GCD 3022 1st Edition Lecture 11 Outline of Last Lecture I Genomic Imprinting a Definition b Three stages c Example Igf 2 in mice II Imprinting and DNA methylation a Imprinting of genes b Methylation III Extranuclear Inheritance a Definition b Mitochondria c Chloroplasts IV Maternal Inheritance a Definition b The four o clock plant Outline of Current Lecture I Maternal Effect a Nurse Cells b Drosophila 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 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 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 Current Lecture I Maternal Effect a Nurse cells able to express both alleles present in the maternal genome The only time this makes a difference in the outcome of the offspring is if the mother is heterozygous b Drosophila a drosophila embryo dies during early embryogenesis due to a recessive maternal effect called bicoid The wild type is bic The genotypes of the embryo s mother and grandmother would have to be bic bic and bic bicrespectively because the grandmother would have to pass on the recessive allele to the mother of the generation in question Since the offspring died that means that it s phenotype reflected the mother s genotype which would have had to have been bic bic c Water Snails A sinistral male dd and dextral female DD mate to produce offspring i What are the genotypes and phenotypes of the F1 generation 1 The result of a cross between two homozygotes DDxdd will result in all Dd offspring Since this is a maternal inheritance pattern the F1 generation will all be dextral ii If the F1 progeny are allowed to mate with one another 1 What are the possible F2 genotypes and what ratios would be expected a The genotypic ratio would e 1DD 2Dd 1dd using a Punnett square of DdxDd 2 What percentage of the F2 snails will have a dextral coiling pattern a Mating the F1 progeny to each other will yield an F2 generation that all have a dextral coiling pattern because all of the F1 females carried a D allele iii If the F2 generation is allowed to interbreed to produce an F3 generation 1 What percentage of sinistral heterozygotes in the F3 generation had a mother with the dd genotype a The genotype of the progeny is not important If the snail is sinistral it s the mother who had to be dd so that answer is 100 II III 2 What is the phenotypic ratio of dextral sinistral in the F3 generation a 3 1 dextral to sinistral Epigenetic Inheritance a Examples of epigenetic inheritance imprinting X inactivation b Genomic imprinting although two copies of the gene in both female and male parents appear wild type by sequence analysis only the maternal allele is expressed in the offspring Igf2 Igf2 and genomic imprinting a Scenario 1 if an Igf2 Igf2 female is crossed to an Igf2 Igf2 male what is the predicted outcome i Half the offspring would be normal and half would be dwarf b Scenario 2 If an Igf2 Igf2 female is crossed to an Igf2 Igf2 male what is the predicted outcome i All the offspring would be dwarf c Imprinted genes provide examples of epigenetic inheritance and are near differentially methylated regions d Scenario 3 you cross a dwarf female mouse with a normal male mouse and get 50 dwarf and 50 normal F1 progeny You then randomy select 10 different pairs of normal sized male and female mice from the F1 generation and mate them to each other to generate an F2 generation In each individual mating you always get a 50 50 ratio of dwarf to normal mice i Note the IGF2 gene is an imprinted gene where the copy inherited from the male is active and the copy from the female is silenced Igf2 is the wild type and Igf2 is the recessive mutant allele 1 What were the genotypes of the male and female mice in the P1 generation a Since half the progeny are dwarf and the male is the only parent that influences how the Igf2 gene is expressed then the male parent must be heterozygous in order to achieve 50 of the F1 progeny as dwarf try making a Punnett square b The female parent is dwarf so she can be either homozygous recessive or heterozygous where she inherits the recessive allele from her father c Try making a table of the possible F1 progeny if the mother was heterozygous and if she was homozygous Count the ratio of phenotypes dwarf normal for both tables and compare to determine the more likely genotype of the mother IV V VI VII VIII Types of inheritance a Example 1 phenotype of the offspring is totally dependent on the genotype of the mother maternal effect b Example 2 phenotype of the offspring follows the phenotype of the mother maternal inheritance c Example 3 phenotype of the offspring is dependent on the genotypes of the mother and the father Mendelian inheritance De novo methylation a In what types of cells would you expect de novo DNA methylation that regulates imprinting to occur i De novo methylation would not occur in somatic cells but would occur in germ cells The Lyon hypothesis a Attempts to explain the molecular mechanism of dosage compensation Xinactivation Barr bodies a The number of Barr bodies in an individual is determined by the number of X chromosomes present An individual with an XXX genotype would have 2 Barr bodies n 1 Reciprocal Cross a Definition two parallel crosses that involve the same genotypes of the two parents but their sexes are opposite in the two crosses For example the reciprocal cross of female BB x male bb is female bb x male BB b A gene is found as a wild type dominant allele B and recessive mutant allele b What would be the expected outcomes genotypic and phenotypic of reciprocal crosses if a true breeding wild type individual was crossed to a true breeding mutant individual i If the gene was inherited in a simple autosomal pattern of inheritance 1 The result of one cross will be the same as the result of the reciprocal cross because both parents influence the genotypes and phenotypes of the offspring ii How would the result of a testcross differ if the gene