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Observing Patterns in Inherited Traits
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Observing Patterns in Inherited Traits

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  • 1. Observing Patterns in Inherited Traits Chapter 11 Impacts, Issues: The Color of Skin Like most human traits, skin color has a genetic basis; more than 100 gene products affect the synthesis and deposition of melanins 11.1 Mendel, Pea Plants, and Inheritance Patterns Recurring inheritance patterns are observable outcomes of sexual reproduction Before the discovery of genes, it was thought that inherited traits resulted from a blend of parental characters Mendel’s Experimental Approach Mendel was a monk with training in plant breeding and mathematics He studied the garden pea (Pisum sativum), which breeds true for a number of traits Terms Used in Modern Genetics Genes •Heritable units of information about traits •Parents transmit genes to offspring •Each gene has a specific locus on a chromosome Diploid cells (chromosome number 2n) have pairs of genes on homologous chromosomes
  • 2. Terms Used in Modern Genetics A mutation is a permanent change in a gene •May cause a trait to change •Alleles are different molecular forms of a gene A hybrid has nonidentical alleles for a trait •Offspring of a cross between two individuals that breed true for different forms of a trait are hybrids Terms Used in Modern Genetics An individual with nonidentical alleles of a gene is heterozygous for that gene An individual with identical alleles of a gene is homozygous for that gene Terms Used in Modern Genetics An allele is dominant if its effect masks the effect of a recessive allele paired with it •Capital letters (A) signify dominant alleles; lowercase letters (a) signify recessive alleles •Homozygous dominant (AA) •Homozygous recessive (aa) •Heterozygous (Aa) Terms Used in Modern Genetics Gene expression •The process by which information in a gene is converted to a structural or functional part of a cell or body •Expressed genes determine traits Terms Used in Modern Genetics
  • 3. Genotype •The particular alleles an individual carries Phenotype •An individual’s observable traits Terms Used in Modern Genetics P stands for parents, F for filial (offspring) F1: First generation offspring of parents F2: Second generation offspring of parents 11.1 Key Concepts Where Modern Genetics Started Gregor Mendel gathered the first experimental evidence of the genetic basis of inheritance His meticulous work gave him clues that heritable traits are specified in units The units, which are distributed into gametes in predictable patterns, were later identified as genes 11.2 Mendel’s Law of Segregation Garden pea plants inherit two “units” of information for a trait, one from each parent Testcrosses Testcross •A method of determining if an individual is heterozygous or homozygous dominant
  • 4. •An individual with unknown genotype is crossed with one that is homozygous recessive (AA x aa) or (Aa x aa) Monohybrid Experiments Monohybrid experiments •Testcrosses that check for a dominance relationship between two alleles at a single locus •May be crosses between true breeding (homozygous) individuals (AA x aa), or between identical heterozygotes (Aa x Aa) Mendel’s Monohybrid Experiments Mendel used monohybrid experiments to find dominance relationships among pea plant traits •When he crossed plants that bred true for white flowers with plants that bred true for purple flowers, all F1 plants had purple flowers •When he crossed two F1 plants, ¾ of the F2 plants had purple flowers, ¼ had white flowers Calculating Probabilities Probability •A measure of the chance that a particular outcome will occur Punnett square •A grid used to calculate the probability of genotypes and phenotypes in offspring Mendel’s Law of Segregation Mendel observed a phenotype ratio of 3:1 in the F2 offspring of his monohybrid crosses •Consistent with the probability of the aa genotype in the offspring of a heterozygous cross (Aa x Aa) This is the basis of Mendel’s law of segregation •Diploid cells have pairs of genes on pairs of homologous chromosomes •The two genes of each pair separate during meiosis, and end up in different gametes
  • 5. 11.2 Key Concepts Insights from Monohybrid Experiments Some experiments yielded evidence of gene segregation: When one chromosome separates from its homologous partner during meiosis, the alleles on those chromosomes also separate and end up in different gametes 11.3 Mendel’s Law of Independent Assortment Mendel’s law of independent assortment •Many genes are sorted into gametes independently of other genes Dihybrid Experiments Dihybrid experiments •Tests for dominance relationships between alleles at two loci •Individuals that breed true for two different traits are crossed (AABB x aabb) •F2 phenotype ratio is 9:3:3:1 (four phenotypes) •Individually, each dominant trait has an F2 ratio of 3:1 – inheritance of one trait does not affect inheritance of the other Mendel’s Law of Independent Assortment Mendel’s dihybrid experiments showed that “units” specifying one trait segregated into gametes separately from “units” for other traits Exception: Genes that have loci very close to one another on a chromosome tend to stay together during meiosis 11.3 Key Concepts Insights from Dihybrid Experiments
  • 6. Some experiments yielded evidence of independent assortment: Genes are typically distributed into gametes independently of other genes 11.4 Beyond Simple Dominance Mendel focused on traits based on clearly dominant and recessive alleles; however, the expression patterns of genes for some traits are not as straightforward Codominance in ABO Blood Types Codominance •Two nonidentical alleles of a gene are both fully expressed in heterozygotes, so neither is dominant or recessive •May occur in multiple allele systems Multiple allele systems •Genes with three or more alleles in a population •Example: ABO blood types Incomplete Dominance Incomplete dominance •One allele is not fully dominant over its partner •The heterozygote’s phenotype is somewhere between the two homozygotes, resulting in a 1:2:1 phenotype ratio in F2 offspring Example: Snapdragon color •RR is red •Rr is pink •rr is white Epistasis Epistasis •Two or more gene products influence a trait •Typically, one gene product suppresses the effect of another Example: Coat color in dogs •Alleles B and b designate colors (black or brown) •Two recessive alleles ee suppress color
  • 7. Pleiotropy Pleiotropy •One gene product influences two or more traits •Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder 11.5 Linkage Groups The farther apart two genes are on a chromosome, the more often crossing over occurs between them Linkage group •All genes on one chromosome •Linked genes are very close together; crossing over rarely occurs between them The Distance Between Genes The probability that a crossover event will separate alleles of two genes is proportional to the distance between those genes 11.6 Genes and the Environment Expression of some genes is affected by environmental factors such as temperature, altitude, or chemical exposure The result may be variation in traits Effects of Temperature on Gene Expression Enzyme tyrosinase, works at low temperatures
  • 8. Effects of Predation on Gene Expression Predators of daphnias emit chemicals that trigger a different phenotype 11.7 Complex Variations in Traits Individuals of most species vary in some of their shared traits Many traits (such as eye color) show a continuous range of variation Continuous Variation Continuous variation •Traits with a range of small differences •The more factors that influence a trait, the more continuous the distribution of phenotype Bell curve •When continuous phenotypes are divided into measurable categories and plotted as a bar chart, they form a bell-shaped curve Regarding the Unexpected Phenotype Phenotype results from complex interactions among gene products and the environment •Enzymes and other gene products control steps of most metabolic pathways •Mutations, interactions among genes, and environmental conditions may affect one or more steps 11.4-11.7 Key Concepts Variations on Mendel’s Theme Not all traits appear in Mendelian inheritance patterns
  • 9. •An allele may be partly dominant over a nonidentical partner, or codominant with it •Multiple genes may influence a trait; some genes influence many traits •The environments also influences gene expression

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