5-1 chapter 5 genetics: a review. copyright © the mcgraw-hill companies, inc. permission required...
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CHAPTER 5 CHAPTER 5 CHAPTER 5 CHAPTER 5
Genetics: A ReviewGenetics: A ReviewGenetics: A ReviewGenetics: A Review
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Gregor Johann Mendel (1822-1884)Gregor Johann Mendel (1822-1884) First person to formulate the principles of First person to formulate the principles of
heredity heredity Mendel chose to work with Mendel chose to work with pure strains of pure strains of
garden peasgarden peas Ex: dwarf and tall varietiesEx: dwarf and tall varieties
The plants were The plants were self-fertilizingself-fertilizing but subject to but subject to experimental experimental cross-fertilization; cross-fertilization; all using all using Meiosis to produce sex cells.Meiosis to produce sex cells.
Mendel’s InvestigationsMendel’s Investigations
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Chromosomal Basis of InheritanceChromosomal Basis of Inheritance
Meiosis: Reduction Division of GametesMeiosis: Reduction Division of Gametes Sex cellsSex cells (gametes) transmit genetic information (gametes) transmit genetic information
from parents to offspring in sexually reproducing from parents to offspring in sexually reproducing organismsorganisms
Chromosomes occur in pairs: Chromosomes occur in pairs: homologshomologs One member is donated by the mother, the other by the One member is donated by the mother, the other by the
fatherfather
HomologsHomologs Contain similar genes encoding the same set of Contain similar genes encoding the same set of
characteristicscharacteristics Usually have the same size and shapeUsually have the same size and shape Example: Chromosome #6 from mom and #6 from dadExample: Chromosome #6 from mom and #6 from dad
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MeiosisMeiosis Special type of nuclear divisionSpecial type of nuclear division
Associated with gamete productionAssociated with gamete production Genetic material replicates once followed by 2 Genetic material replicates once followed by 2
successive nuclear divisionssuccessive nuclear divisions Produces 4 daughter cells (gametes)Produces 4 daughter cells (gametes)
Each with only 1 member of each Each with only 1 member of each homologous chromosome pair or 1 set of homologous chromosome pair or 1 set of chromosomes (haploid) (Ex. Only one #6 chromosomes (haploid) (Ex. Only one #6 chromosome in each gamete)chromosome in each gamete)
Chromosomal Basis of InheritanceChromosomal Basis of Inheritance
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FertilizationFertilization Reestablishes the Reestablishes the diploiddiploid chromosome chromosome
number ( Two #6 chromosomes)number ( Two #6 chromosomes) Union of egg and sperm produces a Union of egg and sperm produces a
zygotezygote
Chromosomal Basis of InheritanceChromosomal Basis of Inheritance
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Meiosis IMeiosis I
Prophase IProphase I The two members of each pair of The two members of each pair of
homologs make side-by-side contacthomologs make side-by-side contact 4 homologous chromosomes lined up is 4 homologous chromosomes lined up is
called a called a tetradtetrad Homologs Homologs cross-overcross-over and exchange and exchange
genetic information to increase variation genetic information to increase variation in animalsin animals
Chromosomal Basis of InheritanceChromosomal Basis of Inheritance
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Crossing Over - during Meiosis
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Metaphase IMetaphase I Homologs line up side by side on the Homologs line up side by side on the
metaphase platemetaphase plate
Anaphase IAnaphase I Homologs are separated and moved to Homologs are separated and moved to
opposite poles of cellopposite poles of cell
Chromosomal Basis of InheritanceChromosomal Basis of Inheritance
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Telophase ITelophase I Nuclear membrane forms around Nuclear membrane forms around
separated homologsseparated homologs Each of these cells (now haploid) enter
Meiosis II No interphase between Meiosis I and
Meiosis II.
Chromosomal Basis of InheritanceChromosomal Basis of Inheritance
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Meiosis IIMeiosis II
Prophase IIProphase II No crossing-over occursNo crossing-over occurs
Metaphase IIMetaphase II All chromosomes line up single file at All chromosomes line up single file at
metaphasic platemetaphasic plate
Chromosomal Basis of InheritanceChromosomal Basis of Inheritance
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Anaphase IIAnaphase II Centromeres of dyads are replicated and Centromeres of dyads are replicated and
single-stranded chromosomes move single-stranded chromosomes move toward each pole toward each pole
Telophase IITelophase II Nuclear membrane forms around Nuclear membrane forms around
separated chromosomesseparated chromosomes Each daughter cell contains one Each daughter cell contains one
complete haploid set of chromosomescomplete haploid set of chromosomes
Chromosomal Basis of InheritanceChromosomal Basis of Inheritance
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Sex Determination in HumansSex Determination in Humans 46 chromosomes (23 pair) in somatic cells46 chromosomes (23 pair) in somatic cells
Pairs 1-22: Pairs 1-22: AutosomesAutosomes (do not determine sex) (do not determine sex) Pair 23: Sex Chromosomes (determine sex)Pair 23: Sex Chromosomes (determine sex)
Two sex chromosomes: X, YTwo sex chromosomes: X, Y Males: XY Females: XXMales: XY Females: XXSome animals Male is X0 and female is XX (there is Some animals Male is X0 and female is XX (there is
no Y chromosome)no Y chromosome)
Chromosomal Basis of InheritanceChromosomal Basis of Inheritance
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Some animals, the absence of a 2nd sex chromosome leads to a male.
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In most animals, an XX or XY distribution determines sex.
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Mendelian Laws of InheritanceMendelian Laws of Inheritance
Mendel’s First LawMendel’s First Law The Law of Segregation:The Law of Segregation:
In the formation of gametes (Meiosis), In the formation of gametes (Meiosis), homologous chromosomes separate so that homologous chromosomes separate so that each gamete receives only one member of the each gamete receives only one member of the pair.pair.
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Testcross - (Punnett Square Used)Testcross - (Punnett Square Used) A A testcross testcross is performed to determine the is performed to determine the
genotypegenotype Crossing an individual of unknown genotype
with a homozygous recessive individual
Mendelian Laws of InheritanceMendelian Laws of Inheritance
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Intermediate Inheritance - (Incomplete Intermediate Inheritance - (Incomplete Dominance)Dominance) Neither allele is completely dominant over Neither allele is completely dominant over
the other the other The heterozygous phenotype is The heterozygous phenotype is distinctdistinct
from those of the parents, often from those of the parents, often intermediate between themintermediate between them
Ex. Red and White flower = Pink flowerEx. Red and White flower = Pink flower Codominance Codominance - Both dominant traits - Both dominant traits
expressed (Ex - white/black spotted cow)expressed (Ex - white/black spotted cow)
Mendelian Laws of InheritanceMendelian Laws of Inheritance
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Intermediate Inheritance or Incomplete Dominance
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Mendel’s Second LawMendel’s Second Law Genes located on homologous Genes located on homologous
chromosomes assort independently chromosomes assort independently during meiosisduring meiosis Pertains to studies of 2 pairs of hereditary Pertains to studies of 2 pairs of hereditary
factors at the same timefactors at the same time Ex. Pea plant - color and pod size Ex. Pea plant - color and pod size
combinations can occur (Yt, YT, yT, yt)combinations can occur (Yt, YT, yT, yt)
Mendelian Laws of InheritanceMendelian Laws of Inheritance
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Multiple AllelesMultiple Alleles Individuals can have no more than 2 Individuals can have no more than 2
alleles at a given locus (designated sight alleles at a given locus (designated sight on chromosome) on chromosome)
Ex:Ex: Freckles Freckles Human ABO blood groupsHuman ABO blood groups
Mendelian Laws of InheritanceMendelian Laws of Inheritance
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Gene InteractionGene Interaction Many different genes can affect a single Many different genes can affect a single
phenotype: phenotype: polygenic inheritancepolygenic inheritance Epistasis:Epistasis: Allele at one locus can mask or Allele at one locus can mask or
prevent the expression of an allele at prevent the expression of an allele at another locus acting on the same traitanother locus acting on the same trait
Quantitative inheritance:Quantitative inheritance: Characters show Characters show variation of 2 extremes (variation of 2 extremes (Ex. Skin pigmentationEx. Skin pigmentation))
Mendelian Laws of InheritanceMendelian Laws of Inheritance
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Sex-Linked InheritanceSex-Linked Inheritance Traits specified by genes located on Traits specified by genes located on sex sex
chromosomeschromosomes X – Linked TraitsX – Linked Traits
Most sex-linked traits are X-linkedMost sex-linked traits are X-linked Genes located on the X sex Genes located on the X sex
chromosome.chromosome. Ex: Ex:
Red – Green Color BlindnessRed – Green Color BlindnessHemophiliaHemophilia
Mendelian Laws of InheritanceMendelian Laws of Inheritance
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Color Blindness
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Blue 1/2 arrow = YNo trait carried on Y
Sex-linkedTrait
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Autosomal LinkageAutosomal 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 InheritanceMendelian Laws of Inheritance
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Chromosomal AbnormalitiesChromosomal Abnormalities 5 out of every 1000 humans are born with 5 out of every 1000 humans are born with
serious genetic defects attributable to serious genetic defects attributable to chromosomal anomalieschromosomal anomaliesChanges in chromosome numberChanges in chromosome number
Monosomy, TrisomyMonosomy, Trisomy
Mendelian Laws of InheritanceMendelian Laws of Inheritance
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EuploidyEuploidy Addition or deletion of whole sets of Addition or deletion of whole sets of
chromosomeschromosomes PolyploidyPolyploidy
Most common type of euploidyMost common type of euploidy The carrying of 3 or more sets of The carrying of 3 or more sets of
chromosomes by an organismchromosomes by an organism More common in plants than animalsMore common in plants than animals
Mendelian Laws of InheritanceMendelian Laws of Inheritance
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Changes in Chromosome StructureChanges in Chromosome Structure InversionInversion
Portion of a chromosome reversedPortion of a chromosome reversed DeletionDeletion
Entire blocks of genes lostEntire blocks of genes lost TranslocationTranslocation
Nonhomologous chromosomes exchange sectionsNonhomologous chromosomes exchange sections DuplicationDuplication
Extra section of chromosomes attached Extra section of chromosomes attached
Mendelian Laws of InheritanceMendelian Laws of Inheritance
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Gene TheoryGene Theory
GENEGENE: Nucleotide sequence that : Nucleotide sequence that encodes a functional polypeptide encodes a functional polypeptide or RNA sequenceor RNA sequence
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Frequency of MutationsFrequency of Mutations Mutation rate for humansMutation rate for humans
About every third person carries About every third person carries approximately one new mutationapproximately one new mutation
Although most mutations are harmful, most are Although most mutations are harmful, most are recessive and not expressedrecessive and not expressed
Sources of Phenotypic VariationSources of Phenotypic Variation