Patterns of Chromosome Inheritance

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Chapter 18

• Patterns of Chromosome Inheritance

• Case Study?
• Mechelle and her peirced ears !
• Whats a keloid?

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Outline

• Introduction
• Chromosomes and the Cell Cycle
• Mitosis
• Phases of Mitosis
• Meiosis
• Stages of Meiosis
• Crossing-Over
• Comparison of Meiosis and Mitosis
• Chromosome Inheritance

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Introduction / Chromosomes and the Cell Cycle

• 3 Factors determining the make-up of an
  individual
• Physical characteristics (genes).
• Environment.
• Spiritual condition.
• Genetics is the study of heredity. . . . .
• the study of how traits are passed from one
  generation to the next.

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• What is the function of cell division?
• Growth gt mitosis
• Repair gt mitosis
• Reproduction gt meiosis
• Chromatin / chromosome ?
• DNA ? 100,000 genes ? ? recipe for you !
• Human Genome Project estimated only 33,000
  genes !!!
• Now say, 20-25,000 genes !!!!!!!!!

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• Each organism has a characteristic of
  chromosomes (humans 46 or 23 pairs) that can be
  identified by a karyotype . . . . .
• an arrangement of all chromosomes within a cell
  by pairs in a fixed order.
• In humans
• 22 pairs of non-sex chromosomes (autosomes)
  
• 1 pair of sex chromosomes. . . .
• XX female, XY male.
• Thus, who determines the childs sex?

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• Prior to cell division
• each chromosome is composed of 2 genetically
  identical parts termed chromatids (or sister
  chromatids) held together at a region called a
  centromere.

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Cell Increase and Decrease

• These represent opposing processes which by their
  very nature maintain balance (homeostasis) in the
  human organism.
• Cell division increases the of body cells
  (somatic cells).
• Cell division mitosis (division of the nucleus)
  and
• Cytokinesis (division of the cytoplasm).
• Decreasing the of cells involves programmed
  cell death (apoptosis).

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Obtaining Fetal Chromosomes

• At times, the doctor and parents may want to view
  an unborn childs chromosomes (karyotype) to see
  if there is a correct number or if a genetic
  disorder may be indicated.
• Syndrome a group of symptoms that always occur
  together.
• Methods to obtain a sample
• Chorionic Villi Sampling (CVS)
• Amniocentesis

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The Cell Cycle

• is an orderly set of stages that take place
  between the time a cell divides and the time the
  resulting cells also divide.
• In order to understand the cell cycle, one must
  recall the structure of the cell. . . . . . . .
• the cell membrane, cytoplasm (containing the
  organelles) and the nucleus.
• Also, one must recall that when a cell is not
  dividing, the DNA (genetic material in the
  nucleus) and associated proteins are a tangled
  mass of thin treads called chromatin rather than
  the distinct rod-like chromosomes seen during
  cell division.

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Cell Cycle

• Cell cycle consists of interphase and mitosis
• Interphase (a period of growth and
  differentiation).
• Mitosis (when the nucleus divides).
• Cytokinesis (when the cytoplasm / cell divides).
• Interphase is the interval of time between cell
  divisions, is the phase the cell is in the
  longest, and is not a time of rest but rather is
  when the cell is active carrying on its functions.

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The Stages of Interphase

• In mammalian cells it last about 20 hours (90 of
  the cell cycle).
• Interphase is divided into 3 stages
• G1 stage cell doubles its organelles,
  accumulates the materials needed for DNA
  synthesis.
• S stage DNA replication occurs, a copy is make
  of all the DNA in the cell.
• G2 stage cell synthesizes the proteins needed
  for cell division (ie. protein in microtubules).

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The Mitotic Stage

• Follows interphase.
• It is called the M stage (for mitotic stage).
• It includes mitosis and cytokinesis.
• Mammalian cells require about 4 hours to complete
  the mitotic stage.

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Mitosis ( duplication division)

• Mitosis occurs in humans when tissues grow or
  when repair occurs, and produces 2 daughter cells
  with the same set (number) of chromosomes as the
  mother cell.
• Before mitosis, the chromatin duplicates and
  shortens into chromosomes, each consisting of 2
  sister chromatids which are held together at the
  centromere.
• Sister chromatids separate, and one of each kind
  of chromosome goes into each daughter cell.

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Mitosis Overview
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Phases of Mitosis

• Prophase.
• Centrioles outside nucleus duplicate and move
  away from each other.
• Spindle fibers appear.
• Chromosomes become distinct, nuclear envelope
  fragments (membrane disappears), nucleolus
  disappears.
• Metaphase.
• Spindle fully-formed.
• Chromosomes align single file at the equator.
• Repel each other and appear X-shaped.

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Stages of Mitosis

• Anaphase.
• Sister chromatids separate and daughter
  chromosomes move to the poles.
• Spindle fibers shorten and pull chromosomes
  towards the poles.
• Telophase.
• When daughter chromosomes arrive at each pole.
• Cytokinesis (via cleavage furrow) occurs.
• Spindle disappears, nucleoli reappear, and the
  nuclear envelopes form.
• Chromosomes become indistinct chromatin.
• Have two identical daughter cells.

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Control of the Cell Cycle

• Is by internal and external chemical signals.
• Growth factors are external signals received at
  the plasma membrane which can cause the cell to
  undergo the cell cycle.
• Steps reception, transduction, activate genes,
  response.
• Cyclin is an internal signal that increases and
  decreases as the cell cycle continues.
• It must be present for the cell to proceed from
  the G2 to the M stage, and from the G1 to the S
  stage.
• If DNA damage occurs, the protein p53 attempts to
  repair the DNA.
• However, if DNA repair is not possible, p53
  brings about cell death (apoptosis).

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Apoptosis

• is often defined as programmed cell death.
• This is because the cell progresses through a
  usual series of events that bring about its
  destruction. . . . . .
• the cell rounds up, looses contact with its
  neighbors ? the nucleus fragments ? the plasma
  membrane develops blisters (blebs) ? the cell
  fragments are engulfed by WBCs or neighboring
  cells.
• Apoptosis is actually facilitated by 2 sets of
  enzymes within the cell, called caspases, which
  include initiators and executioners.

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Video
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Meiosis ( reduction division)

• Meiosis requires two nuclear divisions and
  results in four daughter cells, each with half
  the number of parental chromosomes (haploid, n),
  one of each of the original 23 pair.
• Humans have 23 pairs of homologous chromosomes.
• During meiosis I homologues pair and synapsis
  occurs allowing crossing-over.
• Exchange of genetic material between nonsister
  chromatids of homologous pairs.

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Crossing-Over
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• Next, the homologous chromosomes of each pair
  separate so that one chromosome from each pair
  will be in the daughter cell.
• This reduces the number of chromosomes to half.

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Meiosis

• At the beginning of Meiosis II, the haploid (n)
  number of chromosomes per cell are dyads because
  each is still composed of two sister chromatids
  (attached at the centromere).
• During Meiosis II, sister chromatids separate in
  each of the cells from Meiosis I.
• Each of the resulting four daughter cells has the
  haploid number of chromosomes.

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Meiosis Overview
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Stages of Meiosis

• First Division.
• Prophase I - Spindle appears nuclear envelope
  fragments homologues pair, synapse,
  crossing-over occurs.
• Metaphase I - Tetrads line up at equator.
• Anaphase I - Homologous chromosomes of each pair
  separate and move to opposite poles of the
  spindle, effectively dividing the chromosomes
  number in half (2n ? n).
• Telophase I - Spindle disappears and the nuclear
  envelope reforms briefly.
• Cytokinesis - Plasma membrane furrows and the
  cell waits momentarily during interkinesis.

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Stages of Meiosis

• Second Division.
• Prophase II - Spindle reappears and nuclear
  envelope disassembles.
• Metaphase II - Dyads line up at equator.
• Anaphase II - Sister chromatids separate and move
  towards poles.
• Telophase II - Spindle disappears and nuclear
  envelope reforms.
• Cytokinesis - Plasma membrane furrows.
• Four haploid (n) daughter cells produced.

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Video
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The Importance of Meiosis

• Due to meiosis, the number of chromosomes stay
  the same in each successive generation (Adam
  Eve).
• Also, the process assures that each new
  individual will have a slightly different genetic
  combination than either parent in 3 ways
• Crossing-over recombines genes located on
  homologous chromosomes derived from both parents.
• Each gamete has a different combination of
  chromosomes.
• Fertilization recombines chromosomes.

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Mitosis Compared to Meiosis

• Mitosis
• Occurrence throughout the body.
• Process 1 cell division.
• Daughter cells are diploid (2n) or identical to
  the mother cell.
• Results in somatic cells (body cells)

• Meiosis
• Occurrence only in the reproductive organs
  (testes or ovaries).
• Process - 2 cell divisions.
• Daughter cells - are haploid (n).
• Results in gametes (germ cells, sex cells,
  sperm or ova)

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Concept Maps (Posted on the course webpage)
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Human Life Cycle

• Mitosis ensures every cell has a complete number
  of chromosomes with every cell division.
• Meiosis reduces the chromosome number by half in
  gametes (sex cells, germ cells).
• Sperm and egg (gametes) are haploid (n).
• Somatic cells are diploid (2n).
• Mitosis cell division
• Meiosis reduction division

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Human Life Cycle
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Spermatogenesis and Oogenesis

• Spermatogenesis occurs in the testes of males and
  produces haploid sperm.
• Once started, continues to completion.
• Oogenesis occurs in the ovaries of females, and
  produces haploid eggs.
• Does not necessarily go to completion.

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Considering the topics of the haploid nature of
sex cells and related topics . . . . .
What about the virgin birth of Jesus?
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Chromosome Inheritance

• Remember, humans have 22 pairs of autosomes, and
  one pair of sex chromosomes.
• Abnormal chromosome number or structure often
  leads to a syndrome. . . .
• Syndrome? Techniques to foreworn?
• Amniocentesis and Chorionic Villi Sampling (CVS)
  can be used to obtain a genetic sample to produce
  a karyotype.
• The visual display of chromosomes arranged by
  size, shape, and banding pattern.

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Human Karyotype Preparation
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Syndrome Summary
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Changes in Chromosome Number

• An abnormal number of autosomes or sex
  chromosomes can be due to nondisjunction.
• It occurs
• During Meiosis I when both members of a
  homologous pair go to the same daughter cell.
• During Meiosis II when sister chromatids fail to
  separate and both daughter chromosomes go to the
  same gamete.
• Monosomy vs. trisomy ?
• Barr body?

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Autosomal Syndromes

• Down Syndrome, an Autosomal Trisomy.
• Trisomy 21 (77 female, 23 male).
• Incidence increases with mothers age!
• Associated with mental retardation a number of
  physical traits such as short stature, eyelid
  fold, and stubby fingers and may involve heart
  defects and intestinal defects.
• The genes causing are located on the bottom third
  of chromosome 21.
• In particular, the Gart gene has been identified
  with mental retardation as it leads to a high
  level of purines which may contribute to mental
  impairment.

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Changes in Sex Chromosome Number

• A number of genetic abnormalities can occur when
  an individual inherits too many or too few of the
  sex chromosomes.
• Turner Syndrome.
• XO.
• Is female.
• Is sterile and has certain physical features
  including a bull neck.
• They are of normal intelligence.

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• 47, XXY / Klinefelter Syndrome (KS).
• 1/650 births, 1/1500 births.
• Two or more X chromosomes with a Y (XXY).
• Is a male.
• Gonads are underdeveloped and breasts may
  develop.
• These individuals are usually slow to learn.
• Those 47, XXY who develop more severe symptoms
  as adults are said to have KS.

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• Poly-X Females.
• Occurs in 1/1,500 births.
• More than two X chromosomes (XXX).
• Is female.
• Sometimes referred to as a triple-X female or
  super females.
• Most are completely normal individuals but may
  have menstrual irregularities.
• With XXXX or more more likely to be retarded
  and have various physical abnormalities.

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• Jacobs Syndrome.
• 1/1,000 births.
• XYY.
• Are males.
• Tend to be tall, have persistent acne, and have
  speech and reading problems.

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Sex Chromosomal Syndromes

• Fragile X Syndrome.
• 1/1,000 male births and 1/2,500 female births.
• Involves a nearly broken X chromosome.
• Children with this trait may be autistic or
  hyperactive but appear normal.
• In adulthood, a prominent jaw and large
  protruding ears develop, and there is mental
  retardation.

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HEALTH FOCUS

• When Your Child Is Disabled Getting Help

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Changes in Chromosomal Structure

• .. is a chromosomal mutation.
• A mutation is ..
• a permanent genetic change.
• Chromosome mutations occur when chromosomes
  suffer breaks and do not rejoin properly.
• Due to radiation, organic chemicals, viruses.
• Result in deletions, duplications,
  translocations, or inversions.

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Deletions and Duplications

• Deletion occurs when
• A single break results in the loss of an end
  piece or when two simultaneous breaks results in
  loss of an internal chromosomal segment.
• Duplication occurs when
• a chromosome segment is repeated in the same
  chromosome or in a nonhomologous chromosome
• which results in more that 2 alleles for certain
  traits.

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Williams Syndrome (Deletion)

• Loss of tiny end piece on chromosome 7.
• Pixy-like children with turned-up noses, wide
  mouths, small chin, and large ears.
• Poor academic skills, excellent verbal and
  musical abilities.
• Lack of elastin affects health of the
  cardiovascular system and causes the skin to age
  prematurely.
• Very friendly, but need an ordered life.

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Cri du Chat Syndrome (Deletion)

• An infant with this condition has a moon face,
  small head, and malformed larynx that produces
  the sound like a cat when they cry.
• Severe mental retardation is evident.
• The cause of this condition is a missing portion
  of one chromosome 5.

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• Translocations are exchanges of chromosomal
  segments between nonhomologous chromosomes.
• Example Alagille syndrome.
• Inversions are when a segment of a chromosome is
  turned 180 degrees so the alleles are the
  opposite order.

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BIOETHICAL FOCUS

• Selecting Children

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