Non-Mendelian Inheritance Patterns Sep 8

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X-linked and Non-Mendelian Inheritance Patterns

X-linked Inheritance

X-linked inheritance refers to genetic traits whose genes are located on the X chromosome. These patterns differ from classical Mendelian inheritance due to the unique transmission of sex chromosomes.

X-linked recessive diseases are more common in males because males have only one X chromosome. Examples include color blindness, Duchenne muscular dystrophy, and hemophilia.
Traits are never passed from father to son in X-linked inheritance, as sons inherit their father's Y chromosome.
All sons of an affected female (with two mutant X chromosomes) will be affected.
Carrier mothers can pass the trait to sons (who will be affected) and daughters (who may be carriers).

Example: In a pedigree for X-linked recessive color blindness, affected males are more frequent, and the trait skips generations through carrier females.

X-linked Dominant Inheritance

X-linked dominant diseases are caused by dominant alleles on the X chromosome. These conditions are often more common in females due to the presence of two X chromosomes.

Trait never passed from father to son.
All daughters of an affected male will be affected, as they inherit his X chromosome.
Example: Rett syndrome is an X-linked dominant disorder.

Additional info: X-linked dominant traits can appear in every generation and affect both sexes, but females are often more frequently affected due to the presence of two X chromosomes.

Extensions of Mendelian Genetics

Incomplete (Partial) Dominance

Incomplete dominance occurs when neither allele is completely dominant over the other, resulting in a heterozygous phenotype that is intermediate between the two homozygotes.

Example: In snapdragons, crossing red-flowered (R1R1) and white-flowered (R2R2) plants produces pink-flowered (R1R2) offspring.
F2 ratio:

Additional info: The intermediate phenotype is due to the partial expression of both alleles, not blending.

Codominance

Codominance occurs when two alleles of a single gene both express distinct, detectable phenotypes in heterozygotes.

Example: The M and N forms of a glycoprotein on red blood cells. Individuals with genotype LMLN express both M and N antigens.

Genotype	Phenotype
LMLM	M
LMLN	MN
LNLN	N

Note: Codominance is distinct from incomplete dominance; both alleles are fully expressed, not blended.

Codominance and Multiple Alleles: ABO Blood Group

The ABO blood group system in humans is determined by three alleles (IA, IB, IO) that encode antigens (agglutinogens) on the surface of red blood cells.

Genotype	Antigen	Phenotype
IAIA	A	A
IAIO	A	A
IBIB	B	B
IBIO	B	B
IAIB	A, B	AB
IOIO	Neither	O

Additional info: The Bombay phenotype is a rare condition where individuals lack the H antigen required for A or B antigen expression, resulting in an O phenotype regardless of genotype.

Blood Transfusions and Agglutination

Blood transfusions require compatibility between donor and recipient blood types to prevent agglutination (clumping) of red blood cells.

Blood Type	Receive Blood	Give Blood
O	O only	Any type
A	A, O	A, AB
B	B, O	B, AB
AB	AB, A, B, O	AB only

Type O: Universal donors (no A or B antigens)
Type AB: Universal recipients (no anti-A or anti-B antibodies)

Example: Incompatible transfusions cause agglutination, which can be life-threatening.

Probability in Genetics

Probability calculations are essential for predicting genetic outcomes in offspring.

Example: For a couple with IOIO and IAIB genotypes, the probability that all four children have A blood type is calculated as follows:

Each child has a 0.5 probability of being IAIO (A blood type). For four children:

or 6.25% chance that all four children have A blood type.

Types of Mutations

Mutation Classifications

Mutations are changes in the DNA sequence that can affect gene function in various ways.

Loss-of-function: Allele results in reduction or elimination of the functional activity of the protein or RNA encoded by the affected gene. Complete loss is called a null allele.
Gain-of-function: Allele results in increased expression or activity of the affected gene product.
Neutral: Allele does not detectably change the function of the affected gene.

Additional info: Loss-of-function mutations are often recessive, while gain-of-function mutations are frequently dominant.

Additional Topics for Further Study

Recessive lethal alleles: Alleles that cause death when present in homozygous form.
Genetic linkage: Genes located close together on the same chromosome tend to be inherited together.
Epistasis: Interaction between genes where one gene masks or modifies the effect of another.
Mouse coat color problems: Classic examples for studying genetic interactions and inheritance patterns.