Lecture 4: Extensions of Mendelian Genetics

Question 1

What are the two postulates which are the basic principles of gene transmission?

Answer 1

1. Genes are present on homologous chromosomes
2. Chromosomes segregate and assort independently

Question 2

What does Gene Interaction mean?

Answer 2

Single phenotype is affected by more than one set of genes

Question 3

Describe what X-LINKAGE means

Answer 3

Genes that are present on the X chromosome

Question 4

Can cells tolerate excess gene dosage?

Answer 4

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.

Question 5

Define Alleles

Answer 5

Alternative forms of genes

Question 6

Mutations are the Ultimate source of alleles. New phenotypes result from changes in _____ activity of gene product.

Answer 6

Functional

Question 7

New phenotypes result from changes in functional activity of gene product such as what 3 aspects:

Answer 7

1. Eliminating enzyme function
2. Changing relative enzyme efficiency
3. Changing overall enzyme function

Question 8

What does Wild-type Allele mean?

Answer 8

The “normal” allele which frequently occurs in nature and usually, BUT NOT ALWAYS, dominant

Question 9

What are the 3 different Function Mutations?

Answer 9

1. Loss of Function
2. Gain of Function
3. Neutral Mutations

Question 10

What are the symbols used for Alleles?

Answer 10

1. Dominant: Uppercase D
2. recessive: Italic lowercase letter or group of letters
3. Mutant: italic letter
4. Wild Type: italic letter plus superscript (+)

Question 11

Define Incomplete or Partial Dominance

Answer 11

• Intermediate phenotype
• Neither allele is dominant
• Ex.) Snapdragons (Mix red + white = pink)

Question 12

Define Multiple Alleles

Answer 12

• 3 or more alleles of the same gene
• Resulting mode of inheritance unique
• Can only be studied in populations

Question 13

Explain an example of Incomplete Dominance in humans

Answer 13

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

Question 14

Explain an example of Multiple Alleles in Humans

Answer 14

• Human ABO blood groups
• A and B antigens present on the surface of RBCs
• 3 alleles of a single gene responsible for ABO phenotypes

Question 15

Understand Isoagglutinogen (Antigen) in PowerPoint

Answer 15

Look at Lecture 4 slides!!!

Question 16

What does A and B antigens mean?

Answer 16

Carbohydrate groups bound to lipid groups on red blood cells

Question 17

What is the H substance?

Answer 17

• One or two terminal sugars are added
• O blood types (ii) only have the H substance protruding from red blood cells

Question 18

Explain Bombay Phenotype

Answer 18

• 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

Question 19

Define Essential Genes

Answer 19

• Absolutely required for survival
• Mutations can be tolerated if heterozygous
• Homozygous recessive will NOT survive
• Mutations behave as RECESSIVE LETHAL ALLELE

Question 20

Define Lethal Allele

Answer 20

• Has potential to cause death of organism
• Alleles are result of mutations in essential genes
• Inherited in recessive manner

Question 21

Define Dominant Lethal Allele (provide an example)

Answer 21

• Presence of one copy of allele results in death.
• Ex.) Huntington disease

Question 22

What is Huntington disease?

Answer 22

• Dominant autosomal allele H
• Onset of disease in heterozygous delayed until adulthood
• Characterized by progressive degeneration of nervous system, dementia, and early death

Question 23

Phenotypic characters are influence by _______ _______ ______ and their _______

Answer 23

• Many different genes
• Products

Question 24

What does Gene Interaction mean?

Answer 24

• Several genes influence a particular characteristic
• Cellular function of numerous gene products contributes to the development of common phenotype

Question 25

Define Epistasis

Answer 25

- 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

Question 26

Briefly describe Hereditary deafness

Answer 26

- 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

Question 27

What is Pleiotropy? Provide an example

Answer 27

- 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)

Question 28

Understand Sex Determination. Also, what is the Male XY known as?

Answer 28

- Sex of animals and plants determined by unlike chromosomes X and Y - Hemizygous: Males XY - Homozygous: Females XX

Question 29

Explain X-Linkage

Answer 29

- Genes present on X chromosome exhibit patterns of inheritance - Different from autosomal genes

Question 30

Describe the Y chromosome

Answer 30

- Relatively inert genetically - Male-specific genes on human Y chromosome - Lacks copies of genes found on X chromosome

Question 31

Understand X-Linkage in Drosophila

Answer 31

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

Question 32

Understand Color Blindness as X-linked

Answer 32

Look at slides! ;D

Question 33

Understand Male Pattern Baldness

Answer 33

- Allele B behaves dominant in males and recessive in females - In BB genotypes in females, phenotype is less pronounced

Question 34

Phenotypic expression of a trait is influenced by what 2 things?

Answer 34

Environment and Genotype

Question 35

Define Penetrance

Answer 35

Percentage of expression of the mutant genotype in a population

Question 36

Define Expressivity

Answer 36

- Range of expression of mutant phenotype - Result of genetic background differences and/or environmental effects

Question 37

Dr. K Fact: Understand Microenvironment

Answer 37

Cell inside of an organism is the issue. Not the exterior environment per se. (???)

Question 38

Understand Genetic background: Position effect

Answer 38

- 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)

Question 39

Provide examples of Conditional Mutations: Temperature effects

Answer 39

- Evening primrose: Red at 23°C and White at 18°C - Siamese cats and Himalayan rabbits

Question 40

Define Temperature-sensitive mutations

Answer 40

- 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

Question 41

Define Nutritional mutations

Answer 41

- Prevent synthesis of nutrient molecules in microbes - Auxotrophs (microbe) - Phenotype expressed or not depending upon diet

Question 42

Define Pheylketonuria. What about Galactosemia? What about Lactose Intolerance?

Answer 42

Phenylketonuria: - Loss of enzyme to metabolize amino acid phenylalanine - Severe problems unless low-Phe diet Galactosemia - Cannot metabolize galactose Lactose intolerance - Cannot metabolize lactose

Question 43

Explain Tay-Sachs disease

Answer 43

- Inherited autosomal recessive - Lethal lipid-metabolism disease (hexosaminidase A) - Baby normal for a few months, dies by age 3

Question 44

Explain Lesch-Nyhan syndrome

Answer 44

- Inherited X-linked recessive - Purine salvage enzyme defect (HPRTase) - Normal for about 6 months

Question 45

Explain Duchene muscular dystrophy (DMD)

Answer 45

- X-linked recessive disorder - Diagnosis at 3–5 years old, fatal by age 20

Question 46

Explain Huntington disease

Answer 46

- 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