Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsOverview: Hereditary Similarity and Variation• Living organisms are distinguished by their abilityto reproduce their own kind• Heredity is the transmission of traits from onegeneration to the next• Variation shows that offspring differ in appearancefrom parents and siblings• Genetics is the scientific study of heredity andvariationCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsConcept 13.1: Offspring acquire genes fromparents by inheriting chromosomes• In a literal sense, children do not inherit particularphysical traits from their parents• It is genes that are actually inheritedCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsInheritance of Genes• Genes are the units of heredity• Genes are segments of DNA• Each gene has a specific locus on a certainchromosome• One set of chromosomes is inherited from eachparent• Reproductive cells called gametes (sperm andeggs) unite, passing genes to the next generationCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsComparison of Asexual and Sexual Reproduction• In asexual reproduction, one parent producesgenetically identical offspring by mitosis• In sexual reproduction, two parents give rise tooffspring that have unique combinations of genesinherited from the two parentsLE 13-2Parent0.5 mmBudCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsConcept 13.2: Fertilization and meiosis alternatein sexual life cycles• A life cycle is the generation-to-generationsequence of stages in the reproductive history ofan organismCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsSets of Chromosomes in Human Cells• Each human somatic cell (any cell other than agamete) has 46 chromosomes arranged in pairs• A karyotype is an ordered display of the pairs ofchromosomes from a cell• The two chromosomes in each pair are calledhomologous chromosomes, or homologues• Both chromosomes in a pair carry genescontrolling the same inherited characteristicsLE 13-35 µmPair of homologouschromosomesSisterchromatidsCentromereCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• The sex chromosomes are called X and Y• Human females have a homologous pair of Xchromosomes (XX)• Human males have one X and one Y chromosome• The 22 pairs of chromosomes that do notdetermine sex are called autosomesCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• Each pair of homologous chromosomes includesone chromosome from each parent• The 46 chromosomes in a human somatic cell aretwo sets of 23: one from the mother and one fromthe father• The number of chromosomes in a single set isrepresented by n• A cell with two sets is called diploid (2n)• For humans, the diploid number is 46 (2n = 46)Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• In a cell in which DNA synthesis has occurred,each chromosome is replicated• Each replicated chromosome consists of twoidentical sister chromatidsLE 13-4KeyMaternal set ofchromosomes (n = 3)2n = 6Paternal set ofchromosomes (n = 3)Two sister chromatidsof one replicatedchromosomesTwo nonsister chromatids in a homologous pairPair of homologouschromosomes(one from each set)CentromereCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• Gametes are haploid cells, containing only one setof chromosomes• For humans, the haploid number is 23 (n = 23)• Each set of 23 consists of 22 autosomes and asingle sex chromosome• In an unfertilized egg (ovum), the sexchromosome is X• In a sperm cell, the sex chromosome may beeither X or YCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsBehavior of Chromosome Sets in the Human Life Cycle• At sexual maturity, the ovaries and testes producehaploid gametes• Gametes are the only types of human cellsproduced by meiosis, rather than mitosis• Meiosis results in one set of chromosomes in eachgamete• Fertilization, the fusing of gametes, restores thediploid condition, forming a zygote• The diploid zygote develops into an adultLE 13-5KeyHaploid (n)Diploid (2n)Haploid gametes (n = 23)Ovum (n)Spermcell (n)TestisOvaryMitosis anddevelopmentMulticellular diploidadults (2n = 46)FERTILIZATIONMEIOSISDiploidzygote(2n = 46)Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Variety of Sexual Life Cycles• The alternation of meiosis and fertilization iscommon to all organisms that reproduce sexually• The three main types of sexual life cycles differ inthe timing of meiosis and fertilizationCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• Depending on the type of life cycle, either haploidor diploid cells can divide by mitosis• However, only diploid cells can undergo meiosis• In all three life cycles, chromosome halving anddoubling contribute to genetic variation in offspringCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsConcept 13.3: Meiosis reduces the number ofchromosome sets from diploid to haploid• Like mitosis, meiosis is preceded by the replicationof chromosomes• Meiosis takes place in two sets of cell divisions,called meiosis I and meiosis II• The two cell divisions result in four daughter cells,rather than the two daughter cells in mitosis• Each daughter cell has only half as manychromosomes as the parent cellCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Stages of Meiosis• In the first cell division (meiosis I), homologouschromosomes separate• Meiosis I results in two haploid daughter cells withreplicated chromosomes• In the second cell division (meiosis II), sisterchromatids separate• Meiosis II results in four haploid daughter cellswith unreplicated chromosomesLE 13-7Homologous pairof chromosomesin diploid parent cellInterphaseHomologous pair of replicated chromosomesChromosomesreplicateMeiosis IDiploid cell withreplicatedchromosomesSisterchromatidsMeiosis IIHomologouschromosomesseparateSister chromatidsseparateHaploid cells withreplicated chromosomesHaploid cells with unreplicated chromosomesCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• Which number represents the DNA content of asperm cell?– A. I– B. II– C. III– D. IV–