Chapter 11 Human Heredity

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Chapter 11Human Heredity

• Charles Page High School
• Stephen L. Cotton

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Section 11-1It Runs in the Family

• OBJECTIVES
• Explain how human traits are inherited.

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Section 11-1It Runs in the Family

• OBJECTIVES
• Distinguish between sex chromosomes and
  autosomes.

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Section 11-1It Runs in the Family

• OBJECTIVES
• Discuss the influence of the environment on gene
  expression.

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Section 11-1It Runs in the Family

• Human children inherit characteristics from their
  parents as a result of gene interaction
• The principles of genetics we have discussed so
  far will also apply to humans
• dominant, recessive, etc.

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Section 11-1It Runs in the Family

• Human genetics is fairly complicated however, it
  is rewarding to find out about ourselves
• how some of the disorders and conditions we
  inherit can be cured or prevented

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Section 11-1It Runs in the Family

• A human diploid cell contains 46 chromosomes,
  arranged in 23 pairs (23 is the haploid number)
• Organisms inherit a single copy of each gene from
  each parent
• these are the gametes, or reproductive cells
  (sperm, egg)

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Section 11-1It Runs in the Family

• During fertilization, sperm and egg unite, and a
  zygote (fertilized egg) is produced, with 46
  total chromosomes
• Of the 46 chromosomes, two are the sex
  chromosomes (X and Y)
• Remaining 44 called autosomes

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Section 11-1It Runs in the Family

• Human genes carried on the sex chromosomes (X
  only) are called sex-linked or X-linked
• The phenotype is only partly determined by the
  genotype
• the environment will also influence some traits

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Section 11-1It Runs in the Family

• Environmental factors may include nutrition and
  exercise
• Even though we need to remember that the
  environment is important, only genes are
  inherited, and must have the proper environment
  to work!

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Section 11-2 Inheritance of Human Traits

• OBJECTIVES
• Identify two human traits that are determined by
  multiple alleles.

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Section 11-2 Inheritance of Human Traits

• OBJECTIVES
• Explain how Huntington disease and sickle cell
  anemia are inherited.

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Section 11-2 Inheritance of Human Traits

• OBJECTIVES
• Distinguish between traits determined by multiple
  alleles and polygenic traits.

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Section 11-2Inheritance of Human Traits

• Lets examine just a few of the ways in which
  particular genes affect us
• HUMAN BLOOD GROUPS
• Multiple alleles are three or more alleles of the
  same gene that code for a single trait

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Section 11-2Inheritance of Human Traits

• There are 4 blood types in humans that are
  controlled by the presence or absence of a
  specific chemical substance in the blood
• In 1900, an Austrian physician Karl Landsteiner
  discovered these four groups

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Section 11-2Inheritance of Human Traits

• When mixing blood from different people, he
  noticed the red cells agglutinated, or clumped
  together
• Fig. 11-4, page 230
• Landsteiner determined two antigens A and B

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Section 11-2Inheritance of Human Traits

• The presence of these antigens (A and B) produces
  four possible blood types A, B, AB, and O
• Type A has antigen A
• Type B has antigen B
• Type AB has both antigen A B
• Type O neither antigen A nor B

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Section 11-2Inheritance of Human Traits

• The ABO blood groups are of importance in blood
  transfusions
• A transfusion of the wrong type can cause a
  violent, often fatal, reaction in the body as the
  immune system responds to the antigens not found
  in its own cells

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Section 11-2Inheritance of Human Traits

• Because people with AB blood have both antigens,
  they can receive any type of blood, and are
  called universal recipients
• Since type O blood has neither antigen, it can
  donate to any other blood type, and is called the
  universal donor

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Section 11-2Inheritance of Human Traits

• The three possible alleles identified by
  Landsteiner were IA, IB, and i (sometimes listed
  as Io)
• Note Fig. 11-5, page 231 for the corresponding
  phenotypes
• IA and IB are codominant, and both are dominant
  to i

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Section 11-2Inheritance of Human Traits

• Rh BLOOD GROUPS
• In addition to the ABO antigens, there is another
  antigen on red blood cells that determines
  success or failure of transfusions
• the Rh antigen, named after the Rhesus monkey
  Fig. 11-6, p.231

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Section 11-2Inheritance of Human Traits

• People who have the Rh antigen are said to be Rh
  positive (Rh), and those who do not have it are
  said to be Rh negative (Rh-)
• These two characteristics (ABO and Rh) are
  usually expressed together
• examples A or A- O or O-

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Section 11-2Inheritance of Human Traits

• The gene for the Rh antigen shows simple
  dominance
• HUNTINGTON DISEASE
• So many genes are involved that we do not usually
  notice them until something goes wrong, such as
  in Huntington Disease

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Section 11-2Inheritance of Human Traits

• Huntington disease is caused by a single dominant
  allele (H)
• no symptoms appear until they are in their 30s or
  40s, when the gradual damage to their nervous
  system begins
• painful loss of muscle control and mental
  function then death

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Section 11-2Inheritance of Human Traits

• Huntington disease caused by having a single
  dominant allele, not the normal of two recessive
• Until recently, the only way people knew if they
  carried the gene for Huntington disease was the
  appearance of the disease later in life

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Section 11-2Inheritance of Human Traits

• SICKLE CELL ANEMIA
• 1904 Dr. Herrick had a young patient complaining
  of weakness and dizzy spells, and open sores
• a blood examination revealed the blood cells were
  bent and twisted into shapes like sickles, a farm
  tool used to cut grain

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Section 11-2Inheritance of Human Traits

• These unusually shaped cells were the cause of
  his patients problems
• he gave the disease the name by which we know it
  today sickle cell anemia
• Fig. 11-7, page 232

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Section 11-2Inheritance of Human Traits

• The cause of sickle cell anemia?
• A change in one of the polypeptides found in
  hemoglobin- the protein that carries oxygen in
  red blood cells

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Section 11-2Inheritance of Human Traits

• When deprived of oxygen, either from heavy
  exercise or even anxiety, the hemoglobin
  molecules join and form fibers that cause the
  dramatic change in shape
• these shapes become trapped in the
  capillariesFig. 11-8, p.232

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Section 11-2Inheritance of Human Traits

• When the blood flow is stopped, damage to cells
  and tissues occurs serious injury or even death
  may result
• Normal hemoglobin (A) is codominant with the
  sickle cell gene (S)
• Heterozygous (AS) are carriers

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Section 11-2Inheritance of Human Traits

• Carriers have roughly half of the hemoglobin as
  normal thus these people suffer few ill effects
• Homozygous (SS) are called sickle cell sufferers
• more severely affected by the disease

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Section 11-2Inheritance of Human Traits

• This is caused by a single nucleotide difference
  from the normal hemoglobin
• How can just ONE difference cause so much
  trouble?
• The one that is substituted makes the hemoglobin
  less soluble in water- forms crystals

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Section 11-2Inheritance of Human Traits

• In the U.S., people of African ancestry are the
  most common carriers of the sickle cell trait
• also found in tropical regions of Africa and Asia
• Why is sickle cell anemia so common in some
  regions, and virtually unknown in others?

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Section 11-2Inheritance of Human Traits

• Answer People who are heterozygous (AS) are
  partially resistant to malaria
• This favored an advantage in areas where malaria
  was common, and thus it was favored by natural
  selection- it persisted because it helped against
  malaria

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Section 11-2Inheritance of Human Traits

• Polygenic Traits
• Human traits that are controlled by a number of
  genes are called polygenic- such as height, body
  weight, or skin color
• In humans, at least four different genes control
  skin color

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Section 11-2Inheritance of Human Traits

• The color of human skin ranges from very dark to
  very light, depending upon the amount of melanin-
  a dark colored pigment present in skin cells
• this variation shows what a wonderfully diverse
  species we are

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Section 11-3Sex-Linked Inheritance

• OBJECTIVES
• Describe how sex is determined in humans.

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Section 11-3Sex-Linked Inheritance

• OBJECTIVES
• List two conditions of nondisjunction of sex
  chromosomes.

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Section 11-3Sex-Linked Inheritance

• OBJECTIVES
• Identify some human sex-linked traits.

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Section 11-3Sex-Linked Inheritance

• OBJECTIVES
• Compare sex-linked and sex-influenced traits.

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Section 11-3Sex-Linked Inheritance

• Genes that are located on the sex chromosomes are
  inherited in a sex-linked pattern
• As in many other organisms, the sex in humans is
  also determined by the X and Y chromosomes

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Section 11-3Sex-Linked Inheritance

• Human males normally 46XY, meaning that they
  have a total of 46 chromosomes, which includes
  one X chromosome and one Y chromosome
• Human females normally 46XX

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Section 11-3Sex-Linked Inheritance

• How do the X and Y chromosome patterns determine
  if the zygote will be male or female?
• In Drosophila the absence of a second X produces
  male
• In humans the presence of a single Y produces
  male

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Section 11-3Sex-Linked Inheritance

• Although meiosis is a precise mechanism, errors
  do sometimes take place
• Nondisjunction is the most common error in
  meiosis, and is the failure to separate properly
  during meiosis
• Fig. 11-12, page 235

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Section 11-3Sex-Linked Inheritance

• Nondisjunction can produce
• gametes with two sex chromosomes
• gametes with no sex chromosomes
• Both result in an abnormal number of sex
  chromosomes

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Section 11-3Sex-Linked Inheritance

• The most common result of nondisjunction of the
  sex chromosomes are
• Turner syndrome
• Klinefelter syndrome
• Lets discuss each of these in detail...

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Section 11-3Sex-Linked Inheritance

• Turner syndrome - the result is female in
  appearance, but the sex organs do not develop at
  puberty- thus they are sterile (unable to have
  children)
• Missing a sex chromosome, thus they are 45X (or
  45XO)

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Section 11-3Sex-Linked Inheritance

• Klinefelter syndrome - male in appearance, and
  they are also sterile
• have an extra sex chromosome, thus are 47XXY
• Interesting to note that no reported cases of
  having no X it carries other characteristics

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Section 11-3Sex-Linked Inheritance

• What can we learn from this?
• An X chromosome is essential
• sex is determined by the presence or absence of
  Y, and not the number of X chromosomes
• the Y switches on male growth

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Section 11-3Sex-Linked Inheritance

• Genes that are carried on the X or Y chromosomes
  are said to be sex-linked
• in humans, not many on the Y
• It is particularly easy to spot recessive defects
  in genes on the X occur more in males!

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Section 11-3Sex-Linked Inheritance

• COLORBLINDNESS - is a recessive disorder in which
  a person cannot distinguish between certain
  colors
• usually caused by sex-linked genes on the X
  chromosome
• Fig. 11-13, page 237

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Section 11-3Sex-Linked Inheritance

• Most common is red-green colorblindness- trouble
  distinguishing between the lighter shades of red
  and green
• In Caucasians, about 8 of males are affected,
  but only about 1 of females are
• Note that XCXc is a carrier

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Section 11-3Sex-Linked Inheritance

• A carrier will not be colorblind, but can pass
  the gene for colorblindness to her offspring
• Treatment? NONE
• HEMOPHILIA - another recessive gene located on
  the X
• called bleeders disease
• Fig. 11-14, page 238

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Section 11-3Sex-Linked Inheritance

• In hemophilia, the protein antihemophilic factor
  (AHF) necessary for normal blood clotting is
  missing
• More rare than colorblindness
• affects 1 male in 10,000
• affects 1 female in 100,000,000

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Section 11-3Sex-Linked Inheritance

• Hemophilia is more serious than colorblindness
• people can bleed to death from even a small cut
• internal bleeding (hemorrhaging) can occur from a
  slight bump or bruise

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Section 11-3Sex-Linked Inheritance

• Treatment?
• Remove the AHF from donated blood, and add it to
  the people with hemophilia
• inconvenient, and makes them susceptible to
  diseases carried in the blood

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Section 11-3Sex-Linked Inheritance

• MUSCULAR DYSTROPHY - a sex-linked disease that
  results in the progressive wasting away of
  skeletal muscles
• dystrophy means faulty development
• people rarely survive past early adulthood

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Section 11-3Sex-Linked Inheritance

• Many of the traits that seem to be sex-linked are
  actually caused by genes located on autosomes,
  and not the sex chromosomes
• why then is baldness so much more common in men
  than women?

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Section 11-3Sex-Linked Inheritance

• Male pattern baldness (Fig. 11-15, page 238) is a
  sex-influenced trait
• a trait that is caused by a gene whose expression
  differs in males and females
• the interaction with male sex hormones may be the
  reason

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Section 11-4 Diagnosis of Genetic Disorders

• OBJECTIVES
• Describe how Down syndrome is inherited.

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Section 11-4 Diagnosis of Genetic Disorders

• OBJECTIVES
• Identify two methods of detecting genetic
  disorders during pregnancy.

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Section 11-4 Diagnosis of Genetic Disorders

• Today, for some disorders detection is as simple
  as an examination of a persons chromosomes
• called a karyotype
• Fig. 11-16, page 240

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Section 11-4 Diagnosis of Genetic Disorders

• Nondisjunction can also affect the autosomes, as
  well as the sex chromosomes
• Down syndrome - there is an extra copy of
  chromosome 21 (called trisomy 21) named after
  Dr. Down who worked with this
• Note this on Fig. 11-16, p.240

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Section 11-4 Diagnosis of Genetic Disorders

• Down syndrome
• results in mental retardation that ranges from
  mild to severe
• also characterized by an increased susceptibility
  to many diseases
• affects 1 in 800 babies in U.S.

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Section 11-4 Diagnosis of Genetic Disorders

• PRENATAL DIAGNOSIS -
• Down syndrome and other genetic disorders can be
  diagnosed before birth (prenatal) by analyzing
  cells from the developing embryo
• Several possible methods...

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Section 11-4 Diagnosis of Genetic Disorders

• AMNIOCENTESIS
• requires the removal of a small amount of the
  fluid from the sac surrounding the embryo
• the cells from it are grown in a laboratory a
  chemical stops division, and then examined, to
  make the karyotype

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Section 11-4 Diagnosis of Genetic Disorders

• CHORIONIC VILLUS BIOPSY
• a new alternative to amniocentesis a sample of
  cells removed directly from the membrane
  surrounding the embryo
• results obtained more rapidly than amniocentesis

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Section 11-4 Diagnosis of Genetic Disorders

• Both techniques considered safe for the mother
  and the developing baby
• These techniques have made it possible to detect
  more than 100 genetic disorders from embryonic
  cells

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Section 11-4 Diagnosis of Genetic Disorders

• Ethical considerations?
• The rapid development of these screening
  techniques is forcing a new set of questions on
  society
• Do we favor a society in which diseased
  individuals have no place or do we take efforts
  to preserve all life?