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Chapter 12: Inheritance Patterns and Human Genetics

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Chapter 12: Inheritance Patterns and Human Genetics 12-1 Chromosomes and Inheritance ... Patterns of Inheritance (Autosomal OR Sex-Linked) ... – PowerPoint PPT presentation

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Title: Chapter 12: Inheritance Patterns and Human Genetics


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Chapter 12 Inheritance Patterns and Human
Genetics
12-1 Chromosomes and Inheritance
12-2 Human Genetics
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12-1 Chromosomes and Inheritance
I. Sex Determination (by male NOT female)
  • Sex chromosomes segregate into sex cells during
    meiosis. (XX and XY)

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(A) Sex Linkage (a PATTERN of inheritance)
  • TRAIT is determined by GENES on either SEX
    chromosome (X or Y).

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(1) X-Linked Genes (code for Sex-Linked Traits
e.g., eye color)
  • Located on X chromosome (females 2 copies,
    males get ONE copy)

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(2) Y-Linked Genes (code for MALE traits,
testosterone production)
  • Located on Y chromosome (only MALES receives
    Y-linked genes)

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II. Linkage Groups (a chromosome carries
thousands of genes)
  • Group of genes located on a SINGLE CHROMOSOME
    that TEND to be inherited TOGETHER.

Ex In fruit flies, the genes for BODY COLOR and
WING LENGTH are LINKED (on the SAME chromosome).
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III. Chromosome Mapping (gene locus and
crossing-over BETWEEN genes)
  • Crossing-over DEPENDS on DISTANCE between GENES
    on chromosome (i.e., the CLOSER, the MORE likely
    to be inherited TOGETHER)

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(1) Chromosome Map (used to IDENTIFY inheritance
patterns)
  • Diagram showing the RELATIVE LOCATIONS of genes
    on a chromosome.

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(2) Map Unit (a UNIT in LOCUS DIFFERENCE)
  • 1 map unit is EQUAL to a 1 percent OCCURRENCE of
    crossing-over.
  • (i.e., 2 genes SEPARATED by crossing-over 1
    PERCENT of the time are considered to be 1 MAP
    UNIT apart.)

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IV. Mutation (can occur in ALL cell types)
  • Change in DNA can involve parts of a CHROMOSOME
    or a SINGLE BASE

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Critical Thinking
(1) Biologists have observed that chromosome
mutations often occur during nuclear division.
Why do you think this is so?
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(1) Germ-cell (i.e., sex cell, gamete) Mutations
  • CAN be INHERITED from parent TO offspring
    (i.e., occur in the SEX cells (gametes) of an
    organism)

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(2) Somatic Mutations (body cell mutations)
  • NON-sex cell and can AFFECT the organism ?Types
    of CANCER result from SOMATIC mutations.

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(3) Lethal Mutations (lead to cell death)
  • GENES result in major DEVELOPMENTAL flaws
    ?typically BEFORE birth.

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(A) Chromosome Mutations (LARGE-scale change)
  • Changes that involve the LOSS or ADDITION of an
    ENTIRE chromosome.

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(1) Deletion (of a chromosome)
  • A SEGMENT is LOST due to BREAKAGE (the GENES it
    carried are lost).

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(2) Inversion (of a chromosome)
  • A SEGMENT breaks off AND reattaches UPSIDE-DOWN
    on SAME chromosome.

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(3) Translocation (of a chromosome)
  • A SEGMENT breaks off and REATTACHES to ANOTHER
    chromosome.

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(4) Nondisjunction (results in Monosomy and
Trisomy)
  • Chromosomes FAIL to separate during MEIOSIS (sex
    cells affected).

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(B) Gene Mutation (smaller-scale change)
  • A SINGLE base becomes CHANGED ?(Point Mutations,
    Substitutions, Insertions, Deletions)

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(1) Point Mutation (i.e., mutation occurs at a
SINGLE POINT-3 types)
  • A substitution, insertion, OR deletion of a
    SINGLE BASE in a sequence.

(2) Substitutions (do NOT cause a FRAMESHIFT)
  • 1 base is REPLACED with a DIFFERENT base,
    resulting in a DIFFERENT codon (and possibly a
    DIFFERENT amino acid).

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(3) Sickle-Cell Anemia (inherited disease)
  • Caused by a SUBSTITUTION of an adenine (A) for a
    thymine (T) in a DNA sequence (results in
    DEFECTIVE hemoglobin).

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(4) Frame-Shift Mutation (deletion AND insertion
mutations ONLY)
  • Deletion OR insertion of a base causes REMAINING
    codons to be REGROUPED, (incorrectly)consider
    the following sequence

THE DOG AND THE BAT CAN EAT THE RAT
Knock out the H in the first codon (deletion)
and the entire frame will SHIFT causing a
MISREADING of the base sequence (i.e., the gene).
TED OGA NDT HEB ATC ANE ATT HER AT_
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12-2 Human Genetics
I. Studying Human Inheritance
  • Autosomal OR Sex-Linked trait? Recessive OR
    Dominant trait?
    (NOTE Traits expressed through family
    generations).

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(A) Pedigree Analysis (METHOD to show the PATTERN
of inheritance)
  • Shows HOW a trait is PASSED down through a
    FAMILY. (square
    MALE, circle FEMALE).

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(1) Pedigree (family tree)
  • Displays HOW a trait is inherited OVER
    generations. (GOAL is to discover the PATTERN
    of inheritance).

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(2) Patterns of Inheritance (Autosomal OR
Sex-Linked)
  • Traits may show a predictable PATTERN from
    generation to generation.

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(3) Carriers (Heterozygotes)
  • Do NOT express a recessive trait, but DO carry a
    recessive allele. (i.e., Tt is a
    CARRIER of the short allele)

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Critical Thinking
(2) Individuals who are heterozygous for sickle
cell anemia generally have no symptoms of the
disease. However, they should avoid extreme
conditions that severely reduce the amount of
oxygen available to the body, such as playing
vigorous sports at high elevations. Explain why
this would be advisable.
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II. Genetic Traits and Disorders (inheritable
diseases)
  • An innate etiology result from faulty DNA
    coding (diseased alleles).

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(A) Traits Controlled by a Single Allele
  • 200 human traits are determined by a SINGLE
    allele. (Ex Cystic Fibrosis, Autosomal
    Recessive).

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  1. Huntingtons Disease (Autosomal DOMINANT)
  • Expressed in 40s ?LOSS of neuromuscular control
    AND death. (NOTE Depending on
    WHEN children are born, the disease may be
    UNKNOWINGLY passed from one generation to the
    next).

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(2) Genetic Marker (associated GENE for a disease)
  • SHORT sequence of DNA showing a CORRELATION with
    a DISEASED GENE nearby (an INDICATOR of a trait
    or disease)

NOTE Recently geneticists have discovered a
genetic marker for the HD allele which has a 96
ACCURACY of PREDICTING its onset.
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(B) Traits Controlled by Multiple Alleles
  • Multiple ALLELES code for a SINGLE TRAIT.
  • (Examples include BLOOD TYPE with THREE types of
    alleles, O, A, and B.)

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(C) Polygenic Traits (i.e., multiple GENES code
for a SINGLE TRAIT)
  • MOST human traits are controlled by TWO or MORE
    GENES. (Examples skin color,
    eye color, and height.)

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(D) X-Linked Traits (a.k.a., Sex-Linked, dominant
OR recessive)
  • Determined by GENES found on the X chromosome in
    either sex.

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Critical Thinking
(3) Colorblindness is rare among females. Why
might this be?
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(1) Colorblindness (X-Linked Recessive)
  • Inability to distinguish CERTAIN colors,
    (typically reds from greens). (males ? MORE
    vulnerable for X-linked RECESSIVE disorders)

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(2) Hemophelia (X-Linked Recessive)
  • An INABILITY to produce effective BLOOD-CLOTTING
    proteins. (again, males MORE vulnerable,
    females often protected)

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(3) Duchenne Muscular Dystrophy (X-Linked
Recessive)
  • Results in progressively WEAKENED and eventually
    DESTROYED muscle tissue.

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Critical Thinking
(4) A 20 year old man diagnosed with muscular
dystrophy has a sister who is soon to be married.
If you were the man, what would you tell your
sister?
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(E) Sex-INFLUENCED Traits (NOT Sex-Linkedgenes
are AUTOSOMAL)
  • Presence of SEX HORMONES influences expression
    of a certain trait. (Ex Pattern
    Baldness-B, b)

Ex BB will lose their hair, Bb woman ? keeps
hair, Bb man ? loses hair.
NOTE Differences in gene expression are due to
HIGHER levels of TESTOSTERONE in men, which
INTERACTS with Bb genes ? BALDNESS.
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(F) Disorders Due to Nondisjunction
  • Occurs during MEIOSIS resulting in abnormal sex
    cells (gametes).

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(1) Monosomy (Turners, XO)
  • A gamete contains ONLY 1 copy of chromosome
    instead of 2.

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(2) Trisomy (Klinefelters, XXY and Down
Sydrome-21)
  • A gamete contains 3 copies of a chromosome
    instead of 2.

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(3) Down Syndrome (a.k.a., Trisomy-21)
  • ZYGOTE carries an EXTRA COPY of chromosome 21
    during development (3 copies rather than 2).

NOTE Results in mild to severe mental
retardation and other developmental features
however, still can reproduce and live long,
healthy lives.
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III. Detecting Human Genetic Disorders
  • Detection can occur in UTERO or in the ADULT who
    plans to become a parent (i.e., a screening of
    their genome).

(1) Genetic Screening/Counseling
  • Examination through a KARYOTYPE or other means
    (GENETIC MARKERS).

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(2) Amniocentesis (used to prepare a KARYOTYPE
from FETAL CELLS)
  • Physician REMOVES amniotic fluid from AMNION, a
    sac that surrounds fetus (between week 14-16).

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(3) Chorionic Villi Sampling (CVS)
  • Obtains a sample of chorionic villi, a TISSUE
    that grows BETWEEN the mothers uterus and
    placenta (between week 8-10).

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(4) Phenylketonuria (PKU ? Autosomal Recessive)
  • Inability to BREAK DOWN the amino acid
    phenylalanine. (An
    ACCUMULATION of amino acid may result in severe
    BRAIN DAMAGE).

NOTE If positive for PKU, a phenylalanine-FREE
diet will be REQUIRED from birth THROUGHOUT life.
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Extra Slides AND Answers for Critical Thinking
Questions
(1) Just before nuclear division, chromatin
condenses to form chromosomes, which move
throughout the cell during the different stages
of nuclear division and thus are at higher risk
of breakage.
(2) Because colorblindness is an X-linked
recessive trait, a female would have to receive
two copies of the allele for colorblindness, one
from each parent, in order to exhibit
colorblindness.
(3) Individuals who are heterozygous for sickle
cell anemia have normal and sickled red blood
cells. Since sickled red blood cells cannot
transport oxygen effectively, individuals can
experience circulatory difficulties under
low-oxygen conditions.
(4) Muscular dystrophy is a sex-linked recessive
disorder. The mans sister may be a carrier of
muscular dystrophy and may wish to consult a
genetic counselor before having children.
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