Genetic Deviations and Inheritance Patterns: Mendel, Blood Types, and Epistasis

Deviations from Mendel's Principles

Not all traits follow simple dominant/recessive patterns; many genes show multiple alleles, codominance, incomplete dominance, or gene interactions.

Multiple Alleles

When a gene has more than two allelic forms within a population.

Genotype Formula

The number of possible genotypes = n(n+1)/2, where n = number of alleles.

ABO Blood Group System

Example of multiple alleles with three alleles (IA, IB, i) producing four phenotypes (A, B, AB, O).

Glycosyltransferase

Enzyme coded by ABO gene that adds specific sugars to the H antigen on red blood cells, forming A or B antigens.

IA Allele

Produces an enzyme that adds one specific sugar to the H antigen, forming the A antigen.

IB Allele

Produces an enzyme that adds a different sugar to the H antigen, forming the B antigen.

i Allele

Produces no functional enzyme; red blood cells carry only the H antigen (type O).

Universal Donor

Blood type O; contains only the common H antigen, so it can be given to any recipient.

Universal Recipient

Blood type AB; has both A and B antigens and can receive all blood types.

Complete Dominance

The dominant allele completely masks the recessive allele in heterozygotes.

Incomplete Dominance

Heterozygote phenotype is intermediate between the two homozygotes.

Codominance

Both alleles are expressed equally in the heterozygote (e.g., AB blood type).

Haplosufficient

One functional copy of a gene is enough to produce a normal phenotype.

Loss of Function Allele

Allele that produces no or nonfunctional product; often recessive.

Gain of Function Allele

Allele that produces a new or abnormal function; often dominant.

Essential Gene

Gene required for the survival of the organism.

Lethal Allele

Allele that causes death when present in an organism; can be dominant or recessive.

Tay-Sachs Disease

Recessive lethal disorder caused by mutation in HEXA gene; leads to nervous system deterioration and death by age 3-4.

Gene-Environment Interaction

Phenotype is influenced by both genetic makeup and environmental conditions.

Penetrance

Percentage of individuals with a particular genotype that actually express the expected phenotype.

Incomplete Penetrance

When less than 100% of individuals with a genotype show the expected phenotype.

Expressivity

Degree or intensity with which a particular genotype is expressed in an individual.

Osteogenesis Imperfecta

Disorder showing variable expressivity; may include blue sclera, fragile bones, and deafness.

Age of Onset

The age when a genotype begins to be expressed as a phenotype.

Temperature Effect

Some alleles produce enzymes that function only at specific temperatures, altering phenotype.

Chemical Exposure

Genetic ability to metabolize or clear chemicals can affect phenotype depending on exposure level.

Sex Influence

The sex of the individual can influence the expression of sex-linked, sex-limited, or sex-influenced traits.

Chromosome Theory of Inheritance

Genes are located on chromosomes and are transmitted from parents to offspring via chromosomes.

Homogametic Sex

Produces identical gametes (XX in humans).

Heterogametic Sex

Produces two types of gametes (XY in humans).

Y Chromosome

Contains testis-determining factor, which triggers male development.

Turner Syndrome

45, X karyotype; short stature, webbed neck, infertility, missing one X chromosome.

Klinefelter Syndrome

47, XXY karyotype; underdeveloped testes, sometimes breast development, may have lower intelligence.

X-Linked Gene

Gene located on the X chromosome; males are hemizygous for these genes.

Hemizygous

Having only one copy of a gene, as males do for genes on the X chromosome.

Dosage Compensation

Mechanism that equalizes expression of X-linked genes between males and females.

Barr Body

Inactivated X chromosome in female somatic cells; example of epigenetic regulation.

Epigenesis

Change in gene expression without altering the DNA sequence.

X-Linked Recessive Inheritance

Trait expressed more in males because they have only one X chromosome (e.g., hemophilia).

X-Linked Dominant Inheritance

Trait expressed in both sexes but more severe in females.

Y-Linked Inheritance

Trait passed directly from father to son; appears in every generation.

Epistasis

Interaction between two or more genes where one gene masks or modifies the effect of another.

Recessive Epistasis

Homozygous recessive genotype at one locus masks the expression of another gene; 9:3:4 ratio.

Dominant Epistasis

Dominant allele at one locus masks the effect of another; 12:3:1 ratio.

Agouti Gene (A)

Controls pigment banding in hair; A allele produces agouti pattern, a/a does not.

C Gene

Codes for enzyme in eumelanin pathway; c/c results in albino phenotype (no melanin).

Albino Phenotype

Occurs when C gene is homozygous recessive (c/c); no pigment production regardless of other alleles.

Summer Squash Color

Example of dominant epistasis; white allele (A) masks yellow (B) and green (b) colors.

Take-Home Message

Many genetic traits deviate from simple Mendelian inheritance due to multiple alleles, incomplete dominance, codominance, gene-environment interactions, epistasis, and chromosomal factors.