Genetics Week 2

Incomplete Dominance

The heterozygote has an intermediate phenotype.

How are alleles represented for incomplete or co-dominance?

Alleles are represented by upper case letters, and different alleles are defined by different superscripts.

Monohybrid Cross with Pure-Breeding Parents, Incomplete dominance (g + p ratio and Punnett square)

F1 generation - all progeny resemble neither of the parental strains.
F2 generation -

genotypic ratio: ¼ X¹X¹, ½ X¹X², ¼ X²X²
phenotypic ratio: 1:2:1

Molecular explanation for incomplete dominance in snapdragon flower colour

Normal "red” allele produces enzyme involved in pigment production.

A¹A¹ = 2 normal alleles = red colour
A¹A² = 1 normal allele = pink colour (less enzyme)
A²A² = 0 normal alleles = white colour (no functional enzyme)

Codominance

Contributions from both alleles are visible (distinctly from each other, not as an intermediate) in the phenotype.

Monohybrid Cross with Pure-Breeding Parents, Co-dominance (g + p ratio and Punnett square)

F1 generation - all progeny resemble both of the parental strains.
F2 generation -

genotypic ratio: ¼ X¹X¹, ½ X¹X², ¼ X²X²
phenotypic ratio: 1:2:1

Variation in dominance relations…

does not negate Mendel’s laws of segregation.

Dominance relations between alleles do not affect transmission of alleles.

Wild type

Allele associated with the most common phenotype in a population.
For polymorphic genes, more than one allele could be considered “wild type” but it is more common to refer to these alleles as “common variants”.

Mutant type

Rare alleles (opposite of wild type)

Reciprocal crosses (what does it reveal?)

A reciprocal cross in genetics is a pair of crosses where the sexes of the parents are reversed.
This type of cross helps determine if a trait's inheritance is influenced by the sex of the parent contributing the trait. 

Superscripts for wild-type aleleles

+ Indicates wild-type allele

Otherwise mutant type represented as superscript.

What does a phenotypic ratio of (3:1) or (1:2:1) reveal?

Different alleles exist of the same gene.

Blood type in humans

Gene I encodes enzyme that adds terminal sugar onto polymer chain.
Polymorphic gene - 3 alleles are common:
I^A adds A sugar
I^B adds B sugar
i adds nothing

I^AI^A or I^Ai = “A” sugar is added = blood type A
I^BI^B or I^Bi = “B” sugar is added = blood type B
I^AI^B = “A” and “B” sugar is added = blood type AB

i i = no sugar added = blood type O

Pleitropy

One gene influences multiple traits.

Sickle Cell Disease (example of..?)

Example of pleiotropy - multiple phenotypic effects caused by a single gene.

Mutation in HBB gene causes RBC to stiffen, leading to many effects (i.e. anemia, heart failure, kidney failure, brain damage)

Lethal alleles

Some dominant and recessive alleles that can kill an organism.
(Typically pleiotrophic)

Recessive lethal allele example

Wild-type agouti allele (A) controls hair colour.
A^Y dominant to A and leads to a yellow colour.

Crossing a pure-breeding grey AA mouse with a yellow A^YA mouse, you get 1:1 ratio of grey:yellow mice.
If you cross two yellow mice, you will never get pure-breeding yellow mice.

Monohybrid Cross with Heterozygous Parents, Recessive lethality (g + p ratio and Punnett square)

F1 generation:

genotypic ratio: 2/3 A^YA, 1/3 AA
phenotypic ratio: 2:1 (yellow, grey)
Pure-breeding yellow mice will always die.

What happens if an allele is dominant for lethality?

Offspring will always die either prematurely, before birth, or during birth.

Additive Genes

Two or more genes influence one trait in a cumulative manner.

Dihybrid Cross with Pure-breeding Parents, Additive genes (g + p ratio and Punnett square)

F1 generation - all progeny resemble neither parent (additive phenotype)
F2 generation -
phenotypic ratio: 9:3:3:1 (brown:tan:grey:green)

Biochemical basis for additive effect in lentil colour

AA or Aa activate an enzyme that creates tan pigment.
BB or Bb activate an enzyme that creates grey pigment.
Both pigments combine to form a brown colour.
aa or bb lead to no activation, leading to no pigments and green peas.

Epistasis + related terms + dominant and recessive nature

One gene that hides the effect of another gene.
The allele that does the masking is epistatic to the other gene.
The gene that is masked is hypostatic to the epistatic allele.
Dominant epistasis - one copy of an allele is sufficient to mask the hypostatic gene.
Recessive epistasis - epistatic allele must be homozygous for the effect of the hypostatic gene to be masked.

Dihybrid Cross with Pure-breeding Parents, Dominant epistatic (g + p ratio and Punnett square)

F1 generation - progeny are identical to dominant homozygous, epistatic masking in effect.
F2 generation -
phenotypic ratio: 12:3:1

Gene A:
A_ = yellow
aa = green
Gene B:
B_ = white, regardless of gene A
bb = yellow or green, depending on gene A

B epistatic to A and a

Biochemical explanation for dominant epistasis

AA, Aa activation leads to enzyme converting green pigment to yellow pigment.
aa leads to no activation of gene, pigment remains green.
BB, Bb activation leads to deactivation of pigment depositor. Leads to NO pigment.
bb leads to pigment deposition, leads to yellow or green pigment depending on gene A.

Dihybrid Cross with Pure-breeding Parents, Recessive epistatic (g + p ratio and Punnett square)

F1 generation - progeny are identical to dominant homozygous, epistatic masking not in effect.
F2 generation -
phenotypic ratio: 9:3:4

Gene A:
B_ = black
bb = chocolate
Gene B:
E_ = not yellow
ee = yellow regardless of B gene.

ee is epistatic to any allelic combination of hypostatic gene B.

Biochemical explanation for recessive epistasis

EE, Ee activation leads to protein converting colourless precursor to eumelanin.
ee leads to no activation of eumelanin, so no eumelanin period. Converts to pheomelanin leading to yellow coat.
BB, Bb activation leads to activation of pigment depositor. Leads to black coat.
bb activation leads to activation of pigment depositor. Deposited less densely, leads to chocolate coat.

Bombay allele in blood type

H_ = polymer H added to RBCs.
hh = no polymer H added to RBCs.

If no polymer H, no sugar can be added regardless of I gene allele present.
hh = always resembles blood type O, regardless of gene I.
hh is epistatic to gene I.

Complementary gene action

Two or more genes can work in tandem, in the same biochemical pathway to produce a particular trait.

Dihybrid Cross with Pure-breeding Parents, Complementary gene action (g + p ratio and Punnett square) + heterogenous traits

F1 generation - progeny are identical.
F2 generation -
phenotypic ratio: 9:7

Heterogenous trait - mutation in any one of a number of genes can give rise to the same phenotype.

Purple:
A_
B_
White:
A_ bb
aa B_
aa bb

One functional dominant A and B allele are needed for purple colour.
Flower colour is heterogenous trait with complementary gene action.

Biochemical explanation for recessive epistasis

AA, Aa activation leads to enzyme converting colourless precursor to colourless precursor 2.
bb leads to no activation of enzyme, so no purple pigment. No conversion of precursor 2 to pigment.
aa leads to no activation of enzyme, so no purple pigment. No conversion of precursor to precursor 2.
BB, Bb activation leads to activation of colourless precursor 2 to purple pigment. Leads to purple colour.

Complementation test

One dominant allele for each of these genes is needed for an individual to be hearing = complementary gene action involving many genes.

The observation of complementation (restoration of function) in the offspring of two deaf individuals means that the mutations in the two parents were in different genes.

Observing if parents with the same mutant phenotype can produce offspring with restored function is a complementation test.

Draw complementation vs noncomplementation cross

Complex traits/multifactorial inheritance

A trait is determined by many genes or by the interaction between genes and environment.

Two things that can affect phenotype besides genotype (is this predictable?)

Penetrance: percentage of population with a particular genotype, that demonstrate the expected trait.
Expressivity: degree or intensity with which a particular genotype is expressed in a phenotype within a population.

Expression of phenotypes under such influence are not predictable.

How can individuals with the same genotype vary in phenotype?

In penetrance and expressivity.

Sex-linked traits + examples

Controlled by genes on the X or Y chromosome.
Hemophilia and colour blindness and androgenic alopecia

Sex-limited traits + examples

Affect a structure or process found in only one sex.
Bright plumage in male birds, milk production in females, horns/antlers in males.

Sex-influenced traits + example

Show up in both sexes but their expression may differ between the sexes.

Patterned baldness - androgenic alopecia

Androgenic Alopecia

Loss of hair from forehead and crown.
Affects 70% of men, not women.
Highly heritable, not in a mendelian fashion.
Multiple autosomal and X-chromosome genes are associated.

Examples of how environment can influence phenotype

Temperature, light, altitude.
Himalayan coat pattern in cats and rabbits.
Warmer temperature leads to inactivation of enzyme, colourless precursor does not form into melanin.
Cooler temperature leads to activation of enzyme, colourless precursor forms into melanin.

Conditional Lethality - example of what? + example

Environmental influence.
Allele only lethal in certain conditions:
Permissive conditions: conditions under which the individual survives.
Restrictive conditions: conditions under which the individual dies.

Fruit flies - mutation in shibire gene (codes for protein for nerve signal transmission)
Develop and reproduce normally 18 - 29 degrees.
If temperature increases for a short time, they become reversibly paralyzed.
If temperature is elevated for more than a few hours, they die.