Genetics Flashcards
What is a mutation?
A change in the DNA sequence that may or may not affect an organism’s phenotype.
Mutations can occur in various regions of DNA, influencing gene function and expression.
Why are mutations important?
They can lead to genetic diversity and evolution, and they may cause diseases or affect traits.
Understanding mutations is crucial for fields like genetics, medicine, and evolutionary biology.
What are the different types of mutations?
- Point
- Transition
- Transversion
- Silent
- Missense
- Neutral
- Frameshift
- Inframe
Each type of mutation has distinct characteristics and effects on protein function.
What is a trinucleotide repeat expansion mutation?
A mutation where a sequence of three nucleotides is repeated more times than normal, potentially leading to disease.
This type of mutation is associated with conditions like Huntington’s disease.
How might mutations in the promoter influence protein products?
They can affect the transcription of the gene, leading to changes in the amount or functionality of the protein produced.
Promoter mutations can enhance or reduce gene expression.
What causes cystic fibrosis (CF)?
Mutations in the CFTR gene, which encodes a protein responsible for regulating chloride and sodium ions across epithelial cell membranes.
CF is a genetic disorder that affects the lungs and digestive system.
What protein is responsible for cystic fibrosis?
Cystic Fibrosis Transmembrane Conductance Regulator (CFTR).
The CFTR protein plays a crucial role in maintaining the balance of salt and water on epithelial surfaces.
What mutations are commonly involved in the development of cystic fibrosis?
- F508del
- G551D
- N1303K
F508del is the most common mutation associated with CF.
How are mutations named, such as PHE508DEL?
They are named based on the amino acid change and the specific deletion or alteration in the DNA sequence.
PHE508DEL indicates a deletion of phenylalanine at position 508.
Why might genetic testing for cystic fibrosis not provide an accurate diagnosis?
Not all mutations in the CFTR gene are tested, and some mutations may not be well understood or detectable.
Genetic variability and the presence of atypical mutations can complicate diagnosis.
In what types of cells can mutations occur?
- Germ-line cells
- Somatic cells
Germ-line mutations can be inherited, while somatic mutations affect only the individual.
INHERITANCE PATTERNS OF SINGLE GENES
There are many ways in which two alleles of a single gene may govern
the outcome of a trait
• Simple Mendelian
• Incomplete penetrance
Incomplete dominance
Overdominance
Codominance
…
X-linked
Sex-influenced inheritance
Sex-limited inheritance
Lethal alleles
Exercise
The pattern of inheritance in this allele is
presumably dominant. Is the pedigree analysis
consistent with a dominant disorder?
Why?
Polydactyly:
Additional fingers
and/or toes. Might be
caused by mutant
alleles.
1-2
II-1
II-2
II-3
II-4
II-5
III-1
III-2
III-3
IV-
1
IV-
2
3
III-4
III-5
Inherited the polydactyly allele from his
mother and passed it on to a daughter and son
Does not exhibit the trait himself even
though he is a heterozygote
Incomplete Penetrance
• In some instances
a dominant allele does not influence the outcome of a trait in
a heterozygote individual
Example = Polydactyly
-
-
Autosomal dominant trait
A single copy of the polydactyly allele is usually sufficient to cause this
condition
But
In some cases
the trait
The term Incomplete Penetrance indicates
that a dominant allele does not always
“penetrate” into the phenotype of the
individual
Incomplete Penetrance
• The measure of penetrance is described at the population level
If 60% of heterozygotes carrying a dominant allele exhibit the trait allele
the
trait is 60% penetrant
• Note:
-
In any particular individual
the trait is either penetrant or not
Expressivity
Expressivity is the degree to which a trait is expressed
What is expressivity?
Expressivity is the degree to which a trait is expressed.
How does expressivity vary in polydactyly?
In polydactyly, the number of digits can vary. A person with several extra digits has high expressivity of this trait, while a person with a single extra digit has low expressivity.
What are essential genes?
Essential genes are those that are absolutely required for survival.
What happens in the absence of essential genes?
The absence of their protein product leads to a lethal phenotype.
What proportion of genes are estimated to be essential for survival?
It is estimated that about 1/3 of all genes are essential for survival.
What are nonessential genes?
Nonessential genes are those not absolutely required for survival.
What is a lethal allele?
A lethal allele is one that has the potential to cause the death of an organism, usually inherited in a recessive manner.
What effect do many lethal alleles have?
Many lethal alleles prevent cell division, which can kill an organism at an early age.
What do many lethal alleles prevent?
Many lethal alleles prevent cell division.
When do some lethal alleles exert their effect?
Some lethal alleles exert their effect later in life.
What are examples of diseases caused by lethal alleles?
Huntington disease and Early Onset Alzheimer’s Disease.
What are the characteristics of Huntington disease and Early Onset Alzheimer’s Disease?
They are characterized by progressive degeneration of the nervous system, dementia, and early death.
What is the typical age of onset for Huntington disease and Early Onset Alzheimer’s Disease?
The age of onset of the disease is usually between 30 and 50.
How can a lethal allele affect phenotypic ratios?
A lethal allele may produce ratios that seemingly deviate from Mendelian ratios.
What is an example of a lethal allele in cats?
The Manx cat carries a dominant mutation that affects the spine.
What effect does the Manx cat mutation have?
This mutation shortens the tail.
Why is the Manx allele considered lethal?
This allele is lethal as a homozygote dominant.
What is the expected phenotypic ratio of live offspring when crossing two heterozygous Manx cats?
The expected phenotypic ratio of the live offspring is 2:1.
What is the P generation?
P generation refers to the parental generation in a genetic cross.
What is the F1 generation?
F1 generation is the first filial generation resulting from a cross between the P generation.
What is incomplete dominance?
In incomplete dominance, the heterozygote exhibits a phenotype that is intermediate between the corresponding homozygotes.
What are the alleles for flower color in the four o’clock plant?
CR = wild-type allele for red flower color, CW = allele for white flower color.
What phenotype results from the F1 generation in the four o’clock plant?
The F1 generation exhibits a pink phenotype.
What is the significance of the CR protein in the F1 generation?
In this case, 50% of the CR protein is not sufficient to produce the red phenotype.
What is the phenotypic ratio observed in codominance?
1:2:1 phenotypic ratio
This is different from the 3:1 ratio observed in simple Mendelian inheritance.
What is codominance?
Codominance occurs when two alleles are both expressed in heterozygous individuals.
How many different alleles can influence the same trait?
Three or more different alleles can influence the same trait.
Example: ABO blood group.
What determines the ABO blood group?
The ABO blood group is determined by the type of antigen present on the surface of red blood cells.
What are the alleles that determine the ABO blood group?
The three alleles are:
1. Allele IA, produces antigen A
2. Allele IB, produces antigen B
3. Allele i, no surface antigen is produced.
What is the relationship between allele i and alleles IA and IB?
Allele i is recessive to both IA and IB.
What is the relationship between alleles IA and IB?
Alleles IA and IB are codominant.
What type of allele is recessive to both IA and IB?
The allele is recessive to both IA and IB.
How are alleles IA and IB related?
Alleles IA and IB are codominant.
What happens in a heterozygous individual with alleles IA and IB?
Both IA and IB are expressed in a heterozygous individual.
What is the genotype for blood type A?
The genotype can be pApa or lAi.
What surface antigen is present in blood type A?
Antigen A.
What serum antibodies are present in blood type A?
Against B.
What is the genotype for blood type B?
The genotype can be B/B or Bi.
What surface antigen is present in blood type B?
Antigen B.
What serum antibodies are present in blood type B?
Against A.
What is the genotype for blood type AB?
The genotype is JAB.
What surface antigens are present in blood type AB?
Antigen A and Antigen B.
What serum antibodies are present in blood type AB?
None.
What is the role of glycosyl transferase in blood type A?
It is encoded by the A allele and produces Antigen A.
What is the role of glycosyl transferase in blood type B?
It is encoded by the B allele and produces Antigen B.
What is the carbohydrate tree on the surface of RBCs composed of?
It is composed of three components.
What is the carbohydrate tree on the surface of RBCs composed of?
The carbohydrate tree is composed of three sugars.
What can be added to the carbohydrate tree by the enzyme glycosyl transferase?
A fourth sugar can be added.
What does the i allele encode?
The i allele encodes a defective enzyme.
What is the effect of the i allele on the carbohydrate tree?
The carbohydrate tree is unchanged.
What does the IA allele encode?
IA encodes a form of the enzyme that can add N-acetylgalactosamine to the carbohydrate tree.
What does the IB allele encode?
IB encodes a form of the enzyme that can add galactose to the carbohydrate tree.
How are the A and B antigens recognized?
The A and B antigens are different enough to be recognized by different antibodies.
What must occur for safe blood transfusions?
The donor’s blood must be a ‘match’ with the recipient’s blood.
What happens if a type O individual receives blood from a type A, B, or AB?
Antibodies in the recipient’s blood will react with antigens in the donated blood cells.
What is the result of the reaction between antibodies and antigens during a transfusion?
This causes the donated blood to agglutinate.
What may result from the agglutination of donated blood?
A life-threatening situation may result because of clogging of blood vessels.
What is the composition of the carbohydrate tree on the surface of RBCs?
The carbohydrate tree is composed of three sugars.
What role does the enzyme glycosyl transferase play in the carbohydrate tree?
It can add a fourth sugar to the carbohydrate tree.
What does the i allele encode?
The i allele encodes a defective enzyme.
What is the effect of the i allele on the carbohydrate tree?
The carbohydrate tree is unchanged.
What does the IA allele encode?
IA encodes a form of the enzyme that can add the sugar N-acetylgalactosamine to the carbohydrate tree.
What does the IB allele encode?
IB encodes a form of the enzyme that can add the sugar galactose to the carbohydrate tree.
How do the A and B antigens differ?
The A and B antigens are different enough to be recognized by different antibodies.
What must occur for safe blood transfusions?
The donor’s blood must be a ‘match’ with the recipient’s blood.
What happens if a type O individual receives blood from a type A, B, or AB?
Antibodies in the recipient blood will react with antigens in the donated blood cells, causing agglutination.
What may result from the agglutination of donated blood?
A life-threatening situation may result because of clogging of blood vessels.
What are the ABO blood types?
A, B, AB, O
What is the genotype for blood type A?
JAJA or lAi
What are the serum antibodies for blood type A?
against B
What is the genotype for blood type B?
B/B or /Bi
What are the serum antibodies for blood type B?
against A
What is the genotype for blood type AB?
JAB
What are the serum antibodies for blood type AB?
none
Who is a universal recipient?
Blood type AB
Who is a universal donor?
Blood type O
What is overdominance?
The phenomenon in which a heterozygote is more vigorous than both of the corresponding homozygotes.
What is another name for overdominance?
Heterozygote advantage
What is an example of overdominance?
Sickle-cell anemia
What type of disorder is sickle-cell anemia?
Autosomal recessive disorder
What alleles are involved in sickle-cell anemia?
HbA and HbS
What does HbA encode?
Normal hemoglobin, hemoglobin A
What does HbS encode?
Abnormal hemoglobin, hemoglobin S
What happens to HbSHbS individuals under low oxygen tension?
Their red blood cells deform into a sickle shape.
What is expressivity?
Expressivity is the degree to which a trait is expressed.
What does high expressivity indicate in polydactyly?
A person with several extra digits has high expressivity of this trait.
What does low expressivity indicate in polydactyly?
A person with a single extra digit has low expressivity.
What are essential genes?
Essential genes are those that are absolutely required for survival.
What happens if essential genes are absent?
The absence of their protein product leads to a lethal phenotype.
What proportion of all genes are estimated to be essential for survival?
It is estimated that about 1/3 of all genes are essential for survival.
What are nonessential genes?
Nonessential genes are those not absolutely required for survival.
What is a lethal allele?
A lethal allele is one that has the potential to cause the death of an organism.
How are lethal alleles usually inherited?
Lethal alleles are usually inherited in a recessive manner.
What effect do many lethal alleles have on cell division?
Many lethal alleles prevent cell division, which can kill an organism at an early age.
What do many lethal alleles prevent?
Many lethal alleles prevent cell division.
What is the effect of some lethal alleles?
Some lethal alleles exert their effect later in life.
What diseases are associated with lethal alleles that exert effects later in life?
Huntington disease and Early Onset Alzheimer’s Disease.
What are the characteristics of Huntington disease and Early Onset Alzheimer’s Disease?
They are characterized by progressive degeneration of the nervous system, dementia, and early death.
What is the usual age of onset for Huntington disease and Early Onset Alzheimer’s Disease?
The age of onset is usually between 30 and 50.
How may a lethal allele affect phenotypic ratios?
A lethal allele may produce ratios that seemingly deviate from Mendelian ratios.
What is an example of a lethal allele in cats?
The Manx cat carries a dominant mutation that affects the spine.
What does the Manx mutation do?
This mutation shortens the tail.
Why is the Manx allele considered lethal?
This allele is lethal as a homozygote dominant.
What is the expected phenotypic ratio of live offspring when crossing two heterozygous Manx cats?
The expected phenotypic ratio of the live offspring is 2:1.
Tt
What is the difference?
White
Re
d
P generation
P (parental)
generation
Cross-
fertilize
Purple
White
Pin
F1 generation
Self-fertilize
F1
generation
Purple
Incomplete Dominance
In incomplete dominance the heterozygote exhibits a
phenotype that is intermediate between the corresponding
homozygotes
Red
White
• Example:
Flower color in the four o’clock plant
Two alleles
•
CR = wild-type allele for red flower color
CW = allele for white flower color
P generation
Pink
F
generation
Re
Whit
generation
CWC
W
In this case
50% of
the CR protein is not
cufficiant to produca
Human Pedigree for Duchenne muscular dvstronhv-Affected individuals are shown
with filled symbol $ @ • OTt ? dO>bols.
X-linked Genes
Many species have males and females that
differ in their sex chromosome composition
Certain traits are governed by genes on the sex
chromosomes
Question 1: A X-linked recessive trait that leads to disease will
affect males or females the most? Complete a Punnett Square to
answer this question.
• Question 3: Complete a Punnett Square
Consider a cross between an affected female
golden retriever (XdXd) and unaffected male
(XDY). What offspring would you expect?
Affected offspring?
Carriers?
Reverse the genotype of the aft
does the outcome differ from the scenario
above?
Reciprocal Cross
• Allele / is recessive to both IA and IB
•Alleles IA and IB are codominant
• They are both expressed in a heterozygous individual
Blood type:
Genotype:
Surface antigen:
Serum antibodies:
(a) ABO blood type
RBC
N-acetyl-
galactosamine
ii
neither A or B
against A and B
Antigen A
RBC
pApa orlAi
against B
Antigen B
Antigen A
Antigen B
RBC
B
B/B or Bi
B
against A
Galactose
RBC
AB
JAB
A and B
none
Glycosyl transferase
encoded by A allele
Antigen A
Active
site
RBC
N-acetyl-
galactosamine
Glycosyl transferase
encoded by B allele
RBC
Antigen B
• Active
site
RBC
Galactose
(c) Formation of A and B antigen by glycosyl transferase
RBC
The carbohydrate tree on the surface of RBCs is composed of three
• Allele / is recessive to both IA and IB
•Alleles IA and IB are codominant
• They are both expressed in a heterozygous individual
Blood type:
Genotype:
Surface antigen:
Serum antibodies:
(a) ABO blood type
RBC
N-acetyl-
galactosamine
ii
neither A or B
against A and B
Antigen A
RBC
pApa orlAi
against B
Antigen B
Antigen A
Antigen B
RBC
B
B/B or Bi
B
against A
Galactose
RBC
AB
JAB
A and B
