Vocabulary Concepts of Chapter 13-14

1. adenine:  A purine; a nitrogen-containing base in certain nucleotides.

2. bacteriophage : Category of viruses that infect bacterial cells.

3. cloning:Making a genetically identical copy of DNA or of an organism.

4.    cytosine: Pyrimidine;one of the nitrogen-containing bases in nucleotides.

5.  deoxyribonucleic acid (DNA) : The molecule of inheritance.

6. DNA ligase:  Enzyme that seals new base-pairings during DNA replication.

7.     DNA polymerase: Enzyme of replication and repair that assembles a new strand of DNA on a parent DNA template.

8. DNA repair: Enzyme-mediated process that fixes small-scale alterations in a DNA strand by restoring the original base sequence.

9.  DNA replication:  Any process by which a cell duplicates its DNA molecules before dividing.

10.  guanine: Nitrogen-containing base in nucleotide monomers of DNA or RNA.

11.   nucleotide: Small organic compound with deoxyribose (a five-carbon sugar), a nitrogenous base, and a phosphate group. 

12.   thymine: A nitrogen-containing base; one of the nucleotides in DNA.

13.   x-ray diffraction image: Pattern that forms on film exposed to x-rays that have been directed at a molecule.

14.   anticodon: Series of three nucleotide bases in tRNA; can base-pair with an mRNA codon.

15.  base sequence: Sequential order of bases in a DNA or RNA strand.

16.  base-pair substitution  One amino acid has replaced another during protein synthesis.

17. carcinogen:  Any substance or agent that can trigger cancer.

18.   codon: One of 64 possible base triplets in an mRNA strand.

19.    deletion: At cytological level, loss of a segment from a chromosome. At molecular level, loss of one to a few base pairs from a DNA molecule.

20.   exon:One of the base sequences of an mRNA transcript that will become translated.

21.   gene mutation: A small-scale change in the nucleotide sequence of a DNA molecule.

22.   genetic code: The correspondence between nucleotide triplets in DNA  and specific sequences of amino acids in a polypeptide chain.

23.   insertion: Insertion of one to a few bases into a DNA strand.

24.    intron   A noncoding portion of a pre-mRNA transcript.

25.   ionizing radiation: High-energy wavelengths.

26. mRNA (messenger RNA):  A single strand of ribonucleotides transcribed from DNA, then translated into a polypeptide chain.

27.   mutation rate: Of a gene locus, the probability that a spontaneous mutation will occur during or between DNA replication cycles.

Lecture Outline Chapter 12

12.1   Chromosomes and Inheritance

A.                 Genes and Their Chromosome Locations 

1.                 Genes are units of information about heritable traits.

2.                 Diploid organisms possess pairs of homologous chromosomes, which are alike in length, shape, and gene sequence.

3.                 Alleles are slightly different molecular forms of the same gene, which are shuffled during meiosis.

4.                 Crossing over between homologous chromosomes results in genetic recombination.

5.                 Independent assortment refers to the random alignment of each pair of homologous chromosomes at metaphase I of mitosis, which results in new combinations of genes in offspring.

6.                 A chromosome's structure may change during mitosis or meiosis.

B.                 Autosomes and Sex Chromosomes 

1.                 Sex chromosomes determine gender. 

a.                Human females have two X chromosomes.

b.                Males have one X and one Y.

2.                 Most of the chromosomes are of the same quantity and type in both sexes and are called autosomes (44 in humans).

12.2   Focus on Science: Karyotyping Made Easy

12.3   Sex Determination in Humans

A.                 Each human egg will contain twenty-two autosomes plus one X; but sperm will carry twenty-two autosomes plus either an X or a Y. 

1.                 X-bearing egg plus X-bearing sperm produces female offspring.

2.                 X-bearing egg plus Y-bearing sperm produces male offspring.

B.                 The X chromosome obviously codes for sexual traits, but it also carries many genes for nonsexual traits. 

1.                 The Y chromosome carries a male-determining (SRY) gene which leads to formation of the testes.

2.                 Absence of the male gene in females results in formation of ovaries.

12.4   What Mendel Didn't Know: Crossovers and Recombinations

A.                 Linked genes on specific chromosomes are referred to as linkage groups. 

1.                 In his experiments using fruit flies, Thomas Hunt Morgan confirmed that each gene has a specific location on a chromosome.

2.                 Some of the most intriguing linkages are those of X-linked and Y-linked genes.

B.                 Linkage is the tendency of genes located on the same chromosome to be transmitted together in inheritance. 

1.                 Linkage can be disrupted by crossing over--the exchange of parts of homologous chromosomes. 

a.                Certain alleles that are linked on the same chromosome tend to remain together during meiosis because they are positioned closer together on the chromosome.

b.                This eventually led to the generalization that the probability that a cross over will disrupt the linkage of two genes is proportional to the distance that separates them.

2.                 The careful analysis of recombination patterns in experimental crosses has resulted in linkage mapping of gene locations.

12.5   Human Genetic Analysis

A.                 Human genetics is difficult to study. 

1.                 We live under variable conditions in diverse environments.

2.                 Humans mate by chance and may, or may not, choose to reproduce.

3.                 Humans live as long as those who study them.

4.                 The small family size characteristic of human beings is not sufficient for meaningful statistical analysis.

B.                 Constructing Pedigrees 

1.                 A pedigree is a chart that shows genetic connections among individuals.

2.                 The analysis of family pedigrees provides data on inheritance patterns through several generations.

3.                 Knowledge of probability and Mendelian inheritance patterns is used in analysis of pedigrees to yield clues to a trait's genetic basis.

C.                 Regarding Human Genetic Disorders 

1.                 Genetic abnormality is a term applied to a genetic condition that is a deviation from the usual, or average, and is not life-threatening.

2.                 Genetic disorder is more appropriately used to describe conditions that cause medical problems.

3.                 Genetic disease is applied to those instances where a person's genes increase susceptibility to infection or weakens the response to it.

12.6   Examples of Inheritance Patterns

A.                 Autosomal Recessive Inheritance 

1.                 The characteristics of this condition are: 

a.                Either parent can carry the recessive allele on an autosome.

b.                Heterozygotes are symptom-free; homozygotes are affected.

c.                 Two heterozygous parents have a 50 percent chance of producing heterozygous children and a 25 percent chance of producing a homozygous recessive child. When both parents are homozygous, all children can be affected.

2.                 Galactosemia (the inability to metabolize lactose) is an example of autosomal recessive inheritance in which a single gene mutation prevents manufacture of an enzyme needed in the conversion pathway.

B.                 Autosomal Dominant Inheritance 

1.                 The dominant allele is nearly always expressed and if it reduces the chance of surviving or reproducing, its frequency should decrease; mutations, nonreproductive effects, and postreproductive onset work against this hypothesis.

2.                 If one parent is heterozygous and other homozygous recessive, there is a 50 percent chance that any child will be heterozygous.

3.                 Huntington disease is serious degeneration of the nervous system with an onset from age 40 onward, by which time the gene has (usually) been passed to offspring unknowingly.

4.                 Achondroplasia (dwarfism) is a benign abnormality which does not affect persons to the point that reproduction is impossible so the gene is passed on.

C.                 X-Linked Recessive Inheritance 

1.                 The characteristics of this condition are: 

a.                The mutated gene occurs only on the X chromosome.

b.                Heterozygous females are phenotypically normal; males are more often affected because the single recessive allele (on the X chromosome) is not masked by a dominant gene.

c.                 A normal male mated with a female heterozygote have a 50 percent chance of producing carrier daughters and a 50 percent chance of producing affected sons. In the case of a homozygous recessive female and a normal male, all daughters will be carriers and all sons affected.

2.                 A serious X-linked recessive condition is hemophilia A, (affecting 1/7,000 males), which is the inability of the blood to clot because the genes do not code for the necessary clotting agent(s).

3.                 Males with fragile X syndrome have a defective X chromosome that produces a faulty protein that results in retarded brain development.

12.7   Focus on Health: Progeria--Too Young to Be Old

12.8   Changes in Chromosome Structure

A.                 Major Categories of Structural Change 

1.                 Duplication occurs when a gene sequence is in excess of the normal amount; apparently this is true of chromosome regions that code for polypeptides of hemoglobin and is not harmful.

2.                 An inversion alters the position and sequence of the genes so that gene order is reversed.

3.                 A translocation occurs when a part of one chromosome is transferred to a nonhomologous chromosome as in form of leukemia in which a segment of chromosome 9 is attached to chromosome 22.

4.                 A deletion is the loss of a chromosome segment as when a terminal segment is lost, or when viruses, chemicals, or irradiation cause breaks in a chromosome region; an example is the loss of a portion of chromosome 5 causing a disorder called cri-du-chat with its symptoms of crying and mental retardation.

B.                 Does Chromosome Structure Evolve? 

1.                 Changes in chromosome structure tend to be selected against rather than conserved over evolutionary time.

2.                 However, gene regions for the polypeptide chains of hemoglobin have duplicated to produce different hemoglobins with different oxygen transporting efficiencies.

12.9   Changes in Chromosome Number

A.                 Categories and Mechanisms of Change 

1.                 Aneuploidy is a condition in which the gametes or cells of an affected individual end up with one extra or one less chromosome than is normal.

2.                 Polyploidy is the presence of three or more of each type of chromosome in gametes or cells. It is common in plants but fatal in humans. 

a.                A chromosome number can change during mitotic or meiotic cell division or during the fertilization process.

b.                Tetraploid germ cells can result if cytoplasmic division does not follow normal DNA replication and mitosis.

3.                 Nondisjunction at anaphase I or anaphase II frequently results in a change in chromosome number. 

a.                If a gamete with an extra chromosome (n + 1) joins a normal gamete at fertilization, the diploid cell will be 2n + 1; this condition is called trisomy.

b.                If an abnormal gamete is missing a chromosome, the zygote will be 2n - 1--monosomy.

B.                 Case Study: Down Syndrome 

1.                 Down syndrome results from trisomy 21; 1 in 1,100 liveborns in North America are affected.

2.                 Most children with Down syndrome show mental retardation, and 40 percent have heart defects.

3.                 Down syndrome occurs more frequently in children born to women over age 35.

12.10   Case Studies: Changes in the Number of Sex Chromosomes

A.                 Female Sex Chromosome Abnormalities 

1.                 Turner Syndrome 

a.                Turner syndrome involves females whose cells have only one X chromosome (designated XO).

b.                Affected individuals (1/2,500 to 10,000 girls) are infertile and have other phenotypic problems such as premature aging and shorter life expectancy.

c.                 About 75 percent of the cases are due to nondisjunction in the father; furthermore, about 98 percent of all XO zygotes spontaneously abort.

2.                 XXX Syndrome 

a.                About 1 in 1,000 females inherits 3, 4, or 5 X chromosomes.

b.                Most of these girls are taller and slimmer than average, but are fertile and fall within the normal range of appearance and social behavior.

B.                 Male Sex Chromosome Abnormalities 

1.                 Klinefelter Syndrome 

a.                Nondisjunction results in an extra X chromosome in the cells (XXY) of these affected males (1/500 to 2,000 liveborn males).

b.                About 67 percent of these result from nondisjunction in the mother, 33 percent in the father.

c.                 Sterility, slight mental retardation, and body feminization are symptoms.

2.                 XYY Condition 

a.                The extra Y chromosome in these males (1/1,000) does not affect fertility, but they are taller than average and are slightly mentally retarded.

b.                Erroneous correlations have linked these persons with predisposition to crime.

Ch 11: Pattern of Inheritance

A Smorgasbord of Ears and Other Traits

A.                 The observable traits, such as attached or unattached earlobes, are the result of genetic expression.

B.                 Gregor Mendel was the first person to systematically pursue the questions of genetic.

11.1   Mendel's Insight Into Inheritance Patterns

A.                 Inheritance has always been intriguing to humans. 

1.                 By the late nineteenth century, natural selection suggested that a population could evolve if members showed variation in heritable traits. Variations that improved survival chances would be more common in each generation--in time, the population would change or evolve.

2.                 The theory of natural selection did not fit with the prevailing view of inheritance--blending. 

a.                 Blending would produce uniform populations--such populations could not evolve.

b.                 Many observations did not fit blending--for example, a white horse and a black horse did not produce only gray offspring.

B.                 Mendel's Experimental Approach 

1.                 Gregor Mendel used experiments in plant breeding and a knowledge of mathematics to form his hypotheses.

2.                 Mendel used the garden pea in his experiments. 

a.                 This plant can fertilize itself; true-breeding varieties were available to Mendel.

b.                 Peas can also be cross-fertilized by human manipulation of the pollen.

3.                 Mendel cross-fertilized true-breeding garden pea plants having clearly contrasting traits (example: white vs. purple flowers).

C.                Some Terms Used in Genetics 

1.                 Genes are units of information about specific traits.

2.                 Each gene has a locus on a chromosome.

3.                 Diploid cells have two genes (a gene pair) for each trait--each on a homologous chromosome.

4.                 Alleles are various molecular forms of a gene for the same trait.

5.                 True-breeding lineage occurs when offspring inherit identical alleles, generation after generation; non-identical alleles produce hybrid offspring.

6.                 When both alleles are the same, the condition is called the homozygous condition; if the alleles differ, then it is the heterozygous condition.

7.                 When heterozygous, one allele is dominant (A), the other is recessive (a).

8.                 Homozygous dominant = AA, homozygous recessive = aa, and heterozygous = Aa.

9.                 Genotype is the sum of the genes, and phenotype is how the genes are expressed (what you observe).

10.            P = parental generation; F₁ = first-generation offspring; F₂ = second-generation offspring.

11.2   Mendel's Theory of Segregation

A.                 Predicting Outcomes of Monohybrid Crosses 

1.                 Mendel suspected that every plant inherits two "units" (genes) of information for a trait, one from each parent.

2.                 Mendel's first experiments were monohybrid crosses. 

a.                 Monohybrid crosses have two parents that are true-breeding for contrasting forms of a trait.

b.                 One form of the trait disappears in the first generation offspring (F₁), only to show up in the second generation.

c.                  We now know that all members of the first generation offspring are heterozygous because one parent could produce only an A gamete and the other could produce only an a gamete.

3.                 Results of the F₂ generation required mathematical analysis. 

a.                 The numerical ratios of crosses suggested that genes do not blend.

b.                 For example, the F₂ offspring showed a 3:1 phenotypic ratio.

c.                  Mendel assumed that each sperm has an equal probability of fertilizing an egg. This can be seen most easily by using the Punnett square.

d.                 Thus, each new plant has three chances in four of having at least one dominant allele.

B.                 Testcrosses 

1.                 To support his concept of segregation, Mendel crossed F₁ plants with homozygous recessive individuals.

2.                 A 1:1 ratio of recessive and dominant phenotypes supported his hypothesis.

C.                Mendel's Theory of Segregation 

1.                 The Mendelian theory of segregation states that 2n organisms inherit two genes per trait located on pairs of homologous chromosomes.

2.                 During meiosis the two genes segregate from each other such that each gamete will receive only one gene per trait.

11.3   Independent Assortment

A.                 Predicting Outcomes of Dihybrid Crosses 

1.                 Mendel also performed experiments involving two traits--a dihybrid cross. 

a.                 Mendel correctly predicted that all F₁ plants would show both of the dominant alleles (example: all purple flowers and all tall).

b.                 Mendel wondered if the genes for flower color and plant height would travel together when two F₁ plants were crossed.

2.                 We now know that genes located on nonhomologous chromosomes segregate independently of each other and give the same phenotypic ratio as Mendel observed--9:3:3:1.

B.                 The Theory in Modern Form 

1.                 The Mendelian theory of independent assortment states that during meiosis each gene of a pair tends to assort into gametes independently of other gene pairs located on nonhomologous chromosomes.

2.                 Mendel reported his ideas on heredity to the Brunn Society in 1865 and published them a year later. 

a.                 Few people understood his principles or took note of them.

b.                 He died in 1884 unaware of the revolutionary impact his ideas would have.

11.4   Dominance Relations

A.                 Incomplete Dominance 

1.                 In incomplete dominance, a dominant allele cannot completely mask the expression of another.

2.                 For example, a true-breeding red-flowered snapdragon crossed with a white-flowered snapdragon will produce white flowers because there is not enough red pigment (produced by the dominant allele) to completely mask the effects of the white allele.

B.                 ABO Blood Types: A Case of Codominance 

1.                 In codominance, both alleles are expressed in heterozygotes (for example, humans with both proteins are designated with blood type AB).

2.                 Whenever more than two forms of alleles exist at a given locus, it is called a multiple allele system. In this instance it results in four blood types: A, B, AB, and O.

  

11.5   Multiple Effects of Single Genes

A.                 Sometimes the expression of alleles at one location can have effects on two or more traits; this is termed pleiotropy.

B.                 An excellent example of this phenomenon is the disorder known as Marfan syndrome. 

1.                 The gene for codes for a variant form of fibrillin 1, a protein in the extracellular matrix of connective tissues.

2.                 The altered fibrillin 1 causes a weakening of connective tissues throughout the body.

3.                 Marfan syndrome is characterized by these effects: lanky skeleton, leaky heart valves and weakened blood vessels, deformed air sacs in lungs, pain, lens displacement in the eyes.

11.6   Interactions Between Gene Pairs

A.                 One gene pair can influence other gene pairs, with their combined activities producing some effect on phenotype; this called epistasis.

B.                 Hair Color in Mammals 

1.                 In Labrador retrievers, one gene pair codes for the quantity of melanin produced while another codes for melanin deposition.

2.                 Still another gene locus determines whether melanin will be produced at all--lack of any produces an albino (recessive).

C.                Comb Shape in Poultry 

1.                 Sometimes interaction between two gene pairs results in a phenotype that neither pair can produce alone.

2.                 Comb shape in chickens is of at least four types depending on the interactions of two gene pairs (R and P).

11.7   How Can We Explain Less Predictable Variations?

A.                 Regarding the Unexpected Phenotype 

1.                 Tracking even a single gene through several generation may produce results that are different than expected.

2.                 Camptodactyly (immobile, bent fingers) can express itself on one hand only, both hands, or neither due the possibility that a gene product is missing in one of the several steps along the metabolic pathway.

B.                 Continuous Variation in Populations 

1.                 A given phenotype can vary, by different degrees, from one individual to the next in a population. 

a.                 This is the result of interactions with other genes, and environmental influences.

b.                 In humans, eye color and height are examples.

2.                 Most traits are not qualitative but show continuous variation and are transmitted by quantitative inheritance.

11.8   Environmental Effects on Phenotype

A.                 Fur on the extremities of certain animals will be darker because the enzyme for melanin production will operate at cooler temperatures but is sensitive to heat on the rest of the body.

The color of the floral clusters on Hydrangea plants will vary depending on the acidity of the soil.