Inheritance, Genes, and Chromosomes8

Chapter 8 Inheritance, Genes, and ChromosomesKey Concepts8.1 Genes Are Particulate and Are Inherited According to Mendels Laws8.2 Alleles and Genes Interact to Produce Phenotypes8.3 Genes Are Carried on Chromosomes8.4 Prokaryotes Can Exchange Genetic Material

Chapter 8 Opening QuestionHow is hemophilia inherited through the mother, and why is it more frequent in males?

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsEarly experiments with genetics yielded two theories:Blending inheritancegametes contained determinants (genes) that blended when gametes fused during fertilizationParticulate inheritanceeach determinant was physically distinct and remained intact during fertilization

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsMendel used the scientific method and studied garden peas.Their flowers have both male and female sex organs, pistils, and stamens, to produce gametes.Male organs can be removed to allow fertilization by another flower.

In-Text Art, Ch. 8, p. 145

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsCharacterobservable physical feature (e.g., flower color, seed shape)Traitform of a character (e.g., purple flowers or white flowers, wrinkled seeds)Mendel worked with true-breeding varietieswhen plants of the same variety were crossed, all offspring plants produced the same seeds and flowers.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsMendels crosses:Pollen from one parent was transferred to the stigma of the other parent. Parental generation = PResulting offspring = first filial generation or F1If F1 plants self-pollinate, they produce second filial generation or F2.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsIn Mendels first experiment, he crossed plants differing in just one character (P).This produced monohybrids in the F1 generation.The monohybrids were then allowed to self-pollinate to form the F2 generationa monohybrid cross.Mendel repeated this for seven characters.

Figure 8.1 Mendels Monohybrid Experiments (Part 1)

Figure 8.1 Mendels Monohybrid Experiments (Part 2)

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsOne trait of each pair disappeared in the F1 generation and reappeared in F2 these traits are recessive.The trait that appears in the F1 is the dominant trait.The ratio of dominant traits to recessive traits in the F2 was about 3:1.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsMendels observations rejected the blending theory of inheritance and supported the particulate theory.He proposed that the determinants occur in pairs and are segregated in the gametes.Each plant has two genes for each character, one from each parent.Diploidtwo copies of a geneHaploidone copy of a gene

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsAlleles are different forms of a gene, such as smooth or wrinkled seeds.True-breeding individuals have two copies of the same allelethey are homozygous for the allele (e.g., ss).Heterozygous individuals have two different alleles (e.g., Ss).

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsPhenotypephysical appearance of an organism (e.g., spherical seeds)Genotypethe genetic makeup (e.g., Ss)Spherical seeds can be the result of two different genotypesSS or Ss.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsMendels first law:The law of segregation states that the two copies of a gene separate when an individual makes gametes.Each gamete receives only one copy.

Figure 8.2 Mendels Explanation of Inheritance (Part 1)

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsWhen the F1 self-pollinates, there are three ways to get the dominant trait (e.g., spherical), but only one way to get the recessive (wrinkled)resulting in the 3:1 ratio.Allele combinations can be predicted using a Punnett square.

Figure 8.2 Mendels Explanation of Inheritance (Part 2)

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsA gene is a short sequence on a longer DNA molecule.DNA molecules make up the chromosomes.Different alleles of a gene segregate when chromosomes separate during meiosis I.

Figure 8.3 Meiosis Accounts for the Segregation of Alleles (Part 1)

Figure 8.3 Meiosis Accounts for the Segregation of Alleles (Part 2)

Figure 8.3 Meiosis Accounts for the Segregation of Alleles (Part 3)

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsMendel tested his hypothesis by doing test crosses:He did this to determine whether an individual is homozygous or heterozygous for a trait by crossing it with a homozygous recessive individual.Mendel crossed the F1 with known homozygotes (e.g., wrinkled or ss).

Figure 8.4 Homozygous or Heterozygous? (Part 1)

Figure 8.4 Homozygous or Heterozygous?

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsMendels next experiment involved:Crossing peas that differed in two charactersseed shape and seed colorTrue-breeding parents:SSYYspherical yellow seedsssyywrinkled green seeds

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsF1 generation is SsYyall spherical yellow.Crossing the F1 generation (all identical double heterozygotes) is a dihybrid cross.Mendel asked whether, in the gametes produced by SsYy, the traits would be linked, or segregate independently.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsTwo possibilities included: Alleles could maintain associations seen in parental generationthey could be linkedIf linked, gametes would be SY or sy; F2 would have three times more spherical yellow than wrinkled green.If independent, gametes could be SY, sy, Sy, or sY. F2 would have nine different genotypes; phenotypes would be in 9:3:3:1 ratio.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsOr:The segregation of S from s could be independent of Y from ythe two genes could be unlinkedIf independent, gametes could be SY, sy, Sy, or sY in equal numbers.The F2 generation would have nine different genotypes; and four phenotypes in a 9:3:3:1 ratio.This prediction was supported.

Figure 8.5 Independent Assortment (Part 1)

Figure 8.5 Independent Assortment (Part 2)

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsMendels second law:The law of independent assortment states that alleles of different genes assort independently during gamete formation.This law doesnt always apply to genes on the same chromosome, but chromosomes do segregate independently.

Figure 8.6 Meiosis Accounts for Independent Assortment of Alleles

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsOne of Mendels contributions to genetics was the use of mathematical analysesthe rules of statistics and probability.His analyses revealed patterns that allowed him to formulate his hypotheses.Probability calculations and Punnett squares give the same results.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsProbabilityIf an event is certain to happen, probability = 1If an event cannot possibly happen, probability = 0All other events have a probability between 0 and 1

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsTwo coin tosses are independent events, each will come up heads the time.The probability that both will come up heads is: x = To get the joint probability, multiply the individual probabilities (multiplication rule).

Figure 8.7 Using Probability Calculations in Genetics

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsProbability in a monohybrid crossAfter self-pollination of an F1 Ss, the probability that the F2 offspring will have the genotype SS is x = ; the same for ss offspring.There are two ways to get a heterozygote Ss; the probability is the sum of the individual probabilities (addition rule): + =

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsHuman pedigrees can show Mendels laws.Humans have few offspring; pedigrees do not show the clear proportions that the pea plants showed.Geneticists use pedigrees to determine whether a rare allele is dominant or recessive.

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsPattern of inheritance for a rare dominant allele:Every person with the abnormal phenotype has an affected parent.Either all (if homozygous parent) or half (if heterozygous parent) of offspring in an affected family are affected.

Figure 8.8 Pedigree Analysis and Inheritance (Part 1)

Concept 8.1 Genes Are Particulate and Are Inherited According to Mendels LawsPattern of inheritance for a rare recessive allele:Affected people often have two unaffected parents.In an affected family, one-fourth of children of unaffected parents are affected.

Figure 8.8 Pedigree Analysis and Inheritance (Part 2)

Concept 8.2 Alleles and Genes Interact to Produce PhenotypesDifferent alleles arise through mutation rare, stable, inherited changes in the genetic material.The wild type is the allele present in most of the population. Other alleles are mutant alleles.A gene with a wild-type allele that is present less than 99 percent of the time is called polymorphic.

Concept 8.2 Alleles and Genes Interact to Produce PhenotypesA given gene may have more than two alleles.Multiple alleles increase the number of possible phenotypes and may show a hierarchy of dominance in heterozygotes.One example is the coat color in rabbits.

Figure 8.9 Multiple Alleles for Coat Color in Rabbits

Concept 8.2 Alleles and Genes Interact to Produce PhenotypesSome alleles are neither dominant nor recessive.A heterozygote has an intermediate phenotype in incomplete dominance.Red + white snapdragons = pink in F1Red and white colors reappear in F2 as well as pink.

Figure 8.10 Incomplete Dominance Follows Mendels Laws (Part 1)

Figure 8.10 Incomplete Dominance Follows Mendels Laws (Part 2)

Concept 8.2 Alleles and Genes Interact to Produce PhenotypesCodominancetwo alleles of a gene produce phenotypes that are both present in the heterozygote.Example: ABO blood group system has three alleles of the gene: IA, IB, and IO.

Figure 8.11 ABO Blood Reactions Are Important in Transfusions

Concept 8.2 Alleles and Genes Interact to Produce PhenotypesEpistasisphenotypic expression of one gene is influenced by another geneExample: Coat color in Labrador retrievers: Allele B (black) dominant to b (brown)Allele E (pigment deposition) is dominant to e (no pigment depositionyellow)

Figure 8.12 Genes Interact Epistatically

Concept 8.2 Alleles and Genes Interact to Produce PhenotypesHybrid vigor, or heterosis, is a cross between two different true-breeding homozygotes.It can result in offspring with stronger, larger phenotypes.Most complex phenotypes are determined by multiple genes.Quantitative traits conferred by multiple genes are measured, rather than assessed qualitatively.

In-Text Art, Ch. 8, p. 154

Concept 8.2 Alleles and Genes Interact to Produce PhenotypesGenotype and environment interact to determine the phenotype of an organism.Two parameters describe the effects:Penetrance is the proportion of individuals with a certain genotype that show the phenotype.Expressivity is the degree to which genotype is expressed in an individual.

Concept 8.3 Genes Are Carried on ChromosomesGenes are sequences of DNA that reside at a particular site on a chromosomea locus (plural loci).The genetic linkage of genes on a single chromosome can alter their patterns of inheritance.

Concept 8.3 Genes Are Carried on ChromosomesGenetic linkage was discovered by Thomas Hunt Morgan and students at Columbia University using the fruit fly Drosophila melanogaster.Much genetic research has been done with Drosophila, which is considered a model organism because of its size, ease of breeding, and short generation time.

Concept 8.3 Genes Are Carried on ChromosomesSome crosses performed with Drosophila did not yield expected ratios according to the law of independent assortment.Instead, some genes for body color and wing shape were inherited together.Morgan theorized that the two loci were linked on the same chromosome and could not assort independently.

Figure 8.13 Some Alleles Do Not Assort Independently (Part 1)

Figure 8.13 Some Alleles Do Not Assort Independently (Part 2)

Concept 8.3 Genes Are Carried on ChromosomesSome offspring showed recombinant phenotypes, different from their parents.Genes may recombine during prophase I of meiosis by crossing over.Homologous chromosomes exchange corresponding segments. The exchange involves two chromatids of four in the tetradboth chromatids become recombinant (each ends up with genes from both parents).

Figure 8.14 Crossing Over Results in Genetic Recombination (Part 1)

Figure 8.14 Crossing Over Results in Genetic Recombination (Part 2)

Figure 8.14 Crossing Over Results in Genetic Recombination (Part 3)

Concept 8.3 Genes Are Carried on ChromosomesRecombinant offspring phenotypes (non-parental) appear in recombinant frequencies.To determine the recombinant frequencies, divide the number of recombinant offspring by the total number of offspring.Recombinant frequencies are greater for loci that are farther apart on the chromosome.

Figure 8.15 Recombination Frequencies (Part 1)

Figure 8.15 Recombination Frequencies (Part 2)

Concept 8.3 Genes Are Carried on ChromosomesRecombinant frequencies can be used to make genetic maps showing the arrangement of genes along a chromosome.Recombinant frequencies are converted to map units corresponding to distances between genes.

In-Text Art, Ch. 8, p. 157 (1)

Concept 8.3 Genes Are Carried on ChromosomesThe fruit fly genome has four pairs of chromosomesthree pairs are similar in size, called autosomes.The fourth pair are of different size, the sex chromosomes.Many genes on the X chromosome are not present on the Y chromosome.

In-Text Art, Ch. 8, p. 157 (2)

Concept 8.3 Genes Are Carried on ChromosomesGenes on sex chromosomes dont follow Mendelian patterns.The Y chromosome carries few genes; the X chromosome carries many.Thus, males have only one copy of these geneshemizygous.

Concept 8.3 Genes Are Carried on ChromosomesSex-linked inheritanceinheritance of a gene that is carried on a sex chromosomeOne example is the eye color in Drosophila.

Figure 8.16 A Gene for Eye Color Is Carried on the Drosophila X Chromosome (Part 1)

Figure 8.16 A Gene for Eye Color Is Carried on the Drosophila X Chromosome (Part 2)

Concept 8.3 Genes Are Carried on ChromosomesX-linked recessive phenotypes:They appear much more often in males than females.A male with the mutation can only pass it on to daughters.Daughters who receive one X-linked mutation are heterozygous carriers.Mutant phenotype can skip a generation if it passes from a male to his daughter (normal) and then to her son.

Figure 8.17 RedGreen Color Blindness Is Carried on the Human X Chromosome

Concept 8.3 Genes Are Carried on ChromosomesBesides the genes in the nucleus, mitochondria and plastids contain small numbers of genes.Mitochondria and plastids are inherited only from the mother.The inheritance of organelles and their genes is non-Mendelian and is called maternal or cytoplasmic inheritance.

Figure 8.18 Cytoplasmic Inheritance

Concept 8.4 Prokaryotes Can Exchange Genetic MaterialBacteria exchange genes by bacterial conjugation.Sex pilus is a projection that initiates contact between bacterial cells.Conjugation tube is a cytoplasmic bridge that forms between cells.The donor chromosome fragments and some material enters the recipient cell.

Figure 8.19 Bacterial Conjugation and Recombination (Part 1)

Figure 8.19 Bacterial Conjugation and Recombination (Part 2)

Concept 8.4 Prokaryotes Can Exchange Genetic MaterialBacteria have plasmidssmall circular DNA moleculesbesides the main chromosome.Genes on the plasmids are in categories:Metabolic tasks, breaking down hydrocarbonsInvolved in conjugationAntibiotic resistance

Concept 8.4 Prokaryotes Can Exchange Genetic MaterialPlasmids can move between the cells during conjugation.They can:Replicate independently of the main chromosomeAdd their genes to the recipient cells genome

Figure 8.20 Gene Transfer by Plasmids

Answer to Opening QuestionIn hemophilia, the mutant gene for factor VIII, the clotting factor, is carried on the X chromosome.The affected males inherited their single X chromosome from their mothersif the mutated form of the gene was present, they would develop the disease.Daughters would inherit a normal X chromosome as well and would not express the recessive trait, though could be carriers.

Figure 8.21 Sex Linkage in Royal Families of Europe

**********LINK Review discussion of haploid and diploid organisms in Concept 7.1**LINK The process of meiosis is described in Concept 7.4 *******ANIMATED TUTORIAL 8.1 Independent Assortment of Alleles**See Figures 8.2 and 8.7INTERACTIVE TUTORIAL 8.1 Pedigree Analysis**VIDEO 8.1 Mutant alleles in Drosophila melanogasterSee Chapter 15***See Chapter 31***APPLY THE CONCEPT Alleles and genes interact to produce phenotypes

ANIMATED TUTORIAL 8.2 Alleles That Do Not Assort IndependentlySee Figures 8.3 and 8.6**LINK Review the discussion of crossing over in Concept 7.4*******APPLY THE CONCEPT Genes are carried on chromosomes

****See Chapters 4 and 7LINK The evolutionary consequences of lateral gene transfer are discussed in Concepts 15.6 and 19.1****