Modification of Mendelian Ratios: Extensions and Exceptions to Classic Inheritance

Modification of Mendelian Ratios

Introduction to Modified Ratios

Mendelian genetics is based on the principles that genes are located on homologous chromosomes, which segregate and assort independently during gamete formation. However, classic Mendelian ratios (such as 3:1 and 9:3:3:1) are often modified when gene expression does not follow simple dominant/recessive patterns or when multiple genes influence a single trait.

• Alleles are alternative forms of a gene found at the same locus on homologous chromosomes.

• Gene expression can be influenced by environmental factors, sex chromosomes, and extranuclear DNA.

Allelic Variation and Its Effects

Types of Alleles and Mutations

Alleles can differ in their effects on phenotype, and mutations are the source of new alleles. The wild-type allele is most common in a population and is often, but not always, dominant.

• Loss-of-function mutation: Reduces or eliminates the function of the gene product. A complete loss results in a null allele.

• Gain-of-function mutation: Enhances the function or increases the quantity of the gene product, often by affecting gene regulation.

• Neutral mutation: Does not affect the phenotype.

Symbolizing Alleles

Genetic Nomenclature

Standard conventions are used to symbolize alleles:

• Lowercase italicized letter for recessive alleles (e.g., d for dwarf).

• Uppercase letter for dominant alleles (e.g., D for tall).

• In Drosophila melanogaster, mutant alleles are often denoted by an initial letter or combination (e.g., e for ebony body color).

• When no dominance exists, uppercase italic letters with superscripts are used to denote alternative alleles.

Non-Mendelian Dominance Relationships

Incomplete (Partial) Dominance

In incomplete dominance, neither allele is completely dominant. The heterozygote displays an intermediate phenotype.

• Example: Crossing red and white snapdragons yields pink offspring. The phenotypic and genotypic ratios are identical, as each genotype has a distinct phenotype.

Codominance

In codominance, both alleles in a heterozygote are fully expressed, resulting in a joint phenotype.

• Example: The MN blood group in humans, where both M and N glycoproteins are present on red blood cells if both alleles are inherited.

• Codominance differs from incomplete dominance, where the phenotype is blended rather than jointly expressed.

Multiple Alleles

More Than Two Alleles at a Locus

Multiple alleles refer to the presence of more than two alternative forms of a gene in a population. Each individual still carries only two alleles per gene.

• Example: The ABO blood group system in humans is determined by three alleles: IA, IB, and i.

Genotype	Phenotype (Blood Type)
IAIA or IAi	A
IBIB or IBi	B
IAIB	AB
ii	O

• At the white locus in Drosophila, over 100 alleles exist, producing a range of eye colors.

Lethal Alleles

Essential Genes and Lethality

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

• Recessive lethal alleles: Only lethal in homozygous state; heterozygotes survive.

• Example: In mice, the yellow coat color allele is dominant for color but recessive lethal. Homozygous yellow mice die before birth.

• Dominant lethal alleles: A single copy causes death. Example: Huntington disease in humans, which manifests later in life.

Gene Interactions and Modified Ratios

Combinations of Two Gene Pairs

When two gene pairs with different modes of inheritance are considered together, classic dihybrid ratios (9:3:3:1) can be modified.

• Example: Albinism (autosomal recessive) and ABO blood type (multiple alleles) in humans. The resulting phenotypic ratios are not classic Mendelian.

Gene Interaction and Epistasis

Gene interaction occurs when multiple genes influence a single phenotype. Epistasis is a specific type of gene interaction where one gene masks or modifies the effect of another.

• Epigenesis: The development of complex traits through the interaction of multiple genes (e.g., eye formation, hereditary deafness).

• Epistasis: One gene masks the expression of another. Example: The Bombay phenotype in humans, where a mutation at the FUT1 gene prevents expression of ABO blood group antigens.

Types of Epistasis

• Recessive epistasis: Homozygous recessive genotype at one locus masks expression at another locus. Example: In mice, bb genotype results in albinism regardless of the A allele.

• Dominant epistasis: A dominant allele at one locus masks alleles at a second locus. Example: In summer squash, dominant A allele produces white fruit regardless of the second gene.

• Complementary gene interaction: Both loci must have at least one dominant allele for a particular phenotype to be expressed (e.g., purple flowers in sweet peas).

• Novel phenotypes: Both gene pairs influence a trait equally, producing new phenotypes (e.g., fruit shape in squash: disc, sphere, long).

Pleiotropy

Single Gene, Multiple Effects

Pleiotropy occurs when a single gene affects multiple phenotypic traits.

• Example: Marfan syndrome, caused by a mutation in the gene for fibrillin, affects connective tissue in the eyes, aorta, bones, and more.

X-Linkage and Sex-Linked Inheritance

Genes on the X Chromosome

X-linked genes exhibit unique inheritance patterns because males (XY) have only one X chromosome, while females (XX) have two.

• Hemizygosity: Males are hemizygous for X-linked genes, expressing whatever allele is present.

• Crisscross inheritance: X-linked recessive traits are passed from mothers to all sons, but not to daughters unless the father is also affected.

• Examples: White-eyed mutation in Drosophila, red-green color blindness in humans, Duchenne muscular dystrophy (lethal X-linked recessive disorder).

Sex-Limited and Sex-Influenced Inheritance

Sex-Limited Inheritance

Some traits are expressed in only one sex, even though the genes are present in both sexes. Expression is often dependent on hormone levels.

• Example: Feather plumage in chickens—hen-feathering is dominant and expressed in both sexes, while cock-feathering is recessive and only expressed in males.

Sex-Influenced Inheritance

In sex-influenced inheritance, an allele's dominance varies between sexes.

• Example: Pattern baldness in humans—allele B is dominant in males but recessive in females, leading to different expression patterns.