Biology: Genetics

Question 1

Mendel’s 4 Principals

Answer 1

Gregor Mendel postulated four principles of inheritance.

1) Genes exist in alternative forms (now referred to as alleles). A gene controls a specific trait in an organism.
2) An organism has two alleles for each inherited trait, one inherited from each parent.
3) The two alleles segregate during meiosis, resulting in gametes that carry only one allele for any given inherited trait.
4) If two alleles in an individual organism are different, only one will by fully expressed, and the other will be silent. The expressed allele is dominant, the silent allele, recessive. In genetics problems, dominant alleles are typically assigned capital letters, and recessive alleles are assigned lowercase letters. Organisms that contain two copies of the same allele are homozygous for that trait; organisms that carry two different alleles are heterozygous. The dominant allele is expressed in the phenotype. This is known as Mendel’s Law of Dominance.

Question 2

Dihybrid Cross

Answer 2

The segregation principle provides a satisfactory explanation for the inheritance of a single gene; however, if one wants to follow more than one gene pair, the dihybrid cross also developed by Mendel can be used. The principles of the monohybrid cross can be extended to a dihybrid cross in which the parents differ in two traits, as long as the genes are on separate chromosomes and assort independently during meiosis.

Question 3

Incomplete Dominance

Answer 3

Some progeny phenotypes are apparently blends of the parental phenotypes. The classic example is flower color in snapdragons: Homozygous dominant red snapdragons, when crossed with homozygous recessive white snapdragons, produce 100% pink progeny in the F1 generation. When F1 progeny are self-crossed, the produce red, pink, and white progeny in the ratio of 1:2:1, respectively. The pink color is the result of the combined effects of the red and white genes in heterozygotes. An allele is incompletely dominant if the phenotype of the heterozygote is an intermediate of the phenotypes of the homozygotes.

Question 4

Codominance

Answer 4

Occurs when multiple alleles exist for a given gene and more than one of them is dominant. Each dominant allele is fully dominant when combined with a recessive allele, but when two dominant alleles are present, the phenotype is the result of the expression of both dominant alleles simultaneously.

The classic example of codominance and multiple alleles is the inheritance of ABO blood groups in humans. Blood type is determined by three different alleles Ia, Ib, and i. Only two alleles are present in any single individual, but the popular contains all three alleles. Ia and Ib are both dominant two i. Individuals who are homozygous for Ia and heterozygous Iai have blood type A; individuals who are homozygous Ib or heterozygous Ibi have blood type B; and individuals who are homozygous ii have blood type O. However, Ia and Ib are codominant; individuals who are heterozygous IaIb have a distinct blood type, AB, which combines characteristics of both the A and B blood groups.

Question 5

Sex Determination

Answer 5

The two members of each of the chromosome pairs are identical in shape except for one pair, the sex chromosomes. In sexually differentiated species, most chromosomes exist as pairs of homologues called autosomes, but sex is determined by a pair of sex chromosomes. All humans have 22 pairs of autosomes; additionally, women have a pair of homologous X chromosomes, and men have a pair of heterozygous chromosomes, an X and a Y chromosome. THe sex chromosomes pair during meiosis and segregate during the first meiotic division. Since females can produce only gametes containing the X chromosome, the gender of a zygote is determined by the genetic contribution of the male gamete. If the sperm carries a Y chromosome, the zygote will be male; X, female. 50/50 chance for either.

Question 6

Sex Linage

Answer 6

In humans, women have two X chromosomes, and men have only one. As a result, recessive genes that are carried on the X chromosome will produce the recessive phenotypes whenever they occur in men because no dominant allele is present to mask them. The recessive phenotype will thus be much more frequently found in men. Examples of sex-linked recessives in humans are the genes for hemophilia and for color-blindness.

The pattern of inheritance for a sex-linked recessive is somewhat complicated. Because the gene is carried on the X chromosome, and men pass the X chromosome only to their daughters, affected men cannot pass the trait to their male offspring. Affected men will pass the gene to all their daughters. However, unless the daughter also receives the gene from her mother, she will be phenotypically normal carrier of the trait. Because all of the daughter’s male children will receive their only X chromosome from her, half of her sons will receive the recessive sex-linked allele Thus, sex-linked recessives generally affect only men; they cannot be passed from father to son, but they can be passed from grandfather to grandson via a daughter who is a carrier.

Question 7

Environmental Factors

Answer 7

The environment can often affect the expression of a gene. Interaction between the environment and the genotype produces the phenotype. For example, Drosophila with a given set of genes have crooked wings at low temperatures but straight wings at higher temperatures.

Question 8

Nondisjunction

Answer 8

Is either the failure of homologous chromosomes to separate properly during meiosis I or the failure of sister chromatids to separate properly during meiosis II. The resulting zygote might either have 3 copies of that chromosome, called trisomy (somatic cells will have 2N + 1 chromosomes), or might have a single copy of that chromosome, called monosomy (somatic cells will have 2N - 1 chromosomes). A classic case of trisomy is the birth defect Down’s syndrome, which is called by trisomy of chromosome 21. Most monosomies and trisomie are lethal, causing the embryo to spontaneously abort early in pregnancy.

Nondisjunction of the sex chromosomes may also occur, resulting in individuals with extra or missing copies of the X or Y chromosomes.

Question 9

Chromosomal Breakage

Answer 9

May occur spontaneously of be induced by environmental factors, such as mutagenic agents and X rays. The chromosome that loses a fragment is said to have a deficiency.

Question 10

Mutations

Answer 10

Changes in the genetic information of a cell coded in the DNA. Mutations that occur in somatic cells can lead to tumors in the individual. Mutations that occur in the sex cells (gametes) will be transmitted to the offspring. Most mutations occur in regions of DNA that do not code for proteins and are silent (not expressed in the phenotype). Mutations that do change the sequence of amino acids in proteins are most often recessive and deleterious.

Question 11

Mutagenic Agents

Answer 11

They induce mutations. These include cosmic rays, X rays, ultraviolet rays, and radioactivity, as well as chemical compounds such as colchicine (which inhabits spindle formation, thereby causing polyploidy) or mustard gas. Mutagenic agents are sometimes also carcinogenic.

Question 12

Gene Mutation

Answer 12

In a gene mutation, nitrogen bases are added, deleted, or substituted, thus altering the amino acid sequence; inappropriate amino acids may be inserted into polypeptide chains, and a mutated protein may be produced. Therefore, a mutation is a genetic “error” with the “wrong” base or no base on the DNA at any particular position.

Question 13

Point Mutation

Answer 13

A nucleic acid is replaced by another nucleic acid. The number of nucleic acids substituted may vaery, but generally point mutations involve between one and three nucleotides.

Question 14

Frameshift Mutation

Answer 14

Nucleic acids are deleted or inserted into the genome sequence. This frequency is lethal. The insertion or deletion of nucleic acids throws off the entire sequence of codons from that point on because the genome is “read” in groups of three nucleic acids. Since nucleic acids are inserted or deleted, the length of the genome changes.

Question 15

Sickle-cell anemia

Answer 15

A disease in which red blood cells become crescent-shaped because they contain defective hemoglobin. The sicke-cell hemoglobin carries less oxygen. This disease is caused by a substitution of valine (GUA or GUG) for glutamic acid (GAA or GAG) because of a single base-pair substitution in the gene coding for hemoglobin.

Question 16

Cytoplasmic Inheritance

Answer 16

Heredity systems exist outside the nucleus. For example, DNA is found in mitochondria and other cytoplasmic bodies. These cytoplasmic genes may interact with nuclear genes and are important in determining the characteristics of their organelles. Drug resistance in many microorganisms is regulated by cytoplasmic DNA, known as plasmids, that contain one or more genes.

Question 17

Bacterial Genome

Answer 17

The bacterial genome consists of a single circular chromosome located in the nucleoid region of the cell. Many bacteria also contain smaller circular rings of DNA called plasmids, which contain accessory genes.

Question 18

Bacterial DNA Replication

Answer 18

Replication of the bacterial chromosome begins at a unique origin of replication and proceeds in both directions simultaneously. DNA is synthesized in the 5’ to 3’ direction.

Question 19

Bacterial Genetic Variance

Answer 19

Bacterial cells reproduce by binary fission and proliferate very rapidly under favorable conditions. Although binary fission is an asexual process, bacterial have 3 mechanisms for increasing the genetic variance of a population: transformation, conjugation, and transduction.

Question 20

Transformation

Answer 20

The process by which a foreign chromosome fragment (plasmid) is incorporated into the bacterial chromosome via recombination, creating new inheritable genetic combinations.

Question 21

Conjugation

Answer 21

Described as sexual mating in bacteria; it is the transfer of genetic material between two bacteria that are temporarily joined. A cytoplasmic conjugation bridge is formed between the two cells, and genetic material is transferred from the donor male (+) type to the recipient female (-) type. Only bacteria containing plasmids called sex factors are capable of conjugating. The best studied sex factor is the F factor in E. coli. Bacteria possessing this plasmid are termed F+ cell; those without it are called F- cells. During conjugation between an F+ and an F- cell, the F+ cell replicates its F factor and donates the copy to the recipient, converting it to an F+ cell. Genes that code for other characteristics, such as antibody resistance, may be found on the plasmids and transferred into recipient cells along with these factors.

Sometimes the sex factor becomes integrated into the bacterial genome. During conjugation, the entire bacterial chromosome replicates and begins to move from the donor cell into the recipient cell. The conjugation bridge usually breaks before the entire chromosome is transferred, but the bacterial genes that enter the recipient cell can easily recombine with the genes already present to form novel genetic combinations. These bacteria are called Hfr cells, meaning that they have a high frequency of recombination.

Question 22

Transduction

Answer 22

Occurs when fragments of the bacterial chromosome accidentally become packaged into viral progeny produced during a viral infection. These virions may infect other bacteria and introduce new genetic arrangements through recombination with the new host cell’s DNA. The closer two genes are to one another on a chromosome, the more likely they will be to transduce together. This fact allows geneticists to map genes to a high degree of precision.

Question 23

Recombination

Answer 23

Occurs when linked genes are separated. It occurs by breakage and rearrangements of adjacent regions of DNA when organisms carrying different genes or alleles for the same traits are crossed.

Question 24

Transcription

Answer 24

The regulation of gene expression. It enables prokaryotes to control their metabolism. Regulation of transcription is based on the accessibility of RNA polymerase to the genes being transcribed and is directed by an operon, which consists of structural genes, an operator region, and a promoter region on the DNA before the protein coding genes. Structural genes contain sequences of DNA that code for proteins. The operator is the sequence of nontranscribable DNA that is the repressor binding site. The promoter is the noncoding sequence of DNA that serves as the initial binding site for RNA polymerase. There is also a regulator gene, which codes for the synthesis of a repressor molecule that binds to the operator and blocks RNA polymerase from transcribing the structural genes.

RNA polymerase must also move past the operator to transcribe the structural genes. Regulatory systems function by preventing or permitting the RNA polymerase to pass on to the structural genes. Regulation may be via inducicle systems or repressible systems. Inducible systems are those that require the presence of a substance, called an inducer, for transcription to occur. Repressible systems are in a constant state of transcription unless a corepressor is present to inhibit transcription.

Question 25

Inducible Systems

Answer 25

The repressor binds to the operator, forming a barrier that prevents RNA polymerase from transcribing the structural genes. For transcription to occur, an inducer must bind to the repressor, forming an inducer-repressor complex. This complex cannot bind to the operator, thus permitting transcription. The proteins synthesized are thus said to be inducible. The structural genes typically code for an enzyme, and the inducer is usually the substate, or a derivative of the substate, upon which the enzyme normally acts. When the substrate (inducer) is present, enzymes are synthesized; when it is absent, enzyme synthesis is negligible. Int his manner, enzymes are transcribed only when they are actually needed.

Question 26

Repressible Systems

Answer 26

The repressor is inactive until it combines with the corepressor. The repressor can bind to the operator and prevent transcription only when it has formed a repressor-corepressor complex. Corepressors are often the end-products of the biosynthetic pathways they control. The proteins produced (usually enzymes) are said to be repressible because they are normally being synthesized; transcription and translation occur until the corepressor is synthesized. Operons containing mutations such as deletions or whose regulator genes code for defective repressors are incapable of being turned off; their enzymes, which are always being synthesized, are referred to as constitutive.

Question 27

Bacteriophage

Answer 27

A virus that infects its host bacterium by attaching to it, boring a hole through the bacterial cell wall, and injecting its DNA while its protein coat remains attached to the cell wall. Once inside its host, the bacteriophage enters either a lytic or a lysogenic cycle.

Question 28

Lytic Cycle

Answer 28

THe phage DNA takes control of the bacterium’s genetic machinery and manufacturs numerous progeny. The bacterial cell then lyses, releasing new virions, each capable of infecting other bacteria. Bacteriophages that replicate by the lytic cycle, killing their host cells, are called virulent. If the initial infection takes place on a bacterial lawn (a plated culture), then very shortly a plaque or clearing in the lawn occurs, corresponding to the area of lysed bacteria. The physical characteristics of a plaque are useful in identifying mutant phage strains that may arise.

Question 29

Lysogenic Cycle

Answer 29

If the bacteriophage does not lyse its host cell, it becomes integrated into the bacterial genome in a harmless form (provirus), lying dormant for one or more generations. The virus may stay integrated indefinitely, replicating along with the bacterial genome. However, either spontaneously or as a result of environmental circumstances (e.g. radiation, ultraviolet light, or chemicals), the provirus can reemerge and enter a lytic cycle. Bacteria containing proviruses are normally resistant to further infection (superinfection) by similar phages.

Question 30

Law of Independent Assortment

Answer 30

Genes on the same chromosome will stay together unless crossing over occurs. Crossing over exchanges information between chromosomes and may break the linkage of certain patterns. For example, red hair is usually linked with freckles, but some blonds and brunettes have freckles as well.

This is known as Mendel’s Law of Independent Assortment.

Question 31

Effects of Point Mutation

Answer 31

There are three possible effects on the codon, the sequence of the three nucleotides that determines the identity of the amino acid. First, the new codon may code for the same amino acid (a silent mutation). Second, the new codon may code for a different amino acid (a missense mutation). Finally, the new codon may be a stop codon (a nonsense mutation). Point mutations may not be lethal if they code for the same amino acid or if the amino acid is noit crucial to the functioning of the protein. The length of the genome does not change.

Question 32

Episomes

Answer 32

Episomes are plasmids that are capable of integration into the bacterial genome.