5-1 CHAPTER 5 Genetics: A Review

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-2 Gregor Johann Mendel ( ) Gregor Johann Mendel ( ) First person to formulate the principles of heredity First person to formulate the principles of heredity Mendel chose to work with pure strains of garden peas Mendel chose to work with pure strains of garden peas Ex: dwarf and tall varieties Ex: dwarf and tall varieties The plants were self-fertilizing but subject to experimental cross-fertilization; all using Meiosis to produce sex cells. The plants were self-fertilizing but subject to experimental cross-fertilization; all using Meiosis to produce sex cells. Mendel’s Investigations

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-3 Chromosomal Basis of Inheritance Meiosis: Reduction Division of Gametes Meiosis: Reduction Division of Gametes Sex cells (gametes) transmit genetic information from parents to offspring in sexually reproducing organisms Sex cells (gametes) transmit genetic information from parents to offspring in sexually reproducing organisms Chromosomes occur in pairs: homologs Chromosomes occur in pairs: homologs One member is donated by the mother, the other by the father One member is donated by the mother, the other by the father Homologs Homologs Contain similar genes encoding the same set of characteristics Contain similar genes encoding the same set of characteristics Usually have the same size and shape Usually have the same size and shape Example: Chromosome #6 from mom and #6 from dad Example: Chromosome #6 from mom and #6 from dad

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-4 Meiosis Meiosis Special type of nuclear division Special type of nuclear division Associated with gamete production Associated with gamete production Genetic material replicates once followed by 2 successive nuclear divisions Genetic material replicates once followed by 2 successive nuclear divisions Produces 4 daughter cells (gametes) Produces 4 daughter cells (gametes) Each with only 1 member of each homologous chromosome pair or 1 set of chromosomes (haploid) (Ex. Only one #6 chromosome in each gamete) Each with only 1 member of each homologous chromosome pair or 1 set of chromosomes (haploid) (Ex. Only one #6 chromosome in each gamete) Chromosomal Basis of Inheritance

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-6 Fertilization Fertilization Reestablishes the diploid chromosome number ( Two #6 chromosomes) Reestablishes the diploid chromosome number ( Two #6 chromosomes) Union of egg and sperm produces a zygote Union of egg and sperm produces a zygote Chromosomal Basis of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-7 Meiosis I Meiosis I Prophase I The two members of each pair of homologs make side-by-side contact The two members of each pair of homologs make side-by-side contact 4 homologous chromosomes lined up is called a tetrad 4 homologous chromosomes lined up is called a tetrad Homologs cross-over and exchange genetic information to increase variation in animals Homologs cross-over and exchange genetic information to increase variation in animals Chromosomal Basis of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-8 Crossing Over - during Meiosis

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 5-9 Metaphase I Metaphase I Homologs line up side by side on the metaphase plate Homologs line up side by side on the metaphase plate Anaphase I Homologs are separated and moved to opposite poles of cell Homologs are separated and moved to opposite poles of cell Chromosomal Basis of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Telophase I Telophase I Nuclear membrane forms around separated homologs Nuclear membrane forms around separated homologs Each of these cells (now haploid) enter Meiosis II No interphase between Meiosis I and Meiosis II. Chromosomal Basis of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Meiosis II Meiosis II Prophase II No crossing-over occurs No crossing-over occurs Metaphase II All chromosomes line up single file at metaphasic plate All chromosomes line up single file at metaphasic plate Chromosomal Basis of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Anaphase II Anaphase II Centromeres of dyads are replicated and single-stranded chromosomes move toward each pole Centromeres of dyads are replicated and single-stranded chromosomes move toward each pole Telophase II Telophase II Nuclear membrane forms around separated chromosomes Nuclear membrane forms around separated chromosomes Each daughter cell contains one complete haploid set of chromosomes Each daughter cell contains one complete haploid set of chromosomes Chromosomal Basis of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Sex Determination in Humans Sex Determination in Humans 46 chromosomes (23 pair) in somatic cells 46 chromosomes (23 pair) in somatic cells Pairs 1-22: Autosomes (do not determine sex) Pairs 1-22: Autosomes (do not determine sex) Pair 23: Sex Chromosomes (determine sex) Pair 23: Sex Chromosomes (determine sex) Two sex chromosomes: X, Y Two sex chromosomes: X, Y Males: XY Females: XX Males: XY Females: XX Some animals Male is X0 and female is XX (there is no Y chromosome) Chromosomal Basis of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Some animals, the absence of a 2nd sex chromosome leads to a male.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display In most animals, an XX or XY distribution determines sex.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mendelian Laws of Inheritance Mendel’s First Law Mendel’s First Law The Law of Segregation: The Law of Segregation: In the formation of gametes (Meiosis), homologous chromosomes separate so that each gamete receives only one member of the pair. In the formation of gametes (Meiosis), homologous chromosomes separate so that each gamete receives only one member of the pair.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Testcross - (Punnett Square Used) Testcross - (Punnett Square Used) A testcross is performed to determine the genotype A testcross is performed to determine the genotype Crossing an individual of unknown genotype with a homozygous recessive individual Mendelian Laws of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Intermediate Inheritance - (Incomplete Dominance) Intermediate Inheritance - (Incomplete Dominance) Neither allele is completely dominant over the other Neither allele is completely dominant over the other The heterozygous phenotype is distinct from those of the parents, often intermediate between them The heterozygous phenotype is distinct from those of the parents, often intermediate between them Ex. Red and White flower = Pink flower Ex. Red and White flower = Pink flower Codominance - Both dominant traits expressed (Ex - white/black spotted cow) Codominance - Both dominant traits expressed (Ex - white/black spotted cow) Mendelian Laws of Inheritance

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Mendel’s Second Law Mendel’s Second Law Genes located on homologous chromosomes assort independently during meiosis Genes located on homologous chromosomes assort independently during meiosis Pertains to studies of 2 pairs of hereditary factors at the same time Pertains to studies of 2 pairs of hereditary factors at the same time Ex. Pea plant - color and pod size combinations can occur (Yt, YT, yT, yt) Ex. Pea plant - color and pod size combinations can occur (Yt, YT, yT, yt) Mendelian Laws of Inheritance

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Multiple Alleles Multiple Alleles Individuals can have no more than 2 alleles at a given locus (designated sight on chromosome) Individuals can have no more than 2 alleles at a given locus (designated sight on chromosome) Ex: Ex: Freckles Freckles Human ABO blood groups Human ABO blood groups Mendelian Laws of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Gene Interaction Gene Interaction Many different genes can affect a single phenotype: polygenic inheritance Many different genes can affect a single phenotype: polygenic inheritance Epistasis: Allele at one locus can mask or prevent the expression of an allele at another locus acting on the same trait Epistasis: Allele at one locus can mask or prevent the expression of an allele at another locus acting on the same trait Quantitative inheritance: Characters show variation of 2 extremes ( Ex. Skin pigmentation ) Quantitative inheritance: Characters show variation of 2 extremes ( Ex. Skin pigmentation ) Mendelian Laws of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Sex-Linked Inheritance Sex-Linked Inheritance Traits specified by genes located on sex chromosomes Traits specified by genes located on sex chromosomes X – Linked Traits X – Linked Traits Most sex-linked traits are X-linked Most sex-linked traits are X-linked Genes located on the X sex chromosome. Genes located on the X sex chromosome. Ex: Ex: Red – Green Color Blindness Hemophilia Mendelian Laws of Inheritance

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Autosomal Linkage Autosomal Linkage Genes on the same chromosome Said to be linked Tend to be inherited together However, linkage groups may be broken up during meiosis (crossing over)  distance between the 2 loci,  probability that alleles will be inherited together  distance between the 2 loci,  probability that alleles will be inherited together. Mendelian Laws of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Chromosomal Abnormalities Chromosomal Abnormalities 5 out of every 1000 humans are born with serious genetic defects attributable to chromosomal anomalies 5 out of every 1000 humans are born with serious genetic defects attributable to chromosomal anomalies Changes in chromosome number Changes in chromosome number Monosomy, Trisomy Monosomy, Trisomy Mendelian Laws of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Euploidy Euploidy Addition or deletion of whole sets of chromosomes Addition or deletion of whole sets of chromosomes Polyploidy Polyploidy Most common type of euploidy Most common type of euploidy The carrying of 3 or more sets of chromosomes by an organism The carrying of 3 or more sets of chromosomes by an organism More common in plants than animals More common in plants than animals Mendelian Laws of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Changes in Chromosome Structure Changes in Chromosome Structure Inversion Inversion Portion of a chromosome reversed Portion of a chromosome reversed Deletion Deletion Entire blocks of genes lost Entire blocks of genes lost Translocation Translocation Nonhomologous chromosomes exchange sections Nonhomologous chromosomes exchange sections Duplication Duplication Extra section of chromosomes attached Extra section of chromosomes attached Mendelian Laws of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Gene Theory GENE: Nucleotide sequence that encodes a functional polypeptide or RNA sequence

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display Frequency of Mutations Frequency of Mutations Mutation rate for humans Mutation rate for humans About every third person carries approximately one new mutation About every third person carries approximately one new mutation Although most mutations are harmful, most are recessive and not expressed Although most mutations are harmful, most are recessive and not expressed Sources of Phenotypic Variation