The Human GenomeThe Human GenomeChapter 14

Human HeredityHuman Heredity14-1

NOVA: Cracking the Code of NOVA: Cracking the Code of LifeLifeChapter 1: Instructions for

Making a Human Being

Human ChromosomesHuman Chromosomes

A karyotype is a picture of chromosomes from a cell arranged in homologous pairs.

Human ChromosomesHuman ChromosomesHumans have 46 chromosomes.

◦44 autosomes◦2 sex chromosomes

Normal female: 46 XX Normal male: 46 XY

Sperm: 23 X or 23 Y Egg: 23 X

Human ChromosomesHuman Chromosomes

Human ChromosomesHuman ChromosomesWhat determines the sex of a

child? The sperm……

Human TraitsHuman TraitsTo study the inheritance of

human traits genetic counselors use a pedigree chart.

From this, geneticists can infer genotypes of family members.

Human TraitsHuman TraitsDraw a pedigree chart.

Human TraitsHuman TraitsAssociating an observed human

trait with a gene is difficult. Many human traits are polygenic. The environment influences the

expression of a trait.

Human GenesHuman GenesHumans have 4 blood types

caused by 3 alleles.◦A & B – codominant◦O – recessive

Types: A, B, AB, O

Blood types must be matched for a safe transfusion.

Draw blood type chart…………………..

Human GenesHuman GenesRh factors represent another

group of antigens found on some red blood cells.◦Rh+ = present.◦Rh- = absent.

Human GenesHuman GenesMost genetic disorders are

caused by recessive alleles. To have the disorder an individual must

inherit two recessive alleles. Ex: PKU, Tay-Sachs, Cystic Fibrosis, Albinism

Chromosome # 7

CFTR gene

The most common allele that causes cystic fibrosis is missing 3 DNA bases. As a result, the amino acid phenylalanine is missing from the CFTR protein.

Normal CFTR is a chloride ion channel in cell membranes. Abnormal CFTR cannot be transported to the cell membrane.

The cells in the person’s airways are unable to transport chloride ions. As a result, the airways become clogged with a thick mucus.

Figure 14-8 The Cause of Cystic Fibrosis

Cystic Fibrosis

Human GenesHuman GenesA few are caused by dominant

alleles. Only one allele needs to be inherited to

be expressed. Ex: Huntington’s disease, Dwarfism

Human GenesHuman GenesSickle cell anemia is caused by a

codominant allele. One normal allele and one abnormal

allele are inherited for making the protein hemoglobin. The sickle cell allele is inherited among many

African Americans. Carrying this allele produces resistance to the

Malaria parasite.

Sickle Cell Anemia

NOVA: Cracking the Code of NOVA: Cracking the Code of LifeLifeChapter 2: Getting the Letters

Out

Human ChromosomesHuman Chromosomes14-2

Human Genes and Human Genes and ChromosomesChromosomes

Only 2% of your DNA functions as genes.

Human Genes and Human Genes and ChromosomesChromosomes

Genes located on the same chromosome are linked.◦They tend to be inherited together.◦They can be separated by crossing-

over during meiosis.

Crossing-OverCrossing-Over

Sex-Linked GenesSex-Linked Genes

Sex-linked genes are located on the sex chromosomes.

More genes are on the X than the Y.

Sex-Linked GenesSex-Linked GenesGenes on the X chromosome are always expressed in males even if recessive.◦Males get sex-linked disorders more

often than females. Colorblindness, hemophilia, muscular

dystrophy

X-Chromosome X-Chromosome InactivationInactivationIn females, one X chromosome is

randomly switched off.◦Calico cats are always female.

X-Chromosome X-Chromosome InactivationInactivationThe switched off X chromosome

becomes a dense region in the nucleus known as a Barr body.

Barr bodies aren’t found in males.

Olympics and Barr BodiesOlympics and Barr BodiesSex testing was introduced in

competitive sports in the mid-1960s, amid rumor that some competitors in women's events were not truly female - especially two Soviet sisters who won gold medals at the 1960 and 1964 Olympics, and who abruptly retired when gender verification testing began.

Olympics and Barr BodiesOlympics and Barr BodiesThe first tests, at the European

Championships in 1966 and the Pan-American Games in 1967, required female competitors to undress before a panel of doctors. Other methods used during this period included manual examination or close-up scrutiny of the athlete's genital region.

Olympics and Barr BodiesOlympics and Barr BodiesWhen athletes complained that these

tests were degrading, the IOC at the Mexico City Olympics in 1968 introduced genetic testing in the form of a sex chromatin (Barr body) analysis of cells from a buccal smear. The procedure was further modified at the Barcelona games, using the polymerase chain reaction to amplify the DNA extracted from a specimen to allow detection of a Y chromosome gene, SRY, that codes for male determination.

Journal of the American Medical Association, July 17, 1996, vol. 276, no. 3, pp. 177-178

Chromosomal DisordersChromosomal DisordersCaused by nondisjunction

during meiosis.◦Homologous chromosomes fail to

separate resulting in sex cells with one too many or one too few chromosomes.

Homologous chromosomes fail to separate

Meiosis I:Nondisjunction

Meiosis II

Section 14-2

Nondisjunction

Non disjunction

Chromosomal DisordersChromosomal DisordersDown’s syndrome = 47,XX or

47,XYTurner’s syndrome = 45,XKlinefelter’s syndrome = 47,XXY

Down’s SyndromeDown’s Syndrome

Turner’s SyndromeTurner’s Syndrome

Kleinefelter’s SyndromeKleinefelter’s Syndrome

Human Molecular Human Molecular GeneticsGenetics14-3

Human DNA AnalysisHuman DNA AnalysisEven though the human genome is

over 6 billion bases long, genetic tests exist for detecting the presence of defective recessive genes.

Prospective parents can now be tested to determine if they carry recessive alleles for hundreds of disorders.

Is this a good idea?

DNA FingerprintingDNA FingerprintingAll humans share the same

genes.The 98% of DNA that is not

encoded in genes is what makes us different from each other.

DNA fingerprinting makes use of this difference to identify individuals.

Restriction enzyme

Chromosomes contain large amounts of DNA called repeats that do not code for proteins. This DNA varies from person to person. Here, one sample has 12 repeats between genes A and B, while the second sample has 9 repeats.

Restriction enzymes are used to cut the DNA into fragments containing genes and repeats. Note that the repeat fragments from these two samples are of different lengths.

The DNA fragments are separated according to size using gel electrophoresis. The fragments containing repeats are then labeled using radioactive probes. This produces a series of bands—the DNA fingerprint.

DNA Fingerprinting4-18 DNA Fingerprinting

The Human Genome The Human Genome ProjectProjectIn 2000, the entire human

genome was sequenced.◦Dr. Francis Collins, Dr. Eric Lander

and Dr. Craig Venter (NOVA – Cracking the Code of Life)

Advances in technology allowed this to happen in several years rather than the predicted 20.

Promoter Start signal

Gene Stop signal

Gene Sequence

Rapid Sequencing

Gene TherapyGene TherapyThe process of changing a gene

that causes a disorder.◦An absent or defective gene is

replaced by a normal functioning gene.

Viruses are often used to deliver functional genes to affected cells.

Normal hemoglobin gene

Bone marrow

cellChromosomes

Genetically engineered virus

Nucleus

Bone marrow

Gene Therapy

Ethical Issues…Ethical Issues…Genetic curesDesigner babiesMedical costsResearch costs