Genetics: Allelic Interactions, Incomplete Dominance, and Codominance

A gene provides the potential for trait development, but the extent of this potential depends on interactions with other genes or alleles.
Allelic interactions are the focus, describing how different alleles interact.

Law of Segregation: During gamete formation, allele pairs separate randomly, with each gamete receiving only one allele of each gene.
Law of Independent Assortment: Genes for different traits assort independently during gamete formation.
Law of Dominance: In a heterozygote, one allele (dominant) masks the effect of the other allele (recessive).
These principles typically consider one gene with two alleles exhibiting dominant-recessive inheritance.
Example: Mendel's pea plants with purple (dominant, $P$ ) and white (recessive, $p$ ) flowers.
Pure breeding purple ( $PP$ ) crossed with pure breeding white ( $pp$ ) yields heterozygous F1 generation ( $Pp$ ) with purple flowers.
Self-crossing F1 heterozygotes (Pp
Pp) results in a 3:1 phenotypic ratio (3 purple : 1 white) and a 1:2:1 genotypic ratio ( $1 PP : 2 Pp : 1 pp$ ).

A gene (e.g., $P$ gene) is DNA that codes for a trait, like purple flower color.
The gene codes for a protein product, potentially a purple pigment protein.
Different alleles exist, such as:
- $P$ (normal allele): Codes for functional, purple-colored protein.
- $p$ (defective allele): Codes for a non-purple or white protein.

Mendel's success: The seven traits he studied in pea plants were each located on different chromosomes, allowing for clear observation of segregation and independent assortment.
More complex modes of inheritance involving different allelic interactions exist, including incomplete dominance and codominance.

In heterozygotes, the phenotype is a blend of both traits; an intermediate phenotype is expressed.
Example: Snapdragon flowers
- $RR$ : Red flowers
- $rr$ : White flowers
- $Rr$ : Pink flowers (blend of red and white)
Crossing two heterozygous pink snapdragons (Rr
Rr) yields a 1:2:1 phenotypic and genotypic ratio (1 red: 2 pink: 1 white).
Genotypic and phenotypic ratios are identical because heterozygotes display a distinct intermediate phenotype.
Alternate Notation:
- Using $R$ for the red allele and $W$ for the white allele.
- $RR$ = red, $WW$ = white, $RW$ = pink.
- Or using a color gene designation: $C^RC^R$ = red, $C^WC^W$ = white, $C^RC^W$ = pink.

Gene (e.g., $R$ gene) codes for a protein.
- $R$ allele: Codes for a protein that produces red pigment.
- $W$ allele: Codes for a defective or white protein.
$RR$ : Two copies of red protein leading to red color.
$WW$ : Two copies of defective protein leading to white color.
$RW$ : Some red protein and some defective protein, resulting in a pink color.

Certain chicken breeds: Crossing white and black chickens produces slate blue offspring.
Human hair: Curly hair is incompletely dominant to straight hair, with heterozygous individuals having wavy hair.

Both alleles in a heterozygote are fully expressed; both traits are visible.
Example: Roan cows
- Red cow crossed with a white cow produces roan offspring with both red and white hairs.
- Both red and white traits are equally expressed.
Using $W$ and $R$ for alleles:
- $WW$ : White
- $RR$ : Red
- $WR$ : Roan (both red and white are expressed)
Genotypic ratio of self-crossed heterozygotes remains 1:2:1, but phenotypic ratio also mirrors this because both traits are independently visible.

Mendelian Dominance: Heterozygote expresses the dominant parental phenotype.
Incomplete Dominance: Heterozygote expresses a blend of both parental phenotypes.
Codominance: Heterozygote expresses both parental phenotypes simultaneously.

In incomplete dominance, neither allele is truly recessive. Instead, alleles express distinct phenotypes that blend in the heterozygote.

Work through practice problems to prepare for exams thoroughly.
Sources include textbooks, online resources, and library materials.
Key to solving genetics problems:
- Identify what the problem is asking.
- Determine what information the problem provides.

Two genes involved:
- Feather color, with incomplete dominance.
- Feather presence on legs, with simple Mendelian dominance.
Parental cross: Pure breeding black chicken with feathered legs x White chicken lacking feathers.
Defining alleles:
- Color: $B$ (black), $W$ (white), heterozygote $BW$ is slate blue.
- Feathers: $F$ (presence), $f$ (absence).
Parental genotypes: BB FF
ww ff
F1 generation: All $BW Ff$ (slate blue with feathers).
Phenotypic ratio of F2 generation derived from crossing F1:
- Color: 1 black : 2 slate blue : 1 white.
- Feathers: 3 feathered : 1 no feathers.
Combined phenotypic ratio:
- $3/16$ black with feathers
- $1/16$ black, no feathers
- $6/16$ slate blue with feathers
- $2/16$ slate blue, no feathers
- $3/16$ white with feathers
- $1/16$ white, no feathers.

Horn presence: Simple Mendelian dominance (hornless is dominant).
Coat color: Codominance (red, roan, white).
Cross: Hornless, roan x Horned, roan.
Key instruction: Provide all possible answers, indicating multiple approaches to the problem.
Defining alleles:
- Horns: $A$ (hornless), $a$ (horned).
- Color: $C$ (red), $c$ (white).
Two possibilities for the hornless parent genotype: $AA$ or $Aa$ . Thus, there are two ways to solve this problem.
Possible scenarios:
- Homozygous dominant hornless cattle: AA Cc crossed with aa Cc; can all be heterozygotes.
- Heterozygous hornless allele: Aa Cc crossed with aa Cc; leads to a 1:1 ratio.

When providing ratios, always specify what each number represents to ensure clarity and accuracy.

If a herd of 320 cows results from the mating described earlier, determine how many will be roan and hornless, then select all possible answers.
Using the first cross (AA Cc x aa Cc):
- 1/2 (or 160) exhibit polled and roan characteristics.
Using the second cross (Aa Cc x aa Cc):
- 1/4 (or 80) exhibit polled and roan characteristics.
Both scenarios involve selecting for all probable outcomes given the information provided.