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					Topic 4 and 10
Why sex????
 http://www.pbs.org/wgbh/nova/miracle/program.
  html
Chromosomes
 Chromosomes are composed of DNA and protein (histone)
 Each body cell, called a somatic cell, is composed of 46
  chromosomes (23 pairs).
 Each chromosome consists of two sister chromatids joined
  at the centromere.
Chromosomes
• Two chromosomes composing a pair are called
  homologous chromosomes (or homologues)
  because they both carry genes controlling the same
  inherited characteristics.
• For example, if a gene that determines whether a
  person has freckles is located at a particular place, or
  locus, then the other chromosome of the homologous
  pair also has a gene for freckles at that locus
  – However, the two homologues may have different
    variations of the freckles genes (called alleles), perhaps
    one that promotes freckles and one that does not.
Chromosomes
• Sex chromosome
   – X and Y chromosomes
   – Determine the sex of an individual, and carry genes that
     perform other functions as well.
   – 23rd pair of chromosomes in humans
• The two distinct chromosomes X and Y are important
  exception to the general pattern of homologous
  chromosomes.
• Human females have a homologous pair of X chromosomes
  (XX), but males have one X and one Y chromosome (XY).
   – Only small parts of the X and Y are homologous; most of the
     genes carried on the X chromosome do not have counterparts
     on the tiny Y, and the Y chromosome has genes lacking on the
     X.
Chromosomes
 The other 22 pairs of chromosomes are autosomes, or
  “body chromosomes”.
 For both autosomes and sex chromosomes, we inherit
  one chromosome of each pair form our mother and
  other from our father.
       *a key factor in the human life cycle and in the
       life cycles of all other species that reproduce
       sexually.
Chromosomes
 Any cell with two homologous sets of chromosomes is
 called a diploid cell, and the total number of
 chromosomes is called the diploid number
 (abbreviated 2n)
   For humans, the diploid number is 46; that is 2n=46
   Almost all human cells are diploid
Chromosomes
 The exception are the egg and sperm cells, collectively
 known as gametes.
   Each gamete has a single set of chromosomes: 22
    autosomes plus a single sex chromosome,either X or Y.
   A cell with a single chromosome set is called a haploid
    cell.
   For humans, the haploid number (abbreviated n) is 23;
    that is n=23
Chromosomes
 In humans, sexual intercourse allows a haploid sperm
 cell from the father to reach and fuse with a haploid
 egg cell of the mother in the process of fertilization.
   The resulting fertilized egg is called a zygote, which is
    now diploid.
   It has two haploid sets of chromosomes: one set from the
    mother and a homologous set from the father.
   The life cycle is completed as a sexually mature adult
    develops from the zygote.
   Mitotic cell division ensures that all somatic cells of the
    human body receive copies of all of the zygote’s 46
    chromosomes.
Chromosomes
 All sexual life cycles involve an alternation of diploid
  and haploid stages.
 Having haploid gametes keeps the chromosome
  number from doubling in each generation.
 Gametes are made by a special sort of cell division
  called meiosis, which occurs only in reproductive
  organs (ovaries and testes)
 Whereas mitosis produces daughter cells with the same
  numbers of chromosomes as the parent cell, meiosis
  reduces the chromosome number in half.
   Meiosis
 Meiosis
  Type of cell division that produces haploid gametes
     in diploid organisms.
    Many of the stages of meiosis closely resemble
     corresponding stages in mitosis.
    Meiosis, like mitosis, is preceded by the replication
     of chromosomes.
    However, this single replication is followed by two
     consecutive cell divisions, called Meiosis I and
     Meiosis II.
    These divisions result in four daughter cells, each
     with a single haploid set of chromosomes.
    Thus, meiosis produces daughter cells with only
     half as many chromosomes as the parent cell.
Interphase
 Like mitosis, meiosis begins by interphase, during which the
  chromosomes duplicate.
 Occurs when the cell is between cell division
 Interphase stages:
    G1: Cells grow to mature size
    S: DNA is copied
    G2: Cell prepares for division
 At the end of interphase, each chromosomes consists
  of two genetically identical sister chromatids attached
  together.
 Chromosomes are not yet visible under the
  microscope; they are in a form called chromatin
Prophase I
 Most complex phase of meiosis and typically occupies over 90% of the
  time required for meiotic cell division.
 Chromatin coils up so that individual chromosomes become visible
 A process called synapsis occurs, and homologous chromosomes, each
  composed of two sister chromatids, come together as pairs.
    Resulting structure, consisting of four chromatids, is called a tetrad.
    Chromatids of homologous chromosomes exchange segments in a
     process called crossing over:
      Rearranges genetic information, since homologues may be
        different from each other.
      This genetic shuffling makes an important contribution to the
        genetic variability resulting from sexual reproduction.
Prophase I
Prophase I
 As prophase I continues, the chromosomes condense
  further as the nucleoli disappear.
 Spindle fiber forms
 Nuclear envelope breaks down
Metaphase I
 The chromosome tetrads are aligned on the metaphase
  plate, midway between the two poles of the spindle.
 Each chromosome is condensed and thick, with its
  sister chromatids still attached at their centromeres
 Spindle microtubules are attached at centromeres.
 In each tetrad, the homologous chromosomes are held
  together at sites of crossing over.
 Within each tetrad, the spindle microtubules attached
  to one of the homologous chromosomes from one pole
  of the cell, and the microtubules attached to the other
  homologous chrome come from the opposite pole
   Getting set up to separate the homologous chromsomes!
Metaphase I
Anaphase I
 Sister chromatids remain attached, however, the
  tetrads split up .
 The sister chromatids move to opposite poles
 The cells are now containing half of the genetic
  information from the original parent cell and are thus
  considered HAPLOID!
Anaphase I
Telophase I and Cytokinesis
 Chromosomes arrive at the poles of the cell
 Each pole of the cell has a haploid chromosome set,
  although each chromosome is still in duplicate form at
  this point= each chromosome still consists of two
  sister chromatids.
 Cytokinesis occurs along with telophase I and two
  haploid daughter cells are formed.
Telophase I
Before Meiosis II…
 In some organisms, the chromosomes uncoil and the
  nuclear envelope re-forms, and there is an interphase
  before meiosis II begins.
 IN other species, daughter cells produced during the
  first meiotic division immediately begin preparation
  for the second meiotic division.
 In either case, NO chromosome duplication occurs
  between telophase I and the onset of meiosis II.
Meiosis II

 In organisms having an interphase after meiosis I, the
  chromosomes condense again and the nuclear
  envelope breaks down during prophase II.
 In any case, meiosis II is essentially the same as
  mitosis.
   The key difference is that meiosis II starts with a
    haploid cell.
Prophase II
 A spindle forms and moves the chromosomes toward
 the middle of the cell.
Metaphase II
 Chromosomes are aligned on the metaphase plate as
 they are in mitosis, with the centromeres of the sister
 chromatids pointing towards opposite poles.
  Anaphase II
 centromeres of sister chromatids finally separate
 the sister chromatids of each pair, now individual
 chromosomes, move toward opposite poles of the cell.
Telophase II
 Nuclei form at the cell poles, and cytokinesis occurs at
  the same time.
 There are now four daughter cells, each with the
  haploid number of (single) chromosomes.
Meiosis Animation
 http://www.sumanasinc.com/webcontent/animations
 /content/meiosis.html
Mitosis vs. Meiosis
 Mitosis
   Provides growth, tissue repair, and asexual reproduction
   Produces daughter cells genetically identical to the
    parent cell
   Involves one division of the nucleus, and is usually
    accompanied by cytokinesis, producing two diploid
    daughter cells.
 Meiosis
   Need for sexual reproduction
   Entails two nuclear and cytoplasmic divisions
   Yields four haploid daughter cells, with one member of
    each homologous chromosome pair.
   Form tetrads; crossing over occurs.
Meiosis: Genetic Variation
 We’ve discussed how mutations lead to genetic variation.
 Also, The arrangement of homologous chromosomes pairs
  at metaphase of meiosis I affects the resulting gametes.
    The orientation of homologous pairs in the center of the
     cell is random; thus producing gametes with random
     chromosomes
 For any species, the total number of combinations of
  chromosomes that meiosis can package into gametes is 2n,
  where n is the haploid number.
    For a human, 223, or about 8 million possible chromosome
     combinations
    This means that each gamete you produce contains one of
     roughly 8 million possible combinations of chromosomes
     inherited from your mother and father.
Meiosis: Genetic Variation
 Possibility when a gamete from one individual unites
 with a gamete from another individual in fertilization:
   In humans, the random fusion of a single sperm wit a
    single ovum during fertilization will produce a zygote
    with any of 64 trillion (8 million x 8 million)
    combinations of chromosomes!
Meiosis: Genetic Variation
 Homologous chromosomes
   Bear two different kinds of genetic information for the
    same characterisitic
   The key to what really makes gametes—and therefore
    offspring--different
Meiosis: Genetic Variation
 Crossing Over:
    An exchange of corresponding segments between two
     homologous chromosomes.
    Occurs during prophase I of meiosis.
    Chromosomes are a tetrad—four chromatids, with each
     pair of sister chromatids joined at their centromeres.
    Each gene on each homologue is aligned precisely with
     the corresponding gene on the other homologue
    Sites of crossing over appear as X-shaped regions; each is
     called a chiasma.
        place where two homologous chromatids are attached to each
         other.
    Can produce new combinations of genes= genetic
     recombination!
 Meiosis: Genetic Variation
 Crossing over:
    1. The DNA molecules of two nonsister chromatids—one maternal and
     one paternal—break at the same place.
    2. Immediately, the two broken chromatids join together in a new way .
      In effect, the two homologous segments trade places, or cross over,
        producing hybrid chromosomes with new combinations of maternal
        and paternal genes.
      Called “recombinants”

    3. When the homologous chromosomes separate in anaphase I, each
     contains a new segment originating form its homologues.
    4. Finally, in meiosis II, the sister chromatids separate, each going to a
     different gamete.

   **In meiosis in humans, an average of one to three crossover events occur
     per chromosome pair.
Crossing Over
Meiosis: Genetic Variation
  In summary, there are three sources of genetic
  variability, besides mutations, in sexually reproducing
  organisms:
      1. crossing over during prophase I
      2.independent orientation of chromosomes at metaphase 1
      3.random fertilization
Karyotypes
 The term karyotype refers to the chromosome
  complement of a cell or a whole organism.
 A karyotype is an ordered display of magnified images
  of an individual’s chromosomes arranged in pairs,
  starting with the longest.
 In particular, it shows the number, size, and shape of
  the chromosomes as seen during metaphase of
  mitosis.
 Chromosome numbers vary considerably among
  organisms and may differ between closely related
  species.
Karyotypes
 Karyotypes are prepared from the nuclei of cultured
 white blood cells that are ‘frozen’ at the metaphase
 stage of mitosis.
   Shows the chromosomes condensed and doubled
 A photograph of the chromosomes is then cut up and
  the chromosomes are rearranged on a grid so that the
  homologous pairs are placed together.
 Homologous pairs are identified by their general shape,
  length, and the pattern of banding produced by a
  special staining technique.
Karyotypes
 Male karyotype
    Has 44 autosomes, a single X chromosome, and a Y
     chromosome (written as 44 + XY)
 Female karyotype
    Shows two X chromosomes (written as 44 + XX)
Karyotype- Normal
Karyotype- Down Syndrome
Down Syndrome
 Trisomy 21; named after John Langdon Down, who
  characterized the syndrome in 1866
 47 chromosomes total; there are three number 21
  chromsomes
 In most cases, a human embryo with an abnormal
  number of chromosomes is spontaneously aborted
  (miscarried) long before birth.
 Some chromosome abnormalities upset the genetic
  balance less drastically, and individuals carrying them
  can survive.
Down Syndrome
 Trisomy 21 is the most common chromosome number abnormality.
 Affects about one out of every 700 children born, and is the most
  common serious birth defect in the US
 Symptoms include characteristic
    facial features, notably a round face, a skin fold at the inner corner of
     the eye, a flattened nose bridge
    small, irregular teeth, as well as short stature
    heart defects, and susceptibility to respiratory infections, leukemia,
     and Alzheimer’s disease.
    Exhibit varying degrees of mental retardation.
    Some live to middle age or beyond, and many are socially adept and
     able to hold jobs.
    Most are sexually underdeveloped and sterile
         A few women have had children, however, half of their eggs will have the
          extra chromosome 21, so there is a 50% chance that she will transmit the
          syndrome to her child.
Down Syndrome
 Down Syndrome incidence:
   The incidence of Down Syndrome in the offspring of
    normal parents increases remarkedly with the age of the
    mother.
       Strikes less than 0.05% of children (fewer than one in 2,000)
        born to women under age 30.
       Risk climbs to 1.25% for mothers in their early 30s and is even
        higher for older mothers.
       Because of this relatively high risk, pregnant women over 35 are
        candidates for fetal testing for trisomy 21 and other
        chromosomal abnormalities.
Maternal age and incidence of
Down Syndrome
Errors in Meiosis
 Nondisjunction
   Members of a chromosome fail to separate.
   Can lead to an abnormal chromosome number in any
    sexually reproducing diploid organism.
   For example, if there is nondisjunction affecting human
    chromosome 21 during meiosis I, half the resulting
    gametes will carry an extra chromosome 21.
       Then, if one of these gametes unites with a normal gamete,
        trisomy 21 (Down Syndrome) will result.
Errors is Meiosis
 Two ways that nondisjunction can occur:
    1.a pair of homologous chromosomes does not separate
     during Meiosis I.
       Two of the gametes produced will n + 1(abnormal), and two
        will be n- 1 (abnormal).
   2. meiosis I is normal, but one pair of sister chromatids
    fails to separate during meiosis II.
       Two of the resulting gametes are n + 1 and n-1 (abnormal) and
        two will be n (normal)

       Abnormal gametes that get fertilized, will result in a zygote
        with an extra chromosome.
       Mitosis will then transmit the anomaly to all embryonic cells,
        causing some syndrome linked to abnormal genes.
Errors in Meiosis
 What causes nondisjunction?
   We do not yet know the answer, nor do we fully
    understand why offspring with trisomy 21 are more likely
    to by born as a woman ages.
   We do know, however, that meiosis begins in a woman’s
    ovaries before she is born but is not completed until years
    later, at the time of an ovulation.
        Because only one egg usually matures each month, a cell might
         remain arrested in the mid-meiosis state for decades.
        Some research points to an age-dependent error in one of the
         checkpoints that coordinates the process of meiosis.
Errors in Meiosis
 Nondisjunction can also occur in sex chromosomes.
 For example, Klinefelter’s Syndrome :
    males have an extra X chromosome, making him XXY
    occurs approximately 1 out of every 2,000 live births
    Have male sex organs, but the testes are abnormally small
     and the individual is sterile.
    Often includes breast enlargement and other female body
     characteristics.
    Person is usually of normal intelligence
Errors in Meiosis
 Nondisjunction in sex chromosomes:
   Turner Syndrome
       Females who are lacking an X chromosome= XO, the O
        indicates the absence of a second chromosome
       Characteristic appearance include short stature, and often a
        web of skin extending between the neck and the shoulders
       Sterile; their sex organs do not fully mature at adolescence
       If left untreated, girls will have poorly developed breasts and
        other secondary sexual characteristics.
       Normal intelligence
       *Sole known case where having 45 chromosomes is not fatal.
Errors in Meiosis
 Nondisjunction in sex chromosomes:
   Males with XYY and females with XXX are normal.
Errors in Meiosis
 Abnormalities in chromosome structure:
    Breakage of a chromosome can lead to a variety of
     rearrangements affecting the genes of that chromosome:
       1. deletion: if a fragment of a chromosome is lost.
          Usually cause serious physical and mental problems.

          Deletion of chromosome 5 causes cri du chat syndrome: child
           is mentally retarded, has a small head with unusual facial
           features, and has a cry that sounds like the mewing of a
           distressed cats. Usually die in infancy or early childhood.
       2.duplication: if a fragment from one chromosome joins to a
        sister chromatid or homologous chromosome.
       3.inversion: if a fragment reattaches to the original
        chromosome but in the reverse direction.
          Less likely than deletions or duplications to produce harmful
           effects, because all genes are still present in normal number
Errors in Meiosis
 Abnormalities in chromosome structure:
    Translocation
       The attachment of a chromosomal fragment to a
        nonhomologous chromosome.
       May or may not be harmful
       For example, chromosomal translocation in a somatic cell in
        the bone marrow is associated with chronic myelogenous
        leukemia (CML), which is the most common type of leukemia,
        the cancer that affects the cells that give rise to white blood cells
        (leukocytes)
          Part of chromosome 22 has switched with a small fragment of
           chromosome 9. The chromosome with the cancer-causing gene
           is called the “Philadelphia chromosome”, after the city where it
           was discovered.
Errors in Meiosis
Gregor Mendel
Gregor Mendel
  Background:
     Deduced the fundamental principles of genetics by
      breeding garden peas.
     Known as the “Father of Genetics”
     Was a monk that lived and worked in an abbey in
      Austria.
     In a paper in 1866, Mendel correctly argued that parents
      pass on to their offspring discrete heritable factors.
     In his paper, he stressed that the heritable factors (today
      called genes) retain their individuality generation after
      generation.
Gregor Mendel
 Experiments:
    Chose to study garden peas because he was familiar with
     them from his rural upbringing, they were easy to grow,
     and they came in many readily distinguishable varieties.
    Also, he was able to exercise strict control over pea plant
     matings.
    Due to their anatomical nature (petals of pea flower
     almost completely enclose the stamen and carpel), pea
     plants usually self-fertilize in nature.
        That is, sperm-carrying pollen grains released from the stamens
         land on the egg-containing carpel of the same flower.
        He used a small bag to cover the flower to ensure self-
         fertilization.
Gregor Mendel
 Experiments (continued)
    He was also able to ensure cross-fertilization
        Fertilization of one plant by pollen from a different plant.
   Through his methods, Mendel could always be sure of
    the parentage of new plants
   He chose seven characteristics, that occur in two distinct
    forms ,to study:
        Flower color (purple, white)
        Flower position (axial, terminal)
        Seed color (yellow, green)
        Seed shape (round, wrinkled)
        Pod shape (inflated, constricted)
        Pod color (green, yellow)
        Stem length (tall, dwarf)
Gregor Mendel
 Experiments (continued):
    Mendel worked with his plants until he was sure he had true-
     breeding varieties.
      For instance, he identified a purple-flowered variety that, when
        self-fertilized, produced offspring that all had purple flowers.
    He then asked, What offspring would result if plants with purple
     flowers and plants with white flowers were cross-fertilized?
      The offspring of two different varieties are called hybrids, and the
        cross-fertilization itself is referred to as a hybridization, or simply
        a cross.
      The true-breeding parental plants are called the P generation ( P
        for parental).
      The offspring of the P generation are called the F1 generation (F
        for filial, the Latin word “son”)
      When the F1 self-fertilize or fertilize with each other, their
        offspring are called the F2 generation.
 Gregor Mendel
 Mendel performed many experiments in which he tracked the inheritance of
  characteristics that occur in two forms, such as flower color.
 Monohybrid cross:
    When you’re looking only at one trait (ex, flower color)
    Mendel performed a monohybrid cross between a pea plant with purple flowers
      and one with white flowers.
       The F1 offspring all had purple flowers (not a lighter purple, has predicted by
        a “blending” hypothesis.)
       Was the white gene lost?

          By mating the F1 plants, Mendel found the answer to be NO!

          Out of 929 F2 plants, Mendel found that 705 (about ¾) had purple flowers
           and 224 (about ¼) had white flowers, a ratio of about three plants with
           purple flowers to one with white flowers in the F2 generation (3:1)
          The heritable factor for white flowers did not disappear in the F1 plants,
           but the purple-flower factor was the only one affecting the F1 flower color.
          The F1 plants must have carried two factors for the flower-color
           characteristic, one for purple and one for white.
Gregor Mendel
Gregor Mendel
 Mendel observed these same patterns of inheritance for
  six other pea plant characteristics.
 From these results, he developed four hypotheses, which
  we will describe using modern terminology (such as
  “gene” instead of “heritable factor”):
Gregor Mendel
 Hypothesis 1:
   There are alternative forms of genes that account for
    variations in inherited characteristics.
   For example, the gene for flower color in pea plants exists
    in two forms, one for purple and the other for white.
   The alternative versions of a gene are now called alleles.
Gregor Mendel
 Hypothesis 2:
   For each characteristic, an organism inherits two alleles,
    one from each parent. These alleles may be the same ore
    different.
   An organism that has two identical alleles for a gene is
    said to be homozygous for that gene (and is called a
    homozygote).
   An organism that has two different alleles for a gene is
    said to be heterozygous for that gene (and is called a
    heterozygote)
Gregor Mendel
 Hypothesis 3:
   If the two alleles of an inherited pair differ, then one
    determines the organism’s appearance is called the
    dominant allele; the other has no noticeable effect on
    the organism’s appearance and is called the recessive
    allele.
   We use upper-case letters to represent dominant alleles
    and lowercase letters to represent recessive alleles.
Gregor Mendel
 Hypothesis 4:
   A sperm or egg carries only one allele for each inherited
    trait because allele pairs separate (segregate) from each
    other during the production of gametes.
   This statement is now known as the law of segregation.
   When sperm and egg unite at fertilization, each
    contributes its allele, restoring the paired condition in the
    offspring.
Gregor Mendel
 The right hand side of the diagram on the previous slide explains the
  results of Mendel’s experiment.
    In this example, P represents the dominant allele (for purple
     flowers) and p represents the recessive allele (for white flowers).
    At the top, you see the alleles carried by the parental plants both
     were true-breeding.
    Mendel proposed that one parent had two alleles for purple flowers
     (PP) and the other had two alleles for white flowers (pp)
    Consistent with hypothesis 4, the gametes of Mendel’s parental
     plants each carried one allele; thus, the parental gametes in Figure
     9.3B are either P or p.
    As a result of fertilization, the F1 hybrids each inherited one allele for
     purple flowers and one for white.
    Hypothesis 3 explains why all of the F1 hybrids are (Pp) had purple
     flowers; the dominant P allele has its full effect in the heterozygote,
     while the recessive p allele had no effect on flower color.
Gregor Mendel
 The right hand side of the diagram on the previous
  slide explains the results of Mendel’s experiment
  (continued…)
   Mendel’s hypotheses also explain the 3:1 ratio in the F2
    generation; because the F1 hybrids are Pp, they make
    gametes P and p in equal numbers.
   You can see the possible gamete combinations using a
    Punnett square.
       Used to make predictions about the possible phenotypes and
        genotypes of offspring.
Gregor Mendel
 Genotype vs. Phenotype
    Phenotype
       For example, purple or white flowers.
       Term used to describe an organism’s appearance, or expressed
        physical traits.
       Phenotypic ratio- ratio of the phenotypes of the offspring.
         Example, the ratio of purple flowers to white flowers is 3:1

   Genotype
       For example, PP, Pp, or pp.
       Term used to describe an organism’s genetic makeup.
       Genotypic ratio- ratio of the genotypes of the offspring.
         Example, the 1:2:1 is the ratio of PP, Pp, pp
                  Homologous Chromosomes
Remember, two homologous chromosomes may bear either the
  same alleles or different ones. Thus, we see the connection
between Mendel’s laws and homologous chromosomes: Alleles
    (alternate forms) of a gene reside at the same locus on
                  homologous chromosomes.
Test Cross
 Test Cross:
    a mating between an individual of unknown genotype
     and a homozygous recessive individual.
    Mendel used testcrosses to determine whether he had
     true-breeding varieties of plants.
    Continues to be an important tool of geneticists for
     determining genotypes.
Not so simple…
 Although Mendel’s laws are valid for all sexually
  reproducing organisms, they stop short of explaining
  some patterns of genetic inheritance.
 In fact, for most sexually reproducing organisms, cases
  where Mendel’s laws can strictly account for the
  pattern of inheritance are relatively rare.
 More often, the inheritance patterns are more
  complex…
Incomplete dominance
 Complete dominance
    The dominant allele had the same phenotypic whether
     present in one or two copies.
 Incomplete dominance
    The F1 hybrids have an appearance in between the
     phenotypes of the two parental varieties.
Incomplete Dominance
 For example, red snapdragons crossed with white
 snapdragons produced hybrid flowers in the F1 with
 pink flowers.
   This third phenotype results from flowers of the
    heterozygote having less pigment than the red
    homozygotes.
   The F2 generation would then have a 1:2:1 ratio of red,
    pink, white.
 Incomplete dominance
 In humans, an example involves the condition
  hypercholestrolemia, dangerously high levels of
  cholesterol in the blood, caused by a recessive allele(h),
  due to a lack of LDL receptors.
 hh individuals have about 5 times the normal amount of
  blood cholesterol and may have heart attacks as early as
  age 2.
 Normal individuals are HH.
 Heterozygotes have blood cholesterol about twice
  normal.
   Usually prone to atherosclerosis, the blockage of arteries
    by cholesterol buildup in artery walls, and they may have
    heart attacks from blocked heart arteries by their mid-30s.
Codominance
 Both alleles are expressed in the heterozygous
  individual
 Different from incomplete dominance, which is the
  expression of one intermediate trait
 Can be seen in blood type
Codominance
 The ABO blood group phenotype in humans involves
  three alleles of a single gene.
 These three alleles, in various combinations, produce
  four phenotypes: a person’s blood group may be either
  O, A, B, or AB.
 These letters refer to two carbohydrates, designated A
  and B, that may be found on the surface of red blood
  cells.
 A person’s red blood cells may have carbohydrate A
  (type A blood), carbohydrate B (type B), both (type AB),
  or neither (type O).
Codominance
 Matching compatible blood groups is critical for safe
  blood transfusions.
 If a donor’s blood cells have carbohydrate (A or B) that
  is foreign to the recipient, then the recipient’s immune
  system produces blood proteins called antibodies that
  bind specifically to the foreign carbohydrates and cause
  donor blood cells to clump together, potentially killing
  the recipient.
Codominance
 Four blood groups result from various combinations of
  the three different alleles, symbolized as IA, IB, and i.
 Each person inherits one of these alleles from each
  parent.
 IA and IB are dominant to the i allele, but are
  codominant to each other = both alleles are expressed
  in the heterozygote IAIB , who have the blood type AB
 There are six possible genotypes:
   IAIA and IAi= A
   IBIB and IBi= B
   IAIB= AB
   ii = O
Dihybrid cross
 Dihybrid cross:
   Results from a mating of parental varieties differing in
    two characteristics.
   For example: Mendel crossed homozygous round yellow
    seeds (RRYY) with plants having wrinkled green seeds
    (rryy).
         All of the offspring in the F1 generation had round yellow
          seeds; which raised the question: are the two characteristics
          transmitted from parent to offspring as a package, or was
          each characteristic inherited independently of the other?
         The question was answered when Mendel allowed
          fertilization to occur among the F1 plants the offspring
          supported the idea that the two seed characteristics
          segregated independently.
             Offspring had nine different genotypes, and four different
               phenotypes with 9:3:3:1 ratio.
Dihybrid Cross
 Mendel’s results supported the hypothesis that each
 pair of alleles segregates independently of the other pairs
 of alleles during gamete formation Mendel’s Law of
 Independent Assortment
Pleiotropy
 Most genes influence multiple characteristics, a
  property called pleiotropy.
 An example of pleiotropy in humans is sickle-cell
  disease
   Refer to p. 168
Polygenic Inheritance
 Polygenic inheritance is the additive effects of two or
  more genes on a single phenotypic characteristic
   Examples include human skin color and height
   Different then pleiotropy, in which a single gene affects
    several characteristics
Environmental affects
 Many characteristics result from a combination of heredity
    and environment.
   For example, in humans nutrition influences height,
    exercise alters build, sun-tanning darkens the skin, and
    experience improves performance on intelligence tests.
   It is becoming clear that human phenotypes—such as risk of
    heart disease and cancer and susceptibility to alcoholism
    and schizophrenia—are influenced by both genes and
    environment.
   Simply spending time with identical twins will convince
    anyone that environment, and not just genes, affect a
    person’s traits.
   However, only genetic influences are inherited…cannot pass
    on environmental influences to future generations!
Chromosome Theory of
Inheritance
 The chromosome theory states that genes occupy
  specific loci (positions) on chromosomes and it is the
  chromosomes that undergo segregation and
  independent assortment during meiosis.
 Thus, it is the behavior of chromosomes during meiosis
  and fertilization that accounts for inheritance patterns.
Linked genes
  Genes located close together on the same chromosome
   tend to be inherited together and are called linked
   genes.
  Linked genes generally do not follow Mendel’s law of
   independent assortment.
  Refer to figure 9.19 in book
Crossing over
 As we saw in meiosis, crossing over between
  homologous chromosomes produces new
  combinations of alleles in gametes
 Forms recombinant gametes
Sex Chromosomes
 Sex chromosomes, designated X and Y, determine an
  individual’s sex.
 XX individuals ar e female, and XY individuals are male
 Human males and females both have 44 autosomes
  (nonsex chromosomes)
 As a result of chromosome segregation during meiosis,
  each gamete contains one sex chromosome and a
  haploid set of autosomes (22).
 All eggs contain a single X chromosome; sperm either
  contain an X or Y
   An offspring’s sex is determined by whether the sperm
    cell that fertilizes the egg bears an X or Y
Sex Chromosomes
 The genetic basis of sex determination in humans is
 not yet completely understood, but one gene on the Y
 chromosome plays a crucial role.
   This gene is called SRY (sex-determing region of Y) and
    triggers testis development.
   In the absence of SRY, an individual develops ovaries
    rather than testes.
   SRY codes for proteins that regulate other genes on the Y
    chromosome, which in turn produce proteins necessary
    for testis development.
Sex-linked genes
 Besides bearing genes that determine sex, the sex
  chromosomes also contain genes for characteristics
  unrelated to femaleness and maleness.
 Sex-linked genes are genes located on either sex
  chromosomes, although in humans the term has
  historically referred specifically to a gene on the X
  chromosome. ***Be careful not to confuse the term sex-
  linked gene with the term linked genes!!!***
 Refer to figure 9.23A-D on p. 176
Pedigrees
  Pedigree is a family tree used to study how particular
   human traits are inherited.
  It is analyzed using logic and the Mendelian laws
Goals of Pedigree Analysis
 1. Determine the mode of inheritance: dominant, or
  recessive, sex-linked or autosomal
 2. Determine the probability of an affected offspring
  for a given cross.
Basic symbols
More Symbols
Dominant or Recessive?
 Is it a dominant pedigree or a recessive pedigree?
 1. If two affected people have an unaffected child, it must be a
  dominant pedigree: D is the dominant mutant allele and d is the
  recessive wild type allele. Both parents are Dd and the normal child is
  dd.
 2. If two unaffected people have an affected child, it is a recessive
  pedigree: R is the dominant wild type allele and r is the recessive
  mutant allele. Both parents are Rr and the affected child is rr.
 3. If every affected person has an affected parent it is a dominant
  pedigree.
Dominant Autosomal Pedigree
  I

                          1           2




 II

          1           2           3           4       5           6




III

      1       2   3           4       5   6       7       8   9       10
Assigning Genotypes for
Dominant Pedigrees
 1. All unaffected are dd.
 2. Affected children of an affected parent and an unaffected parent
  must be heterozygous Dd, because they inherited a d allele from the
  unaffected parent.
 3. The affected parents of an unaffected child must be heterozygotes
  Dd, since they both passed a d allele to their child. (also called
  carriers)
 4. If both parents are heterozygous Dd x Dd, their affected offspring
  have a 2/3 chance of being Dd and a 1/3 chance of being DD.
Recessive Autosomal Pedigree
Assigning Genotypes for
Recessive Pedigrees
 1. all affected are rr.
 2. If an affected person (rr) mates with an unaffected person, any
    unaffected offspring must be Rr heterozygotes, because they got a r
    allele from their affected parent.
   3. If two unaffected mate and have an affected child, both parents must
    be Rr heterozygotes.
   4. Recessive outsider rule: outsiders are those whose parents are
    unknown. In a recessive autosomal pedigree, unaffected outsiders are
    assumed to be RR, homozygous normal.
   5. Children of RR x Rr have a 1/2 chance of being RR and a 1/2 chance of
    being Rr. Note that any siblings who have an rr child must be Rr.
   6. Unaffected children of Rr x Rr have a 2/3 chance of being Rr and a 1/3
    chance of being RR.
Outsider Rules
 In any pedigree there are people whose parents are unknown. These
  people are called “outsiders”, and we need to make some assumptions
  about their genotypes.
 Sometimes the assumptions are proved wrong when the outsiders have
  children. Also, a given problem might specify the genotype of an
  outsider.
 Outsider rule for dominant pedigrees: affected outsiders are assumed
  to be heterozygotes. (or carriers)
 Outsider rule for recessive pedigrees: unaffected (normal) outsiders
  are assumed to be homozygotes.

 Both of these rules are derived from the observation that mutant alleles
  are rare.
Autosomal Dominant
 All unaffected individuals are homozygous for the normal
  recessive allele.
AutosomalDominant
Look for:
 Trait in every generation
   Once leaves the pedigree does not return
 Every person with the trait must have a parent with
  the trait
 Males and females equally affected
Autosomal Dominant
Autosomal Dominant
Autosomal Recessive
 The recessive gene is located on 1 of the autosomes
 Letters used are lower case ie bb
 Unaffected parents (heterozygous) can produce affected offspring (if
    they get both recessive genes ie homozygous)
   Inherited by both males and females
   Can skip generations
   If both parents have the trait then all offspring will also have the trait.
    The parents are both homozygous.
   E.g. cystic fibrosis, sickle cell anaemia, thalassemia
Autosomal Recessive
Look for:
 Skips in generation
 Unaffected parents can have affected children
 Affected person must be homozygous
 Males and females affected equally
Autosomal Recessive
AutosomalRecessive
Sex Linked Inheritance
 Genes are carried on the sex chromosomes (X or
  Y)
 Sex-linked notation
   XBXB normal female
   XBXb carrier female
   XbXb affected female
   XBY normal male
   XbY affected male
    Sex-linked dominant
 Mothers pass their X’s to both sons and daughters
      If the mother has an X- linked dominant trait and is
        homozygous (XAXA) all children will be affected
       If Mother heterozygous (XAXa) 50% chance of each child being
        affected
   Fathers pass their X to daughters only.
       Affected males pass to all daughters and none of their sons
         Genotype= XAY
   Normal outsider rule for dominant pedigrees for females, but for sex-
    linked traits remember that males are hemizygous and express whichever
    gene is on their X.
   XD = dominant mutant allele
   Xd = recessive normal allele
   E.g. dwarfism, rickets, brown teeth enamel.
Sex-Linked Dominant
Look for:
 More males being affected
 Affected males passing onto all daughter
  (dominant) and none of his sons
 Every affected person must have an affected
  parent
Sex-Linked Recessive
 males get their X from their mother
 fathers pass their X to daughters only
 females express it only if they get a copy from both parents.
      Females can only inherit if the father is affected and mother is a
       carrier (hetero) or affected (homo)
   expressed in males if present
      More males than females affected (males inherit X from
       mother)
   recessive in females
      An affected female will pass the trait to all her sons
         Daughters will be carriers if father is not affected
   Outsider rule for recessives (only affects females in sex-linked situations):
    normal outsiders are assumed to be homozygous.
   Males cannot be carriers (only have 1 X so either affected or not)
   Can skip generations
   E.g. colour blindness, haemophilia, Duchene muscular dystrophy
Sex Linked Recessive
 Look for:
 More males being affected
 Affected female will pass onto all her sons
 Affected male will pass to daughters who will be a
  carrier (unless mother also affected)
 Unaffected father and carrier mother can produce
  affected sons
Sex-linked recessive
Sex-linked recessive

				
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