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Chapter 4 Genetics: From Genotype to Phenotype

Chapter 4 Genetics: From Genotype to Phenotype. From Genotype to Phenotype. Structural Genes. Regulatory Genes. From Genotype to Phenotype (cont ’ d). Genotype. Phenotype. From Genotype to Phenotype (cont ’ d). The ABO Blood Type System Recessive, dominant and co-dominant alleles

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Chapter 4 Genetics: From Genotype to Phenotype

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  1. Chapter 4Genetics: From Genotype to Phenotype

  2. From Genotype to Phenotype StructuralGenes RegulatoryGenes

  3. From Genotype to Phenotype (cont’d) Genotype Phenotype

  4. From Genotype to Phenotype (cont’d) • The ABO Blood Type System • Recessive, dominant and co-dominant alleles • Obesity: A Complex Interaction • Genes, environment, and phenotype

  5. Mendelian Genetics • Gregor Mendel (1822-1884) • Experiments in particulate inheritance • Discovered dichotomous variation • Developed a series of postulates about inheritance

  6. Gregor Mendel • A monk living in what is now the Czech Republic • Crossed different strains of purebred plants and studied their progeny. • His work illustrates the basic rules of inheritance.

  7. Mendel’s Postulates • Hereditary characteristics are controlled by particulate unit factors that exist in pairs in individual organisms • When an individual has two different unit factors responsible for a characteristic, only one is expressed and is said to be dominant to the other, which is said to be recessive

  8. Mendel’s Postulates (cont’d) • Mendel’s Law of Segregation: during the formation of gametes, the paired unit factors separate or segregate randomly so that each sex cell receives one or the other with equal likelihood • Mendel’s Law of Independent Assortment: during gamete formation, segregating pairs of unit factors assort independently of each other

  9. Hybrids • Offspring of parents that differ from each other with regard to certain traits or certain aspects of genetic makeup; heterozygotes.

  10. Traits Mendel Studied in Peas

  11. Principle of Segregation • Discrete units, or genes, occur in pairs because chromosomes occur in pairs. • During gamete production, members of each gene pair separate so each gamete contains one member of a pair. • During fertilization, the full number of chromosomes is restored and members of a gene or allele pairs are reunited.

  12. Results When One Trait (Height) Is Considered at a Time

  13. Dominance • Dominant traits are governed by an allele that can be expressed in the presence of another, different allele. • Dominant alleles prevent the expression of recessive alleles in heterozygotes.

  14. Recessiveness • Recessive traits are not expressed in heterozygotes. • For a recessive allele to be expressed, there must be two copies of the allele.

  15. Alleles • Alternate forms of a gene. • Alleles occur at the same locus on a pair of chromosomes and influence the same trait. • However, because they are slightly different, their action may result in different expressions of that trait. • The term is sometimes used synonymously with gene.

  16. Locus • The position on a chromosome where a given gene occurs. • The term is sometimes used interchangeably with gene, but this usage is technically incorrect.

  17. Phenotypes • The observable or detectable physical characteristics of an organism; the detectable expressions of genotypes, frequently influenced by environment.

  18. Principle of Independent Assortment • The distribution of one pair of alleles into gametes does not influence the distribution of another pair. • The genes controlling different traits are inherited independently of one another.

  19. Random Assortment • The chance distribution of chromosomes to daughter cells during meiosis; along with recombination, a source of genetic variation (but not new alleles) from meiosis.

  20. Mendelian 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.

  21. Mendelian Inheritance in Humans • Over 19,000 human traits are known to be inherited according to Mendelian principles (Online Mendelian Inheritance in Man www.ncbi.nlm.nih.gov/omim/) • The human ABO blood system is an example of a simple Mendelian inheritance. • The A and B alleles are dominant to the O allele. • Neither the A or B allele are dominant to one another; they are codominant and both traits are expressed.

  22. Codominance • The expression of two alleles in heterozygotes. • In this situation, neither allele is dominant or recessive; thus, both influence the phenotype.

  23. What Dominance Doesn’t Mean • Complete determinant of phenotype • Better or stronger

  24. Polygenic Inheritance • Polygenic traits, or continuous traits, are governed by alleles at two or more loci, and each locus has some influence on the phenotype. • Hair, eye and skin color are polygenic traits

  25. Linkage and Assortment • Linkage: genes found on the same chromosome are said to be linked. The closer together two genes are, the greater the linkage • Crossing Over: makes possible the independent assortment of linked genes

  26. X-Linked Disorders Genetic conditions that result from mutations to genes on the X chromosome. They are almost always expressed in males, who have only one copy of the X chromosome.

  27. Mendelian Genetics in Humans • Over the past century, hundreds of human disorders and diseases have been cataloged, which can be explained in terms of Mendelian genetic transmission • Earlobe form • Phenylthiocarbamide (PTC) tasting or non-tasting

  28. Mendelian Genetics in Humans (cont’d) • The Online Mendelian Inheritance in Man (OMIM) Web site: (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM ) provides an extraordinary database on genetic conditions in humans, from the most innocuous to the most lethal

  29. Genetics Beyond Mendel • Qualitative variation • Phenotypic variation that can be characterized as belonging to discrete, observable categories • Quantitative variation • Phenotypic variation that is characterized by the distribution of continuous variation within a population

  30. Genetics Beyond Mendel (cont’d) • Polygenic Traits, the Phenotype, and the Environment • Heritability • The proportion of total phenotypic variability observed for a given trait that can be ascribed to genetic factors

  31. Genetics Beyond Mendel (cont’d) Variability caused by genetics Variability caused by genetics + Variability caused by the environment

  32. Genetics Beyond Mendel (cont’d) • Heritability and IQ Test Score Performance • IQ Test Score Performance • Exhibit continuous variation in human populations, with a normal distribution • Genetics is an important factor in producing the variation observed within populations • Both genetics and environment effect IQ score variation

  33. Phenylketonuria: Illustrating Mendelian and Post-Mendelian Concepts • Autosomal recessive condition that leads to the accumulation of large quantities of the amino acid phenylalanine, which causes mental retardation and other phenotypic abnormalities • Screening in newborns • Nutritional prognosis

  34. Genes and Environments • Environment • From a gene’s perspective the “environment” is made up of other genes • The genetic environment is just as critical to the production of phenotypes as any other kind of environment

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