LECTURE 19 Principle of Segregation Each gamete carries only one allele for seed shape because the alleles have segregated during meiosis When meiosis occurs homologous chromosomes separate in anaphase of meiosis I so each cell at the end of meiosis I is haploid has only one copy of each distinct kind of chromosome This means that gametes have only one allele for each gene SEPERATION SEGREGATED OF HOMOGOUS CHROMOSOMES IN MEOSIS I inherits one copy of a particular trait recessive trait may not be expressed in an individual but may still be passed on and reappear in the next generation USE OF A PUNETT SQUARE Law of Independent Assortment the genes for seed shape and seed color assort independently because they are located on different chromosomes Chromosomes line up independently of each other because non homologous chromosomes line up in the center of the cell independently of each other in metaphase of meiosis I INDEPDENT ALLIGNMENT IN MEOSIS I Chromosome Theory of inheritance mendel s rules can be explained by the independent alignment and separation of homolougous chromosomes at meiosis I Diploid 2 alleles for each trait having two type of each chromosome Haploid 1 allele gametes are haploid 1 type of each chromosome Sickle cell anemia autosomal reccesive disorder Heterzygous resistant to malaria no sickle cell Homozygous recessive sickle cell anemia Homozygous dominant do not have sickle cell anemia not resistant to malaria Balanced Polymorphism heterozygous individuals have an advantage over individuals who are homozygous for the dominant allele Discovery of Sex Chromosomes Nettie Stevens discovered them Worked with meal worms Y small necessary to become male TH Morgan s Fly WHY FLYS Red eyes mutant white eyes gene name white started tradition of naming genes after their mutant allele o Small size easy to culture in lab o Short generation time o Produce lots of offspring 100s per mating Experiment Does the sex of the parent affect the inheritance of white eye color in flies o Hypothesis the sex of the parent does affect the inheritance of white eyes o o Sex chromosomes pair at meiosis I o X linked inheritance or X linkage X chromosome carries the gene for something X linkage is generally indicated when reciprocal crosses give different results Other sex linked traits o Color blindness males with recessive X trait will always express disorder Recombination occurs when the combination of alleles on the chromosomes of progeny is different from the combinations of alleles present in the parental generation OCCURS FROM CROSSING OVER in MEIOSIS I Linkage Mapping distance between genes dictates the frequency of crossing over between them longer distance more crossing over Exceptions to Mendel s Laws sex chromosomes genetic linkage genes that do not assort independently recombination crossing over Polygenic Inheritance single trait controlled by 2 or more sets of alleles LECTURE 18 Heredity the transmission of traits from one generation to the next Genetics the scientific study of heredity and of hereditary information Early Ideas about inheritance Preformation hypothesis an individual already exists in microscopic form in either the egg or sperm and only needs to unfold during development Blending inheritance hypothesis sperm and eggs of parent organisms contain a sampling of the parent s essence that is blended to give offspring intermediate results Inheritance of Acquired Characters traits present in parents are modified through use and these modifcations are passed on to offspring Gregor Mendel 1822 1884 Why Study Peas Many true breeding varieties available Can easily control matings Peas self pollinate Short generation time Produce many offspring Many discrete traits characters o Determined by 1 action of 1 gene o Phenotypes call into only a few distinct catagories Phenotype expressed observable trait Genotype genetic makeup of the individual Monohybrid cross cross between two heterozygous parents 1 trait Dihybrid cross heterozygous for 2 traits between 2 parents Dominat trait that is expressed in the phenotype of a heterzygote R Reccesive trait that is not expressed r Test Cross method used to establish if a individual is heterozygous or homozygous dominant by mating with a homozygous reccesive Dependent assortment alleles of different genes stay together when gametes form Independent assortment alleles of different genes don t stay together when gametes form