Lecture Outline - Chapter 23

23.0. Introduction (Fig. 23.1) (p. 440)

1. The particulate model of heredity is based on genes that control a trait; genes are sections of a chromosome and are represented by paired letters designating particular spots (loci) on a homologous pair of chromosomes.
2. Alleles are alternative forms of a particular gene having the same position on a pair of homologous chromosomes and affecting the same trait. Example: G and G; R and r.

1. Gregor Mendel (Austrian monk) in 1860
   • a. Asserted that garden pea traits are controlled by two factors.
   • b. One of the factors could be dominant (tall) over the other, which was recessive (short).
   • c. The Law of Segregation states that each organism contains two factors for each trait; the factors segregate during formation of gametes and each gamete contains only one factor from each pair of factors.
   • d. Fertilization gives each new individual two factors for each trait.
2. Inheriting a Single Trait (p. 442)
   • a. A capital letter indicates a dominant allele; it is expressed when present alone; example is W for widow's peak.
   • b. A lowercase letter indicates a recessive allele; it is only expressed in absence of a dominant allele; example is W for a continuous hairline. (Fig. 23.2)
3. Compare Genotype and Phenotype
   • a. Genotype
     • i. Refers to the genes of a particular individual.
     • ii. Is represented by two letters or a short descriptive phrase. (Table 23.1)
     • iii. Homozygous means both alleles for a trait are same; for example, WW stands for homozygous dominant or ww for homozygous recessive.
     • iv. Heterozygous indicates members of allelic pair are different; for example, Ww.
   • b. Phenotype
     • i. Refers to physical or observable characteristics of the individual.
     • ii. Includes microscopic and metabolic characteristics.
     • iii. Note that genotypes WW and Ww show same phenotype (widow's peak).
4. Gametes: One Allele Per Trait (p. 442)
   • a. While genotype of individual has two alleles for each trait, a gamete has only one allele for each trait due to meiosis; for example, a W or a w.
   • b. In coding genotype of gamete, no two letters should be the same.
5. One-Trait Crosses in a Square (Fig. 23.4)
   • a. A Punnett square allows calculation of probability of genotypes among offspring.
   • b. First must determine genotypes of P (parental) generation, the gametes available from each parent.
   • c. All possible combinations of sperm are aligned vertically, eggs horizontally (or vice versa).
   • d. Merged combinations for children (F = filial) are inside the intersecting boxes and represent likely proportions of offspring of each genotype and phenotype.
   • e. "Chance has no memory"; the illustrated proportions are chance occurrences between millions of sperm and the eggs.
6. One-Trait Crosses and Probability (p. 444)
   • a. The probability that two or more independent events will occur together is product (multiplication) of their chances occurring separately.
   • b. Where parents genotype is Ww, chance of gamete carrying either W or w is 1/2;
     • i. Therefore, chance of WW is 1/2 x 1/2 = 1/4
     • ii. Chance of forming Ww or wW is also 1/2 x 1/2 = 1/4.
     • iii. By summing all three possibilities, chance of an offspring with widow's peak (dominant) is 3/4 or 75%.
     • iv. Chance of an offspring with continuous hairline (recessive) is remaining 1/4 or 25% (or also probability of 1/2 x 1/2).
7. One-trait Testcross: Who's Heterozygous? (p. 444)
   • a. Testcross is made when an individual with dominant phenotype (either homozygous or heterozygous) is crossed with individual having recessive phenotype.
   • b. Examining offspring ratio reveals if dominant phenotype is homozygous or heterozygous; reveals hidden gene in heterozygous parent.
   • c. Example: Aa and AA express same phenotype and are not distinguishable.
     Cross of AA x aa yields only dominant offspring.
     Cross of Aa x aa yields likely 50% recessive homozygous offspring.
8. Inheriting Many Traits (p. 445)
   • a. Genes occurring on the same chromosome form a linkage group.
   • b. Alleles for different traits occurring on different chromosomes are not linked.
9. Assorting Independently
   • a. Mendel's second law of independent assortment states that each pair of factors segregates or assorts independently of other pairs and that all possible combinations of factors can occur in gametes.
   • b. Both law of segregation and law of independent assortment reflect random arrangement of homologous pairs of chromosomes at the equator during metaphase I; thus homologous pairs separate independently of one another.
10. Two-Trait Crosses in a Square (Fig. 23.7)
    • a. In a two-trait cross, genotypes of the parents require four letters with an allelic pair for each trait.
    • b. Gametes of parents will contain one letter of each kind in every possible combination.
    • c. To calculate probability of offspring, all possible matings are presumed to occur.
    • d. When a dihybrid (WwSs) reproduces with another dihybrid also heterozygous for these two traits, four possible gametes are formed with 16 possible offspring, producing a 9:3:3:1 phenotypic ratio.
11. Two-Trait Crosses and Probability (Fig. 23.8)
    • a. Where the monohybrid probability for either widow's peak or short fingers alone is 3/4, then the dihybrid probability of an individual having both widow's peak and short fingers is 3/4 x 3/4 = 9/16.
    • b. Probability of continuous hairline and long fingers is 1/4 x 1/4 = 1/16.
12. Two-Trait Testcross: Who's Heterozygous?
    • a. This testcross involves an individual with dominant phenotype (unknown as to whether homozygous or heterozygous) for two traits crossed with a homozygous recessive for both traits.
    • b. If any offspring express the recessive phenotype, parent with dominant phenotype must be heterozygous or carry the recessive trait.

1. Many human disorders are genetic in origin.
2. Pedigree charts can determine if pattern is inherited.
   • a. Males are represented by squares, females by circles.
   • b. Line between square and circle indicates a union.
   • c. Vertical lines extend down to children.
3. Autosomal Dominant Genetic Disorders (Fig. 23.9) (p. 448)
   • a. In this pattern, a child and at least one parent is affected; caused by a dominant allele on an autosomal (nonsex) chromosome.
   • b. Neurofibromatosis
     • i. One of most common genetics disorders(one in 3,000); also called von Reckinghausen disease.
     • ii. Small benign tumors, made up largely of nerve cells, occur under skin or on various organs; has variable expression and often mild.
     • iii. Gene isolated in 1990; found on chromosome 17.
     • iv. Gene is nested gene (three smaller genes); acts as tumor suppressor gene that controls cell division but mutates to form benign tumor.
   • c. Huntington Disease
     • i. Affects one in 20,000 persons in U.S.
     • ii. Individuals appear normal until middle-age; then have progressive degeneration of brain tissue with no treatment known.
     • iii. Due to gene on chromosome 4; gene (located in 1993) contains many more repeats of base triplets AGC than in normal persons, the more repeats, the earlier the onset of the disease.
     • iv. Triplet repeat disorders are more likely to be inherited from only the father or only the mother, a hypothesis called genomic imprinting.
4. Some Disorders Are Recessive (Fig. 23.10)
   • a. In this pattern, child is affected but neither parent is affected.
   • b. Parents are usually heterozygous (monohybrids) "carriers."
   • c. Disorder results from recessive alleles on homologous chromosomes.
   • d. Tay-Sachs Disease (p. 450)
     • i. Most common among United States Jewish people originating from central and eastern European descent.
     • ii. Affected infant appears normal; neurological impairment appears four to eight months old; blindness, seizures, paralysis usually lead to death by age three or four.
     • iii. Direct cause is lack of enzyme hexosaminidase A (Hex A); accumulation of glycosphingolipid in lysosomes mainly in brain.
     • iv. Test of tears or blood can detect lowered Hex A activity.
   • e. Cystic Fibrosis (Fig. 23.12)
     • i. Most common lethal genetic disease among Caucasians in U.S.; about one in twenty is a carrier.
     • ii. Due to chloride ions unable to pass through plasma membrane channel proteins; lack of water that follows the chloride causes mucus production to be thick and viscous which in turn impedes respiration.
     • iii. Gene is located on chromosome 7; has been inserted into lungs of living animals and may be possible to insert normal genes by inhaler in future.
   • f. Phenylketonuria (PKU)
     • i. Occurs once in 5,000 births.
     • ii. Caused by lack of enzyme to break down the amino acid phenylalanine.
     • iii. Resulting phenylketone accumulates in urine resulting in severe mental retardation.
     • iv. Universal urine test detects phenylketone in hospital.
     • v. Diet low in phenylalanine prevents damage while brain develops.
     • vi. PKU gene is on chromosome 12; prenatal DNA test can detect mutation.

1. Certain traits that do not follow the simplicity of Mendelian laws.
2. Genes That Add Up (Fig. 23.13)
   • a. Two or more sets of alleles may affect the same trait in an additive fashion.
   • b. Creates a bell-shaped curve for various characteristics; the more genes involved, the more smooth the curve.
3. Inheriting Skin Color
   • a. A couple with very dark and very light skin have children with medium brown skin.
   • b. A couple with medium brown skin have children ranging in skin color from very dark to very light.
   • c. Assuming skin color is contributed by two or more pairs of alleles, the distribution of offspring forms a bell-shaped curve.
4. Polygenic Disorders
   • a. Polygenes are probably involved in: cleft lip and palate, clubfoot, congenital dislocations of hip, hypertension, diabetes, schizophrenia, allergies and cancers.
   • b. Environmental factors also involved; poses scientific puzzle of "nature versus nurture."
5. When Multiple Alleles Control a Trait (p. 452)
   • a. Within a population, there may be three or more alleles that affect a particular trait.
   • b. Each person can have only two of alleles for that trait.
6. ABO Blood Types
   • a. Types
     - A has blood type A antigen on surface of RBC
     - B has type B antigen on RBC
     - O has no antigens on RBC
     - AB blood has both type antigens on RBC

     b. Inheritance:

     - AB has alleles A and B

     - type A has AA or AO

     - type B has BB or BO

     - type O has two O alleles and is recessive to A and B

     c. Blood test cannot prove paternity, but can be used to exclude a certain man from possible paternity lawsuits.
7. Rh Factor
   • a. Inherited separately from ABO blood type.
   • b. If a person is Rh⁺, there is a antigen on surface of RBCs; if Rh^-, it is absent.
   • c. Rh⁺ appears dominant over Rh^-.
   • d. If Rh^- woman marries an Rh⁺ man, fetus stands a possible chance of being Rh⁺.
   • e. Mother exposed to RBC of child through bleeding or childbirth, builds up antibodies that may cross the placenta to destroy future baby's RBCs.
8. Degrees of Dominance: (p. 453)
   • a. Incomplete dominance and codominance occurs when neither member of an allelic pair is dominant over other.
   • b. Curly-haired person reproduces with straight-haired person producing children with wavy-hair (Fig. 23.14).
9. Sickle-Cell Disease (Fig. 23.15)
   • a. Abnormal hemoglobin causes RBCs to be disk-shaped, clog vessels and break down, resulting in poor circulation, poor resistance to infection, and anemia.
   • b. Genotype Hb^SHb^S has sickle-cell disease, normal person has Hb^AHb^A, and Hb^AHb^S causes sickle-cell trait, a handicap under stress and oxygen depletion.
   • c. In region of origin in Africa, infants with sickle-cell die and normal infants are at high risk of malaria; the trait provides better chance of survival because malaria agent within RBC dies when potassium leaks out as cells become sickle-shaped.
   • d. Therefore, evolution favors preserving high numbers of heterozygotes in population.