Monohybrid and Dihybrid Crosses: Mendelian Ratios

Understanding Mendel's Experimental Approach

Mendel's experiments with pea plants established the foundational principles of inheritance. By performing monohybrid and dihybrid crosses, he revealed predictable ratios for the transmission of traits.

• Monohybrid Cross: Involves one gene; F2 ratio is 1:2:1 for genotypes and 3:1 for phenotypes.

• Dihybrid Cross: Involves two genes; F2 ratio is 9:3:3:1 for phenotypes, demonstrating independent assortment.

• Trihybrid Cross: Extends Mendel's principles to three genes, showing that inheritance patterns apply to multiple traits.

Mathematical Rules in Genetics

Simple mathematical rules help predict the outcomes of genetic crosses involving independent assortment.

Probability and Pedigree Analysis

Laws of Probability in Genetics

Probability laws are essential for predicting genetic outcomes:

• Product Rule (Multiplication): Used for simultaneous independent events.

• Sum Rule (Addition): Used when considering mutually exclusive events.

Pedigree Analysis

Pedigrees are diagrams that reveal inheritance patterns of human traits. They are crucial for tracking traits through generations and for identifying modes of inheritance (dominant, recessive, X-linked, etc.).

Extensions to Mendelian Genetics

Alleles and Phenotypes

Understanding alleles is fundamental to genetics. An allele is an alternate form of a gene. The most common allele in a population is called the wild-type allele, which is often, but not always, dominant. Mutant alleles arise from mutations and may result in altered gene products.

Types of Mutant Alleles

• Loss-of-function allele: Reduces or eliminates gene function.

• Null allele: Complete loss of function.

• Gain-of-function allele: Increases gene activity or confers a new function.

• Neutral mutation: No effect on phenotype or function.

Symbols for Alleles

• Dominant alleles: Italic uppercase letter (e.g., D for dominant, d for recessive).

• Recessive alleles: Italic lowercase letter or with a + superscript for wild-type (e.g., Wr+).

• No dominance: Use uppercase letters with superscripts (e.g., R1, R2).

Non-Mendelian Inheritance Patterns

Incomplete Dominance

Incomplete dominance occurs when the heterozygote displays an intermediate phenotype between the two homozygotes. Neither allele is completely dominant.

• Example: In snapdragons, crossing red and white flowers yields pink offspring.

• Genotype ratio equals phenotype ratio (1:2:1).

Codominance

Codominance is the joint expression of both alleles in a heterozygote, resulting in both gene products being detectable.

• Example: MN blood group in humans, where both M and N antigens are expressed in heterozygotes.

Multiple Alleles

Some genes have more than two alleles in a population. The classic example is the ABO blood group system in humans, controlled by three alleles: IA, IB, and i.

• Phenotypes: A, B, AB, and O blood types.

• Inheritance patterns can be predicted by examining parental genotypes.

Bombay Phenotype and Epistasis

The Bombay phenotype is an example of epistasis, where a mutation in the FUT1 gene prevents the synthesis of the H substance, masking the expression of A and B antigens. Individuals with the Bombay phenotype appear as blood type O, regardless of their ABO genotype.

Gene Interactions and Epistasis

Gene Interactions

Many phenotypes are influenced by interactions between multiple genes. Epistasis occurs when one gene masks or modifies the effect of another gene at a different locus.

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

• Dominant epistasis: Dominant allele at one locus masks expression at another (e.g., fruit color in squash).

• Complementary gene action: Both genes must have at least one dominant allele to express a phenotype (e.g., flower color in peas).

Lethal Alleles and Pleiotropy

Lethal Alleles

Lethal alleles cause death when present in certain genotypes. Recessive lethal alleles are tolerated in heterozygotes but lethal in homozygotes. Dominant lethal alleles cause death even in heterozygotes, but are rare because they often prevent reproduction.

• Example: Yellow coat color in mice is dominant for color but recessive lethal.

Pleiotropy

Pleiotropy occurs when a single gene influences multiple phenotypic traits. Examples include Marfan syndrome (affecting connective tissue, skeleton, eyes, and cardiovascular system) and variagate porphyria (affecting skin and nervous system).

Sex-Linked, Sex-Limited, and Sex-Influenced Inheritance

X-Linked Genes

Genes located on the X chromosome exhibit unique inheritance patterns, as males have only one X chromosome. The inheritance of white-eye color in Drosophila is a classic example of X-linkage.

Sex-Limited and Sex-Influenced Traits

Sex-limited traits are expressed in only one sex, while sex-influenced traits are expressed differently in males and females due to hormonal differences, despite being autosomal.

• Example (sex-limited): Cock feathering in chickens occurs only in males.

• Example (sex-influenced): Pattern baldness in humans is dominant in males but recessive in females.

Genetic Background, Environment, and Conditional Mutations

Penetrance and Expressivity

Penetrance is the percentage of individuals with a genotype that show the expected phenotype. Expressivity is the degree to which a phenotype is expressed among individuals with the same genotype.

Environmental Effects and Conditional Mutations

Environmental factors such as temperature can influence gene expression. Conditional mutations (e.g., temperature-sensitive mutations) are only expressed under certain environmental conditions and are useful for studying essential genes.

Genetic Anticipation

Anticipation in Genetic Disorders

Genetic anticipation refers to the phenomenon where certain genetic disorders appear at an earlier age and with increased severity in successive generations. This is often due to the expansion of unstable DNA repeats, as seen in myotonic dystrophy.

• Example: The DMPK gene on chromosome 19 contains unstable CTG repeats; longer repeats correlate with earlier onset and increased severity.

Summary Table: Key Genetic Ratios and Interactions

Additional info: These notes integrate foundational Mendelian genetics with extensions and exceptions, providing a comprehensive overview for exam preparation in a college-level genetics course.