BIO102 Ch14c

Definition: When a gene exists in more than two alleles within a population; this can result in many variations of a trait.
Example: Human blood types, which include three alleles (IA, IB, i)

Definition: A situation where a heterozygote expresses both alleles equally, resulting in a phenotype that shows both traits.
Example: In human blood types, individuals with genotype IAIB express both A and B antigens on their red blood cells.

Definition: A form of inheritance where the phenotype of heterozygotes is intermediate between those of the two homozygotes.
Example: Four-o'clocks produce pink flowers (F1) when red (RR) and white (rr) flowered plants are crossed. In the F2 generation, the ratio of red, pink, and white flowers is 1:2:1.

Definition: A single gene influences multiple traits.
Example: Marfan syndrome is caused by mutations in a single gene that affects several connective tissue characteristics, including:
- Nearsightedness
- Scoliosis
- Heart valve defects

Essential Genes: Required for survival; mutations may lead to lethal phenotypes.
- Dominant Lethal Alleles: Causes death in both homozygous and heterozygous individuals (e.g., Huntington’s disease).
- Recessive Lethal Alleles: Causes death only in homozygous recessive individuals (e.g., Tays-Sachs disease).

Two Genes Affecting One Trait: Interaction between different nonallelic genes can produce new phenotypes.
Example: In chickens, comb shape is influenced by two genes (R and P), leading to variations in their phenotype while following Mendelian ratios.

Definition: One gene locus masks or modifies the phenotype of another locus, resulting in altered phenotypic ratios.
Example: Coat color in rodents where certain alleles can mask others, leading to ratios like 9:3:4.
Types of Epistasis:
- Recessive Epistasis: Homozygous recessive at one locus affects the phenotype of another locus (e.g., white coat in mice).
- Dominant Epistasis: A dominant allele at one locus masks the expression of alleles at a different locus, resulting in ratios like 12:3:1.

Phenotype Expression: Many traits result from interactions between genes and environmental factors.
Penetrance: The proportion of individuals with a given genotype that exhibit the associated phenotype.
- Complete Penetrance: 100% of individuals show the phenotype.
- Incomplete Penetrance: Less than 100% show the phenotype (e.g., in some cancers).
Expressivity: The degree to which a penetrant gene manifests itself phenotypically. It can vary among individuals even with the same genotype.

Factors influencing phenotypes can include:
- Age
- Sex
- Temperature
- Chemical Exposures

Involves genes not located in the nucleus, such as those in mitochondria and chloroplasts.
Mitochondrial genes often exhibit maternal inheritance patterns, affecting energy production in cells.

Pedigrees are used to track traits through generations, helping identify recessive, dominant, autosomal, and sex-linked traits.
Autosomal Recessive Traits: Traits that often skip generations; affected individuals have unaffected parents who are carriers.
Autosomal Dominant Traits: Traits that do not skip generations and affected individuals usually have at least one affected parent.
Sex-Limited Traits: Traits that are only expressed in one sex, such as milk production in females.
X-Linked Recessive Traits: Typically skip generations and affect males more than females, as males only need one recessive gene to be affected.
X-Linked Dominant Traits: Affects both males and females but cannot be passed from father to son.

Contain genes primarily responsible for male sexual development; very few known traits are associated with the Y chromosome.