Modification of Mendelian Genetics: Alleles, Dominance, and Complex Inheritance Patterns

Chapter 4: Modification of Mendelian Genetics

4.1: Alleles Alter Phenotype in Different Ways

Alleles are alternative forms of the same gene, and their presence can lead to variations in phenotype. The study of alleles is fundamental to understanding genetic diversity and inheritance.

• Wild-type (normal) allele: The allele that occurs most frequently in a population; often, but not always, dominant.

• Mutation: The source of new alleles. A mutant allele contains modified genetic information and often produces an altered gene product.

Types of Mutation Effects on Phenotype

• Loss-of-function mutation: Causes a change in the enzyme's conformation, reducing or eliminating its activity. If the loss is complete, the mutation results in a null allele.

• Gain-of-function mutation: Enhances allelic function, often by increasing the quantity of gene product or altering regulation.

• Neutral mutation: The gene product presents no change to the phenotype.

4.2: Geneticists Use a Variety of Symbols for Alleles

Standard conventions are used to name and symbolize alleles, aiding in clear communication of genetic crosses and inheritance patterns.

• Recessive trait: Lowercase letter (e.g., d for recessive).

• Dominant trait: Uppercase letter (e.g., D for dominant).

• Species-specific conventions: For Drosophila melanogaster, the initial letter or combination from the mutant phenotype is used (e.g., e for ebony mutant, e+ for wild-type gray).

• No dominance: Italic letters and superscripts are used to denote alleles when neither is dominant.

• Gene naming examples: cdk for cyclin dependent kinase, leu for leucine biosynthesis mutation, BRCA1 for breast cancer gene.

4.3: Neither Allele is Dominant—Incomplete or Partial Dominance

Incomplete dominance occurs when the heterozygote exhibits a phenotype intermediate between those of the two homozygotes.

• Example: Crossing red and white snapdragons yields pink flowers in the F1 generation. F2 generation shows a 1:2:1 ratio of red:pink:white.

• Each genotype has its own distinct phenotype.

4.4: Codominance—Both Alleles Expressed in Heterozygotes

Codominance occurs when both alleles in a heterozygote are fully expressed, resulting in a phenotype that displays both traits distinctly.

• Example: MN blood group in humans. The M and N antigens are glycoproteins on red blood cells; individuals may express M, N, or both (MN).

• Crosses between MN heterozygotes yield all three blood types (M, MN, N).

• Codominance differs from incomplete dominance, which produces a blended phenotype.

Genotype	Phenotype
MM	M antigen
MN	M and N antigens
NN	N antigen

4.5: Multiple Alleles of a Gene in a Population

Some genes have more than two allelic forms, leading to complex inheritance patterns that can only be studied at the population level.

• Example: ABO blood group in humans. Three alleles (IA, IB, i) determine blood type.

• IA and IB produce their respective antigens and are codominant; both are dominant to i, which produces no antigen.

Genotype	Antigen	Phenotype
IAIA or IAi	A	Type A
IBIB or IBi	B	Type B
IAIB	A and B	Type AB
ii	None	Type O

4.6: Lethal Alleles Represent Essential Genes

Lethal alleles are mutations in essential genes that can cause death when present in certain genotypes.

• Recessive lethal allele: Homozygous recessive individuals do not survive; heterozygotes are viable.

• Dominant lethal allele: Presence of one copy results in death (e.g., Huntington's disease).

• Example in mice: The yellow coat color allele (AY) is dominant for color but recessive lethal. Crosses between yellow mice yield a 2:1 ratio of yellow to agouti offspring; homozygous AYAY individuals do not survive.

4.7: Combinations of Two Gene Pairs—Modified Mendelian Ratios

When two gene pairs with different modes of inheritance interact, the classic 9:3:3:1 dihybrid ratio is often modified, resulting in new phenotypic ratios.

• Gene interactions can produce ratios such as 9:7, 12:3:1, or 9:3:4, depending on the type of interaction (e.g., epistasis, complementary gene action).

4.8: Epistasis—Gene Interaction Modifies Phenotype

Epistasis occurs when one gene masks or modifies the effect of another gene at a different locus.

• Recessive epistasis: Homozygous recessive condition at one locus masks the expression of alleles at another locus (e.g., coat color in mice).

• Dominant epistasis: Dominant allele at one locus masks alleles at a second locus (e.g., fruit color in squash).

• Complementary gene action: Two genes interact such that both must have at least one dominant allele for a particular phenotype to be expressed (e.g., flower color in sweet peas).

Type	Example	Modified Ratio
Recessive Epistasis	Mouse coat color	9:3:4
Dominant Epistasis	Squash fruit color	12:3:1
Complementary Genes	Sweet pea flower color	9:7

4.9: Complementation Analysis—Determining Gene Relationships

Complementation analysis helps determine whether mutations with similar phenotypes are in the same gene or in different genes.

• Cross two mutant strains and analyze the F1 generation.

• If the F1 shows the wild-type phenotype, mutations are in different genes (complementation).

• If the F1 is mutant, mutations are in the same gene (no complementation).

4.10: Expression of a Single Gene—Multiple Effects (Pleiotropy)

Pleiotropy occurs when a single gene influences multiple phenotypic traits.

• Example: Marfan syndrome, caused by a dominant mutation in the gene encoding fibrillin, affects eyes, aorta, bones, and other tissues.

4.11: X, Y System of Sex Determination

Sex determination in many animals and plants is governed by the presence of X and Y chromosomes.

• Females: XX

• Males: XY (single copy of genes encoded by X chromosome)

4.12: X-Linkage—Genes on the X Chromosome

X-linked inheritance describes genes located on the X chromosome, with distinct inheritance patterns due to the difference in sex chromosomes between males and females.

• Hemizygosity: Males have only one X chromosome and are hemizygous for X-linked genes.

• Example: White-eyed mutation in Drosophila and color blindness/hemophilia in humans.

• Carrier females can pass recessive alleles to sons, who will express the trait if they inherit the mutant allele.

4.13: Sex-Limited and Sex-Influenced Inheritance

Some autosomal genes are expressed differently depending on the individual's sex, often due to hormonal influences.

• Sex-limited inheritance: Trait is expressed in only one sex (e.g., cock-feathering in chickens).

• Sex-influenced inheritance: Trait is expressed differently in males and females (e.g., pattern baldness in humans).

Genotype	Phenotype (Females)	Phenotype (Males)
BB	Bald	Bald
Bb	Not bald	Bald
bb	Not bald	Not bald

Additional info: These notes expand on the original slides and handwritten content, providing definitions, examples, and tables for clarity and completeness. All equations and ratios are described in text or tables as appropriate for genetics study.