chapter 5

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CHAPTER 5 CHAPTER 5 CHAPTER 5 CHAPTER 5

Genetics: A ReviewGenetics: A ReviewGenetics: A ReviewGenetics: A Review

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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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)



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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



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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



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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



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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.



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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



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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



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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)



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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



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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)



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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)



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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



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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))



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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



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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.



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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



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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



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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



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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

chapter 5

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