GCD 3022 1st Edition Lecture 12 Outline of Last Lecture I Maternal Effect a Nurse Cells b Drosophila c Water Snails i F1 genotypes and phenotypes ii F2 genotypes and phenotypes iii Percentage of F2 that are dextral iv F3 generation v Phenotypic ratio of F3 generation II Epigenetic Inheritance a Examples of epigenetic inheritance b Genomic imprinting III Igf2 and Igf2 and genomic imprinting a Scenario 1 b Scenario 2 c Imprinted genes d Scenario 3 IV Types of Inheritance These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute a Example 1 b Example 2 c Example 3 V De novo DNA methylation VI The Lyon Hypothesis VII Barr Bodies VIII Reciprocal Cross a Definition b Reciprocal cross scenario c Maternal inheritance Outline of Current Lecture I Intro to linkage a Linkage Groups b Recombination II Recombination Crossing Over a Discovery b Process III Determining if genes are linked a Chi square test b Steps IV Evidence of Linked Genes a Experiment b Result c Hypotheses V Genetic mapping in plants and animals a Definition b Equation c Testcross d Map units VI Three factor crosses a Step 1 b Step 2 c Step 3 d Step 4 e Step 5 Current Lecture I Linkage genes on the same chromosome can be inherited as a group instead of individually This means that they do not assort independently violate Mendel s law of independent assortment a Linkage group also referred to as chromosomes because most genes on a chromosome are linked Number of linkage groups is the number of types of chromosomes in a species ex humans have 22 autosomal linkage groups and an X and Y linkage group b Recombination also known as crossing over Occurs when genes on the same chromosome combine to form new genotypic combinations The closer the genes are the less likely the event of crossover but genes that are far apart may assort independently from one another II Recombination a Discovery discovered by Bateson and Punnett in 1905 using sweet pea plant crosses and observing the traits of pollen shape and flower color The dihybrid cross should have produced a 9 3 3 1 ratio according to Mendelian inheritance III IV but the parental phenotypes were more common than expected This was due to recombination during meiosis b Process occurs during prophase I of meiosis Replicated sister chromatids from two homologous chromosomes associate as bivalents and exchange DNA segments i Cells with nonparental genetic information are called nonparental or recombinant cells Determining if genes are linked a Chi square test compares expected values with observed values to determine if the result of the experiment is due to random chance alone accept null hypothesis or the findings are statistically significant reject null hypothesis b Steps to the chi square test i Propose a hypothesis 1 Ex the genes for flower color and pollen shape assort independently This hypothesis allows us to calculate the expected values ii Based on the hypothesis calculate the expected values of each of the four phenotypes an independent assortment hypothesis will predict that each phenotype has an equal probability of occurring iii Apply the chi square formula see earlier chapters Accept or reject the hypothesis depending on the P value from the table Evidence of linkage a Experiment i Morgan provided evidence of linkage through study of several X linked genes in drosophila ii Traits were body color eye color and wing length b Results i A much higher proportion of combinations of traits found in the parental generation were found in the F2 generation ii For example if one of the parents had gray body red eyes and long wings there was a very large proportion of offspring that had that same phenotype iii Morgan also observed that there were a significant number of nonparental phenotypes in the F2 generation and that there was a quantitative difference between the parental and nonparental combinations c Hypotheses i The genes for body color eye color and wing length are all located on the X chromosome alleles will tend to be inherited together ii Due to crossing over the homologous X chromosomes in females can exchange pieces of chromosomes V VI iii The likelihood of crossing over depends on the distance between the two genes more likely to occur when genes are farther apart from each other Genetic mapping in plants and animals a Definition also known as gene mapping or chromosome mapping Purpose is to determine the linear order of linked genes along the same chromosome b Equation Map distance of recombinant offspring total of offspring x100 Units are called map units mu also referred to as centiMorgans cM one map unit is equivalent to 1 recombination frequency in a testcross c Testcross used in genetic mapping experiments by mating a heterozygote for two or more genes to an individual that is homozygous recessive for the same genes Expected to yield a maximum of only 50 recombinant offspring anything over 50 map units is considered unreliable Three factor crosses provides information about map distance and gene order a Step 1 cross two true breeding strains that differ with regard to the three alleles b Step 2 perform a test cross by mating F1 female heterozygotes to male flies that are homozygous recessive for all three alleles c Step 3 collect data for the F2 generation double crossovers may occur but only appear as the least frequent phenotypes Genes that are farther apart will show higher rates of crossover than genes that are close together d Step 4 Calculate the map distance between pairs of genes i Start by regrouping the data according to pairs of genes parental and non parental combinations ii The regrouped data will allow us to calculate the map distance between the two genes e Step 5 construct the map based on map unit calculation i Location of the genes are mapped relative to the centromere