Chapter 14-1: Human Heredity

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Chapter 14-1Human Heredity
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Human chromosomes

• Most of our cells contain 23 pairs of
  chromosomes, for a total of 46 chromosomes.
• These cells are called somatic cells or body
  cells.
• Two exceptions are the gametes (sex cells), sperm
  and eggs, which have one copy of each chromosome.
  Sperm and egg cells each have 23 chromosomes.

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Human chromosomes

• There are two types of chromosomes.
• Autosomes Of the 46 chromosomes, 44 of them (22
  pairs of chromosomes) are called autosomes
  (non-sex chromosomes).
• Sex chromosomes The last two chromosomes are
  called the sex chromosomes because they determine
  the sex of the person. Females have two X
  chromosomes (XX) and males have one X and one Y
  chromosome (XY).

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• Gametes
• All gametes are haploid. In humans, that means
  each egg cell and each sperm cell has 1 copy of
  each chromosome for a total of 23 chromosomes.
• Egg cells All human egg cells carry 23
  chromosomes, one of which is a single X
  chromosome. This is written as 23, X.
• Sperm cells In males, there are two types of
  sperm cells- one carries an X chromosome (23, X)
  and one carries a Y chromosome (23, Y).

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• When a sperm and egg cell combine, half of the
  time the fertilized eggs (also called zygotes)
  are female (46, XX) and half of the time they are
  male (46, XY).

X
X
eggs
X
XX
XX
female
female
Y
XY
XY
male
male
sperm
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1. Scientists can analyze chromosomes by taking a
   picture of cells during mitosis. It is easiest
   to view chromosomes during mitosis because they
   are condensed. From a picture of chromosomes,
   scientists can cut and paste to arrange the
   chromosomes in pairs to form a karyotype. They
   are arranged from largest (pair 1) to smallest
   (pair 22). The last pair (23) is the sex
   chromosomes.

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Karyotype
Y
X
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Male or Female?
Male
Female
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Human Traits

• A pedigree is similar to a family tree- both are
  used to show relationships in a family.
• Pedigrees can be used to demonstrate how traits
  are passed from one generation to another.
• Genetic counselors use pedigrees to follow how
  genetic disorders are inherited.
• People who are heterozygous for a recessive
  genetic disorder (they are unaffected) are called
  carriers.

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female
male
children
marriage
parents
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Add to your notes Sometimes, but not always,
carriers of traits (heterozygotes) may be
represented as a half-shaded shape or a shape
with a dot in the middle.
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Human Genes Blood typing

• In humans, blood type is determined by the Rh
  blood group and the ABO blood group.
• The Rh blood group determines if your blood is
  positive or negative.
• There are two Rh alleles the Rh allele is
  dominant to the Rh- allele.
• Your blood is positive if you are Rh /Rh or
  Rh/Rh-. Your blood is negative if you are Rh-/Rh-

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• The ABO group is more complicated. There are
  three alleles IA, IB, and i.
• The IA and IB alleles are codominant. The IA and
  IB alleles cause expression of carbohydrate
  chains called antigens on surface of red blood
  cells. They help your body identify the cells.
• The i allele is recessive to the other two
  alleles. The i allele O does not produce
  antigens.

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• The ABO blood group is important in blood
  transfusions.
• If the blood recipient has never been exposed to
  an antigen (A or B) and that antigen enters the
  body it will cause an immune reaction. This can
  cause death.

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• In emergency rooms when there isnt time to
  figure out the blood type of the patient, which
  type of blood will the patient receive? Type O
  because these blood cells have no A or B
  antigens. People with Type O blood are called
  universal donors.
• Who is the universal recipient that can receive
  blood from any donor? Type AB.

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Chapter 14-2Human Chromosomes
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• Most genetic disorders are caused by mutations on
  autosomes, or non-sex chromosomes.
• Examples include
• Autosomal recessive disorders albinism, cystic
  fibrosis, Tay-Sachs disease
• Autosomal dominant disorders most common form
  of dwarfism (achondroplasia) and
  Hypercholesterolemia (high cholesterol)
• Codominant disorders sickle-cell disease

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Albinism
Achondroplasia
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• Some genetic disorders are caused be genes on
  the sex chromosomes.
• Most of these genes are on the X chromosome
  because the Y chromosome is very small and has
  few genes. The genes on X chromosome are
  different from the genes on the Y chromosome.

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• Because females are XX they have two copies of
  the genes on the X chromosome. For sex-linked
  traits, females can be homozygous dominant,
  heterozygous, or homozygous recessive.

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• Because males are XY they have only one copy of
  the genes on the X chromosome this is called
  hemizygous. In males, only one recessive allele
  on the X chromosome is necessary for the
  recessive phenotype to be expressed because there
  is not another allele for this gene on the Y
  chromosome. Some sex-linked (also known as
  X-linked) genetic conditions include
• Color blindness- the inability to distinguish
  certain colors
• Hemophilia- missing protein important for blood
  clotting
• Duchenne Muscular Dystrophy- progressive
  weakening of skeletal muscles

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Colorblindness- Test A
Everyone should be able to see a circle, star,
and square in the demonstration picture.
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Colorblindness- Test B
Colorblind individuals should see the yellow
square. Color normal individuals should see the
yellow square and a "faint" brown circle.
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Colorblindness- Test C
Colorblind individuals should see nothing. Color
normal individuals should see a "faint" brown
boat.
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• Example of a sex-linked Punnett square
• XBXb (heterozygous female with normal vision)
  crossed to XBY (hemizygous male with normal
  vision)

XBY
Y
XB
XB XB
XBY
XB
XB Xb
XbY
XB Xb
Xb
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Chromosomal Disorders

• Remember that meiosis is the reductional cell
  division that divides one diploid cell to produce
  four haploid gametes (sex cells, sperm or egg).
  Normally gametes have one copy of each
  chromosome.
• Sometimes chromosomes might not separate properly
  during meiosis this is called nondisjunction.
• If nondisjunction occurs, abnormal numbers of
  chromosomes (usually one is missing or there is
  an extra copy of one) are found in gametes and
  disorders of chromosomal numbers may result.

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gametes
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1. Trisomy Some chromosomal disorders are caused
   by having three copies of one chromosome. This
   is called trisomy. In trisomies, the gamete of
   one parent donated two of one type of chromosome
   to the child and the gamete of the other parent
   donated one chromosome (like normal).
2. Monosomy Chromosomal disorders characterized by
   missing one chromosome are called monosomies. In
   monosomies, the gamete of one parent donated one
   chromosome and the other did not donate any.

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• Some examples of chromosomal disorders resulting
  from nondisjunction
• Down syndrome- Trisomy 21
• Klinefelters syndrome- XXY (male)
• Turners syndrome- XO (female)
• Add to your notes
• As long there is at least one Y chromsome, the
  karyotype is male.

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Human Genome Project

• The human genome project sequenced the human
  genome. Now the code is being analyzed and
  scientists are finding genes for many traits and
  genetic disorders. In gene therapy, a gene that
  has been mutated and does not work properly is
  replaced by a normal, working copy of the gene.
  Gene therapy is a work in progress.