Non-Mendelian Inheritance and Gene Linkage

Non-Mendelian Inheritance

• Incomplete dominance: Traits are neither dominant nor recessive; a blend of traits occurs. Example: crossing a red flower with a white flower produces pink flowers. Offspring phenotype ratio of a cross between heterozygous pink flowers is 1:2:1 (red:pink:white).
• Co-dominance: Both traits are expressed. Example: black and white parents can produce black and white offspring.
• Multiple alleles: Many genes affect a trait; examples include height and blood type.
• Polygenic inheritance: Many genes contribute to a characteristic (e.g., skin color, hair color, height). This variation in genotype results in a bell-shaped curve in the population.

Gene Linkage

• Genes located on the same chromosome are linked and tend to be inherited together.
• The closer genes are on a chromosome, the stronger the linkage.
• In a dihybrid cross, linked genes deviate from a 9:3:3:1 phenotypic ratio.

Sex-Linked Genes

• Carried on the sex chromosomes (primarily the X chromosome).
• X-linked traits:
  • Females inherit one X from each parent. A recessive mutated gene requires two copies for expression; a single copy makes the female a carrier.
  • Males inherit one X from their mother and a Y from their father. If the X chromosome has a mutated gene, the male will express the trait; males are not carriers.
• Examples of X-linked recessive traits: color blindness and hemophilia.
• X-linked genes are passed from fathers to daughters only.
• Males are affected more often by sex-linked traits than females.

Sex-Influenced Traits

• Genes present in both sexes but appear more frequently in one sex due to hormonal influences.
• Example: Baldness.

Karyotypes

• Laboratory procedure analyzing chromosome size, shape, and number during metaphase.
• Humans have 46 chromosomes (44 autosomes + 2 sex chromosomes).
• Male: XY, Female: XX.
• Gametes (sperm and ova) have 23 chromosomes.

Non-Disjunction

• Error during meiosis where homologous chromosomes fail to separate.
• Results in aneuploidy (abnormal chromosome number).
• Trisomy: having three copies of a chromosome. Down syndrome (trisomy 21) is an example.
• Triploid (3n): organism with three sets of chromosomes.
• Polyploidy: organism with more than three sets of chromosomes.

Gene Defects

• Hemophilia: Inability of blood to clot.
• Phenylketonuria (PKU): Deficiency in a specific enzyme.
• Sickle cell anemia: Substitution of glutamic acid with valine in hemoglobin, leading to abnormal RBC shape and impaired oxygen transport.
• Tay-Sachs disease: Mutation in a lysosomal enzyme.

Blood Groups

• Blood type inheritance: A and B are dominant; AB is co-dominant; O is recessive.