JC Genetics Chapter 4

Mendelian inheritance: Patterns that obey Mendel’s laws of inheritance.
- Laws of Segregation: Alleles segregate during gamete formation.
- Laws of Independent Assortment: Genes for different traits can segregate independently.

Involves a single gene with two alleles.
Alleles show a dominant/recessive relationship.
This chapter explores traits that deviate from simple relationships while still following Mendelian laws.

Examination of inheritance to predict outcomes and understand gene-trait relationships.

Simple Mendelian Inheritance: Two alleles obey Mendel’s laws with strict dominant/recessive traits.
- Functional allele: 50% of protein produced in a heterozygote is sufficient for the trait.
Incomplete Penetrance: Dominant phenotype not expressed despite the presence of a dominant allele (e.g., polydactyly).
Incomplete Dominance: Heterozygotes exhibit an intermediate phenotype (e.g., pink flowers from red and white parents).
Heterozygote Advantage: Heterozygotes may have an advantage in reproductive success (e.g., sickle cell trait).
Codominance: Both alleles expressed simultaneously (e.g., blood type AB).
X-linked Inheritance: Genes on the X chromosome, affecting males and females differently due to hemizygosity.

Sex-influenced Inheritance: Dominance varies by sex (e.g., traits that are dominant in females but recessive in males).
Sex-limited Inheritance: Traits occur in one sex only (e.g., sperm production in males).
Lethal Alleles: Alleles that cause organism death; often recessive and can alter expected Mendelian ratios.

Wild-type alleles: Common alleles in a population that code for normal functioning proteins.
Mutant alleles: Altered by mutation, often recessive, and lead to dysfunctional proteins.
In simple dominant/recessive relationships, the phenotypic effects of recessive alleles are masked in heterozygotes, often due to sufficient normal protein production.

Phenylketonuria: Inability to metabolize phenylalanine; dietary management prevents cognitive impairment.
Albinism: Lack of pigmentation due to tyrosinase mutation.
Cystic Fibrosis: Chloride transporter mutation causes thick mucus and organ malfunctions.

Much rarer; examples include gain-of-function, dominant-negative effects, and haploinsufficiency.

A dominant trait may not always manifest; presence of the allele does not guarantee expression (e.g., polydactyly).
Measured at the population level and varies among individuals.

Environmental conditions can greatly impact phenotypes (e.g., arctic fox's seasonal coat variation).
The reaction norm reflects how various environments influence traits.

Incomplete Dominance: Heterozygote shows an intermediate phenotype; phenotypic ratios differ from classic Mendelian ratios.

Heterozygotes resistant to malaria while affected individuals have health issues due to sickle-shaped cells.

Interactions between different genes influence single trait outcomes, affecting classic Mendelian inheritance patterns.
Epistasis: One gene masks the effect of another.

Gene modification: One gene's allele modifies another gene's phenotypic effects (e.g., pigment production in parakeets).
Gene redundancy: Loss-of-function alleles may not affect phenotype due to functional compensation by other genes.

The complexities of Mendelian inheritance extend beyond simple patterns of dominance and recessivity, influenced by multiple factors including environment and gene interaction.