Chapter 14: The Human Genome 14-1: Human Heredity 14-2: Human Chromosomes 14-3: Human Molecular Genetics

14-1: Human Heredity Objective: How is sex determined? How do small changes in DNA cause genetic disorders?

Human Chromosomes Cell biologists Photograph cells in mitosis Chromosomes are fully condensed Easy to see Cut out the chromosomes from the photographs Group them in pairs karyotype: set of photographs of chromosomes grouped in order in pairs

Human Chromosomes Cont’d Humans have 46 chromosomes 23 from each parent One of the 23 chromosomes given is a sex chromosome sex chromosome: determine an individual’s sex Females can only give an “X” chromosome Large chromosome Males can give an “X” or “Y” chromosome Ratio is 50:50 “Y” chromosomes are small The other chromosomes are called autosomes autosomes:chromosome that is not a sex chromosome

QUICK QUESTION What is a karyotype?

QUICK ANSWER A picture of chromosomes arranged in pairs

Human Traits Human genes are inherited according to Mendle’s Principles Biologists must identify an inherited trait controlled by a single gene determine that the trait is inherited and not environmentally influenced study how the trait is passed from one generation to the next

Human Traits Cont’d Genetic counselors analyze pedigree charts to infer the genotypes of family members pedigree: shows the relationships within a family

Human Traits Cont’d Some of the most obvious human traits are almost impossible to associate with single genes many traits are polygenic many personal traits are determined by genes and environment 2 types nutrition exercise Not inherited Genes may be denied a proper environment in which to reach full expression

Group Activity Complete the “Problem Solving” pedigree on page 343.

Human Genes The human genome our complete genetic information tens of thousands of genes specify many characteristics Exploration of the human genome sequence of the human genome has been a major scientific undertaking identify genes that directly control a single human trait blood type

Blood Group Genes Human blood Variety of blood groups ABO Rh Receiving the wrong type of blood can be fatal

Blood Group Genes cont’d Rh blood group Named for the “rhesus monkey” in which this factor was found Determined by a single gene 2 possible alleles Positive (dominant) Negative (recessive)

Blood Group Genes cont’d ABO blood group 3 alleles for this gene IA (codominant) IB (codominant) I (recessive)

Blood Group Genes cont’d Blood types Type A Make the A antigen IAIA IAi Type B Make the B antigen IBIB IBi Type AB Make the A and B antigens IAIB Type O Make no antigens ii

Recessive Alleles Autosomal recessive The presence of a normal, functioning gene is revealed only when an abnormal or nonfunctioning allele affects the phenotype

Recessive Alleles cont’d PKU Lack the enzyme that is needed to break down an amino acid found in milk and many other foods Build up of this amino acid can cause mental retardation Tay-Sachs disease Found mostly in Jewish families of central and eastern European ancestry Nervous system breakdown and death in the first few years of life

Dominant Alleles Autosomal dominant A dominant allele for a genetic disorder Example Achondroplasia Dwarfism Nervous system disorder Huntington’s disease Progressive loss of muscle control and mental function until death

Codominant Alleles Diseases and disorders caused by a codominant allele (both alleles are dominant) Example Sickle cell disease 1 out of 5 African Americans

Quick Question What type of allele causes Huntington’s disease?

Quick Answer An autosomal dominant allele

From Gene to Molecule Scientists are still working to find the answer for How DNA sequences in genes affect phenotype The link between the DNA bases in the allele for a genetic disorder Example In both cystic fibrosis and sickle cell disease, a small change in the DNA of a single gene affects the structure of a protein, causing a serious genetic disorder

Cystic Fibrosis Common genetic disease Most common among people whose ancestors came from Northern Europe Caused by a recessive allele on chromosome 7 Deletion of 3 bases in the middle of a sequence for a protein Cells do not transport the protein to the cell membrane Misfolded protein is destroyed Serious digestive problems Produce a thick, heavy mucus that clogs their lungs and breathing passageways Tissues throughout the body malfunction

Sickle Cell Disease A common genetic disorder found in African Americans Ancestry traced to west central Africa Malaria is common Heterozygous people are resistant to malaria

Sickle Cell Disease cont’d Characterization Bent and twisted shape of the red blood cells Decrease in blood oxygen levels Body destroys the sickled cells and malaria parasite More rigid Gets stuck in capillaries Damages cells, tissues and organs Produces physical weakness Damage to the brain, heart, and spleen Can be fatal

Dominant or Recessive? Deciding if an allele is dominant, recessive, or codominant depends on the nature of a gene’s protein product and its role in the cell.

Exit Ticket Answer the following questions. Use complete sentences. You may NOT use your book. You MAY use your notes. This is a quiz grade. How is sex determined? How do small changes in DNA cause genetic disorders?