Biology Summary Chapter 4 4.1 Traits are distinguishing characteristics that make living organisms a unique individual Inherited is having certain physical traits that are transmitted from one generation to the next Genetics deals with the principles of variation and inheritance in animals and plants - study of genetics gives us greater understanding of how we can determine the likelihood of inheriting certain traits 4.2 - Gregor Mendel was the first to explain the inheritance of characteristics He succeeded in discovering the basis of inheritance when others before him had failed because: 1. chose the appropriate organism to study - chose pea plant to study - it’s commercially available - easy to grow - can self-pollinate so Mendel could control which plants reproduced - different varieties of the pea had different traits that could be observed easily from one generation to the next 2. designed and performed the experiment correctly - obtained purebred plants for the trait he wanted to study - Mendel produced purebred varieties of pea plants through selective breeding to produce plants that were true breeding 3. analyzed the data properly True Breeding describes organisms that are homozygous for a particular trait or sets of traits and produce like offspring Purebred organism s descended from ancestors of a distinct type or breed P generation = parent generation Hybrids are results of a cross between different purebred plants and are heterozygous Monohybrid is a cross of two heterozygous individuals that differ in one trait Dominant trait is a characteristic that is always expressed Recessive trait is a characteristic that is latent (present but inactive) and therefore not usually expressed Principle of dominance states that when individuals with contrasting traits are crossed, the offspring will express only the dominant trait Mendelian ratio is the ratio of dominant phenotype (homozygous dominant genotype and heterozygous genotypes) to recessive phenotype (homozygous recessive genotype) Law of segregation states that inherited traits are determined by pairs of alleles from each parent. The alleles separate during gamete formation giving each offspring only one allele from each parent Genes are parts of the chromosome that governs the expression of a particular trait Alleles are alternate form of a gene located at a particular place on a chromosome Homozygous is having two alleles for a trait that are the same, as is the result of pure breeding ie tt Heterozygous is having two alleles for a trait that are different ie Tt Probability is the chance or likelihood of a particular outcome usually expressed as a ratio - the probability that two or more independent events will occur one after another is the product of their individual probabilities of occurring alone - ie the probability of flipping a coin twice and getting tail both times is ¼ - the chance of getting one tail is ½, the chance of getting the second tail is also ½ - the chance of getting tail one after another is ½ x ½ = ¼ - the law of probability forms the basis of solving genetic problems Punnett square is a simple grid used to illustrate all possible combinations of gametes from a given set of parents Genotype is the genetic make-up of an organism Phenotype is the appearance of the trait in an organism Mendelian ratio for genotype is the ratio of homozygous dominant genotype to heterozygous genotypes to homozygous recessive genotype Ie 1:2:1, there is 1 homozygous dominant genotype, 2 heterozygous genotypes and 1 homozygous recessive genotype Pedigree is a diagram that illustrates the genetic relationships among a group of individuals - careful analysis of different pedigrees has shown that certain human traits are inherited as simple dominant traits Simple dominant traits is trait that has only two possible alleles (dominant or recessive) and is not influenced by the inheritance of other simple dominant traits Carrier is an organism that is heterozygous for the given trait but does not show the recessive trait Test cross is used to determine the genotype of an individual - involves crossing an individual of unknown genotype with a homozygous recessive individual - the offspring will exhibit certain phenotypes that will allow you to determine if the unknown is homozygous dominant or heterozygous - test crosses are valuable for determining whether a particular trait is passed by one, two, or more pairs of alleles - it’s important to know how many pairs of alleles govern a particular trait because multiple allele inheritance is often influenced by environmental factors 4.3 Dihybrid cross is a cross of two heterozygous individuals that differ in two traits ie AaBb x AaBb Law of Independent Assortment is the second law of inheritance states that the inheritance of alleles for one trait does not affect the inheritance of alleles for another trait - - a test cross may also be used to determine the genotype of an individual for two traits a two trait test cross involves crossing an individual that shows the dominant phenotype for two traits with an individual that is homozygous recessive for the same two traits the individual showing the dominant phenotype for both traits may be either heterozygous or homozygous dominant a homozygous recessive individual is used in the cross because it provides the best chance of producing an offspring that is homozygous recessive for both traits 4.4 - not all traits are purely dominant or purely recessive in some cases neither of the alleles controlling the trait are dominant Incomplete dominance is an inheritance pattern where heterozygote exhibits an intermediate phenotype between the two homologous phenotypes; neither allele for the trait is completely dominant - examples can be found in many species of plants ie snapdragon - in some cases both alleles for a trait may be dominant Co-dominant is the case when both alleles for a trait are dominant Multiple alleles is a pattern of inheritance when a gene may have more than two alleles for any given trait