Bio 1B Lecture Outline please print and bring along Fall 2007 B D Mishler Dept of Integrative Biology 2 6810 bmishler berkeley edu Evolution lecture 10 Mendelian genetics Hardy Weinberg Nov 28th 2007 ch 14 251 270 and ch 23 454 458 in 7th ed ch 14 247 266 and ch 23 445 449 in 6th ed Summary of topics Use Mendel s first law independent segregation to predict genotypes and phenotypes resulting from a given cross Genes on different chromosomes assort independently Mendel s second law independent assortment Apply Mendel s principles to examples with incomplete dominance and codominance relationships of alleles and multiple alleles Use the method of gene allele counting to determine allele frequencies when heterozygotes can be distinguished from the homozygote state Describe the Hardy Weinberg law explain the conditions that must be met for it to hold true and determine if a population is in HW proportions including multiple allele cases Use the HW expectations to calculate allele frequencies for recessive dominant traits or diseases estimate the frequency of heterozygous carriers for a recessive trait and explain why the majority of mutant alleles are carried in heterozygous individuals for rare recessive traits Timeline of Mendelian genetics you do not need to memorize all these just the three with asterisks 1858 Darwin and Wallace independently propose the mechanism of evolution natural selection 1859 Darwin published On the Origin of Species 1865 Gregor Mendel discovered the basic laws of genetic inheritance Mendel s laws were unknown to Darwin 1900 Mendel s results are rediscovered ABO blood group system in humans are discovered and shown to be an example of Mendelian inheritance 1944 DNA is the genetic material 1953 Watson and Crick discover the molecular structure of DNA 1970 Gene mapping in humans essentially limited to the X chromosome based on the specific pattern of inheritance 1983 Genetic linkage of Huntington disease to a chromosomal location 1989 Cystic fibrosis gene identified 1990 Human Genome project initiated a handful of human disease genes had been identified 1993 The Huntington disease gene identified 1994 The familial breast ovarian cancer gene BRCA1 was identified 1997 The first cloning of a mammal a sheep named Dolly Evolution 10 pg 1 The future Documenting genetic variation of human and other genomes at the population level identifying the genes involved in complex diseases understanding the genetic basis of development and much more Many ethical issues will arise Overview of Mendelian genetics Gregor Mendel 1865 discovered basic laws of genetics by focusing on particulate inheritance going against previous approaches to heredity that focused on quantitative characters gene sequence of DNA coding for a protein or in some cases part of a protein allele a variant of a single gene inherited at a particular genetic locus A and a genotype the genetic constitution of an individual in diploid individuals it is the set of two alleles at a locus possessed by an individual AA Aa and aa phenotype an observable trait in an organism it can be determined by genotype and environment and interaction between the two see Fig 14 6 7th Fig 14 5 6th homozygote individual having two copies of the same allele at a genetic locus AA and aa heterozygote individual having two different alleles at a genetic locus Aa dominant an allele A is dominant if the phenotype of the heterozygote Aa is the same as that of the homozygote AA but differs from the homozygote aa recessive an allele a is recessive if the phenotype of the homozygote aa differs from that of the heterozygote Aa and the homozygote AA which are the same Mendel s life and work lonely life in a monastery in Brno in current Czech Republic Peas and bees and financial headaches eased by beer Mendel s Experimental Quantitative Approach Advantages of pea plants for genetic study There are many varieties with distinct heritable features or characters such as color character variations are called traits Mating of plants can be controlled Each pea plant has sperm producing organs stamens and egg producing organs carpels Cross pollination fertilization between different plants can be achieved by dusting one plant with pollen from another Evolution 10 pg 2 In a typical experiment Mendel mated two contrasting true breeding varieties a process called hybridization The true breeding parents are the P generation The hybrid offspring of the P generation are called the F1 generation When F1 individuals self pollinate the F2 generation is produced When Mendel crossed contrasting true breeding white and purple flowered pea plants all of the F1 hybrids were purple When Mendel crossed the F1 hybrids many of the F2 plants had purple flowers but some had white Mendel discovered a ratio of about three to one purple to white flowers in the F2 generation Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids Mendel called the purple flower color a dominant trait and white flower color a recessive trait Mendel observed the same pattern of inheritance in six other pea plant characters each represented by two traits What Mendel called a heritable factor is what we now call a gene Mendel s Model Mendel developed a hypothesis to explain the 3 1 inheritance pattern he observed in F2 offspring Four related concepts make up this model below These concepts can be related to what we now know about genes and chromosomes The first concept is that alternative versions of genes account for variations in inherited characters For example the gene for flower color in pea plants exists in two versions one for purple flowers and the other for white flowers These alternative versions of a gene are now called alleles Each gene resides at a specific locus on a specific chromosome The second concept is that for each character an organism inherits two alleles one from each parent Mendel made this deduction without knowing about the role of chromosomes The two alleles at a locus on a chromosome may be identical as in the true breeding plants of Mendel s P generation Alternatively the two alleles at a locus may differ as in the F1 hybrids The third concept is that if the two alleles at a locus differ then one the dominant allele determines the organism s appearance and the other the recessive allele has no noticeable effect on appearance In the flower color example the F1 plants had purple flowers because the allele for that trait is
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