Non Mendelian Genetics

Genetics Overview

  • Understanding traits requires knowledge of specific laws governing inheritance.

Laws of Inheritance

  • Law of Independent Assortment: Chromosomes align randomly at the metaphase plate, leading to various combinations of maternal and paternal characteristics in offspring.

  • Law of Segregation: Duplicated chromosomes separate, ensuring only one allele for each trait goes into a gamete, preventing the offspring from getting two alleles for the same trait (e.g., eye color).

Non-Mendelian Genetics

  • Inheritance can be more complex than Mendelian principles (dominant and recessive traits).

Degrees of Dominance

  • Incomplete Dominance: Phenotype is a blend of the parents. Example: Black (B) and white (W) rabbits producing gray (BW) offspring.

  • Codominance: Both dominant traits are expressed fully. Example: Black (B) and white (W) rabbits producing offspring with both black and white fur.

Multiple Alleles

  • Some traits are controlled by more than two alleles, resulting in various phenotype expressions.

  • Example: Blood groups with A (IA), B (IB), and O (i) alleles, leading to possible combinations (AA, AB, AO, etc.).

Polygenic Inheritance

  • Traits controlled by two or more genes, leading to a range of phenotypes.

  • Examples include height and skin color, where environmental factors also contribute to the expression of these traits.

Sex-Linked Traits

  • Genes located on the X or Y chromosome. Most sex-linked traits are X-linked due to the larger number of genes on the X chromosome.

Inheritance Pattern

  • Fathers pass X-linked alleles to daughters but not to sons (sons receive Y from father).

  • Mothers can pass X-linked alleles to both daughters and sons.

Carrier Status

  • Carriers have one normal allele and one affected allele for a recessive trait, allowing them to pass on the trait without expressing it themselves.

  • Males are hemizygous for X-linked traits; they express the trait if they inherit the affected X allele from their mother.

Common X-Linked Disorders

  • Conditions such as color blindness and hemophilia often appear in inheritance pattern questions.

X Inactivation

  • During development, one of the X chromosomes in females becomes inactive, forming a Barr body, which regulates gene expression.

  • The choice of which X is inactivated is random.

Linked Genes

  • Genes located close together on the same chromosome tend to be inherited together, showing parental phenotypes in higher proportions than recombinant phenotypes.

Recombinant vs. Parental Types

  • Parental Types: Offspring that resemble the parents.

  • Recombinant Types: Offspring that do not resemble either parent due to crossing over events during meiosis.

  • If recombination frequency exceeds 50%, genes are likely on different chromosomes (unlinked).

Genetic Variation Sources

  • Independent assortment, crossing over during meiosis, and the random fertilization of egg and sperm contribute to genetic diversity.