HUMAN GENETICS AND INHERITANCE PATTERNS

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HUMAN GENETICS AND INHERITANCE PATTERNS

• Biology
• Corsicana High School

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Linkage

• Genes are located all along chromosomes
• A gene is a group of base triplets on the DNA
  molecule
• Therefore, there are hundreds of genes on each
  chromosome

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Linkage (continued)

• During meiosis, one of each homologous pair of
  chromosomes goes into each of the gametes formed
• All the genes on that chromosome are carried with
  it
• These genes are linked---they are distributed to
  the same gamete

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Linkage (example)

• If genes for seed shape (R, r) and stem length
  (T, t) were carried on same chromosome
• Genes on the same chromosome do not separate
• Only 2 possible gametes RT and rt

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Linkage (continued)

• linkage---when two genes are carried on the same
  chromosome, they do not separate independently
  when forming gametes
• Law of Independent Assortment was modified to
  account for this

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Law of Independent Assortment

• When the gene pairs on a given pair of
  chromosomes are separated, they are distributed
  to the gametes completely independently of the
  way other gene pairs on other chromosomes are
  distributed.

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Human Genetics is Hard to Study Because

• humans have a long generation span
• humans produce relatively few offspring
• the influence of the environment is strong on
  many traits
• many possible gene combinations
• you cannot do experimental crosses on humans

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Population Genetics

• based on finding gene frequencies---how often a
  particular gene occurs in a given population
• Much of the study of human genetics is population
  genetics

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Population Sampling

• testing a cross-section of a population for a
  given trait
• in certain populations, there are high gene
  frequencies for certain traits

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Gene Pool

• all the genes present in a given population
• any individual member of the population is
  considered to have a random sample of genes from
  the gene pool
• examples

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Changes in Gene Pools

• gene pools change as populations shift
• individuals move into or out of the population
• new combinations of genes occur

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Pedigree

• a chart which traces the inheritance of a trait
  in a family
• another way to study human genetics

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Sex Chromosomes

• Humans have 22 pairs of autosomes---regular
  chromosomes
• Humans have 1 pair of sex chromosomes---determine
  if individual is male or female
• Male XY
• Female XX

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Sex Determination Cross
P1 XY x XX
Genotypes 2 XX, 2 XY
X
X
X
X
X
X
X
Phenotypes 2 females, 2 males
Y
X
Y
X
Y
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Which Parent Determines the Sex of a Baby?

• the gamete from the father does (the sperm)
• The sperm can contribute either
• an X chromosome (for a female)
• or a Y chromosome (for a male)
• The egg from the mother can contribute only an X
  chromosome

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Sex Linkage

• Some traits can be found mostly in one sex or the
  other
• These traits are carried on the sex chromosomes

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Hemophilia

• free-bleeders
• lack a clotting factor in the blood
• can result in great loss of blood from a minor
  wound
• most hemophiliacs are male (a sex-linked trait)

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Genetics of Hemophilia

• gene for hemophilia is a recessive gene carried
  on the X chromosome
• gene for normal blood (with clotting factor) is
  dominant
• there is no gene for this trait on the Y
• Males get only one gene for the trait (on X)
  females get two (two X chromosomes)

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Genetics of Hemophilia (continued)

• XhY---hemophiliac male
• he has only one gene for the trait
• there is no gene on the Y
• XHY---normal male

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Genetics of Hemophilia (continued)

• XHXh---carrier female
• she has normal blood because of dominant gene
• she carries the recessive gene for hemophilia,
  and can pass it on
• XHXH---normal female
• XhXh---hemophiliac female (not likely)

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Sample CrossSex Linkage
normal male x carrier female
XHY
XHXh
XH
Xh
XH
XH
XH
XH
Xh
Y
Y
Y
XH
Xh
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Genotypes 1 XHXH, 1 XHXh, 1 XHY, 1 XhY
Phenotypes 1 normal female, 1 carrier female 1
normal male, 1 hemophiliac male
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Sex-Influenced Traits

• the presence of male or female hormones
  influences the expression of the trait
• example pattern baldness--dominant in males and
  recessive in females
• due to the influence of the male hormone
  testosterone

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Polygenic traits

• determined by more than one set of genes
• phenotypes results from the combination of all
  the sets of genes
• examples hair color, eye color, skin
  pigmentation

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Human Blood Types

• determined by genes
• important in blood transfusions
• a transfusion with the wrong blood type can cause
  blood cells to clump inside blood vessels, which
  can be fatal

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Blood Groups

• there are many blood groups Rh, MN, etc.
• ABO blood group is the most familiar

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Genetics of ABO Blood Types

• multiple alleles---more than two alleles for a
  trait
• three alleles in this group
• A and B are codominant
• O is recessive

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ABO Blood Types

• There are four blood types in this group
• Type A genotypes AA, AO
• Type B genotypes BB, BO
• Type AB genotype AB
• Type O genotype OO

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Antigens

• genes result in formation of proteins in the red
  blood cells called antigens
• People with type A blood have A antigen, those
  with type B have B antigen
• Those with AB blood have both antigens
• Those with type O have neither

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Blood Type Cross
type AB x type O
AB
OO
Genotypes 2 AO, 2 BO
O
O
A
A
A
O
O
Phenotypes 2 type A, 2 type B
B
B
B
O
O
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Twins
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Fraternal Twins

• dizygous
• formed when two different sperms fertilize two
  different eggs
• Genetically, they are no more alike than regular
  siblings

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Identical Twins

• monozygous
• one sperm fertilizes one egg. Later, the mass of
  cells formed from the zygote splits into 2
  masses, each growing up into a separate
  individual
• Genetically identical

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Genetic Disorders

• autosomal recessive---examples sickle-cell
  anemia, cystic fibrosis
• autosomal dominant---example Huntingtons
  disease
• sex-linked---carried on X chromosome. example
  hemophilia

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Nondisjunction

• inherited disorder resulting from failure of
  homologous chromosomes to separate properly in
  meiosis
• gametes formed have too few chromosomes or extra
  chromosomes

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Downs Syndrome

• individual has an extra 21st chromosome in body
  cells
• also called trisomy-21
• causes mental retardation, weakness, etc.
• more common in older mothers

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Other Conditions from Nondisjunction

• Turners syndrome---only one X chromosome
  (X_)---results in a short, sterile female
• Klinefelters syndrome---XXY---sterile male,
  mentally retarded
• Trisomy X---XXX---female, some mentally retarded,
  some show male traits

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Genetic Screening

• examining a persons genetic makeup
• karyotype---a picture of an individuals
  chromosomes
• genetic counseling---informs people of genetic
  disorders that could affect offspring

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Amniocentesis

• a test for detecting some genetic defects in
  unborn babies
• sample of amnionic fluid is taken, loose cells
  are cultured and studied

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Mutation

• a change in a gene---results in a new trait that
  can be inherited
• most mutations are harmful (some are lethal)
• mutations are nearly always recessive

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Types of Mutations

• somatic mutation---occurs in body cells. Is not
  passed on to offspring
• germ mutation---occurs in reproductive cells.
  May be passed on to offspring

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Cancer

• uncontrolled growth of cells
• caused by disorders in gene expression, which
  result in cell division disorders
• oncogene (onco cancer)---genes that, when
  expressed, can cause a normal cell to become
  cancerous

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Tumor

• an abnormal mass of cells resulting from
  uncontrolled cell division
• benign---cells remain withing the mass no threat
  to life
• malignant---cells break away and cause new tumors
  in other locations (metastasis)