Selective breeding a practice whereby animal or plant breeders choose which individual animals or plants will be allowed to mate based on the traits For a long time before the 19th century it was common belief that traits seen in offspring resulted from the blending of parental traits Performed various experiments in monastery garden leading him to investigate how physical traits could be expressed in plant hybrids Hybrids offspring of parents who differ from each other with regard to certain traits of certain aspects of genetic makeup also known as heterozygotes Heredity and Evolution Anne Boleyn Having extra digits fingers toes is called polydactyly they hope to produce in the offspring The Genetic Principles Discovered by Mendel Gregor Mendel Lived in an abbey what is now the Czech Republic Studied botany at University of Vienna cid 127 Worked on garden peas Concentrated on seven different traits each expressed in two ways Segregation cid 127 Mendel grew groups of pea plants that had different traits between each group Example one group had all tall plants other group all short plants Cross tall plants with short plants calling them the parental generation The hybrid offspring F1 generation were all tall Not intermediate height like traditional views thought cid 127 Mendel let the F1 plants self fertilize to produce a second generation f2 generation cid 127 Only 3 quarters of offspring were tall the rest were short 3 1 ratio Results led to Mendels first principle of inheritance aka Principle of Segregation seperate so that each gamete contains one member of each pair Explained today as meiosis Principle of segregation genes alleles occur in pairs because chromosomes occur in pairs During gamete formation the members of each pair of alleles Paired chromosome and genes they carry seperate from each other and up in different gametes In the zygote the full complement of chromosomes is restored and both members of each chromosome pair are present in the offspring Dominance and Recessiveness Still there just not expressed The unit for the absent characteristic shortness in the f1 plants didn t dissappear Recessive a trait that isn t expressed in heterozygotes An individual must have two copies of a recessive allele for it to be expressed Dominant a trait governed by an allele that s expressed in the presence of another allele Dominant alleles prevent the expression of recessive alleles in heterozygotes Height in garden peas is controlled by 2 different alleles at the same genetic locus height locus Allele specifying tall is dominant to the allele for short In Mendels experiment all the parent plants had two copies of the same allele dominant or recessive depending on if they were tall or short Homozygous having the same allele at the same locus on both members of a pair of chromosomes All tall parents were homozygous for the dominant allele All short parent plants were homozygous for the recessive allele All plants in parent generation had same allele Lacked genetic variation at height locus All the F1 plants hybrids inherited one allele from each parent plant A tall allele from one parent and a short allele from another Inheriting two different alleles at the height locus Heterozygous having different alleles at the same locus on members of a pair of chromosomes T allele for tallness t allele for shortness The same symbols are combined to describe an individual s actual genotype Genotype genetic makeup of an individual An organisms genetic makeup alleles at a particular locus Refers to the alleles at a specific genetic locus in an organism cid 127 Genotypes of plants in Mendels experiments were TT homozygous tall plants Tt heterozygous tall plants tt homozygous short plants environmental factors Phenotypes the observable or detectable physical characteristics of an organism the detectable expressions of genotypes frequently influenced by cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 The only way a recessive allele can be expressed is if it occurs with another recessive allele Homozygous recessive at particular locus Independent Assortment Different characteristics aren t necessarily inherited together Plant height and seed color are independent of each other Any tall pea plant had 50 50 chance of producing yellow or green peas controlling different traits are inherited independently of one another Figure 4 6 Principle of independent assortment the distribution of one pair of alleles into gametes does not influence the distribution of another pair The genes Happens because the genes that control plant height and seed color are located on different nonpartner chromosomes and during meiosis the chromosomes travel to newly forming cells independently of one another in a process called random assortment Random assortment the chance distribution of chromosomes to daughter cells during meiosis Along with recombination random assortment is an important source of genetic variation cid 127 Genes on the same chromosome aren t independent of each other They usually stay together during meiosis cid 127 Mendels results published in 1866 cid 127 Overlooked unappreciated at the time Acceptance of his work in 1900 after discoveries and understanding of chromosomes and cell division Mendelian Inheritance in Humans Mendelian traits aka discrete traits characteristics that are influenced by alleles at only one genetic locus Examples include many blood types such as ABO many genetic disorders including sickle cell anemia and Tay Sachs disease are also mendelian traits listings currently list more than 21 000 human characteristics that are inherited according to mendelian principles The majority of Mendelian traits are biochemical in nature and many genetic disorders result from harmful alleles inherited in Mendelian fashion A number of genetic disorders are caused by dominant alleles If a person inherited one copy of a harmful dominant allele the condition will be present regardless of the presence of a different recessive allele on the partner chromosome Recessive conditions are commonly associated with the lack if a substance usually an enzyme For someone to have a recessive disorder they must have two copies of the recessive allele causing it Having one copy of a harmful