Mendelian Genetics: Laws, Patterns, and Variations

Mendelian Genetics

Mendel's Laws and Hypotheses

Gregor Mendel's experiments with pea plants led to foundational principles in genetics. He proposed four key hypotheses to explain patterns of inheritance:

• Hypothesis 1: There are alternative versions of genes (alleles) that account for variations in inherited characteristics.

• Hypothesis 2: For each character, an organism inherits two alleles, one from each parent.

• Hypothesis 3: If two alleles differ, the dominant allele determines the organism's appearance; the recessive allele has no noticeable effect.

• Hypothesis 4: Each gamete carries only one allele for each character because the two alleles for a character segregate during gamete production.

Example: Mendel's pea plant crosses for flower color demonstrated these principles, showing how traits are inherited across generations.

Law of Segregation

The law of segregation states that during gamete formation, the two alleles for a character separate (segregate) so that each gamete carries only one allele for each gene.

• Definition: Each egg or sperm carries only one allele for each inherited gene because allele pairs separate during gamete production.

• Application: Explains why offspring can inherit traits not visible in the parents (reappearance of recessive traits).

Example: In Mendel's pea plants, crossing purple-flowered (PP) and white-flowered (pp) plants resulted in all purple F1 offspring (Pp), but the F2 generation included both purple and white flowers (PP, Pp, pp).

Law of Independent Assortment

The law of independent assortment states that alleles of different genes assort independently of one another during gamete formation.

• Definition: Each pair of alleles segregates independently of other pairs during gamete formation.

• Application: The inheritance of one character does not affect the inheritance of another.

Example: Pea plant traits such as seed color and seed shape are inherited independently.

Genetic Crosses and Punnett Squares

Monohybrid Crosses: Angus Cattle Example

In Angus cattle, hair color is controlled by two alleles: B (dominant, black) and b (recessive, red). Punnett squares help predict offspring genotypes and phenotypes.

B	B
Bb	Bb

Genotypes: 100% Bb Phenotypes: 100% black

B	b
Bb	bb

Genotypes: 50% Bb, 50% bb Phenotypes: 50% black, 50% red

Dihybrid Crosses: Pea Plant Color Example

Pea plants have two alleles for color: Y (yellow, dominant) and y (green, recessive).

Y	y
Yy	yy
Yy	yy

Genotypes: 25% YY, 50% Yy, 25% yy Phenotypes: 75% yellow, 25% green

Pedigrees and Inheritance Patterns

Pedigree Analysis

Pedigrees are diagrams that show inheritance patterns across generations. Squares represent males, circles represent females, and shaded symbols indicate individuals affected by a trait.

• Dominant Trait: Trait appears in every generation; affected individuals have at least one affected parent.

• Recessive Trait: Trait can skip generations; affected individuals may have unaffected parents.

Example: If two parents without the trait have a child with the trait, the trait is likely recessive.

Types of Dominance

Type	Description	Example
Complete dominance	At least one dominant allele is present and that trait is expressed in the phenotype.	Black fur is dominant over brown fur in Labrador dogs.
Incomplete dominance	The phenotype of a heterozygote is intermediate between the two homozygotes.	Red flowers and white flowers produce pink flowers when crossed.
Codominance	In the heterozygote, both alleles are fully expressed.	Blood type AB: both A and B carbohydrates are present on the surface of the same cell.

Blood Type Inheritance

Human blood types are determined by multiple alleles (IA, IB, i). The possible phenotypes and genotypes for offspring depend on parental genotypes.

IA	i
IAi	ii

Genotypes: IAi (50%), ii (50%) Phenotypes: Type A (50%), Type O (50%)

Variations on Mendel's Laws

Pleiotropy

Description	How does it deviate from Mendel's Laws?	Example
When one gene influences several characters.	Mendel's laws focus on the impact of one gene on one character instead of one gene on multiple characters.	Sick Cell Disease, phenylketonuria, albinism, Marfan syndrome.

Polygenic Inheritance

Description	How does it deviate from Mendel's Laws?	Example
Additive effects of two or more genes on a single phenotypic character (continuous or spectrum).	Mendel's laws focus on the impact of one gene on one character instead of multiple genes on one character.	Eye color, skin color, and height.

Epigenetic Inheritance

Description	How does it deviate from Mendel's Laws?	Example
Transmission of traits by mechanisms not directly involving DNA sequence; there are external modifications to DNA that turn genes on or off.	Epigenetic inheritance accounts for the effects of external (environmental) factors such as stress and diet on the genome, whereas Mendel's laws focus strictly on gene sequences.	Epigenetic changes (such as histone modification or DNA methylation) may be passed on to future generations if the changes occur in sperm or egg cells. Transgenerational epigenetic inheritance is still being studied, especially in imprinting, the amount of mouse and environmental impact on inheritance (exposure to toxins and chronic stress).

Key Equations and Concepts

• Law of Segregation: $\text{Each gamete receives only one allele for each gene}$

• Law of Independent Assortment: $\text{Alleles of different genes assort independently during gamete formation}$

• Punnett Square Probability: $\text{Probability} = \frac{\text{Number of desired genotype}}{\text{Total number of possible genotypes}}$