Transcript PPT
th 4 Exam is Thursday, December 9 Review session will be at 5:00 PM Wednesday, December 8 Final Exam Final exam will be Dec. 16, 8:00-10:00 AM Yellow Sheets: You will be allowed to put whatever you want onto one side of one yellow sheet of paper (to be handed out in class). You will attach your yellow sheet to your exam and hand it in when you are finished. Final Exam The format and style of the final will be similar to the regular exams. Bring a calculator Applications of the H-W Law: Calculate Heterozygote Frequency If you know the frequency of any one genotype, you can calculate the frequency of the other genotypes. This is especially useful if you are interested in identifying the frequency of carriers for a specific condition like cystic fibrosis. Cystic fibrosis is a recessive condition occurring in about 1 of every 2,500 people (1/2500 = 0.0004) Calculating Carrier Frequency Remember, p + q = 1.0 The incidence of cystic fibrosis is 0.0004, so q2 = 0.0004 q = 0.0004 = 0.02 p + 0.02 = 1.0 p = 1.0 – 0.02 = 0.98 Calculating Heterozygote Frequency In the H-W equation, heterozygote frequency = 2pq, so 2pq = 2(0.98)(0.02) = 0.04 or 4%, about 1 in 25 people Factors that influence allele and genotype frequencies These are the influences that keep a population from reaching H-W equilibrium. 1. Natural Selection 2. Mutation 3. Migration 4. Genetic Drift 5. Nonrandom Mating Natural Selection The H-W assumption is that all individuals have an equal chance of survival to reproductive age and equal chance of reproductive success. Any difference in survival or ability to reproduce is called natural selection. Natural selection is the strongest force that alters allele frequencies and is one of the most important factors inducing genetic changes. Natural Selection Selection occurs whenever some individuals within a population have an advantage in survival or reproduction over other individuals. These advantages ultimately translate into increased contribution to future generations, or fitness. Genotypes with high rates of reproductive success are said to have high fitness. Natural Selection and Quantitative Traits Most phenotypic traits are controlled by multiple loci and the environment. Polygenic traits also respond to selection, which can be classed into three types: 1. Directional 2. Stabilizing 3. Disruptive Directional Selection Directional selection: Selection of desirable traits. Selected traits usually represent phenotypic extremes. Used by plant and animal breeders, but also occurs in nature. In nature, the selective agent is usually an environmental change. Stabilizing Selection Stabilizing selection favors intermediate types, meaning both phenotypic extremes are selected against. Tends to keep a population adapted to its environment. Disruptive Selection Disruptive selection is selection against intermediates and for phenotypic extremes. Occurs in populations with a heterogeneous environment. Mutation Because the number of possible genotypes is so large, at any given time, a population will only represent a small fraction of the possible genotypes. Mendelian assortment and recombination produce new allele combinations, but do not produce new alleles. Mutation Mutational events occur at random, i.e., without regard to possible benefits or detriments to the individual. However, by creating new alleles, mutation can be a significant force causing allele frequencies to change. Migration Populations may be geographically separated and respond to different selective pressures such that their allele frequencies differ. When individuals from one population move into the other, allele migration occurs. Genetic Drift Genetic drift is the random fluctuation in allele frequencies caused by chance deviation. Accurately predicting the genetic ratios (1:1, 3:1, 1:2:1 etc.) requires large numbers of individuals and mating pairs. If only small numbers are used, the probability of deviation from expected ratios increases. Genetic Drift Small populations are created when they are separated from the larger population. 1. 2. Disruptive event, like a war or epidemic Migration Non-Random Mating Non-random mating does not alter allele frequencies, but does alter genotype frequencies. The most important form of non-random mating is inbreeding, or mating between relatives. Inbreeding Inbreeding increases the percentage of individuals homozygous for recessive alleles previously concealed in heterozygous individuals. Developmental Genetics Developmental Genetics Genes control development of a fertilized egg into a cohesive individual composed of millions of cells organized into tissues and organs with distinct functional and structural properties. Basic Events 1. 2. 3. 4. Cytoplasmic localization Cell-cell interaction Determination Differentiation Cytoplasmic Localization Following fertilization, initial cell divisions produce cells with different distributions of maternal cytoplasm contributed by the oocyte. In different cells, the cytoplasm exerts different influences on the genetic material of the different cells, ultimately resulting in differences in transcription. Cell-Cell Interaction Physical contact and signal molecules produced by one cell influence another cell. Cells in immediate proximity create a microenvironment that influence development within that environment and the ultimate structure and function. Determination Determination is the point at which the developmental fate of a cell becomes fixed. Determination Differentiation is the process by which a cell achieves final form and function. Differentiation follows determination. Master Regulators Master regulators are genes that act as switches. When the switch is flipped, the number of developmental pathways is reduced. The switch commits the cell to move along a specific pathway. Most master regulators are binary, meaning there are only two possible alternatives. When the switch is activated, there is only one. These genes are called binary switch genes Binary Switch Most binary switch genes are identified by isolation of non-functional mutations. The binary switch gene triggering differentiation of the eye in Drosophila is called eyeless. The eyeless gene is expressed in all embryonic cells that will give rise to the eye. When eyeless is lost, cells normally destined to become part of the eye degenerate later in development and die. Binary Switch Genes In the absence of eyeless, eyes do not form. If eyeless is expressed in other tissues such as the leg, wing or antenna, eyes will form in those locations. Therefore, eyeless is a master regulator directing eye development. Maternal Effect Genes Maternal effect genes deposit mRNA and/or proteins into the oocyte cytoplasm. Maternal effect genes may be distributed evenly throughout the cytoplasm or may be concentrated in particular areas. Distribution can affect the concentration of maternal effect products in embryonic cells. Gradients Concentration of maternal effect gene products in cells produces a gradient that provides positional information and directs formation of the anteriorposterior orientation and body segments. Genes that Control Development Gap genes: transcription factors that activate pair rule genes. Pair-rule genes: products divide the embryo into smaller regions about two segments wide and in turn, activate segment polarity genes. Segment polarity genes: divide segments into anterior and posterior compartments. Collectively, these genes define the fields of action for selector genes that specify the identity of each segment.