-45% of the biomass of the planet. Metabolically diverse. Essential for eukaryotic life: Nutrient cycling (biogeochemical cycles) Symbionts (It?s fair to say that prokaryotes make eukaryotic life possible) Darwin?s Logic Darwin took common observations made by other naturalists, and really by everyone, and put them together in attempting to answer the question of the origin of new species. -Humans have 46 chromosomes (23 pairs). -The number of chromosome pairs is called N, the haploid number. -The total number of chromosomes is 2N, the diploid number. Human Life Cycle: Sexual Reproduction Two basic kinds of sexual Reproduction: In sexual reproduction, reproduction and genetic recombination are coupled. Meiosis produces genetically unique haploid cells: gametes (or spores) Working with peas Mendel: Identified predictable patterns of the inheritance of traits from parents to offspring. Predicted the existence of genes, alleles, chromosomes, haploid, diploid, and meiosis even though none of those things were known about at the time. Most genes code for: Proteins PROTEINS and GENES Most genes code for proteins via messenger RNAs (mRNA). The others code for two types of RNA: ribosomal RNAs (rRNA) & transfer RNAs (tRNA) Humans have genes for 25,000 to 30,000 different proteins. Any particular cell will have 5,000 to 20,000 different proteins. Individuals in a population will have several thousand heterozygous genes, and other than identical twins, no two individuals will have the identical genetic makeup. Mutations Mutations are the ultimate source of variation in populations: mutations create new alleles. Different alleles, as they produce different phenotypes, are what selection acts upon. Most mutations are harmful (are selected against). Some have no effect (that is, are not selected for or against in a particular environment). Some mutations are selectively favored in particular environments. March 2 & 4 Notes Mutation: The origin of variation Mutation rates vary somewhat across organisms? genomes but is in the range of 10-4 to 10-6 mutations per base pair per generation. Mutations can be: *Silent: A mutation that does not result in an amino acid change. *Those that do result in an amino acid change: *Deleterious: The change alters normal functioning of the gene product (protein). *Neutral: The change that does not alter normal functioning of the protein. *Beneficial: The change alters normal functioning of the protein in a way that is selectively favored in some environments. Variation Intraspecific variation: -Variation within a species/population is the result of gene mutations. -Variation in a population is mixed each generation by genetic recombination (sexual reproduction). -Intraspecific variation is maintained, reduced, or increased by evolutionary processes. -Intraspecific variation is the raw material of evolutionary change. -Intraspecific variation can be discontinuous (phenotypes determined by different alleles of a single gene) or continuous (due to multiple genes and/or environmental influence on phenotype) Populations evolve- not individuals. A population that is not evolving is in genetic equilibrium: allele and genotype frequencies do not change from generation to generation. Conditions for Genetic Equilibrium: No mutation. Population is VERY large. Population is isolated from others. Gene has no effect on survival or reproduction = No natural selection. Mating is random. If one or more of the above does not hold, the allele frequencies are changing across generations, and the population is evolving. Evolution and Natural Selection: Natural selection = Evolution Evolution involves: 1. Origin of variation: Mutation + recombination. 2. Change in the frequency of alleles and genotypes: Natural selection and genetic drift. Natural selection is the differential survival and reproduction (fitness) of individuals that differ in one or more heritable traits. Natural selection: Natural selection is different from evolution by natural selection; because natural selection can have no evolutionary effect unless phenotypes differ in genotypes. A feature cannot evolve by natural selection unless it makes a positive contribution to the reproduction or survival of individuals that have it. Natural selection proceeds independently on traits determined by different genes. Fitness and Adaptation: Fitness: Differential survival & reproduction of individuals that differ in one or more traits - a relative measure. (If females with genotype AA and Aa have on average 100 offspring and females with genotype aa have on average 60 the fitness of AA and Aa genotypes is 1 and of the aa genotype is 60/100 = 0.6) Adaptation: The process and products of natural selection: fit between organisms and their environment. An environmentally dependent concept. Three Modes of Selection Life at High Elevation: Studies have shown that some humans living at high altitudes exhibit the following anatomical and physiological differences relative to populations at sea level: 1. 30% higher red blood cell counts. 2. Larger right ventricles of the heart (right ventricle pumps deoxygenated blood to the lungs via the pulmonary artery). 3. Lower birth weights, but larger placentas. 4. Delayed sexual maturity. 5. Higher infant mortality rates, mainly resulting from respiratory infections. Sexual Dimorphism Male characteristics a consequence of female mate choice and male-male competition. Biological Evolution Has Two Outcomes: Adaptation: The result of natural selection. (But note that sexual selection may result in traits that are non-adaptive). Lineage divergence: Speciation arising from reproductive isolation. ?Micro-? and ?Macroevolution?: ?Microevolution?: Changes in allele frequencies from generation to generation, resulting from mutation, selection, gene flow, and/or genetic drift. Is occurring all of the time! ?Macroevolution?: Lineage splitting and speciation; allele frequency change accompanied by reproductive isolation. Occurs over long periods of time, or can occur very rapidly.
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