Every trait is controlled by a single gene model proposed by Mendel.
Traits represented by letters, e.g., dominant allele 'A' and recessive allele 'a'.
Only two alleles exist in the population for a particular trait.
Example: Normal pigmentation (A) vs. Albinism (a).
Dominant relationship:
Inherit 'A': Normal coloration.
Inherit 'aa': Albinism phenotype.
Albinism in Animals
Albinism observed across various animal species.
Recessive phenotype: Shows albinism only if genotype is 'aa'.
Dominant phenotype: Exhibits normal coloration if at least one dominant allele is present.
Limitations of Mendel's Rules
Many traits do not strictly follow Mendelian genetics.
Mendel's laws remain useful for fundamental inheritance understanding and Punnett square applications.
Need to explore non-Mendelian inheritance types for complexity.
Polygenic Inheritance
Many human traits (e.g., skin color, hair color, eye color, height) are governed by multiple genes.
Phenotypic differences arise from cumulative effects of several alleles:
Continuous variation observed in a normal distribution (bell curve).
Most people exhibit average traits, with fewer at extremes (very tall or very short).
Examples of Polygenic Traits
Height: Wide range indicating multiple gene interactions.
Skin Color: Also polygenic, showing a spectrum of pigmentation.
Eye Color: Controlled by multiple genes, but fewer combinations than height or skin color.
Melanin and Coloration
Color produced by melanin:
No melanin: Gray/blue eyes.
Low melanin: Green/hazel eyes.
High melanin: Dark brown eyes.
Variations arise due to genetic combinations affecting melanin production.
Pleiotropy
A single gene can influence multiple phenotypic traits.
Example: Cystic fibrosis arises from a single gene mutation affecting a protein responsible for salt ion transport, impacting:
Respiratory system.
Reproductive system.
Digestive system.
Pleiotropy vs. Polygenic Inheritance:
Polygenic: Many genes influence one trait.
Pleiotropy: One gene influences many traits.
Dominance and Lethal Alleles
Some dominant alleles lead to lethal homozygous conditions, e.g., achondroplasia (dwarfism).
Example: Manx gene in cats (tailless cats).
Taillessness (T) is heterozygous. Homozygous (TT) is lethal.
Multiple Alleles in Populations
A gene may have more than two alleles present in the population.
Each individual has two alleles for any given gene (diploid).
Example: Fruit flies with multiple eye color alleles, despite having only two alleles in one organism.
Blood Types as a Case Study
Human blood types illustrate multiple alleles and codominance.
Three alleles: A, B, and O.
A and B are codominant, O is recessive (represented as 'i').
Genotypes and Phenotypes in Blood Types
Type A: Genotypes AA or Ai.
Type B: Genotypes BB or Bi.
Type AB: Must be genotype AB (express both sugars).
Type O: Genotype ii (homozygous recessive).
Blood Compatibility and Transfusions
Blood type compatibility is crucial for safe transfusions:
Example: Type A has anti-B antibodies; transfusion from type B would trigger an immune response and attack resulting in possibly fatal clots.
Type O as the universal donor: Lacks surface antigens, hence not recognized as foreign.
Incomplete Dominance and Codominance
Incomplete Dominance: There is a blending of traits.
Example: Snapdragons produce red, white, or pink flowers from homozygous red (RR), homozygous white (WW), and heterozygous (RW).
In F2 generation: Phenotypic ratio 1:2:1.
Codominance: Both traits expressed simultaneously.
Example: Cattle with red and white fur; heterozygous phenotype is roan (both red and white displayed).
Conclusion
Understanding these genetic concepts beyond Mendelian rules is essential for comprehending the complexity of inheritance patterns in various organisms.