Chapter 8 Bio

Chapter 8:  

•  Introduction: 

  • Genetics is the study of heredity 

  • Johann Mendel set the framework for genetics long before chromosomes or genes had been identified 

  • Genes, carried on chromosomes: are the basic functional units of heredity with the ability to be replicated, expressed, or mutated. 

• 8.1  

  • Mendel’s experiments: 

    • Model system: a system with convenient characteristics that is used to study a specific biological phenomenon to gain understanding to be applied to other systems 

    • Continuous variation: is the range of small differences we see among individuals in a characteristic like human height. 

    • Discontinuous variation: is the variation seen among individuals when each individual shows oneof two– or a very few — easily distinguishable traits  

• 8.2 

  • Laws of inheritance 

    • Gene variants that arise by mutation and exist at the same relative locations in homologous chromosomes are called alleles 

  • Phenotypes and genotypes 

    • Phenotype: The observable traits expressed by an organism  

    • Genotype: An organism’s underlying genetic makeup, consisting of both the physically visible and the non-expressed alleles 

  • Laws of dominance: 

    • Mendel’s law of dominance states that in a heterozygote, one trait will conceal the presence of another trait for the same characteristic. 

  • Monohybrid cross and the Punett Square 

    • Monohybrid cross: When fertilization occurs between two true-breeding parents that differ by only the characteristic being studied 

    • The probability of an event is calculated by the number of times the event occurs divided by the total number of opportunities for the event to occur

    • Punett square: devised by the British geneticist Reginald Punnett, is useful for determining probabilities because it is drawn to predict all possible outcomes of all possible random fertilization events and their expected frequencies  

  • Law of segregation:

    • Law of segregation: 

      • This law states that paired unit factors(genes) must segregate equally into gametes such that offspring have an equal likelihood of inheriting either factor 

    • The physical basis of Mendel’s law of segregation is the first division of meiosis  in which the homologous chromosomes with their different versions of each gene are segregated into daughter cells 

    • Test cross: the dominant expressing organism is crossed with an organism that is homozygous recessive for the same characteristic 

  • Law of independent assortment 

    • Mendel’s law of independent assortment states that genes do not influence each other with regard to the sorting of alleles into gametes, and every possible combination of alleles for every gene is equally likely to occur. 

    • Dihybrid cross: a cross between two true-breeding parents that express different traits for two characteristics 

    • The physical basis for the law of independent assortment also lies in meiosis I 

• 8.3 

  • Alternatives to dominance and recessiveness 

    • Recessive alleles can be “ carried” and not expressed by individuals. 

    • Incomplete dominance 

      • Incomplete dominance: one of the alleles appears in the phenotype in the heterozygote but to the exclusion of other which can also be seen 

    • Codominance 

      • Codominance: in which both alleles for the same characteristic are simultaneously expressed in the heterozygote  

  • Linked genes violate the law of independent assortment 

    • Genes that are located on separate, non-homologous chromosomes will always sort independently  

    • Linkage: in which genes that are located physically close to each other on the same chromosome are more likely to be inherited as a pair