Ch. 4 - Extensions to Mendelian

Question 1

Despite the complexity that will be discussed, what are the two basic postulates of gene transmission that still are correct?

Answer 1

1. genes are present on homologous chromosomes

2. chromosomes segregate and assort independently

Question 2

What are the 3 things that gave rise to basic nomenclature?

Answer 2

• -> alleles
• -> mutation
• -> wild-type (wt) allele

Question 3

Define allele.

Answer 3

alternative forms of a gene (they need not be restricted to two alternative forms)

Question 4

What is mutation the ultimate source of?

Answer 4

alleles.

new phenotypes result from changes in functional activity, expression or stability of gene product.

for example,

• -> eliminating enzyme function
• -> changing relative enzyme efficiency
• -> changing overall enzyme function
• -> change stability of the enzyme

Question 5

Where does wild-type (wt) allele occur mostly in?

–> is the allele dominant?

Answer 5

nature and is often, but not always, dominant

Question 6

Does formal genetic nomenclature differ from different organisms?

Answer 6

yes, it’s incredibly complex.

Question 7

What are the symbols used to simplify communication in genetics?

Give an example.

Answer 7

–> dominant alleles (indicated by either an italic uppercase letter (D) or letters (Wr)

–> recessive alleles (indicated by either an italic lowercase letter (d) or group of letters(wr)

Example
–> drosophila melanogaster.

Question 8

Define incomplete dominance.

Give an example.

Answer 8

A form of dominance occurring in heterozygotes in which the dominant allele is only partially expressed, and usually resulting in an offspring with an intermediate phenotype

phenotype in intermediate = neither allele is dominant

Example given in class
–> red snapdragon crossed with white snapdragon

Question 9

Define codominance.

Give examples.

Answer 9

• -> No dominance or recessiveness
• -> No incomplete dominance or blending
• -> Distinct expression of both alleles in a heterozygote

Example
–> blood group antigens

Question 10

What are essential genes?

Answer 10

They are absolutely required for survival

Question 11

What is lethal allele?

Answer 11

• -> Has potential to cause death of organism
• -> Alleles are result of mutations in essential genes
• -> Inherited in recessive manner if gene product in a heterozygote exceeds the threshold level

Question 12

What is dominant lethal allele?

Give an example.

Answer 12

Presence of one copy of allele results in death

Example
–> huntington disease

Question 13

Example of dominant lethal allele: Huntington disease.

Describe it?

Answer 13

Dominant autosomal allele H

Onset of disease in heterozygous delayed until adulthood

Characterized by progressive degeneration of nervous system, dementia, and early death

Question 14

Describe the concept of gene interactions.

Answer 14

Phenotypic characters may be influenced by manydifferentgenesand their products.

–> ‘several genes’ influence a particular characteristic

*** doesn’t necessarily mean genes / gene products are interacting but rather that cellular function of numerous gene products contributes to development of common phenotype.

Question 15

Example of gene interaction.

–> What is epistasis? Give an example.

Answer 15

Expression of one gene masks/modifies effect of another gene pair.

***see notebook for diagram

Example
–> Bombay phenotype

Question 16

What is recessive epistasis?

Give an example.

Answer 16

When bb masks the expression of the A locus.

Example
–> mouse

Question 17

What is dominant epistasis?

Give an example.

Answer 17

Dominant allele at one locus masks an allele at second locus.

Example
–> summer squash fruit color

Question 18

What is complementary gene interaction?

Answer 18

…

Question 19

What is pleiotropy?

Give two examples.

Answer 19

expression of single gene has multiple phenotypic effects

Example

• -> marfan syndrome
• -> porphyria variegata

Question 20

Incomplete dominance
–> since examples of incomplete dominance at the phenotypic level are fairly rare, give an example at the molecular level.

Answer 20

There are many examples at the molecular level.

Example
–> human disorders in which the mutation leads to deficiency in enzyme activity. A heterozygote (Aa) does not exhibit the disorder, but if you look at the amount of enzyme it is one half the normal activity (i.e. normal enzyme is only produced by the wild type allele).

We say that a thresholdlevel of enzyme is necessary to produce the normal phenotype. That level is usually less than 50%