Inheritance and Mutations

Question 1

What is a Mutation?

Answer 1

Mutation is the source of all genetic variation and the term “mutation” refers to:

• A change in the genetic material
• The process by which the change occurs

Question 2

What are the types of mutations?

Answer 2

• Changes in chromosome number and structure

- Point mutations - changes at specific sites in a gene (substitution, insertion, or deletion)

Question 3

What does “Mutant” mean?

Answer 3

• An organism that exhibits a novel phenotype

Question 4

Why are mutations important?

Answer 4

• Mutations are heritable changes in the genetic material that provide the raw material for evolution
• Recombination mechanisms rearrange genetic variability into new combinations.
• Natural selection preserves the combinations best adapted to the existing environment.

Question 5

What are the steps in identifying a mutation?

Answer 5

• Firstly, is it Somatic or Germinal
• Secondly, is it Spontaneous or Induced
• Thirdly, is it usually Random & Non-adaptive
• Fourth, Stationary Phase, Mutagenesis in Bacteria
• Lastly, is it Reversible

Question 6

How do you identify whether a mutation is Somatic or Germinal?

Answer 6

• Germinal mutations occur in germ-line cells and will be transmitted through the gametes to the progeny.
• Somatic mutations occur in somatic cells; the mutant phenotype will occur only in the descendants of that cell and will not be transmitted to the progeny.

Question 7

How do you identify whether a mutation is Spontaneous or Induced?

Answer 7

• Spontaneous mutations occur without a known cause due to inherent metabolic errors or unknown agents in the environment
• Induced mutations result from exposure of organisms to mutagens, physical and chemical agents that cause changes in DNA, such as ionizing irradiation, ultraviolet light, or certain chemicals

Question 8

What is the likeliness of getting a spontaneous or induced mutation?

Answer 8

• Spontaneous mutations are infrequent:
  • Bacteria and phage: 10–8 to 10–10 per nucleotide pair per generation
  • Eukaryotes: 10–7 to 10–9 per nucleotide pair per generation, or 10–4
  to 10–7 per gene per generation
• Treatment of bacteria with mutagens can increase the mutation frequency to > 1% per gene for induced mutations

Question 9

Is mutation random or directed by the environment?

Answer 9

• Mutation usually is a non-adaptive process in which an environmental stress simply selects organisms with preexisting, randomly occurring mutations, therefore, mutations are usually random and non-adaptive

Question 10

What is one of the basic features of mutations?

Answer 10

• Environmental stress does not cause mutations but selects for mutants that are best adapted to the environmental stress

Question 11

What is the stationary phase of a mutation?

Answer 11

• It is a phase that provides a selective advantage to the mutant organism in a certain environment and occurs when populations of bacteria stop growing (in stationary growth phase)

Question 12

What is Mutagenesis?

Answer 12

• The production of genetic mutations.

Question 13

How are Adaptive and non-adaptive mutations created?

Answer 13

• Adaptive (along with non adaptive) mutations result from stressed induced increase in mutation rates
• Mutagenesis due to induction of error prone DNA repair processes
• Appears to be widespread in bacteria

Question 14

What is the difference between a forward mutation and a reverse mutation?

Answer 14

• Forward mutation - mutation of a wild-type allele to a mutant allele
• Reverse mutation (reversion) - a second mutation that restores the original phenotype.

Question 15

What are the types of reverse mutation?

Answer 15

• Back mutation - a second mutation at the same site

- Suppressor mutation - a second mutation at a different location in the genome

Question 16

What is the difference between a wild-type allele and a mutant allele?

Answer 16

• The allele that encodes the phenotype most common in a particular natural population is known as the wild type allele. It is often designated, in genetic shorthand, as “+”. Any form of that allele other than the wild type is known as a mutant form of that allele.

Question 17

How do mutant genes revert back to wild-type?

Answer 17

• Some mutants can revert to wild-type by both back and suppressor mutations
• To distinguish between the two, backcross the phenotypic revertant with the wild-type
• Back mutation - all progeny will have the wild-type phenotype
• Suppressor mutation - some of the progeny will have the mutant phenotype

Question 18

Study the diagrams for the Stationary Phase of Mutation, Reversible Mutations and for the Backcross to Wild-Type

Answer 18

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

Question 19

What is the range for effect of a mutation?

Answer 19

• The effects of mutations on phenotype range from no observable change to lethality

Question 20

What are the types of mutations?

Answer 20

• Isoalleles
• Null alleles
• Recessive lethal mutations
• Neutral mutations
• Mutations may be dominant or recessive: In diploids most recessive mutations will not be recognised; X-linked recessive mutations are an exception

Question 21

What are Isoalleles and Null alleles?

Answer 21

• Isoalleles have no effect on phenotype or small effects that can be recognized only by special techniques
• Null alleles result in no gene product or totally nonfunctional gene products

Question 22

What are Recessive Lethal mutations and neutral mutations?

Answer 22

• Recessive lethal mutations affect genes required for growth of the organisms and are lethal in the homozygous state
• Because of the degeneracy and order in the genetic code, many mutations have no effect on the phenotype of the organism. These are called neutral mutations

Question 23

Why are X-linked recessive lethal mutations significant?

Answer 23

• X-linked Recessive Lethal Mutations Alter the Sex Ratio
• For example, If you have a woman with a lethal mutation on one of her x chromosomes then she breeds with a normal male, the punnet square will show that only fifty percent of male children will survive due to this mutation.

Question 24

How do Recessive Mutations block metabolic pathways?

Answer 24

• Each step is catalysed by a specific enzyme encoded by one or more genes
• Alterations in the base-pair sequence of genes leads to changes in the amino acid sequence of the enzyme
• These changes are usually deleterious

Question 25

Study the diagram for how Recessive mutations often block metabolic pathways

Answer 25

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

Question 26

What is a single point mutation?

Answer 26

• Where a single nucleotide is mutated from the normal wild-type

Question 27

How does the expression of Wild-type and Mutant alleles work?

Answer 27

https://docs.google.com/document/d/1Nzo4FTzXCbwOZjpoc_J_4IF3gsOXPcoyC2BowELmx0U/edit?usp=sharing

Question 28

What are the two chains that hemoglobin has?

Answer 28

• Adult hemoglobin (Hemoglobin A) contains two a chains and two b chains

Question 29

How does Hemoglobin A differ from Hemoglobin S?

Answer 29

• Hemoglobin in patients with sicklecell anemia (Hemoglobin S) differs from Hemoglobin A at only one position
• The 7th amino acid in the b chain is glutamic acid in Hemoglobin A and is valine in Hemoglobin S. This substitution is caused by mutation of a single base pair

Question 30

What is Tay-Sachs Disease?

Answer 30

• Tay-Sachs disease is an autosomal recessive disease
• Mutation causing Tay-Sachs disease is in the gene encoding hexosaminidase A
• 1 in 30 Ashkenazi Jews are a recessive carrier. 1 in every 3,500 births
• The organization Dor Yeshorim carries out an anonymous screening program so that couples with Tay–Sachs disorder can avoid conception

Question 31

What are Phenylalanine and Tyrosine?

Answer 31

• Phenylalanine and Tyrosine are essential amino acids not synthesised de novo in humans

Question 32

What muatants occur due to the mutation of Phenylalanine and Tyrosine?

Answer 32

• Phenylketonuria is caused by the absence of phenylalanine hydroxylase: Patients placed on diet low in phenylalanine
• Albinism is caused by a mutation in tyrosinase
• Alkaptonuria is caused by a mutation homogentisic acid oxidase: Garrod’s (Module 1)
• Tyrosinosis is caused by a mutation in tyrosine transaminase
• Tyrosinemia is caused by a mutation in phydroxyphenylpyruvic acid oxidase

Question 33

What are conditional lethal mutations?

Answer 33

• The most useful for genetics studies
• Conditional lethal mutations are: Lethal in the restrictive condition but viable in the permissive condition
• Mutants with conditional lethal alleles can be propagated under the permissive condition, and the phenotype can be studied under restrictive condition

Question 34

What are the three major classes of conditional lethal mutants?

Answer 34

• Auxotrophs
• Temperature-sensitive mutants
• Suppressor-sensitive mutants

Question 35

What are Auxotrophs?

Answer 35

• A type of Conditional lethal mutantion
• They are unable to synthesize an essential metabolite that is synthesized by prototrophs. Auxotrophs can grow only when the essential metabolite is supplied in the medium

Question 36

What are Temperature-sensitive mutants?

Answer 36

• A type of Conditional lethal mutantion

- They will grow at one temperature but not at another

Question 37

What are Suppressor-sensitive mutants?

Answer 37

• A type of Conditional lethal mutantion

- They are viable only when a second genetic factor, a suppressor, is present

Question 38

How are conditional lethal mutations used to investigate biological processes?

Answer 38

• Intermediate Y immediately converted to Product B, therefore difficult to identify
• Intermediate Y accumulates allowing identification
• An appropriate mutation will eliminate the activity of a single polypeptide, thus allowing a complex process to be broken down to their individual components

Question 39

What was on the summary page?

Answer 39

• Mutations occur in both germ-line and somatic cells, but only germ-line mutations are transmitted to progeny.
• Mutations can occur spontaneously or be induced by mutagenic agents in the environment.
• Mutation usually is a nonadaptive process in which an environmental stress simply selects organisms with preexisting, randomly occurring mutations.
• Adaptive, or stationary phase, mutagenesis occurs in bacteria that have been exposed to an environmental stress such as starvation.
• Restoration of the wild-type phenotype in a mutant organism can result from either back mutation or a suppressor mutation.
• The effects of mutations on the phenotypes of living organisms range from minor to lethal changes.
• Most mutations exert their effects on the phenotype by altering the amino acid sequences of polypeptides, the primary gene products.
• Eg. mutant polypeptides, in turn, cause blocks in metabolic pathways.
• Conditional lethal mutations provide powerful tools with which to dissect biological processes