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Chapter 15. The Chromosomal Basis for Inheritance. Genetics. Late 1800s- Mitosis and Meiosis described Early 1900s- Scientists noticed similarities between chromosome behavior and Mendel’s “heritable factors”

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Chapter 15

Chapter 15

The Chromosomal Basis for Inheritance


Genetics
Genetics

  • Late 1800s- Mitosis and Meiosis described

  • Early 1900s- Scientists noticed similarities between chromosome behavior and Mendel’s “heritable factors”

  • Chromosome Theory of Inheritance: genes occupy specific loci on chromosomes and it is the chromosomes which undergo segregation and independent assortment during meiosis

    • The behavior of chromosomes during meiosis can account for Mendel’s laws of segregation and independent assortment


Genetics1
Genetics

The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene


Morgan s experimental evidence
Morgan’s Experimental Evidence

  • Thomas Hunt Morgan

    • Experiments with fruit flies provided first evidence associating a specific gene with a specific chromosome

  • Several characteristics make fruit flies a good organism for genetic studies

    • Produce many offspring

    • A generation can be bred every two weeks

    • Four pairs of chromosomes

  • Different phenotypes in fly populations

    • Wild type phenotypes= “normal” trait, most common in the fly populations

    • Mutant phenotypes= Traits alternative to the wild type


Eye color in fly populations
Eye color in fly populations

Wild type= red eyes

Mutant= white eyes


Correlating behavior of a gene s alleles with behavior of a chromosome pair
Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair

  • In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)

    • The F1 generation all had red eyes

    • The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes


EXPERIMENT a Chromosome

Morgan determined that the white-eyed mutant allele must be located on the X chromosome

PGeneration

All offspringhad red eyes.

F1Generation

RESULTS

F2Generation

Supported the chromosome theory of inheritance


Figure 15 4b

CONCLUSION a Chromosome

Figure 15.4b

w

w

PGeneration

X

X

Y

X

w

w

Sperm

Eggs

F1Generation

w

w

w

w

w

Sperm

Eggs

w

w

w

F2Generation

w

w

w

w

w


Sex linked characteristics
Sex-Linked Characteristics a Chromosome

  • Sex chromosomes

    • Determine sex of offspring

      • Genes can have other functions too

  • Mammals= Males have XY chromosomes, Females have XX chromosomes

    • Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosome

  • Gametes carry one copy of sex chromosome

    • Females= 100% X

    • Males= 50% X, 50% Y

  • X

    Y


    Sex linked genes have unique patterns of inheritance
    Sex-linked genes have unique patterns of inheritance a Chromosome

    • Sex-linked gene= genes located on sex chromosomes

      • X chromosome has many genes unrelated to sex

      • Y chromosome has very few genes

        • The SRY gene on the Y chromosome codes for a protein that directs the development of male anatomical features

    • Females- 2 copies of X chromosome

      • Typical dominant-recessive relationship

      • Females can be carriers of sex-linked disorders

    • Males- 1 copy of X chromosome

      • Only needs one copy of allele to express trait

      • Sex-linked disorders tend to affect mostly males


    Sex linked genes have unique patterns of inheritance1
    Sex-linked genes have unique patterns of inheritance a Chromosome

    • Cross a white-eyed female fruit fly with a red-eyed male fruit fly

    • Parent Genotypes= XrXr x XRY

    Xr

    Xr

    XR

    Y


    Sex linked genes in humans
    Sex-linked genes in humans a Chromosome

    Disorders caused by recessive alleles on the X chromosome in humans

    • Color blindness (mostly X-linked)

    • Duchenne muscular dystrophy

      • Progressive weakening of muscles and loss of coordination

      • Caused by absence of key muscle protein

    • Hemophilia

      • Absence of one or more proteins involved in blood clotting


    Red green colorblindness
    Red-green colorblindness a Chromosome

    • Normal vision woman- XNXN

    • Color-blind woman- XnXn

    • Woman Carrier- XNXn

    • Normal vision man- XNY

    • Color-blind man- XnY

    XN

    Xn

    XN

    Y


    X inactivation
    X Inactivation a Chromosome

    • Mammalian females= one of the two X chromosomes in each cell is randomly inactivated during embryonic development

      • Inactive X condenses into a Barr body

    • If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character

      • Fur color in female mammals


    Linked genes
    Linked Genes a Chromosome

    • Each chromosome has hundreds or thousands of genes (except the Y chromosome)

    • Genes located on the same chromosome that tend to be inherited together are called linked genes

    • Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters

      • Crossed flies that differed in traits of body color and wing size


    Figure 15 9 4

    EXPERIMENT a Chromosome

    P Generation (homozygous)

    Figure 15.9-4

    Double mutant(black body,vestigial wings)

    Wild type(gray body, normal wings)

    b b vg vg

    b b vg vg

    Double mutant

    F1 dihybrid(wild type)

    TESTCROSS

    b b vg vg

    b b vg vg

    Testcrossoffspring

    Eggs

    b vg

    bvg

    bvg

    b vg

    Wild type(gray-normal)

    Black-normal

    Black-vestigial

    Gray-vestigial

    bvg

    Sperm

    b bvg vg

    bbvgvg

    b bvgvg

    bbvg vg

    PREDICTED RATIOS

    :

    1

    If genes are located on different chromosomes:

    :

    1

    1

    1

    :

    If genes are located on the same chromosome andparental alleles are always inherited together:

    1

    0

    1

    0

    :

    :

    :

    RESULTS

    :

    944

    185

    :

    965

    :

    206


    Linked genes1
    Linked Genes a Chromosome

    • Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes)

      • These genes do not assort independently

        • Must be on the same chromosome

    • However, nonparental phenotypes were also produced

    • Genetic recombination:production of offspring with combinations of traits differing from either parent


    Genetic recombination unlinked genes
    Genetic Recombination: Unlinked genes a Chromosome

    • Mendel observed that combinations of traits in some offspring differ from either parent

      • Parental Types= Offspring with a phenotype matching one of the parental phenotypes

      • Recombinant types (recombinants)= Offspring with nonparental phenotypes (new combinations of traits)

        • A 50% frequency of recombination is observed for any two genes on different chromosomes


    Gametes from yellow-round a Chromosome dihybrid parent (YyRr)

    yr

    Yr

    yR

    YR

    Gametes from green-wrinkled homozygousrecessive parent (yyrr)

    yr

    Yyrr

    yyrr

    yyRr

    YyRr

    Recombinant offspring

    Parental-type offspring

    For unlinked genes, genetic recombination occurs due to the independent assortment of chromosomes


    Genetic recombination linked genes
    Genetic Recombination: Linked Genes a Chromosome

    • Morgan discovered that genes can be linked, but the linkage was incomplete, because some recombinant phenotypes were observed

    • He proposed that some process must occasionally break the physical connection between genes on the same chromosome

      • Crossing over of homologous chromosomes


    Gray body, normal wings a Chromosome (F1 dihybrid)

    Black body, vestigial wings(double mutant)

    Testcrossparents

    bvg

    bvg

    bvg

    bvg

    Replicationof chromosomes

    Replicationof chromosomes

    bvg

    bvg

    bvg

    bvg

    bvg

    bvg

    bvg

    bvg

    Meiosis I

    bvg

    Meiosis I and II

    bvg

    bvg

    bvg

    Meiosis II

    Recombinantchromosomes

    bvg

    bvg

    bvg

    bvg

    bvg

    Eggs

    Sperm


    Recombinant a Chromosome chromosomes

    bvg

    bvg

    bvg

    bvg

    Eggs

    206Gray-vestigial

    944Black-vestigial

    185Black-normal

    965Wild type(gray-normal)

    Testcrossoffspring

    bvg

    bvg

    bvg

    bvg

    bvg

    bvg

    bvg

    bvg

    bvg

    Sperm

    Parental-type offspring

    Recombinant offspring

    391 recombinants2,300 total offspring

    Recombinationfrequency

     100  17%


    Genetic recombination
    Genetic Recombination a Chromosome

    • Recombinant chromosomes bring alleles together in new combinations in gametes

    • Random fertilization increases even further the number of variant combinations that can be produced

    • This abundance of genetic variation is the raw material upon which natural selection works


    Mapping the distance between genes using recombination data
    Mapping the Distance Between Genes Using Recombination a Chromosome Data

    • Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome

    • Predicted= farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency


    Mapping the distance between genes using recombination data1
    Mapping the Distance Between Genes Using Recombination a Chromosome Data

    • Linkage map= genetic map of chromosome based on recombination frequencies

      • Distances between genes can be expressed as map units

        • 1 map unit (centimorgan)represents a 1% recombination frequency

        • Map units= relative distance and order, not precise locations of genes


    Mapping the distance between genes using recombination data2
    Mapping the Distance Between Genes Using Recombination a Chromosome Data

    • Genes that are far apart on the same chromosome can have a recombination frequency near 50%

      • Such genes are physically linked, but genetically unlinked

        • Behave as if found on different chromosomes

    • Sturtevant used recombination frequencies to make linkage maps of fruit fly genes

    • Using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes

      • Cytogenetic maps=positions of genes with respect to chromosomal features


    Alteration of chromosomal structure
    Alteration of Chromosomal Structure a Chromosome

    Breakage of a chromosome can lead to four types of changes in chromosome structure

    • Deletion removes a chromosomal segment

    • Duplication repeats a segment

    • Inversion reverses orientation of a segment within a chromosome

    • Translocation moves a segment from one chromosome to another

  • If occurs in sex cells= congenital disorders

  • If occurs in somatic cells= non-inherited, can lead to cancer


  • Figure 15 14

    A a Chromosome

    B

    C

    D

    E

    F

    G

    H

    A

    B

    C

    E

    F

    G

    H

    A

    B

    C

    D

    E

    F

    G

    H

    A

    B

    C

    B

    C

    D

    E

    F

    G

    H

    A

    B

    C

    D

    E

    F

    G

    H

    A

    D

    C

    B

    E

    F

    G

    H

    A

    B

    C

    D

    E

    F

    G

    M

    N

    O

    P

    Q

    R

    A

    B

    P

    Q

    R

    M

    N

    O

    C

    D

    E

    F

    H

    (a) Deletion

    Figure 15.14

    A deletion removes a chromosomal segment.

    (b) Duplication

    A duplication repeats a segment.

    (c) Inversion

    An inversion reverses a segment within a chromosome.

    (d) Translocation

    H

    A translocation moves a segment from onechromosome to a nonhomologous chromosome.

    G


    Disorders in humans caused of alteration of chromosome structure
    Disorders in Humans Caused of Alteration of Chromosome Structure

    • The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5

    • A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood


    Disorders in humans caused of alteration of chromosome structure1
    Disorders in Humans Caused of Alteration of Chromosome Structure

    • Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes

      • Portion of chromosome 22 switches places with portion of chromosome 9

      • Gene creates a hybrid protein that stimulates cell division, leading to tumors


    There are two normal exceptions to mendelian genetics
    There are two normal exceptions to Mendelian Structuregenetics

    • One exception involves genes located in the nucleus

    • Other exception involves genes located outside the nucleus

      • Extranuclear genes (or cytoplasmicgenes)are found in organelles in the cytoplasm

        • Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules

    • In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance


    Genes located in nucleus
    Genes located in nucleus Structure

    • For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits

      • This type of variation in phenotype is called genomic imprinting

      • Genomic imprinting involves the silencing of certain genes that are “stamped” with an imprint during gamete production


    Figure 15 17

    Normal StructureIgf2 alleleis expressed.

    Figure 15.17

    Paternalchromosome

    Maternalchromosome

    Normal-sized mouse(wild type)

    Normal Igf2 alleleis not expressed.

    (a) Homozygote

    Mutant Igf2 alleleinherited from mother

    Mutant Igf2 alleleinherited from father

    Dwarf mouse (mutant)

    Normal-sized mouse (wild type)

    Normal Igf2 alleleis expressed.

    Mutant Igf2 alleleis expressed.

    Mutant Igf2 alleleis not expressed.

    Normal Igf2 alleleis not expressed.

    (b) Heterozygotes


    Genes located in nucleus1
    Genes located in nucleus Structure

    • The difference in mouse size is thought to be caused by imprinting from the methylation (addition of –CH3) of cysteine nucleotides

    • Genomic imprinting is thought to affect only a small fraction of mammalian genes

    • Most imprinted genes are critical for embryonic development


    Genes located outside of nucleus
    Genes located outside of nucleus Structure

    • Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg

    • First evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant

    • Some defects in mitochondrial genes prevent cells from making enough ATP

      • result in diseases that affect the muscular and nervous systems


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