Heredity: Meiosis

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Asexual vs sexual reproduction
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Meiosis

• Process of producing haploid cells
• Gametogenesis

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Meiosis

• Heredity transmission of heritable
  characteristics (traits) from one generation to
  the next
• Continuity
• Variation inherited differences among
  individuals
• Evolution

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Meiosis

• Genetics the scientific study of heredity
• DNA sequence of nucleotides stores the genetic
  code for making structural and functional
  proteins
• Genes sections of DNA that are the codes for
  specific proteins
• Chromatin raw uncoiled DNA in the nucleus
  helix wrapped around histone proteins

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Meiosis

• Chromosome chromatin that has coiled up during
  prophase of mitosis and meiosis
• Locus specific location on the chromosome that
  contains a gene

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Asexual Reproduction

• Asexual (vegetative) reproduction
• Vegetative nonreproductive cells
• Only mitosis (diploid)
• ex. New plant from cutting clone

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Asexual Reproduction

• Single parent
• All genes passed on
• Genetically identical offspring Cloning
• Few mutations - no genetic variation bad or
  weak genes also passed along
• Genes susceptible to environment
• Very rapid, energy efficient method

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Sexual Reproduction

• Fertilization uniting of egg sperm (2 haploid
  cells gametes)
• Zygote diploid organism formed by fertilization

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Sexual Reproduction

• Sexual reproduction
• Requires more energy
• Slower
• Advantage of
• Genetic variation
• Diploid - 2 copies of genes less opportunity
  for problems

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Sexual Life Cycles

• Somatic cell any cell other than gamete
• Gamete sex cell haploid only good for sex
• Karyotype karyon kernel
• Display of chromosomes in sequence
• Lymphocytes in metaphase
• Screen for abnormalities

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Sexual Life Cycles

• Homologous chromosomes (homologues) - a pair of
  chromosomes that have the same size, centromere
  location and staining pattern
• Same genetic loci
• Autosome non-sex chromosome
• Sex chromosome chromosome that determines
  gender dissimilar
• Females XX Males XY

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Sexual Life Cycles

• Humans 22 pairs of autosomes
• 1 pair of sex chromosomes
• 1 homologue is inherited from each parent
  Diploid
• Most animals are diploid (2n)
• Gametes are haploid (1n)

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Variety in Sexual Life Cycles Animals

• Gametes are the only haploid cells
• Meiosis gametogenesis

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Variety in Sexual Life Cycles Fungi

• Zygote is diploid
• Meiosis occurs immediately after fertilization
• Multicellular haploid organism
• Gametes are formed by mitosis

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Variety in Sexual Life Cycles Plants

• Alternation of Generations
• Alternate between multicellular haploid and
  multicellular diploid generations

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Plant Life Cycles

• Multicellular diploid Sporophyte
• Diploid sporophyte undergoes meiosis to produce
  haploid spores
• Haploid spores undergo mitosis to become a
  multicellular haploid organism

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Plant Life Cycles

• Haploid multicellular organism produces gametes
  (1n) by mitosis
• Fertilization
• Diploid sporophyte

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Heredity Variety in Sexual Life Cycles

• Sporophyte multicellular, 2n reproduces by
  producing 1n spores
• Gametophyte multicellular, 1n reproduces by
  producing gametes which unite (fertilization) to
  form 2n sporophyte

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Heredity Meiosis

• Similar to mitosis
• Preceded by replication of chromosomes/DNA
• Chromosomes appear similar
• Same phases

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Heredity Meiosis

• Mitosis
• 4 (5) stages
• 2 identical daughter cells
• Diploid (2n)

• Meiosis
• 8 (10) stages
• 4 non identical cells
• Haploid (1n)

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Interphase I
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Prophase I
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Crossing over
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Metaphase I
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Anaphase I
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Telophase I
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Meiosis II
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Heredity Meiosis

• Meiosis
• Homologous chromosomes come together as pairs
  (Synapsis)
• Homologous pairs form tetrads
• Crossing over may occur

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Genetics Inheritance Patterns

• Mendels Law of Independent Assortment
• Dihybrid cross 2 traits
• Each allele segregates independently of other
  gene pairs during gametogenesis

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Genetics Inheritance Patterns

• Hypothesis
• If the 2 characters segregate together, then you
  will get a 31 ratio

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If the 2 characters separate independently the
F1 hybrids will show 9331 pattern
YR
yr
X
YyRr
4 types of Gametes YR, Yr, yR, yr
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Genetics Inheritance Patterns

• 9 yellow, round
• 3 yellow, wrinkled
• 3 green, round
• 1 green, wrinkled
• Repeated dihybrid crosses demonstrated
  independent assortment

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Genetics Inheritance Patterns

• Independent Assortment
• Color inheritance in budgies
• 1 gene outer pigmentation yellow (d) or
  colorless (r)
• 1 gene core pigmentation melanin (d) or no
  melanin (r)

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Green Y_B_
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Blue yyB_
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Yellow Y_bb
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White yybb
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Genetics Inheritance Patterns

• What is the probability that a trihybrid cross
  between two organisms with genotypes AaBbCc and
  AaBbCc will produce an offspring aabbcc?

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Genetics Inheritance Patterns

• Aa x Aa aa ¼
• Bb x Bb bb ¼
• Cc x Cc cc ¼
• ¼ x ¼ x ¼ 1/64

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Genetics Inheritance Patterns

• 3 types of inheritance patterns
• Complete dominance
• Incomplete dominance
• Codominance

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Genetics Inheritance Patterns

• Incomplete dominance dominant is not fully
  expressed in the offspring
• Red crossed with a white produces a pink
  offspring (intermediate between the 2 parents)

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Genetics Inheritance Patterns

• Codominance both alleles are expressed
• 3 phenotypes
• Red, white, roan cattle, horses
• ABO blood types

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Inheritance Patterns

• Multiple alleles locus on a chromosome with
  more than 2 alleles for a gene.
• ABO blood types
• Inherit only 2 of the 3

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Inheritance Patterns

• Multiple alleles
• A A antigen (peripheral proteins)
• B B antigens
• O no antigens

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Genetics Inheritance Patterns

• A A antigens B antibodies
• B B antigens A antibodies
• O no antigens A, B antibodies
• Antigens cause antibodies to attack the cells
• Blood agglutination (glue)

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Genetics Inheritance Patterns

• Rh simple dominance
• Presence () or absence (-) of surface protein
• Rhesus monkey

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Genetics Inheritance Patterns

• Pleiotropy 1 gene can have multiple effects
• Symptoms caused by sickle cell
• Cross-eyed-ness in blue-eyed cats

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Genetics Inheritance Patterns

• Epistasis a gene at one locus can alter the
  expression of another gene
• Ex. Color in mice
• Pigment deposition must be expressed (on)
  before pigment will be expressed

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Environment Affects Phenotype

• External environment causes phenotypic
  expression T, pH, salinity, etc.
• Color of hydrangea
• Color of skin

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Polygenic Traits

• Multiple genes
• Skin, hair, eye color

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Polygenic Traits

• AaBbCcDdEe x aabbCcDdee
• aabbccddee
• Aa x aa ½
• Bb x bb ½
• Cc x Cc ¼
• Dd x Dd ¼
• Ee x ee ½

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Polygenic Traits

• ½ x ½ x ¼ x ¼ x ½ 1/128
• 1128
• AABBCCDDEE x aabbccddee
• Probability of producing? AaBbccDdee
• 0

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Testcross

• To test to see if a dominant phenotype is
  homozygous (pure) or heterozygous (carrier,
  hybrid)
• Cross a homozygous (pure) recessive with the
  phenotype

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Nature vs Nurture

• Norm of reaction range of phenotypic expression
  produced by environmental conditions
• May be limited (ABO blood types)
• May have wide range of possibilities (blood cell
  counts, skin, hair color, behavior/learning

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Mendelian Inheritance in Humans

• Difficult to study
• Few offspring
• Long periods of time to generate offspring
• Too many variables

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Mendelian Inheritance in Humans

• Pedigrees family tree diagramming relationships
  through generations showing inheritance patterns
• Allows predictions of genetic problems

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Mendelian Inheritance in Humans

• Recessive disorders
• Usually a defective version of the normal allele
• Heterozygotes are phenotypically normal

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Recessive Disorders

• Phenotypes only in homozygous recessive
• Lethal or non-lethal (albinism)
• Born to parents that are normal but are carriers

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Recessive Disorders Cystic Fibrosis

• Caucasians 4 are carriers, lack a membrane
  protein that pumps chloride ions out of cells,
  chloride accumulates abnormally in the cell
  causing osmotic uptake of water from the
  interstitial fluid, leaving the mucus very thick.

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Cystic fibrosis

• Symptoms mucus around pancreas, lungs,
  digestive tract
• Can be treated

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Recessive Disorders Tay-Sachs

• 1 in 3600 births mostly in Ashkenazic Jews
  (central Europe)
• Brain cells of babies are unable to metabolize
  lipids (lack of enzyme)
• Lipids accumulate
• Seizures, blindness, loss of motor, lethal after
  a few years

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Recessive Disorders Sickle Cell

• African Americans 1 in 400
• Single amino acid substitution in hemoglobin code
• Hemoglobin molecules crystallize when O2 content
  is low (exercise)
• Blood cells form abnormal crescent shape clogs
  capillaries

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Recessive Disorders Sickle Cell

• Heterozygotes are carriers and suffer
  (co-dominant)
• Homozygotes can be lethal
• Heterozygote advantage heterozygous people in
  the population do not suffer from malaria

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Recessive Disorders PKU

• Phenylketonuria inability to metabolize
  (enzyme) phenylalanine
• Chromosome 12
• Low diet of phenylalanine until brain develops

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Genetic Disorders

• Consanguinity parents are closely related
• More likely to be homozygous for recessive
  traits- most are lethal
• Some human populations show no effects

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Dominant Disorders

• Polydactyly multiple digits
• Achondroplasia dwarfism (homos spontaneous
  abortion)
• Huntingtons chorea shows up in later life
  nervous disorder tip of 4

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Genetic Screening, Counseling

• Pedigrees
• Carrier recognition some tests available to
  check for Tay-Sachs, CF, sickle-cell
• Fetal testing

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Genetic Screening, Counseling Fetal Testing

• Amniocentesis extracts amniotic fluid
• Test fluids for chemicals
• Grow cells in culture, karyotypes
• Chorionic villi sampling stem cells from the
  chorionic villi of the placenta
• Karyotyping, chemical
• Can be done earlier

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Genetic Screening, Counseling

• Newborn screening PKU
• Multifactorial disorders have both genetic and
  environmental causes heart disease, cancers,
  alcoholism, mental illness
• Often polygenic

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Chromosomal Basis of Inheritance

• Chromosomes Are the Cause of Inheritance

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Chromosomal Theory of Inheritance

• 1860s Mendel factors segregate and assort
  independently during gametogenesis pea plants
• 1875 mitosis
• 1890 meiosis
• 1902 Sutton noticed parallels with chromosomes
  and Mendels factors

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Chromosomal Theory of Inheritance

• Mendelian factors (genes) are located on
  chromosomes
• Chromosomes segregate and assort independently
• Morgan confirmed using fruit flies

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Morgan

• Mendel called inheritance factors
• Had no idea WHY traits were inherited
• Lucky that the traits he studied in pea plants
  were all simple dominant

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Morgan

• Embryologist early 1900s
• Fruit flies breed quickly, produce lots of
  offspring, easy to maintain, only have 4
  chromosomes
• Proved that Mendels factors were on chromosomes

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Fruit Flies Jargon

• Wild type most common or normal ()
• Mutant phenotypes eye colors, wing shapes, etc
• Wild red eyes, normal wings

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Fruit Flies

• Use recessive initial to represent recessive ie
  w white eyes v vestigial wing shape
• Use to indicate wild type ww red eyes
  vv normal wings

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Fruit Flies

• Linked genes genes that are located on the same
  chromosome (loci are close together)
• Sex-linked - white-eyed males hemophilia in
  humans
• If the genes are linked, some phenotypes should
  not show up
• Reason for unusual ratio is
• CROSSING OVER

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Crossing Over and Genetic Maps

• The frequency of recombinations indicates how
  close the loci are to the end of the chromosome
• Sturtevant student of Morgan

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Crossing Over and Genetic Maps

• How to determine the sequence of genes using
  frequency map units
• Cn cinnabar eye shape
• Vg vestigial wings
• B black body

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Crossing Over and Genetic Maps

• Frequency of inheriting b and vg 17
• Frequency of inheriting cn and b 9
• Frequency of inheriting cn and vg 9.5

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1. Relative position of 2 genes farthest apart
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vg
b
2. Frequency between 1st and 3rd
9
b
cn
3. Consider placement of 3rd gene
cn
cn
b
vg
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4. Frequency between 2nd and 3rd cn-vg 9.5
cn
9
9.5
vg
b
17
5. Sequence b cn vg
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Crossing Over and Genetic Maps

• Only gives RELATIVE position, not absolute

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Sex Chromosomes and Sex Linkage

• Gender is determined by the presence of certain
  chromosomes
• Heterogametic sex produces 2 types of gametes
  determines gender (X, Y)
• Homogametic sex produces 1 type of gamete

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Sex Chromosomes and Sex Linkage

• Male or female depends upon presence of Y
  chromosome
• Triggers testicular development
• Sry gene - code for regulating other genes

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Types of sexual reproduction
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Sex Chromosomes and Sex Linkage Disorders

• More genes on the X than Y
• Most X-linked genes have no homologous loci on
  the Y
• Most Y-genes have no homologous gene on the X
  (Male-determining factors)

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Sex Chromosomes and Sex Linkage Disorders

• Ex. Color blindness
• Duchennes Muscular Dystrophy
• Hemophilia
• Fathers cannot pass sex-linked traits to their
  sons, only daughters

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Sex Chromosomes and Sex Linkage Disorders

• Mothers donate the X to their sons
• Mothers pass x-linked traits to sons and
  daughters
• Many more males have sex-linked recessive traits
  than females

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Sex Chromosomes and Sex Linkage X-Inactivation

• Females have only one active X chromosome
• Each stem cell inactivates one of the X
  chromosomes
• Barr body dense mass in the nucleus that is an
  inactive X chromosome

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Sex Chromosomes and Sex Linkage Barr Bodies

• Female mammals are a mosaic of both types of
  cells
• Which of the 2 Xs becomes active is random
  chance
• If female is heterozygous for a trait, then she
  could express both traits
• Ex. Calico cats, human sweat glands

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Calico Cats

• Epistasis gene for color must be present color
  or no color (white)
• Sex-linkage - color is either orange or black
• Barr bodies one of the two X chromosomes is
  randomly inactivated so fur can be orange or
  black in patches

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Chromosome Alterations

• Deletions lose a piece of the chromosome Cri
  du chat
• Duplication lost piece joins someplace else
  (homologous chromosome)
• Translocation join to a non-homologous
  chromosome leukemia

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Chromosome Alterations

• Inversion rejoin in the reverse order
• CROSSING OVER source of deletions and
  duplications
• Positional effect expression of the gene altered
  due to sequence

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Chromosome Alterations Human Disease

• Nondisjunction failure of chromosomes to
  separate during meiosis I
• Results in zygote receiving 3 copies or 1 copy of
  a chromosome instead of 2 (diploid)

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Lethal
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Chromosome Alterations Human Disease

• Aneuploidy abnormal number of chromosomes due
  to nondisjunction
• Usually lethal (spontaneous abortion)
• May cause a syndrome (set of characteristics)

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Chromosome Alterations Human Disease

• Downs syndrome trisomy of No. 21 extra
  muscular tissue, tongue, eye pads (diamond-shaped
  eyes), mental retardation, heart defects
• Linked to age
• First meiotic division as a fetus, meiosis II
  during ovulation

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Chromosome Alterations Human Disease

• Autosomal aneuploidies are less severe fewer
  genes on the Y and only one X
• Aneuploidy of 23 (trisomy)
• Absence of Y is required for femaleness 1 Y
  causes maleness

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Chromosome Alterations Human Disease Sex Genes

• Males
• Klinefelters XXY small testis, breasts,
  sterile, feminine contours
• XYY normal, taller than average

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Chromosome Alterations Human Disease Sex Genes

• Females
• Metafemales XXX limited fertility, retardation
• Turners XO sterile, short, no secondary sex
  characteristics, no mature ovaries

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