Lecture 4: Extensions of Mendelian Genetics Flashcards
Exam 2 Genetics
What are the two postulates which are the basic principles of gene transmission?
- Genes are present on homologous chromosomes
- Chromosomes segregate and assort independently
What does Gene Interaction mean?
Single phenotype is affected by more than one set of genes
Describe what X-LINKAGE means
Genes that are present on the X chromosome
Can cells tolerate excess gene dosage?
NO! Remember that n=haploid and 2n=diploid. This also demonstrates that only one gene should be expressed (DOMINANT) and then there is recessive traits which remain silent.
Define Alleles
Alternative forms of genes
Mutations are the Ultimate source of alleles. New phenotypes result from changes in _____ activity of gene product.
Functional
New phenotypes result from changes in functional activity of gene product such as what 3 aspects:
- Eliminating enzyme function
- Changing relative enzyme efficiency
- Changing overall enzyme function
What does Wild-type Allele mean?
The “normal” allele which frequently occurs in nature and usually, BUT NOT ALWAYS, dominant
What are the 3 different Function Mutations?
- Loss of Function
- Gain of Function
- Neutral Mutations
What are the symbols used for Alleles?
- Dominant: Uppercase D
- recessive: Italic lowercase letter or group of letters
- Mutant: italic letter
- Wild Type: italic letter plus superscript (+)
Define Incomplete or Partial Dominance
- Intermediate phenotype
- Neither allele is dominant
- Ex.) Snapdragons (Mix red + white = pink)
Define Multiple Alleles
- 3 or more alleles of the same gene
- Resulting mode of inheritance unique
- Can only be studied in populations
Explain an example of Incomplete Dominance in humans
Example: Tay-Sachs disease
* Homozygous recessives affected by fatal lipid-storage
disorder
* Disorder fatal for neonates
* Hexosaminidase A activity absent
* Enzyme involved in lipid metabolism
* Normal heterozygotes: one copy of mutant gene
* 1/2 w t enzyme activity compared to homozygous
normal noncarriers
Explain an example of Multiple Alleles in Humans
- Human ABO blood groups
- A and B antigens present on the surface of RBCs
- 3 alleles of a single gene responsible for ABO phenotypes
Understand Isoagglutinogen (Antigen) in PowerPoint
Look at Lecture 4 slides!!!
What does A and B antigens mean?
Carbohydrate groups bound to lipid groups on red blood cells
What is the H substance?
- One or two terminal sugars are added
- O blood types (ii) only have the H substance protruding from red blood cells
Explain Bombay Phenotype
- Female found to be homozygous for FUT1 at the fucosyl transferase locus
– Prevents her from producing H substance
– No substrate to make A or B antigens
– Results functionally in type O - rare blood type that occurs when someone lacks the H antigen on their red blood cells
- anti-H antibodies in their plasma
- They appear to be type O in ABO typing
- The Bombay group (Oh) results from the inheritance of two rare recessive h genes
Define Essential Genes
- Absolutely required for survival
- Mutations can be tolerated if heterozygous
- Homozygous recessive will NOT survive
- Mutations behave as RECESSIVE LETHAL ALLELE
Define Lethal Allele
- Has potential to cause death of organism
- Alleles are result of mutations in essential genes
- Inherited in recessive manner
Define Dominant Lethal Allele (provide an example)
- Presence of one copy of allele results in death.
- Ex.) Huntington disease
What is Huntington disease?
- Dominant autosomal allele H
- Onset of disease in heterozygous delayed until adulthood
- Characterized by progressive degeneration of nervous system, dementia, and early death
Phenotypic characters are influence by _______ _______ ______ and their _______
- Many different genes
- Products
What does Gene Interaction mean?
- Several genes influence a particular characteristic
- Cellular function of numerous gene products contributes to the development of common phenotype
Define Epistasis
- Expression of one gene masks/modifies effect of another gene pair
- Gene masks phenotypic effects of another gene
- Each step of development increases complexity of organ
- Under control and influence of many genes
Briefly describe Hereditary deafness
- Ear forms as result of many genes
- Genes interact to produce common phenotype
- Mutations interrupt development = hereditary deafness
- Mutant phenotypes: Heterogenous trait where many genes are involved
What is Pleiotropy? Provide an example
- Expression of single gene has multiple phenotypic effects
- Ex: Marfan syndrome
- Single autosomal dominant mutation in the gene that encodes protein fibrillin results in multiple phenotypic effects
- Ex.) Porphyria variegata (inability to get rid of unwanted substances such as waste, toxins, etc)
Understand Sex Determination. Also, what is the Male XY known as?
- Sex of animals and plants determined by unlike chromosomes X and Y
- Hemizygous: Males XY
- Homozygous: Females XX
Explain X-Linkage
- Genes present on X chromosome exhibit patterns of inheritance
- Different from autosomal genes
Describe the Y chromosome
- Relatively inert genetically
- Male-specific genes on human Y chromosome
- Lacks copies of genes found on X chromosome
Understand X-Linkage in Drosophila
Look at slides ;)
- Sergio’s Fun Fact: reciprocal crosses is when you do the same genes but on the opposite genders to see if pattern changes. Helps to see if a gene is X-linked
Understand Color Blindness as X-linked
Look at slides! ;D
Understand Male Pattern Baldness
- Allele B behaves dominant in males and recessive in females
- In BB genotypes in females, phenotype is less pronounced
Phenotypic expression of a trait is influenced by what 2 things?
Environment and Genotype
Define Penetrance
Percentage of expression of the mutant genotype in a population
Define Expressivity
- Range of expression of mutant phenotype
- Result of genetic background differences and/or environmental effects
Dr. K Fact: Understand Microenvironment
Cell inside of an organism is the issue. Not the exterior environment per se. (???)
Understand Genetic background: Position effect
- Physical location of gene influences expression (specific loci on gene)
- Translocation or inversion events modify expression
- Gene relocated to condensed or genetically inert chromosome (heterochromatin)
Provide examples of Conditional Mutations: Temperature effects
- Evening primrose: Red at 23°C and White at 18°C
- Siamese cats and Himalayan rabbits
Define Temperature-sensitive mutations
- Viruses, bacteria, fungi, and Drosophila
- Mutant allele expresses mutant phenotype at one temperature, wild-type phenotype at another
- Useful when studying phage (bacterial virus) mutants
- easily induced and
isolated in viruses
Define Nutritional mutations
- Prevent synthesis of nutrient molecules in microbes
- Auxotrophs (microbe)
- Phenotype expressed or not depending upon diet
Define Pheylketonuria. What about Galactosemia? What about Lactose Intolerance?
Phenylketonuria:
- Loss of enzyme to metabolize amino acid phenylalanine
- Severe problems unless low-Phe diet
Galactosemia
- Cannot metabolize galactose
Lactose intolerance
- Cannot metabolize lactose
Explain Tay-Sachs disease
- Inherited autosomal recessive
- Lethal lipid-metabolism disease
(hexosaminidase A) - Baby normal for a few months, dies by age 3
Explain Lesch-Nyhan syndrome
- Inherited X-linked recessive
- Purine salvage enzyme defect (HPRTase)
- Normal for about 6 months
Explain Duchene muscular dystrophy (DMD)
- X-linked recessive disorder
- Diagnosis at 3–5 years old, fatal by
age 20
Explain Huntington disease
- Variable age of onset in humans
- Autosomal dominant disorder
- Affects frontal lobes of cerebral
cortex - Progressive cell death—brain
deterioration - Age range 30–50 years old
