Chapter 13: Extensions of and Deviations from Mendelian Genetic Principles

Question 1

Multiple alleles?

Answer 1

• in a population of individuals a given gene can have multiple alleles (usually one wild type and the rest mutant) not just two as previously we’ve been studying.

Question 2

Wild type allele?

Answer 2

• most common in the population

Question 3

Multiple allele example?

Answer 3

• blood types

Question 4

Multiple alleles:

• Blood Types

L> two possible antigens?

L> four possible blood groups ?

L> rules for each blood group?

Answer 4

L> A and B

L> AB, A, B, O (phenotypes)

L> do not have antibodies against self antigens - do not have antibodies against non self antigens

Question 5

What are the genotypes of O, A, B and AB?

Answer 5

• ii
• I^AI^A or I^AI
• I^BI^B or I^BI
• I^AI^B

Question 6

For the blood genotypes what kind of expression goes on?

Answer 6

I^A andI^B are dominant to i (no surface antigens)

I^A and I^B are codominant with each other!

Question 7

I^A and I^B code for what?

Answer 7

• glycosyl transferase…each enzyme modifies the H structure on the red blood cell surface in a different way.

Question 8

Since I^A and I^B are codominant what does this mean for their structure?

Answer 8

• Both H structure mods are present on the RBC

Question 9

In RBC the i allele produces what kind of enzyme?

Answer 9

• nonfunctional

Question 10

Multiple Alleles…..Codominance example with Drosophila!

• X-linked eye colour gene:

W+= red eye colour

W= white eye colour

W^e= eosin eye colour

ranking?

Answer 10

W+ is dominant to W^e and W

W^e is dominant to W

(see slides for run through example of this)

Question 11

Incomlete dominance?

Answer 11

• when one allele of a gene is not completely dominant to another allele of the same gene….the phenotype of a heterozygote lies within the range of the phenotypes of the homozygotous parents….this phenotype is called an intermediate

Question 12

Incomplete dominance?

• this is called ___ dominance
• Heterozygous pheno is an ____ with repsect to the phenotype of the homozygous for either allele.
• An F2 ratio of _:_:_ from mating of heterozygous F1’s.

Answer 12

• intermediate
• 1:2:1

Question 13

Codominance?

Answer 13

• the heterozygote exhibits phenotypes of both homozygotes

Ex: blood type I^AI^B

Question 14

Complete dominance?

• this is called a ___ condition. Meaning?

Answer 14

• heterozygous condition
• Haplosufficient: 1/2 production sufficient for normal cell phenotype; dominant phenotype
• dominant allele is transcribed twice as frequently to result in an increase of protein production

(involves one gene)

Question 15

Incomplete dominance?

• _____ condition
• called?

L>meaning?

Answer 15

• heterozygous condition
• haploinsufficient: 1/2 product is insufficent for normal cell phenotype; resulting in an intermediate phenotype
• gene product from one allele results in a mixed intermediate.

Question 16

Gene interaction?

Answer 16

• interaction between genes that control the same character….. two genes, four alleles.
• interactions of different genes in which an allele of one gene masks the expression of alleles of another gene

Question 17

Example of phenotypes of combs in chickens for gene interaction…

• rose, pea, walnut, single
• cross rose with single

L> F1? F2?

• cross pea with single

L> F1? F2?

• cross pea with rose?

Answer 17

• rose

L> 3 rose: 1 single

• all pea

L> 3 pea: 1 single

• all walnut

Parents: R/R p/p (rose) and r/r P/P (pea)

F1: R/r P/p via

                   rP

Rp R/r P/p

** Rp= gamete of one parent ….rP=gamete of another parent

F2 geneotypes:

R/-,P/- = walnut dom for R and P

R/-, p/p = R dominant allele=rose

r/r,P/- = P dominant allele= pea

Question 18

Explain the chicken comb question for gene interaction ….for recessive, dominant alleles!

Answer 18

Recessive: no influence on the basic phenotype of single comb

dominant R allele: may produce gene product that interacts with product of genes controlloing single comb to get rose comb

• Dominant P allele: acts like R to get Pea comb
• Dominant R and P alleles: may produce gene products that interact to produce the comb variation of walnut.

Question 19

Epistasis??

Answer 19

• form of gene interaction
• one gene masks the phenotypic epression of another
• NO NEW PHENOS ARE PRODUCED

Question 20

epistatic gene?

hyostatic gene?

Answer 20

• gene that does the masking
• gene that has its phenotypic expressionmasked

Question 21

Recessive Epistasis?

Answer 21

• epistasis due to presence of homozygous alleles of one gene pair

Question 22

Recessive epistasis ex:

Coat colour in mice

• Explanation first:
• Agouti: normal “wild type coat” A/-
• Black coat : recessive…. a/a
• at a second locus “c” permits coat colour when it is dominant..C/-….it is not permitted when it is in the form c/c

Q. Double homozygous Agouti parent is crossed Albino Parent (that is homozygote for Black coat)

Answer 22

P1: AACC x aacc

              agouti                     albino(but should be brown) 

F1: all AaCc (100% agouti)

F2: AaCc x AaCc

           3/4 C-             9/16 A-C- agouti 

3/4 A- 1/4 cc 3/16 A-cc albino

1/4 aa 3/4 C- 3/16 aaC- black

             1/4 cc               1/16 aacc    albino 

9 : 3 : 4

agouti; black; white

Question 23

Lethal allele?

L> essential gene?

Answer 23

• an allele that results in the death of an organism

L> genes that when mutated can result in a lethal phenotype…

Question 24

Dominant lethal allele?

Answer 24

• mutation of an essential gene is caused by a dominant lethal allele…both homozygous and heterozygotes for that allele show the lethal phenotype

Question 25

Recessive lethal allele?

Answer 25

- if the mutation in the essential gene is caused by a recessive allele, only homozygotes for that allele have the lethal phenotype.

Question 26

What does one mean by essential genes exactly?

Answer 26

- the gene product is essential for normal functioning of the organism

Question 27

Example of an Essential gene question: 1. Mice coat colour yellow variety never breed true (essential gene) Cross two Yellow coated mice.

Answer 27

A^Y/A x A^YA ## Footnote gametes: A^Y and A A^Y and A A^Y A A^Y                  A^YA^Y  A^YA A A^Y A AA F1 dies: 1/4 A^YA^Y   1/2 A^YA = yello 1/4 A/A = nonyellow

Question 28

Example of an Essential gene question: 1.Mice coat colour yellow variety never breed true (essential gene) Cross Yellow coated mouse with a non-yellow coated mouse! L\> cross the F1's?

Answer 28

A^Y/A^Y x A/A ## Footnote Gametes: A^Y and A A and A A A A^Y                A^YA A^YA A AA AA two yellow, two non yellow.....results are similar to a test cross for the F1 generation! 1:1 2. Cross F1's A^YA x AA 2: 1 ratio?

Question 29

Explain the logic behind the yellow mice coat corsses

Answer 29

yellow allele is dom wrt coat colour when heterozygotes are crossed, a 2:1 ratio is indicative of a recessive lethal allele L\> lethality? only homozygotes die ie A^YA^Y - raly gene is deleted and its promoter is next to the agouti -yellow allele controlling tis expression

Question 30

Gene manifestation L\> Penetrance??

Answer 30

- % of individuals that show at least some degree of expression of the mutant genotype/ Not all genotypes penetrate the phenotype..complete or incomplete \*\*basically what you see in phenotype wrt genotype in a pop

Question 31

Gene manifestations: Expressivity??

Answer 31

- range of expression of the mutant phenotype - due to other genes modifying the phenotype - due to environmental factors ex: retina blastoma 90% penetrance \*\* generally lethal alleles are not common in a pop unless via mutation or if its dom past reproduction stage.

Question 32

Temperature effects? L\> Conditional?(breaks down into two)

Answer 32

- mutations affected by temp L\> functional at one temp = permissive L\> non functional at different temp = restrictive for the enzyme because its not working

Question 33

Temperature effects? -Protein denaturaiton occurs?

Answer 33

- dark patches due to temperature sensitive alleles --\> tyrosinase gene: permits melanin synthesis - active at lower temps \>30 C - leaving a white/creame coloured fur - inactive at higher temps \<25 C

Question 34

Phenocopy??

Answer 34

- non-hereditary phenotype modification - mimics a similar phenotype caaused by a gene mutation L\> copy of a phenotype with no genetic basis..only mimics showing phenotypically but not genotypically

Question 35

Examples of phenocopy?

Answer 35

- Rickets (Vit D deficiency) L\> due to malnutrition or x-linked alleles -Phocomelia L\> due to thalidamide or recessive alleles L\> causes suppression of long bone development....