CHAPTER 14 THE HUMAN GENOME - PowerPoint Presentation

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CHAPTER 14 THE HUMAN GENOME

14-1 Human Heredity

A

. Human chromosomes

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chromosomes are analyzed by taking a photograph of condensed chromosomes during mitosis

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the chromosomes are then cut out of the photograph and grouped together in pairs

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a picture of chromosomes arranged this way is known as a

karyotype

(See Fig 14-2 pg. 341)

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Normal human karyotype

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normal karyotypes show

46 chromosomes

which are labeled 1-23

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the

last set (23) are the sex chromosomes

which determine the person’s sex

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the rest

(1-22) are autosomal

chromosomes or autosomes

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biologists summarize the total number of chromosomes in a human cell as 46XX (female) and 46XY (male)

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all egg cells carry a single X chromosome (23X) and half of all sperm carry an X (23X) or a Y (23Y)

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each chromosome contains a single, double stranded DNA molecule

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B

. Human Traits

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human genes are inherited according to the same principles discovered with

Gregor

Mendel’s peas

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biologists must identify an inherited trait controlled by a single gene(not easy) by establishing that it is an inherited trait and not the result of environmental influences

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they then study how the trait is passed from generation to generation

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to do this, they use a chart called a

pedigree

which shows the relationship in families of certain traits (See Fig 14-3)

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many traits are

polygenic

(shapes of ears and eyes)

- many traits are strongly influenced by the environment or

nongenetic

factors such as nutrition and exercise; environmental effects are not inherited, but genes are

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C

. Human Genes

The first human genes to be identified were those that control blood type

1

. Blood Group Genes

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a number of genes are responsible for human blood groups but the best known are the ABO blood groups and the Rh blood groups

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a

. Rh- - determined by a single gene with two alleles (+ and -) - Rh+ is dominant, so those who are heterozygous (Rh+/Rh-) are said to be Rh+ and those with 2 recessive alleles are Rh-

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b

. ABO

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more complicated

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there are

three alleles for this gene

; I

A

, IB, and i

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these alleles produce

antigens

on the surface of the red blood cell which are recognized by the immune system

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IA and IB are

codominant

; the i allele is recessive and produces no antigen (See Fig 14-4)

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2

. Recessive alleles (autosomal)

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we have learned about many genes by studying genetic disorders

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Fig 14-6 pg. 345 lists some of the more common disorders

a

. phenylketonuria (PKU)

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the enzyme needed to break down the amino acid phenylalanine is missing

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the amino acid is found in milk, and many other foods

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if phenylalanine builds up in the tissues of a newborn, it leads to mental

retardation

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caused by an autosomal recessive allele on chromosome 12

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treated with a low phenylalanine diet

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Boy with PKU who was treated reacting with his sister (normal)

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b

.

Tay

-Sachs disease

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caused by an autosomal recessive allele

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found mostly in Jewish families of central and eastern Europe ancestry

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results in nervous system breakdown and death within the first few years of life

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no treatment, but there is a way to test for the allele letting parents know if their children would be at risk for the disorder

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fats build up in the nerve cells

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affects chromosome #15

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Tay Sachs Disease

Normal and affected sibling diagnosed at 71/2 months of age

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3

. Autosomal dominant allelesIf you have a dominant allele for a genetic disorder, it will be expresseda. achondroplasia- a form of dwarfism whereby there is a defect in the formation of cartilage in long bones

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b

. Huntington’s chorea - a nervous system disorder - progressive loss of muscle control and mental function until death occurs - symptoms do not show up until around 40 years of age when nervous system deterioration begins

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4

. Codominant autosomal alleles Sickle cell disease - sickle-shaped red blood cells causing tissue damage

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D

. From Gene to Molecule

In two disorders, sickle cell disease and cystic fibrosis, a small change in a single gene affects protein structure, causing a serious genetic disorder

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. cystic fibrosis

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a common, fatal genetic disease in people whose ancestors were from northern Europe

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caused by a recessive allele on chromosome 7

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children develop serious digestive problems and produce a thick mucus that clogs their lungs and breathing passageways

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only ½ of those affected survive into their 20s

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results from the deletion of 3 bases in the middle of a sequence for a protein

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the protein allows chlorine ions to pass across biological membranes

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the missing amino acid results in the protein folding improperly, it cannot be transported to the cell membrane, and it gets destroyed

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being unable to transport chlorine ions, tissues in the body malfunction

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those with one normal allele are unaffected

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2

. sickle cell disease

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commonly found in African Americans

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red blood cells are bent, twisted, and more rigid than normal red blood cells and get stuck in capillaries

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results in blood ceasing to move through blood vessels, and cells and tissues beyond the blockage become damaged

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produces physical weakness and damage to the brain, heart, and spleen; can be fatal

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the gene that codes for the protein hemoglobin (carries oxygen in the blood) differs by 1 DNA base in the sickle cell allele causing the insertion of the amino acid

valine

instead of glutamic acid

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the abnormal hemoglobin is less soluble than normal hemoglobin - decreases in blood oxygen levels cause hemoglobin molecules to come out of solution and stick together forming long chains and fibers - there is a relationship between sickle cell in African-Americans of West Central Africa and resistance to malaria

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3

. Dominant or Recessive?

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what makes alleles dominant, recessive, or

codominant

is the nature of the gene’s protein product and its function in the cell

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in cystic fibrosis, because only one allele supplies enough chlorine channel proteins to function normally, it is considered

dominant recessive allele

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the sickle cell alleles are believed to be

codominant

because in the heterozygous genotype, the individual phenotype is different from someone with only normal alleles in that they are resistant to malaria

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14-2 Human Chromosomes

A

. Human Genes and Chromosomes

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chromosomes 21 and 22 are the smallest human autosomes

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they were the first two whose sequence were determined

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their structural features are like the other human chromosomes

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genetic disorders of chromosome #22 include an allele that causes a form of leukemia and one associated with neurofibromatosis ( tumor-causing disease of the nervous system)

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Neurofibromatosis

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-there

are also many long stretches of repetitive DNA that do not code for protein and are unstable sites where rearrangements can occur - chromosome 21 is similar with one gene associated with amyolateral sclerosis (ALS or Lou Gehrig’s disease) - there are also regions with no genes

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B

. Sex-linked Genes

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there is a special pattern of inheritance for genes located on the X and Y chromosomes

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genes located on them are said to be sex-linked because they are formed on the chromosome that determines sex

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more than 100 sex-linked disorders have been mapped out on the X chromosome

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the smaller Y chromosome has only a few genes

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1

. colorblindness

- the

inability to distinguish certain colors

- in

males, a defect in any of the 3 genes for color vision located on the X chromosome results in colorblindness

- the

most common form in red-green colorblindness (1/10 males)

- colorblindness

in females is rare (1/100)

***males have just one X chromosome, therefore

, all x-linked alleles are expressed in males even if they are recessive

***to be expressed

in females, two copies of the allele must be present, one on each chromosome

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the recessive phenotype of a sex-linked disorder is more common in males than females

- also, men pass their X chromosome to their daughters and the sex-linked disorder may show up in the sons of those daughters

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2

. hemophilia

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another sex-linked disorder

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involves 2 genes carried on the X chromosome that help control blood clotting

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recessive allele in either gene causes hemophilia where a protein necessary for normal blood clotting is missing

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hemophiliacs can bleed to death from minor cuts or bleed internally from bumps or bruises

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treated with injections of normal clotting proteins

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1/10,000 males are affected

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Hemophilia

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3

.

Duchenne

Muscular Dystrophy

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a sex-linked disorder that results in progressive weakening and loss of skeletal muscle

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fatal; affected individuals rarely live past early adulthood

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affects 1/3000 males

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caused by a defective version of a gene that codes for a muscle protein

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treatment or cure is being attempted by inserting a normal gene into muscle cells of DMD patients

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C

. X Chromosome Inactivation - females have 2 X chromosomes, but one is randomly switched off forming a dense region in the chromosome known as a Barr body - Barr bodies are not found in males because with only one X chromosome, it is still active - Occurs in other mammals as well Ex. fur color in female cat

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D

. Chromosomal Disorders - most common disorder is when chromosomes fail to separate during meiosis; this is called nondisjunction (See Fig 14-15) - this results in abnormal numbers of chromosomes in gametes and disorders of chromosome number can occur

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1

. Down Syndrome - a condition of trisomy, where two copies of an autosomal chromosome fail to separate and an individual may be born with 3 copies of chromosome #21 - affects 1/800 babies - results in mild to severe mental retardation and an increased susceptibility to many diseases and higher incidence of birth defects

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2

. Sex Chromosome disorders a. Turner’s Syndrome - nondisjunction in females who inherit only one X chromosome (XO) - sterile; sex organs do not develop at pubertyLymphedema seen in newborns with Turners

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b

.

Klinefelter’s

Syndrome

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nondisjuntion

in males

who inherit two X chromosomes (

XXY

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the extra X chromosome interferes with meiosis and

prevents individuals from reproducing

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some cases have shown genotypes of XXXY or XXXXY

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no babies are born without an X chromosome, indicating the X chromosome

contains

genes necessary for development

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sex chromosome abnormalities show the role of the Y chromosome in sex determination

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a small region of the Y chromosome

is

necessary to produce male sexual development, even if combined with several X chromosomes

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if the sex determining region is missing from the Y chromosome, the child develops as a female

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14-3 Human Molecular Genetics

A

. Human DNA analysis

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there are about 6 million base pairs in our DNA

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biologists must use special techniques to read DNA sequences to look for information they need

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1

. Testing for alleles

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used when looking for disorders that contain DNA sequences that are slightly different from the normal condition

Ex

. cystic fibrosis and

Tay

-Sachs disease

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DNA probes are used to find specific sequences in disease-causing alleles

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Some tests look for changes in restriction enzymes or differences in length of normal and abnormal alleles

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There are genetic tests for many disorders to determine if parents risk passing abnormal alleles to their children

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DNA testing can tell exactly what the genetic problem causing the disorder is, making it easier to treat

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2

. DNA Fingerprinting

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no individual is exactly like any other genetically; except identical twins

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DNA fingerprinting is

a powerful tool used to identify individuals

by analyzing segments of DNA that have little or no known function, but that vary widely among individuals (See Fig 14-18)

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Used to determine whether blood, semen, or other material left at a crime scene matches the DNA from a suspect

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Used since late 1980s in U.S. ; very reliable

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B

. The Human Genome Project

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an

attempt to sequence all human DNA

begun in 1990

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the genome of several other organisms was also completed including yeast and the fruit fly

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the human genome was completed in June of 2000

Searching

for Genes

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the hunt for genes continues today through 24 volumes of information (22 autosomal and 2 sex chromosomes)

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A Breakthrough for Everyone

Genome research data is available to everyone and can be found on the internet

(

See www.phschool.com for direct link to latest information)

C

. Gene Therapy

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an absent or faulty gene is replaced

by a normal, working gene

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can be

used to correct genetic disorders

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the normal gene can make the correct protein or enzyme and eliminate the cause of the disorder

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viruses are modified so they do not cause disease and genetically engineered to carry the normal gene into the individual’s cells (See Fig 14-21)

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not totally successful; it remains a high-risk and experimental procedure

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D

. Ethical Issues in Human Genetics

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guidelines must be instituted as to what should and should not be done with the human genome

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this is

a societal issue and does not remain solely in the hands of researchers