Chromosomal Inheritance
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Drosophila Chromosomes. Outline. X-Linked Alleles Human X-Linked Disorders Gene Linkage Crossing-Over Chromosome Map Changes in Chromosome Number Changes in Chromosome Structure Human Syndromes. Chromosomal Inheritance. Humans are diploid (2 chromosomes of each type)

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Drosophila Chromosomes

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Drosophila chromosomes

Chromosomal Inheritance


Drosophila chromosomes

Drosophila Chromosomes


Outline

Outline

  • X-Linked Alleles

    • Human X-Linked Disorders

  • Gene Linkage

    • Crossing-Over

    • Chromosome Map

  • Changes in Chromosome Number

  • Changes in Chromosome Structure

    • Human Syndromes


Chromosomal inheritance

Chromosomal Inheritance

  • Humans are diploid (2 chromosomes of each type)

    • Humans have 23 different kinds of chromosomes

    • Arranged in 23 pairs of homologous chromosomes

    • Total of 46 chromosomes (23 pairs) per cell

  • One of the chromosome pairs determines the sex of an individual (The sex chromosomes)

  • The other 22 pairs of chromosomes are autosomes

  • Autosomal chromosomes are numbered from smallest (#1) to largest (#22)

  • The sex chromosomes are numbered as the 23rd pair


Sex determination in humans

Sex Determination in Humans

  • Sex is determined in humans by allocation of chromosomes at fertilization

  • Both sperm and egg carry one of each of the 22 autosomes

  • The egg always carries the X chromosome as number 23

  • The sperm may carry either and X or Y

    • If the sperm donates an X in fertilization, the zygote will be female

    • If the sperm donates a Y in fertilization, the zygote will be male

    • Therefore, the sex of all humans is determined by the sperm donated by their father


X linked alleles

X-Linked Alleles

  • Genes carried on autosomes are said to be autosomally linked

  • Genes carried on the female sex chromosome (X) are said to be X-linked (or sex-linked)

  • X-linked genes have a different pattern of inheritance than autosomal genes have

    • The Y chromosome is blank for these genes

    • Recessive alleles on X chromosome:

      • Follow familiar dominant/recessive rules in females (XX)

      • Are always expressed in males (XY), whether dominant or recessive

      • Males said to be monozygous for X-linked genes


Eye color in fruit flies

Eye Color in Fruit Flies

  • Fruit flies (Drosophila melanogaster) are common subjects for genetics research

  • They normally (wild-type) have red eyes

  • A mutant recessive allele of a gene on the X chromosome can cause white eyes

  • Possible combinations of genotype and phenotype:


X linked inheritance

X-Linked Inheritance


Human x linked disorders red green color blindness

Human X-Linked Disorders:Red-Green Color Blindness

  • Color vision In humans:

    • Depends three different classes of cone cells in the retina

    • Only one type of pigment is present in each class of cone cell

      • The gene for blue-sensitive is autosomal

      • The red-sensitive and green-sensitive genes are on the X chromosome

      • Mutations in X-linked genes cause RG color blindness:

        • All males with mutation (XbY) are colorblind

        • Only homozygous mutant females (XbXb) are colorblind

        • Heterozygous females (XBXb) are asymptomatic carriers


Red green colorblindness chart

Red-Green Colorblindness Chart


X linked recessive pedigree

X-Linked Recessive Pedigree


Human x linked disorders muscular dystrophy

Human X-Linked Disorders:Muscular Dystrophy

  • Muscle cells operate by release and rapid sequestering of calcium

  • Protein dystrophin required to keep calcium sequestered

  • Dystrophin production depends on X-linked gene

  • A defective allele (when unopposed) causes absence of dystrophin

    • Allows calcium to leak into muscle cells

    • Causes muscular dystrophy

  • All sufferers male

    • Defective gene always unopposed in males

    • Males die before fathering potentially homozygous recessive daughters


Human x linked disorders hemophilia

Human X-Linked Disorders:Hemophilia

  • “Bleeder’s Disease”

  • Blood of affected person either refuses to clot or clots too slowly

    • Hemophilia A – due to lack of clotting factor IX

    • Hemophilia B – due to lack of clotting factor VIII

  • Most victims male, receiving the defective allele from carrier mother

  • Bleed to death from simple bruises, etc.

  • Factor VIII now available via biotechnology


Hemophilia pedigree

Hemophilia Pedigree


Human x linked disorders fragile x syndrome

Human X-Linked Disorders:Fragile X Syndrome

  • Due to base-triplet repeats in a gene on the X chromosome

  • CGG repeated many times

    • 6-50 repeats – asymptomatic

    • 230-2,000 repeats – growth distortions and mental retardation

  • Inheritance pattern is complex and unpredictable


Gene linkage

Gene Linkage


Gene linkage1

Gene Linkage

  • When several genes of interest exist on the same chromosome

  • Such genes form a linkage group

    • Tend to be inherited as a block

    • If all genes on same chromosome:

      • Gametes of parent likely to have exact allele combination as gamete of either grandparent

      • Independent assortment does not apply

    • If all genes on separate chromosomes:

      • Allele combinations of grandparent gametes will be shuffled in parental gametes

      • Independent assortment working


Linkage groups

Linkage Groups


Constructing a chromosome map

Constructing a Chromosome Map

  • Crossing-over can disrupt a blocked allele pattern on a chromosome

  • Affected by distance between genetic loci

  • Consider three genes on one chromosome:

    • If one at one end, a second at the other and the third in the middle

      • Crossing over very likely to occur between loci

      • Allelic patterns of grandparents will likely to be disrupted in parental gametes with all allelic combinations possible

    • If the three genetic loci occur in close sequence on the chromosome

      • Crossing over very UNlikely to occur between loci

      • Allelic patterns of grandparents will likely to be preserved in parental gametes

  • Rate at which allelic patterns are disrupted by crossing over:

    • Indicates distance between loci

    • Can be used to develop linkage map or genetic map of chromosome


Crossing over

Crossing Over


Complete vs incomplete linkage

Complete vs. Incomplete Linkage


Chromosome number polyploidy

Chromosome Number:Polyploidy

  • Polyploidy

    • Occurs when eukaryotes have more than 2n chromosomes

    • Named according to number of complete sets of chromosomes

    • Major method of speciation in plants

      • Diploid egg of one species joins with diploid pollen of another species

      • Result is new tetraploid species that is self-fertile but isolated from both “parent” species

      • Some estimate 47% of flowering plants are polyploids

    • Often lethal in higher animals


Chromosome number aneuploidy

Chromosome Number:Aneuploidy

  • Monosomy (2n - 1)

    • Diploid individual has only one of a particular chromosome

    • Caused by failure of synapsed chromosomes to separate at Anaphase I (nondisjunction)

  • Trisomy (2n + 1) occurs when an individual has three of a particular type of chromosome

    • Diploid individual has three of a particular chromosome

    • Also caused by nondisjunction

    • This usually produces one monosomic daughter cell and one trisomic daughter cell in meiosis I

    • Down syndrome is trisomy 21


Nondisjunction

Nondisjunction


Trisomy 21

Trisomy 21


Chromosome number abnormal sex chromosome number

Chromosome Number:Abnormal Sex Chromosome Number

  • Result of inheriting too many or too few X or Y chromosomes

  • Caused by nondisjunction during oogenesis or spermatogenesis

  • Turner Syndrome (XO)

    • Female with single X chromosome

    • Short, with broad chest and widely spaced nipples

    • Can be of normal intelligence and function with hormone therapy


Chromosome number abnormal sex chromosome number1

Chromosome Number:Abnormal Sex Chromosome Number

  • Klinefelter Syndrome (XXY)

    • Male with underdeveloped testes and prostate; some breast overdevelopment

    • Long arms and legs; large hands

    • Near normal intelligence unless XXXY, XXXXY, etc.

    • No matter how many X chromosomes, presence of Y renders individual male


Turner and klinefelter syndromes

Turner and Klinefelter Syndromes


Chromosome number abnormal sex chromosome number2

Chromosome Number:Abnormal Sex Chromosome Number

  • Ploy-X females

    • XXX simply taller & thinner than usual

    • Some learning difficulties

    • Many menstruate regularly and are fertile

    • More than 3 Xs renders severe mental retardation

  • Jacob’s syndrome (XYY)

    • Tall, persistent acne, speech & reading problems


Abnormal chromosome structure

Abnormal Chromosome Structure

  • Deletion

    • Missing segment of chromosome

    • Lost during breakage

  • Translocation

    • A segment from one chromosome moves to a non-homologous chromosome

    • Follows breakage of two nonhomologous chromosomes and improper re-assembly


Deletion translocation duplication and inversion

Deletion, Translocation,Duplication, and Inversion


Abnormal chromosome structure1

Abnormal Chromosome Structure

  • Duplication

    • A segment of a chromosome is repeated in the same chromosome

  • Inversion

    • Occurs as a result of two breaks in a chromosome

      • The internal segment is reversed before re-insertion

      • Genes occur in reverse order in inverted segment


Inversion leading to duplication and deletion

Inversion Leading toDuplication and Deletion


Abnormal chromosome structure2

Abnormal Chromosome Structure

  • Deletion Syndromes

    • Williams syndrome - Loss of segment of chromosome 7

    • Cri du chat syndrome (cat’s cry) - Loss of segment of chromosome 5

  • Translocations

    • Alagille syndrome

    • Some cancers


Williams syndrome

Williams Syndrome


Alagille syndrome

Alagille Syndrome


Review

Review

  • X-Linked Alleles

    • Human X-Linked Disorders

  • Gene Linkage

    • Crossing-Over

    • Chromosome Map

  • Changes in Chromosome Number

  • Changes in Chromosome Structure

    • Human Syndromes


Drosophila chromosomes

Chromosomal Inheritance


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