Chromosomal Basis of Inheritance

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Chromosomal Basis of Inheritance

• Chapter 3

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• Chromosomes and Cellular Reproduction

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Eukaryote Chromosomes

• Multiple linear chromosomes
• Many have two of each type
• Diploid zygote
• Fusion of two haploid gametes
• Called homologous chromosomes
• Autosomes
• Sex chromosomes

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Eukaryote Chromosomes

• Differ in size and morphology
• Specific to each species
• Centromere - constriction
• Metacentric, submetacentric, acrocentric,
  telcentric
• Karyotype complete chromosome complement

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Human Karyotype

• Genes ID is chromosome and loci band on that
  chromosome
• Also p or q arm
• BRCA1 is at 17q21

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Mitosis

• Cell reproduction includes growth, mitosis and
  cytokinesis
• Cell cycle includes mitotic phase and interphase
  between division
• Mitosis is a process for growth, repair, and
  asexual reproduction
• Interphase G1, S and G2

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Interphase

• G1- presynthesis
• Cell prepares for DNA synthesis and chromosome
  replication
• S DNA synthesis and chromosome replication
• G2- postsynthesis
• Cell prepares for division

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S Phase

• During interphase chromosomes unwound
• Chromosomes are replicated
• Sister chromatids
• Centromeres replicated but not separated
• Become daughter chromosomes

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Mitosis

• Length of process varies
• Continuous process
• 4 Phases
• Prophase
• Metaphase
• Anaphase
• Telophase

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Prophase

• Chromosomes coil
• Spindle forms
• Centriole in animals
• Nuclear membrane breaks down
• Nucleolus disappears
• Kinetochore forms

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Metaphase and Anaphase

• Metaphase
• Microtubules orient the chromosomes
• Along plane of cell
• Metaphase plate
• Anaphase
• Centromeres separate
• Daughter chromosomes move toward poles

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Telophase and Cytokinesis

• Telophase
• Chromosomes uncoil
• Nuclear envelope forms
• Spindle disappears
• Nucleolus reforms
• Cytokinesis
• Separation of cytoplasm
• Different in plants and animals

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Meiosis

• One replication, two divisions
• Reduction in number of chromosomes
• Occurs at certain points in a life cycle
• Only in certain tissues
• Makes gametes
• Gametogenesis
• Four haploid daughter cells

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Meiosis I Prophase I

• Chromosomes have duplicated
• 5 substages
• Leptotene stage chromosomes coil
• Zygotene stage homologous pairs align in
  synapsis
• Synaptonemal complex - Aligns perfectly base pair
  to base pair
• Telomeres move and align chromosomes
• Pachytene stage crossing-over takes place
• Recombinant chromosome

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Meiosis I Prophase I

• 5 substages
• Diplotene stage synaptonemal complex
  disassembles
• Chiasma are formed
• Diakinesis nucleus and nuclear envelope
  disintegrate

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Meiosis I Metaphase I and Anaphase I

• Metaphase I
• Homologous pairs line up on plate
• Spindle is completely formed
• Kinetochores formed
• Anaphase I
• Homologous pairs separate
• Move toward opposite poles
• Chiasma separate
• Sister chromosomes remain joined

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Meiosis I Telophase I

• Nuclear envelope forms
• Cytokinesis

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Meiosis II

• Similar to mitosis
• Prophase II chromosomes condense
• Metaphase II sister chromosomes line up
• Anaphase II sister chromatids toward poles
• Telophase II nuclear envelope reforms,
  chromosomes no longer visible

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Gene Segregation in Meiosis

• Results of meiosis
• Haploid cells with half of the of chromosomes
• In independent assortment
• Each pair of chromosomes sorts its maternal and
  paternal homologues into daughter cells
  independently of the other pairs
• 2n-1 where n of chromosomes
• Crossover increases variation

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Meiosis vs Mitosis
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Meiosis vs. Mitosis
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Meiosis in Animals

• Diploid for most of their life
• Haploid gametes fuse to restore
• Sexual reproduction
• Gametes produced by spermatogenesis and oogenesis

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Spermatogenesis

• In the testes
• 1 Spermatogonia
• 2 Spermatogonia
• 1 Spermatocytes
• Go through Meiosis I
• 2 Spermatocytes
• Go through Meiosis II
• Spermatids

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Oogenesis

• In the ovaries
• 1 Oogonia
• 2 Oogonia
• 1 Oocytes
• Go through Meiosis I and unequal cytokinesis
• 2 Oocyte and first polar body
• Go through Meiosis II
• Ovum and 2nd polar body

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Meiosis in Plants

• Gametophyte and sporophyte stage
• Alternation of generations
• Flower is sexual structure
• Can be bisexual or unisexual
• Stamens and pistils
• Pollen from anther
• Ovule contains egg cells

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Alternation of Generations

• Multicellular gametophyte
• Meiosis then mitosis BEFORE fertilization
• Plants fungi and algae

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• Chromosome Theory of Inheritance

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Chromosome Theory of Inheritance

• Correlation between traits and separation of
  chromosomes in meiosis
• First called inheritance factors genes
• Chromosome varies

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Sex Chromosomes

• Autosomes
• Sex chromosomes represented differently in the
  two sexes
• When gametes are formed one sex chromosome goes
  to each
• In humans male determines sex
• Some animals have unpaired chromosomes

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Sex Chromosomes

• The X-O system
• The Z-W system
• Chromosome Number

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Sex Chromosomes

• Drosophila melanogaster
• Heterogametic and homogametic sexes
• Same size but different shape
• Genes create characteristics of male and female

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Sex Linkage

• Sex chromosomes also carry genes, any gene
  located on the sex chromosome is called a
  sex-linked gene
• Most are found on X chromosomes
• Hemizygous
• X-linked Allele Animation

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Sex Linkage
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Nondisjunction of Chromosomes

• Homologous or sister chromosomes do not separate
• Can involve either autosomes or sex
• In sex chromosomes can lead to either 2 X
  chromosomes or no X chromosomes
• X chromosome nondisjunction
• Aneuploidy and polyploidy
• Primary and secondary nondisjunction (XXY)

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Nondisjunction of Chromosomes
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Secondary Nondisjunction
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• Sex Chromosomes and Sex Determination

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Genotypic Sex Determination

• Sex chromosomes play a role in inheritance and
  sex determination
• Placentals sex determined by Y
• Y carries genes toward male sex determination
• Testis-determining factor gene
• Factor causes gonad tissue to become testes
  instead of ovaries

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Sex Chromosome Nondisjunction

• XO individuals
• Turner Syndrome
• Genetically female
• At puberty, develop poorly
• XXY individuals
• Klinefelter syndrome
• Genetically male
• Several variations XXY, XXXY, XXYY
• Symptoms include underdeveloped testes, taller
  and some breast tissue

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Turner Syndrome
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Klinefelter Syndrome
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Dosage Compensation with Sex Chromosomes

• Cannot have an unequal dosage of genes from
  duplicate sex chromosomes
• Can be lethal if they are not equal
• Barr body
• Condensed and inactive X chromosome
• Has become lyonized
• Only in females
• X independently chosen from cell to cell
• Epigenetic silencing of one chromosome

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Barr Bodies
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Genic Sex Determinations

• Allelic differences determine sex
• Mating types in yeast
• a and a
• Same morphologies, but mating only occurs between
  different type
• Same in some basidiomycetes

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• Analysis of Sex-Linked Traits in Humans

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X-Linked Recessive Inheritance

• Trait resulting from recessive mutant allele
• More than 100 traits
• Hemophilia A
• Blood lacks clotting factor
• Queen Victoria
• Carrier daughters son with hemophilia
• Females must be homozygous to express

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X-Linked Recessive Inheritance
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X-Linked Dominant Inheritance

• Traits resulting from a dominant mutant allele
  carried on the X chromosome
• Very few
• Faulty tooth enamel
• Heterozygous females express the trait
• Milder in females than in males

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Y-Linked Inheritance

• Trait resulting from mutant gene that is carried
  on the Y chromosome
• Easily recognizable
• Every male in the family has it
• No females express it
• Hairy ears trait
• Hehe!!!