2Chromosomes are matched in homologous pairs MEIOSIS AND CROSSING OVERChromosomes are matched in homologous pairsSomatic cells of each species contain a specific number of chromosomesHuman cells have 46, making up 23 pairs of homologous chromosomesChromosomesCentromereSister chromatidsFigure 8.12
3Homologous Chromosomes Humans have 23 pairs of homologous chromosomes22 pairs – autosomes – found in both males and females1 pair – sex chromosomes, XX = female,XY= male
4Homologous Chromosomes Matched pairs of chromosomesSimilar in size, shape, and banding patternBoth carry genes controlling the same inherited characteristics (the version of the gene may be different)The genes are located at the same locusOne chromosome of each pair is inherited from the mother, the other from the father
5Multicellular diploid adults (2n = 46) Mitosis and development The human life cycleHaploid gametes (n = 23)Egg cellSperm cellMEIOSISFERTILIZATIONDiploid zygote (2n = 46)Multicellular diploid adults (2n = 46)Mitosis and developmentFigure 8.13
6Human Life CycleDiploid cells (2n) – cells that contain both homologous chromosomes. In humans the diploid number is 46.Haploid cells (n) – cells with one copy of each homologous chromosome. The gametes (egg and sperm) are haploid. In humans the haploid number is 23.
7MeiosisMeiosisThe division that reduces the number of chromosomes by half.In animals, meiosis results in the formation of haploid egg and sperm cells.
10Two nuclear divisions occur: Meiosis I During prophase I homologous chromosomes pair – synapsisDuring prophase I the paired chromosomes exchange chromosome parts – crossing overHomologous chromosomes are separated2 cells produced each containing one copy of each homologous chromosome
18MeiosisOogenesisFormation of an egg by meiosisOccurs in special cells (oogonia) in the ovariesUnequal divisions of the cytoplasm during meiosis I and meiosis II result in the formation of 1 haploid egg and 3 haploid polar bodiesOnly the egg can be fertilized
22Independent Assortment of Chromosomes The large number of possible arrangements of chromosome pairs at metaphase I of meiosis leads to many different combinations of chromosomes in gametesThis results in 2n possible combinations of gametesFor humans 2n = 223 = 8 million possible combinationsRandom fertilization also increases variation in offspring
23C E C E C E c e c e c e Coat-color genes Eye-color genes Brown Black WhitePinkTetrad in parent cell (homologous pair of duplicated chromosomes)Chromosomes of the four gametesFigure 8.17A, B
24Homologous chromosomes carry different versions of genes The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene at corresponding loci
26How crossing over leads to genetic recombination Coat-color genesEye-color genesHow crossing over leads to genetic recombinationTetrad (homologous pair of chromosomes in synapsis)1Breakage of homologous chromatids2Joining of homologous chromatidsChiasmaSeparation of homologous chromosomes at anaphase I3Separation of chromatids at anaphase II and completion of meiosis4Parental type of chromosomeRecombinant chromosomeRecombinant chromosomeParental type of chromosomeFigure 8.18BGametes of four genetic types
27Crossing over a closer look Meiosis II Animation(must insert Miller and Levine lecture cd ch11)Meiosis 1Crossing over a closer lookMeiosis IIFigure 8.19
28Genetic Recombination Crossing overThe exchange of genetic information between 2 homologous chromosomes.Occurs during prophase I.Random fertilizationDepends on which sperm cell and its chromosome combinations fertilizes which egg
29Preparation of a karyotype FixativePacked redAnd whiteblood cellsHypotonic solutionBlood cultureStainWhiteBlood cellsCentrifuge321FluidCentromereSister chromatidsPair of homologous chromosomes45Figure 8.19
30A karyotype is a photographic inventory of an individual’s chromosomes ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTUREA karyotype is a photographic inventory of an individual’s chromosomesTo study human chromosomes microscopically, researchers stain and display them as a karyotypeA karyotype usually shows 22 pairs of autosomes and one pair of sex chromosomes
318.21 Accidents during meiosis can alter chromosome number Abnormal chromosome count is a result of nondisjunctionEither homologous pairs fail to separate during meiosis INondisjunction in meiosis INormal meiosis IIGametesn + 1n + 1n – 1n – 1Number of chromosomesFigure 8.21A
32Or sister chromatids fail to separate during meiosis II Normal meiosis INondisjunction in meiosis IIGametesn + 1n – 1nnNumber of chromosomesFigure 8.21B
33Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome Egg celln + 1Zygote 2n + 1Sperm celln (normal)Figure 8.21C
34Connection: An extra copy of chromosome 21 causes Down syndrome This karyotype shows three number 21 chromosomesAn extra copy of chromosome 21 causes Down syndromeFigure 8.20A, B
35The chance of having a Down syndrome child goes up with maternal age Figure 8.23
36Alterations of Chromosomes In most cases abnormal chromosome number results in spontaneous abortion long before birth.Nondisjunction in the sex chromosomes has less of an affect on survival
37Connection: Abnormal numbers of sex chromosomes do not usually affect survival Nondisjunction can also produce gametes with extra or missing sex chromosomesUnusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes
40Connection: Alterations of chromosome structure can cause birth defects and cancer Chromosome breakage can lead to rearrangements that can produce genetic disorders or cancerFour types of rearrangement are deletion, duplication, inversion, and translocation
41Duplication – the fragment joins to a homologous chromosome. DeletionDeletion – a chromosome breaks and a fragment is lost. Seems to have the greatest affect.DuplicationDuplication – the fragment joins to a homologous chromosome.Homologous chromosomesInversion – the fragment reattaches to the original chromosome but in reverse orientation. Least likely to produce harmful affects.InversionReciprocal translocationTranslocation– attachment of a chromosome fragment to a nonhomologous chromosome. May/may not be harmful.Nonhomologous chromosomesFigure 8.23A, B
42Alterations of Chromosomes Abnormalities in the structure of the chromosome may cause disorders (Figure 8.23A)Deletion – a chromosome breaks and a fragment is lost. Seems to have the greatest affect.Duplication – the fragment joins to a homologous chromosome.
43Alterations of Chromosomes Inversion – the fragment reattaches to the original chromosome but in reverse orientation. Least likely to produce harmful affects.Translocation (Figure 8.23B) – attachment of a chromosome fragment to a nonhomologous chromosome. May/may not be harmful.
44Chromosomal changes in a somatic cell can cause cancer A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemiaChromosome 9Reciprocal translocationChromosome 22“Philadelphia chromosome”Activated cancer-causing geneFigure 8.23C