Chapter 12: Inheritance Patterns and Human Genetics 12-1 Chromosomes and Inheritance 12-2 Human Genetics

I. Sex Determination (by male NOT female) Sex chromosomes segregate into sex cells during meiosis. (XX and XY) 12-1 Chromosomes and Inheritance

(A) Sex Linkage (a PATTERN of inheritance) TRAIT is determined by GENES on either SEX chromosome (X or Y).

(1) X-Linked Genes (code for Sex-Linked Traits; e.g., eye color) Located on X chromosome (females = 2 copies, males get ONE copy)

(2) Y-Linked Genes (code for MALE traits, testosterone production) Located on Y chromosome; (only MALES receives Y-linked genes)

II. Linkage Groups (a chromosome carries thousands of genes) Group of genes located on a SINGLE CHROMOSOME that TEND to be inherited TOGETHER. Ex: In fruit flies, the genes for BODY COLOR and WING LENGTH are LINKED (on the SAME chromosome).

III. Chromosome Mapping (gene locus and crossing-over BETWEEN genes) Crossing-over DEPENDS on DISTANCE between GENES on chromosome (i.e., the CLOSER, the MORE likely to be inherited TOGETHER)

(1) Chromosome Map (used to IDENTIFY inheritance patterns) Diagram showing the RELATIVE LOCATIONS of genes on a chromosome.

(2) Map Unit (a UNIT in LOCUS DIFFERENCE) 1 map unit is EQUAL to a 1 percent OCCURRENCE of crossing-over. (i.e., 2 genes SEPARATED by crossing-over 1% PERCENT of the time are considered to be 1 MAP UNIT apart.)

IV. Mutation (can occur in ALL cell types) Change in DNA can involve parts of a CHROMOSOME or a SINGLE BASE

(1) Biologists have observed that chromosome mutations often occur during nuclear division. Why do you think this is so? Critical Thinking

(1) Germ-cell (i.e., sex cell, gamete) Mutations CAN be INHERITED from parent TO offspring (i.e., occur in the SEX cells (gametes) of an organism)

(2) Somatic Mutations (body cell mutations) NON-sex cell and can AFFECT the organism  Types of CANCER result from SOMATIC mutations.

(3) Lethal Mutations (lead to cell death) GENES result in major DEVELOPMENTAL flaws  typically BEFORE birth.

(A) Chromosome Mutations (LARGE-scale change) Changes that involve the LOSS or ADDITION of an ENTIRE chromosome.

(1) Deletion (of a chromosome) A SEGMENT is LOST due to BREAKAGE (the GENES it carried are lost).

(2) Inversion (of a chromosome) A SEGMENT breaks off AND reattaches UPSIDE-DOWN on SAME chromosome.

(3) Translocation (of a chromosome) A SEGMENT breaks off and REATTACHES to ANOTHER chromosome.

(4) Nondisjunction (results in Monosomy and Trisomy) Chromosomes FAIL to separate during MEIOSIS (sex cells affected).

(B) Gene Mutation (smaller-scale change) A SINGLE base becomes CHANGED  (Point Mutations, Substitutions, Insertions, Deletions)

(1) Point Mutation (i.e., mutation occurs at a SINGLE POINT-3 types) A substitution, insertion, OR deletion of a SINGLE BASE in a sequence. (2) Substitutions (do NOT cause a FRAMESHIFT) 1 base is REPLACED with a DIFFERENT base, resulting in a DIFFERENT codon (and possibly a DIFFERENT amino acid).

(3) Sickle-Cell Anemia (inherited disease) Caused by a SUBSTITUTION of an adenine (A) for a thymine (T) in a DNA sequence (results in DEFECTIVE hemoglobin).

(4) Frame-Shift Mutation (deletion AND insertion mutations ONLY) Deletion OR insertion of a base causes REMAINING codons to be REGROUPED, (incorrectly)…consider the following sequence: THE DOG AND THE BAT CAN EAT THE RAT Knock out the “H” in the first codon (deletion) and the entire frame will SHIFT causing a MISREADING of the base sequence (i.e., the gene). TED OGA NDT HEB ATC ANE ATT HER AT_

I. Studying Human Inheritance Autosomal OR Sex-Linked trait? Recessive OR Dominant trait? (NOTE: Traits expressed through family generations) Human Genetics

Shows HOW a trait is PASSED down through a FAMILY. (square = MALE, circle = FEMALE). (A) Pedigree Analysis (METHOD to show the PATTERN of inheritance)

(1) Pedigree (family tree) Displays HOW a trait is inherited OVER generations. (GOAL is to discover the PATTERN of inheritance).

(2) Patterns of Inheritance (Autosomal OR Sex-Linked) Traits may show a predictable PATTERN from generation to generation.

(3) Carriers (Heterozygotes) Do NOT express a recessive trait, but DO carry a recessive allele. (i.e., “Tt” is a “CARRIER” of the short allele)

(2) Individuals who are heterozygous for sickle cell anemia generally have no symptoms of the disease. However, they should avoid extreme conditions that severely reduce the amount of oxygen available to the body, such as playing vigorous sports at high elevations. Explain why this would be advisable. Critical Thinking

II. Genetic Traits and Disorders (inheritable diseases) An innate etiology; result from faulty DNA coding (diseased alleles).

(A) Traits Controlled by a Single Allele human traits are determined by a SINGLE allele. (Ex: Cystic Fibrosis, Autosomal Recessive).

(1)Huntington’s Disease (Autosomal DOMINANT) Expressed in 40’s  LOSS of neuromuscular control AND death. (NOTE: Depending on WHEN children are born, the disease may be UNKNOWINGLY passed from one generation to the next).

(2) Genetic Marker (associated GENE for a disease) NOTE: Recently geneticists have discovered a genetic marker for the HD allele which has a 96% ACCURACY of PREDICTING its onset. SHORT sequence of DNA showing a CORRELATION with a DISEASED GENE nearby (an INDICATOR of a trait or disease)

(B) Traits Controlled by Multiple Alleles Multiple ALLELES code for a SINGLE TRAIT. (Examples include BLOOD TYPE with THREE types of alleles, O, A, and B.)

(C) Polygenic Traits (i.e., multiple GENES code for a SINGLE TRAIT) MOST human traits are controlled by TWO or MORE GENES. (Examples: skin color, eye color, and height.)

(D) X-Linked Traits (a.k.a., Sex-Linked, dominant OR recessive) Determined by GENES found on the X chromosome in either sex.

(3) Colorblindness is rare among females. Why might this be? Critical Thinking

(1) Colorblindness (X-Linked Recessive) Inability to distinguish CERTAIN colors, (typically reds from greens). (males  MORE vulnerable for X-linked RECESSIVE disorders)

(2) Hemophelia (X-Linked Recessive) An INABILITY to produce effective BLOOD-CLOTTING proteins. (again, males MORE vulnerable, females often protected)

(3) Duchenne Muscular Dystrophy (X-Linked Recessive) Results in progressively WEAKENED and eventually DESTROYED muscle tissue.

(4) A 20 year old man diagnosed with muscular dystrophy has a sister who is soon to be married. If you were the man, what would you tell your sister? Critical Thinking

(E) Sex-INFLUENCED Traits (NOT Sex-Linked—genes are AUTOSOMAL) Presence of SEX HORMONES influences expression of a certain trait. (Ex: Pattern Baldness-B, b) Ex: BB will lose their hair, Bb woman  keeps hair, Bb man  loses hair. NOTE: Differences in gene expression are due to HIGHER levels of TESTOSTERONE in men, which INTERACTS with Bb genes  BALDNESS.

(F) Disorders Due to Nondisjunction Occurs during MEIOSIS resulting in abnormal sex cells (gametes).

(1) Monosomy (Turners, XO) A gamete contains ONLY 1 copy of chromosome instead of 2.

(2) Trisomy (Klinefelters, XXY and Down Sydrome-21) A gamete contains 3 copies of a chromosome instead of 2.

ZYGOTE carries an EXTRA COPY of chromosome 21 during development (3 copies rather than 2). (3) Down Syndrome (a.k.a., Trisomy-21) NOTE: Results in mild to severe mental retardation and other developmental features; however, still can reproduce and live long, healthy lives.

III. Detecting Human Genetic Disorders Detection can occur in UTERO or in the ADULT who plans to become a parent (i.e., a screening of their genome). (1) Genetic Screening/Counseling Examination through a KARYOTYPE or other means (GENETIC MARKERS).

(2) Amniocentesis (used to prepare a KARYOTYPE from FETAL CELLS) Physician REMOVES amniotic fluid from AMNION, a sac that surrounds fetus (between week 14-16).

Obtains a sample of chorionic villi, a TISSUE that grows BETWEEN the mother’s uterus and placenta (between week 8-10). (3) Chorionic Villi Sampling (CVS)

Inability to BREAK DOWN the amino acid phenylalanine. (An ACCUMULATION of amino acid may result in severe BRAIN DAMAGE). (4) Phenylketonuria (PKU  Autosomal Recessive) NOTE: If positive for PKU, a phenylalanine-FREE diet will be REQUIRED from birth THROUGHOUT life.

Extra Slides AND Answers for Critical Thinking Questions (1) Just before nuclear division, chromatin condenses to form chromosomes, which move throughout the cell during the different stages of nuclear division and thus are at higher risk of breakage. (2) Because colorblindness is an X-linked recessive trait, a female would have to receive two copies of the allele for colorblindness, one from each parent, in order to exhibit colorblindness. (3) Individuals who are heterozygous for sickle cell anemia have normal and sickled red blood cells. Since sickled red blood cells cannot transport oxygen effectively, individuals can experience circulatory difficulties under low-oxygen conditions. (4) Muscular dystrophy is a sex-linked recessive disorder. The man’s sister may be a carrier of muscular dystrophy and may wish to consult a genetic counselor before having children.