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Exploring Non-Mendelian Inheritance and DNA

Apr 25, 2025

Lecture 34: Non-Mendelian Inheritance and Introduction to DNA Structure

Overview

This lecture covers various forms of non-Mendelian inheritance and briefly introduces the structure and function of DNA.

Non-Mendelian Inheritance

Polygenic Inheritance

  • Definition: Multiple genes contribute to a single phenotype, resulting in continuous variation.
  • Example: Human traits like skin tone, hair color, eye color.
  • Key Term: Continuous variation - a range of phenotypic possibilities.

Incomplete Dominance

  • Definition: Heterozygotes have an intermediate phenotype between the two homozygotes.
  • Example: Snapdragon flowers - red (RR), white (rr), and pink (Rr).

Codominance

  • Definition: Both alleles are fully expressed in a heterozygote.
  • Example: Human blood types - A, B, and AB show codominance.

Multiple Alleles

  • Definition: More than two alleles exist for a given gene.
  • Example: Blood types in humans, involving alleles A, B, and O.

Recessive Lethal

  • Definition: An allele that is lethal in its homozygous form.
  • Example: Manx cats - homozygous recessive leads to embryonic lethality.

X-Linked Traits

  • Definition: Traits where genes are located on the X chromosome.
  • Examples: Red-green color blindness, Duchenne muscular dystrophy, and SCID X1.

Epistasis

  • Definition: Interaction where one gene masks or interferes with the expression of another.
  • Example: Coat color in mice involves two genes, one for pigment color and another for pigment production.

Introduction to DNA Structure and Function

DNA Composition

  • Nucleotides: Consist of a five-carbon sugar, a phosphate group, and a nitrogenous base.
  • Bases in DNA: Adenine (A), Guanine (G), Thymine (T), Cytosine (C).

DNA Structure

  • Double Helix: Two strands form this structure, held together by hydrogen bonds between bases.
  • Base Pairing: A pairs with T, and G pairs with C.
  • Antiparallel Strands: Strands run in opposite directions; one 5’ to 3’, another 3’ to 5’.

Important Concepts

  • Phosphate Backbone: 5’ phosphate connects to 3’ OH of the next nucleotide.
  • 5’ and 3’ Ends:
    • 5’ end: Free phosphate group.
    • 3’ end: Free OH group.

Conclusion

  • This lecture ends the discussion on genetics and begins the exploration of DNA in preparation for understanding its function in future lectures.

Full transcript