Chapter 8 - Genetics

Question 1

What is genetics the study of?

Answer 1

Genetics is the study of how traits are inherited from one generation to the next

Question 2

What is the basic unit of heredity?

Answer 2

The gene

Question 3

What are genes composed of and where are the located?

Answer 3

DNA and on chromosomes

Question 4

What is it called when a gene exists in more than one form?

Answer 4

Alleles

Question 5

What is a genotype?

Answer 5

Genetic makeup of an individual

Question 6

What is a phenotype?

Answer 6

Physical makeup of an individual

- Some phenotypes correspond to a single genotype, whereas some correspond several different genotypes

Question 7

Who developed the basic principles of genetics? Through what?

Answer 7

Gregor Mendel

- Through his garden pea experiments

Question 8

What did Mendel do during these garden pea experiments?

Answer 8

Genetic Crosses
- Took true breeding individual with different traits, mated them, and statistically analyzed the inheritance of the traits in the progeny

Question 9

What is Mendels First Law?

Answer 9

Law of Segregation - 4 Principles:

1) Genes exist in alternative forms (alleles); a gene controls a specific trait in an organism
2) An organism has 2 alleles for each inherited trait; one inherited from each parent
3) Two alleles segregate during meiosis, resulting in gametes that only carry one allele for any given inherited trait

4) If two alleles in an individual are different, only one will be fully expressed and the other will be silent
- Dominant is expressed
- Recessive is silent
- Homozygous: contain 2 copies of same allele
- Heterozygous: contain 2 different alleles
- I.e. Mendel’s Law of Dominance

Question 10

What is a monohybrid cross?

Answer 10

A cross between two true breeding parents

Question 11

What is a punnett square?

Answer 11

A grid to predict phenotype and genotype of progeny

Question 12

What is a test cross?

Answer 12

Diagnostic tool to determine genotype of organism

• Only with a recessive phenotype can genotype be predicted with 100% accuracy
• Homozygous recessive alway breed true

Question 13

What is Mendel’s Second Law?

Answer 13

Law of Independent Assortment

• Dihydrid crosses extend to parents who differ in two traits
• Crossing over exchanges info between chromosomes
• Heterozygote crosses have a 9:3:3:1 ratio

Question 14

What are the 6 non-mendelian inheritance patterns?

Answer 14

1) Incomplete Dominance
- Some progeny phenotypes are apparently blends of parental phenotypes
- An allele is incompletely dominant if the phenotype of the heterozygote is an intermediate of the phenotypes of the homozygotes

2) Co-dominance
- 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 expression of both dominant alleles simultaneously
- Differs from #1 because both alleles in the genotype are expressed at the same time without blending phenotype
- Ex. ABO-Blood Groups

3) Sex Determination
- In sexual differentiated species most chromosomes exist are pairs of homologues called autosomes but sex is determined by a pair of sex chromosomes
- Humans have 22 autosomes; women have homologues X chromosomes and men have heterologous XY chromosomes
- These chromosomes pair during meiosis and segregate during first meiotic division
- Gender is determined by male (50% chance M or F)
- Genes located here are called sex-lined (mostly located on F)

4) Sex Linkage
- Recessive genes carried on X and therefore found more frequently in men (ex. hemophilia and colour-blindness)
- Since carried on X, men cannot pass down their male offspring but only female
- Can be passed from father to grandson via daughter who is a carrier (skips a generation)

5) Environmental Factors
- Temperature influences hair follow (ex. Hares)

6) Cytoplasmic Inheritance
- Heredity systems exist outside nucleus
- DNA in mitochondria and cytoplasmic bodies
- May interact with nuclear genes and important to determine characteristics of organelles

Question 15

What are genetic problems? What does it result in?

Answer 15

Chromosome number and structure alterations by abnormal cell division during meiosis or by mutagenic agents
- Results in appearance of abnormal characteristics in offspring

Question 16

What is non-disjunction? What does it result in?

Answer 16

Either the failure of homologues chromosomes to separate properly during meiosis 1 or the failure of sister chromatids to separate properly during meiosis 2

• May result in the zygote having 3 copies of a chromosomes (trisomy - ex. down syndrome) or a single copy of that chromosome (monosomy)
• Potential abortion of fetus
• Can also occur on sex chromosomes giving an extra or missing X or Y chromosome

Question 17

What is a chromosomal breakage?

Answer 17

Chromosomal breakage occurs spontaneously or be induced by environmental factors (ex. x-rays)

Question 18

What are mutations?

Answer 18

Changes in the genetic info coded in the DNA of a cell

Question 19

What if a mutation occurs in a somatic cell?

Answer 19

May lead to tumors

Question 20

What if a mutation occurs in the sex cells?

Answer 20

Mutation will be passed down to offspring

Question 21

What are mutagenic agents?

Answer 21

They induce mutations

• X-rays
• UV rays
• Mustard gas
• Carcinogens

Question 22

What are the types of mutations? What does it do?

Answer 22

Nitrogen bases are:

1) Added
2) Deleted
3) Substituted

This can cause the amino acid sequence to alter

Question 23

Describe the types of mutations:

Answer 23

1) Point mutation: Nucleic acid is replace by another nucleic acid - three possible effects:
a) Silent = no change
b) Mis-sense = codes for a different amino acids
c) Non-sense = codes for a stop codon (often lethal)

2) Frame Shift Mutation: Nucleic acids are deleted or inserted into the sequence
- Throws off entire sequence of codons

Question 24

What are 2 examples of genetic disorders?

Answer 24

1) Phenyketonuria (PKU)
- Molecular disease caused by inability to produce proper enzyme for the metabolism of phenylalanine

2) Sickle-cell anemia
- Disease in which RBC become crescent shaped because they contain defective hemoglobin
- Caused via substitution of valine for glutamic acid

Question 25

What is a bacterial genome? What is a plasmid? What is an episome?

Answer 25

Consists of a singular circular chromosome located in the nucleoid region of the cells

• Contain smaller circular rings of DNA (plasmids) that contain accessory genes
• Episomes are plasmids capable of integration into bacterial genome

Question 26

How does bacteria replicate?

Answer 26

Begins at unique origin of replication and proceeds in both direction simultaneously (5’ to 3’ direction)

Question 27

Describe bacterial genetics in relation to genetic variance:

Answer 27

Bacterial cells reproduce via binary fission and proliferate very rapidly under favourable conditions

Question 28

What are the 3 mechanisms bacteria have to increase genetic variance?

Answer 28

1) Transformation: Foreign chromosome fragment (plasmid) is incorporated into the bacterial chromosome via recombination, creating new inheritable genetic combination

2) Conjugation (sexual mating): Transfer of genetic material between two bacteria that are temporarily joined
- Cytoplasmic bridge formed between the two and genetic material is transferred from donor male (+) to recipient female (-)
- Only bacterial with plasmids (sex factors) can do so
- If sex factor is integrated into the bacterial genome, during conjugation, the entire bacterial chromosome replicate and moves from donor to recipient cell… the bridge usually breaks before entire chromosome is transferred but the bacterial genes that enter recipient cell easily recombine with genes present (Hfr cells - high frequency of recombination)

3) Transduction: Occurs when fragements of the bacterial chromosome become packaged into the viral progeny produced
- Bacteriophase is a virus that infect host by attaching to bacterium, boring a hole through bacterial cell wall, and injected its DNA
- These virons may infect other bacteria and introduce new genetic arrangements through recombination within the host
- Closer two genes are to one another on a chromosome… the more like they will be transducer together - this allows geneticists to map genes to a high degrees of precision

Question 29

What is recombination?

Answer 29

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

Question 30

Describe gene regulation (transcription):

Answer 30

Regulation of transcription enables prokaryotes to control their metabolism

• Regulation of transcription is based on 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 promotor region on the DNA before the protein coding genes
• Structural genes contain sequences of DNA that code for proteins
• Operator is the sequence of non-transcribable DNA that is the repressor binding site
• Promoter is the non-boding sequence of DNA that serves as the initial binding site for RNA polymerase
• Regulator genes code for the synthesis of a repressor molecule that binds to the operator and blocks RNA polymerase from transcribing the structural genes
• RNA polymerase must move past operator to transcribe structural genes
• Regulatory systems function by preventing or permitting RNA polymerase to pass on to structural genes

Question 31

What are Inducible vs. Repressible Systems?

Answer 31

1) Inducible: Those that require the presence of a substance for transcription to occur
- Repressor binds to operator forming a barrier to prevent RNA polymerase from transcribing structural genes
- For transcription to occur, an inducer must bind to the repressor (inducer-repressor complex)
- This complex cannot bind to the operator, and thus removing it as a barrier and permitting transcription
- Proteins synthesized are said to be inducible
- Structural genes code for an enzyme and the inducer is usually the substrate, upon which the enzyme normally acts
- When substrate is present, enzymes are synthesized
- When substate is absent, enzymes synsytheis is negligible
- Therefore, enzymes are only transcribed when needed

2) Repressible: In a constant state of transcription less a co-repressor is present to inhibit transcription
- Repressor is active until it combines with co-repressor
- Repressor can bind to operator and prevent transcription only when it has formed repressor-corepressor complex
- Co-repressors are often the end products of the pathways they control
The proteins produced are repressible because they are normally being synthesizes; transcription and translation occur until co-repressor is synthesized
- Operons containing mutations are incapable of being turned off, their enzymes are always being synthesized (constitutive)