Human Genetics Genetic Mutations

Mutations – changes in DNA Types of changes permanent changes to nucleotide base sequence Extra genetic material Missing genetic material Results of changes increase the amount of variation among offspring. Harmful – cancer Helpful – antibiotic resistant bacteria (bad for us good for them) Neutral – many occur on introns so they are not expressed by proteins

Two main categories Germ Cell Mutations Occur during meiosis Take place in sex cells Affects offspring Typically doesn’t affect parent Ex. Aneuploidy – irregular number of chromosomes; too many or not enough. Somatic Mutations Take place in body cells Is not passed to offspring Typically localized to a single or small group of cells Ex. Skin cancer from UV radiation exposure

Gene Mutation - Changes in the sequence of Nucleotides Point Mutations (2 types) Base –pair substitutes: may have no effect, change amino acid; or change to stop codon sickle-cell disease is caused by a single point mutation (a missense mutation)

Frameshift Mutations Base-pair insertion or deletion Result is a nonfunctional protein Alters the reading frame of mRNA triplets Specific diseases caused by point mutations Cystic fibrosis Cancer Neurofibromatosis

III. Chromosome Mutations – changes in chromosome structure Deletion: removal of a chromosomal segment Duplication: repeats a chromosomal segment Inversion: segment reversal in a chromosome Translocation: movement of a chromosomal segment to another non-homologous chromosome

Causes of Mutations Chance errors in DNA replication Errors during meiosis Mutagen - environmental factor that damages DNA Ex. UV light (sun; skin cancer), cigarette tar, asbestos, virus, radiation

V. Karyotypes Shows chromosomes paired by size, shape, and appearance in metaphase. Cells are treated, photographed, sorted and arranged by homologous pairs. Chorionic villi sampling, and amniocentesis can be used to diagnose chromosomal abnormalities.

VI. Sex Determination Thomas Hunt Morgan Experiments with Drosophila (fruit fly) Sex Chromosomes Determine Gender Autosomes are non-sex chromosomes Sex chromosomes differ between males and females. Normal Human female is XX Normal Human male is XY.

VII. Irregular Chromosome Number (Aneuploidy) Nondisjunction = Failure of chromosomes to separate during Prophase I of meiosis Result is extra, or missing chromosomes

B. Nondisjunction Syndromes Monosomy (1 missing) A single chromosome is missing (45 instead of 46) Nearly all monosomies are fatal. XO females (Turner syndrome)- short, webbed neck, no puberty. Cri du chat (partial deletion of Chromosome 5) – infant’s cry sounds like kitten meowing, excessive drooling, speech and behavioral problems, small head and jaw.

Turner syndrome affects approximately 1 out of every 2,500 female live births worldwide. It embraces a broad spectrum of features, from major heart defects to minor cosmetic issues. Some individuals with Turner syndrome may have only a few features, while others may have many. Almost all people with Turner syndrome have short stature and loss of ovarian function, but the severity of these problems varies considerably amongst individuals.

One extra single chromosome is present (47 instead of 46) Trisomy One extra single chromosome is present (47 instead of 46) Many are fatal Examples Klinefelter Syndrome = XXY; male, some retardation, low fertility (rare cases (48,XXXY) or (49,XXXXY) Jacob syndrome (XYY males) - tall, acne, not overly aggressive like people once thought.

Triple X syndrome (XXX) – usually no physical abnormalities Triple X syndrome (XXX) – usually no physical abnormalities. Rare complications include mental retardation, increased width between eyes, ovarian failure. Fragile X Syndrome – X chromosome broken; males; hyperactive or autistic, delayed speech Down Syndrome = extra 21st chromosome; mental retardation, fold of skin above eyes, weak muscles, and other neurological, sense and physical abnormalities.

OUT: Compare and contrast gene mutations and chromosome mutations using a Venn Diagram: