Extensions of Mendelian Genetics

Question 1

Limitations of Mendel’s Studies

Answer 1

1) Used a model that had a strict “either/or” trait type (only 2 alleles)

2) Characters that were studied were controlled by only ONE gene

–> His model was too simple to account for all the complexities of genetics

Question 2

Inheritance of characters determined by a single gene can differ from Mendelian genetics patterns when: (3)

Answer 2

1) Alleles are not completely dominant or recessive
–> Degrees of dominance

2) A particular gene has MORE than 2 alleles

3) A single gene affects more than one phenotype

Question 3

Dominance

Answer 3

The relationship between the alleles of one gene in which the phenotype of one allele “masks” the other

Question 4

Complete Dominance

Answer 4

When the phenotype of a heterozygote and homozygous dominant are indistinguishable

–> In a heterozygote, the dominant phenotype is expressed

(Essentially what Mendel studied)

Question 5

Incomplete Dominance

Answer 5

The phenotype of the heterozygote is an INTERMEDIATE between those of the 2 homozygotes –> A mix of both allele phenotypes

Question 6

Tay Sachs Disease is an example of…

Answer 6

Incomplete Dominance

Question 7

Tay Sachs Disease

Answer 7

Neurodegenerative autosomal recessive disease

–> Due to a recessive lethal mutation

Question 8

Tay Sachs Disease: what is mutated?

Answer 8

An enzyme: Hexoseaminidase –> An enzyme that breaks down lipids in nerve cells

Question 9

TS Homozygous Recessive

Answer 9

= deficiency in hexoseaminidase = Disease Phenotype

Question 10

TS Homozygous Dominant

Answer 10

= Normal # hexoseaminidase = Normal Phenotype

Question 11

TS Heterozygous

Answer 11

= 1/2 # hexoseaminidase = Normal Phenotype

–> Incomplete dominance at biochemical level (intermediate amount of the enzyme is produced)

–> Complete dominance at physical level

Question 12

Incomplete Dominance: Carnation Flower Color Example

Answer 12

Homo Dominant = Red Flower
Hetero Recessive = White Flower
Heterozygous = PINK flower –> Intermediate color

Question 13

Co-Dominance

Answer 13

Two different phenotypes BOTH get expressed in the heterozygote

–> The 2 alleles both affect the phenotype in separate, distinguishable manners

Question 14

What part does the I-Gene play in blood typing?

Answer 14

I-Gene = Encodes for the enzyme glycosyltransferase

== Enzyme that adds sugar onto its substrate (for blood it’s protein that gets secreted by rbcs and then resides on their surfaces)

Question 15

Blood Type Alleles

Answer 15

3 of them: Ia, Ib, i

Question 16

Type A Blood Genotype

Answer 16

Ia + Ia OR Ia + i

Question 17

Type B Blood Genotype

Answer 17

Ib + Ib OR Ib + i

Question 18

Type AB Blood Genotype

Answer 18

Ia + Ib

Question 19

Type O Blood Genotype

Answer 19

i + i

Question 20

Type AB blood is a form of….

Answer 20

Co-dominance

–> Both the A and B sugars are attached to the RBCs

–> Both the Ia and Ib alleles are being expressed at the same time

Question 21

Multiple Alleles

Answer 21

Within a population there may be many different alleles of a particular gene –> Most genes have > 2 alleles

Question 22

No matter how many alleles exist within a population…

Answer 22

a single individual will only have TWO alleles per gene

Question 23

Common Misconceptions of Dominance (2)

Answer 23

1) The dominant allele is the “normal/wild type” and the recessive is the mutant (not normal)

2) The dominant allele is the one found most frequently in a population

–> These may be true in many cases but NOT all

Question 24

Polydactyl

Answer 24

The dominant allele (P) leads to the growth of an extra digit (in humans and cats)

–> HOWEVER, most of the population is recessive = no extra digit

–> Although the trait is dominant, it is not the trait most frequently found in the population: It’s found in every 1 in 1000 people

Question 25

Extensions that Impact Phenotypic Expression (4)

Answer 25

1) Epistasis
2) Pleiotropy
3) Environment
4) Lethal Dominant Allele

Question 26

Lethal Dominant Alleles

Answer 26

Homozygous lethal dominant alleles can disrupt the phenotypic ratios: Changes the probability of the offspring

Question 27

Manx Cat

Answer 27

Homozygous Dominant (MM) = Early Embryonic Death
–> Insufficient spinal development

Heterozygous (Mm) = Short/No tail (manx)

Homozygous Recessive (mm) = Normal tail (non-manx)

Question 28

Epistasis

Answer 28

A gene at one locus alters the phenotypic expression of another gene at a second locus

–> Two genes contribute to an overall single phenotype

Question 28

Hair Color and Baldness Example

Answer 28

Baldness gene is epistatic to hair color gene
–> If you have the baldness allele, the hair color won’t even matter because it won’t get the chance to be phenotypically expressed

–> ***Baldness affects the expression of the hair color phenotype

Question 29

Albino Mice Example

Answer 29

2 genes control hair coat color:

1) Pigment color gene (brown or black)
2) Pigment deposition gene –> Homo. Rec. = No pigment deposited

Any mouse with “cc” for the 2nd gene = ALBINO
(because no pigment deposition occurs)

–> Prevents hair color phenotype from being expressed

Question 30

Pleiotropy

Answer 30

The ability of a gene to affect an organism’s phenotype in multiple ways (affecting more than one trait)

Question 31

Deaf-White Cats Example

Answer 31

Most cats with albino (white) fur and blue eyes are also DEAF

–> ***The gene for pigmentation is also involved in ear development

–> White cats with only one blue eye are deaf only in one ear

Question 32

Environmental Impacts

Answer 32

NATURE vs NURTURE

–> We are not just a product of our genes, but a product of our environment as well

Question 33

Sickle Cell Anemia Example

Answer 33

A single point mutation affects all the following:

1) Hemoglobin
2) Organ Function
3) Respiration
ETC……

(Affects many aspects of human physiology = pleiotropy)

Question 33

A genotype is not generally associated with…

Answer 33

a rigidly defined phenotype, but rather with a RANGE of phenotypic possibilities due to environmental impacts

Question 34

Siamese Cats Example

Answer 34

Phenotype depends on climate!

Temp. sensitive (TS) tyrosinase = enzyme that catalyzes production of dark pigment
–> Only works at temp < 33C

== Extremities tend to be colder, thus the cats have dark pigment in their extremities

Question 35

Siamese Cats in Warm Climate

Answer 35

Tend to have lighter coats than those in colder climates

Question 36

If a patch of white fur is removed from a siamese cat and then an icepack is placed over the patch…

Answer 36

The fur will grow back in dark

Question 37

An organism’s overall phenotype is composed of/is a

Answer 37

1) Physical appearance, internal anatomy, physiology, and behavior

2) Reflection of its overall genotype and unique environmental history