Biology - Chp 14 - Human Heredity - PowerPoint
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Transcript of Biology - Chp 14 - Human Heredity - PowerPoint
Chapter 14Human Heredity
14-1 Human Heredity
• Scientists once knew much less about humans then about other “model” organisms such as fruit flies and mice
• With the completion of the Human Genome Project scientists are on verge of understanding human genetics at least as well as they understand that of some other organisms
Human Chromosomes• To analyze chromosomes, cell
biologists photograph cells in mitosis
• Chromosomes are fully condensed and easiest to see during metaphase
Karyotype• A picture of chromosomes
arranged in order
• We all began life when a haploid sperm fertilized a haploid egg carrying just ________ chromosomes each.
• The _________________________ zygote or fertilized egg contained the full complement of __________ chromosomes
23
Diploid
46
Sex Chromosomes• Determine an individuals sex
Females: XX
Males: XY
• The regular 44 chromosomes are known as autosomes
Q: Why are males & females born in a roughly 50:50 ratio?
A: All egg cells carry a single _______ chromosome. However, half of all sperm carry an _______ chromosome, the other half carry a ________ chromosome
XX
Y
Human Traits• In order to apply Mendelian
genetics to humans, biologists must identify an inherited trait controlled by a single gene
• Then, they have to study how the trait is passed from one generation to the next
Pedigree• A chart which shows the
relationship within a family
• Most human traits are not coded for by single genes
• Also, many traits are strongly influenced by environmental factors–Ex.) average height has
increased 10cm in the United States and Europe since 1800’s
Polygenic Traits• Traits coded for by many genes
• Ex.) skin color, hair color
The Human Genome• Our complete set of genetic
information
• Includes tens of thousands of genes
• Until recently the identification of a human gene took years of work
• Long generation times
• Complex life cycle
• Produce few offspring
Humans aren’t easy test subjects
Blood Group Genes• A number of genes are responsible
for human blood groups, but the best known are the ABO and Rh blood groups
• Rh blood groups
• Rh+ (dominant)
• Rh- (recessive)
Q: How do scientists identify recessive alleles that cause these disorders
A: Compare affected persons genotype to normal persons
PKU – phenylketonuria • Lack enzyme needed to break down
phenylalanine• Found in milk and other foods• If newborn has PKU, phenylalanine may build
up in the tissues during severe mental retardation
• If newborns are tested early, they can be placed on a low phenylalanine diet which prevents most of the affects
• PKU is caused by a recessive allele carried on chromosome 12
PKU
Tay Sachs• Autosomal recessive• Found mostly in Jewish families of
central and eastern European ancestry• Results in nervous system breakdown
and death in the first few years of life• There is no treatment, but there is a
test prospective parents can take
Achondroplasia - dwarfism
• Never reach 4 feet 4 inches
• Cartilage forms in such a way that the arms and legs end up being disproportionately short
• 1 in every 10,000 is affected
Huntington's• Progressive loss of muscle
control and mental function until death occurs
• People with disease show no symptoms until they are in their 30’s and 40’s
Huntington's affect on the brain
Codominant• Sickle cell disease
• Affects 1 in 500 African Americans
From Gene to Molecule
Cystic Fibrosis (CF)• Most common among people whose
ancestors come from northern Europe
• Caused by a recessive allele on chromosome 7
• Produce a thick heavy mucus that clogs their lungs and breathing passageways
• Serious digestive problems
• Only half survive into their 20’s
Sickle Cell Disease• Characterized by the bent and twisted shape of
the red blood cell• Sickle shaped red blood cell tend to get stuck in
the capillaries• Produce physical weakness and damage to the
brain, heart and spleen• Sometimes fatal• Change in just one DNA base• This change inserts amino acid valine in place of
glutamic acid• Hemoglobin molecules stick together and form
long chains that produce the characteristic shape of sickled cells
Q: Why do so many African Americans carry the sickle cell allele?
• A: Many African Americans have West Central African ancestry where malaria is a serious problem
• People who are heterozygous for the sickle cell allele don’t get sickle cell and they don’t get malaria
Where malaria is common Where sickle cell is common
Dominant or Recessive• It all depends on the nature of a
genes protein product and its role in the cell
• Ex.) In CF, one copy of the normal allele can supply cells with enough chloride channel proteins to function therefore the normal CF allele is considered ____________________Dominant
14-2 Human Chromosomes
• A human diploid cell contains more than 6 billion nucleiotide pairs of DNA
• Despite its size, all of this information is neatly packed into the 46 chromosomes present in every diploid cell
• Each chromosome is like a library containing hundreds or even thousands of books
• Biologists are many decades away from mastering the contents of these books, but they are learning just how many books there are and what they deal with
Sex Linked Genes
• Genes located on the sex chromosomes
• Most found on the ________ chromosomeX
Colorblindness• Gene associated with color vision
are located on the X chromosome
• Affects 1 in 10 males
• Affects 1 in 100 females
Q: Why the difference between the sexes?
A: Males have ____________________________ X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive
• In order for a woman to be affected, there must be __________________________ of the allele
just one
Two copies
Hemophilia• A protein necessary for normal
______________________________is missing
• 1 in 10,000 males• People with hemophilia can bleed to
death from minor cuts and may suffer from internal bleeding
• Treated with normal clotting factors
Blood clotting
Duchenne Muscular Dystrophy
• Results in progressive weakening and loss of skeletal muscle
• Rarely live past early adulthood
• In the U.S. 1 in 3000 males is born with Duchenne muscular dystrophy
• Caused by a defective version of the gene that codes for a muscle protein
X-Chromosome Inactivation
Q: If all you need is one X chromosome, what happens to the extra X chromosome in females?
A: one chromosome is randomly switched off
• Forms a barr body – dense region in the nucleus
Calico Cats• Fur color is on the X chromosome. Some
areas have one color switched on, and others its switched off
nondisjunction• When homologous chromosomes
fail to separate – most common error in meiosis
• If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes
Down Syndrome• When nondisjunction happens and a
baby is born with 3 copies of chromosome 21
• Trisomy 21
• 1 in 800 in U.S.
• Mild to severe retardation
• Susceptible to many diseases
• Increased frequency of birth defects
Down Syndrome
Turners Syndrome (female)
• Only inherit one X chromosome, and no Y
• Genotype = XO
• Women with Turner syndrome are sterile, their sex organs don’t properly develop at puberty
Klinefelters Syndrome (males)
• Inherit extra X chromosomes• Genotype = XXY• The extra X interferes with meiosis and
usually prevents them from reproducing• Some cases XXXY or XXXXY• These abnormalities show us the role of Y in
sex determination– Even in combination with several X’s, the Y
makes them male• But if this Y is absent, the embryo develops
into a female
14-3 Human Molecular Genetics
Human DNA Analysis• Way too much DNA to search
through
• Biologists search the volumes of the human genome using DNA sequences
Testing for alleles• If two prospective parents suspect
they might be carrying recessive alleles for a genetic disorder they can now get a test to determine the risk of passing that trait on to their children– Use labeled DNA probes to detect
specific sequences found in disease causing alleles
– Looking at changes in restrictive enzyme cutting sites
– Looking at differences in lengths of alleles
DNA fingerprinting• Analyzes sections of DNA that
have little or no known function but vary widely from one individual to another
• Used to settle paternity disputes
• Convict criminals and overturn convictions
The Human Genome Project• Advances in DNA sequencing technologies at the close
of the twentieth century made it possible to begin sequencing entire genomes
• At first, biologists worked on small genomes of viruses and bacteria
• In 1990, scientists in the United States and other countries began the Human Genome Project– An attempt to sequence all human DNA
• In 2000 scientists announced that the DNA sequence of the human genome was essentially complete
• We estimate as little as 31,000 genes– Fruit fly – 14,000– C. elegans worm 20,000
• Now the task is to figure out how so few genes make an organism as complex as us
Promoter• Section of DNA that signals the start of
a gene• Researchers are looking for genes
that provide useful clues to some of the basic properties of life
• Also looking for genetic information that may be useful in developing new drugs and treatment of disease
A Breakthrough for Everyone
• Data from the human genome project is posted on the internet on a daily basis
• www.genome.gov
Gene Therapy• Replacing an absent or faulty
gene by a normal working gene
Ethical Issues in Human Genetics
• There are many questions which science will rapidly force society to come to grips with
• If it’s just as easy to manipulate genes for personal preference then it is to cure a disease should we do it?
• Our society will have to develop a thoughtful and ethical conscious of what should and shouldn’t be done with the human genome