Genetics Study Guide: Language and Structure of DNA, Mendelian Inheritance, and Allelic Variation

Language and Structure of DNA

Introduction to DNA

DNA (Deoxyribonucleic acid) is the hereditary material in almost all living organisms. Its structure and function are central to genetics, as it encodes the information necessary for the development, functioning, and reproduction of life.

• Nucleosides: Consist of a sugar and a nitrogenous base.

• Nucleotides: Consist of a sugar, a nitrogenous base, and a phosphate group.

• DNA is a polymer of nucleotides joined by phosphodiester bonds between the 3' carbon of one sugar and the 5' carbon of the next.

• DNA strands have polarity: a free 5' end and a free 3' end.

Single-Stranded and Double-Stranded DNA

• Single-stranded DNA is a chain of nucleotides (monomer units).

• Double-stranded DNA forms a double helix structure, with two antiparallel strands held together by hydrogen bonds between complementary bases (A=T, C≡G).

• Base stacking forces (hydrophobic interactions) contribute to the stability of the double helix.

DNA Helix Conformations

• B form: Right-handed, most common under physiological conditions.

• Z form: Left-handed, occurs in special sequences (alternating purine-pyrimidine).

• A form: Right-handed, can be induced by dehydration or protein binding.

X-ray Diffraction and DNA Structure

• X-ray diffraction patterns (e.g., Rosalind Franklin's photo 51) were crucial in determining the double helix structure of DNA.

Grooves in DNA

• DNA double helix has major and minor grooves that contribute to sequence-specific protein binding.

• Proteins such as transcription factors interact with specific DNA sequences via these grooves.

DNA Helix Stability and Flexibility

• B-DNA has general measurements: major groove (2.2 nm), minor groove (1.2 nm), 10 base pairs per turn, 0.34 nm between bases.

• Local parameters can differ based on sequence and environment, allowing flexibility for protein binding.

Mendelian Inheritance

Gregor Mendel's Experiments

Gregor Mendel, through his work with pea plants, established the foundational principles of inheritance. He studied seven traits, each controlled by a single gene with two alleles.

• Traits: Seed color, seed shape, flower color, pod shape, pod color, flower position, stem length.

• Each trait had two alleles: dominant (wildtype) and recessive (mutant, loss of function).

Mendel's Laws of Inheritance

• First Law (Law of Segregation): Each individual has two alleles for each gene, which segregate equally during gamete formation. Each gamete has a 50% chance of inheriting either allele. Equation: $P(A) = 0.5$, $P(a) = 0.5$

• Second Law (Law of Independent Assortment): Alleles of different genes assort independently, producing a 9:3:3:1 phenotypic ratio in dihybrid crosses if genes are on different chromosomes.

Modern Understanding of Mendelian Laws

• Genes are carried by chromosomes.

• Alleles are separated during meiosis when homologous chromosomes segregate.

• For independent assortment, genes must be on different chromosomes; linked genes do not assort independently.

Exceptions and Extensions

• Genes on the same chromosome are linked and do not assort independently, resulting in fewer recombinant gametes.

• New combinations (e.g., Ac, aC) are less frequent due to linkage.

Allelic Variation and Genotype-Phenotype Relationships

Alleles and Mutation

• Allele: A variant form of a gene, resulting from mutations such as small changes in DNA sequence, or partial/complete gene deletions.

• Sources of mutation: Mutagens (UV light, chemicals), replication errors (higher fidelity in animals, lower in viruses).

• Most mutations are neutral; some are beneficial or deleterious.

Population Genetics: Multiple Alleles

• A population can have multiple alleles for a gene (e.g., ABO blood types).

• A diploid individual can carry only two alleles: homozygous (identical alleles) or heterozygous (different alleles).

Genotype to Phenotype Translation

• The phenotype produced by an allele depends on the protein's quantity and function.

• Changes in DNA sequence can affect protein amount, sequence, or function, leading to different phenotypes.

• Alleles can be categorized as wild type, gain of function, loss of function, dominant, or recessive.

Interaction of Alleles

• Both alleles interact to control a trait; the effect depends on the changes in protein activity.

• Dominant allele: Manifests its phenotype regardless of the other allele.

• Recessive allele: Only manifests in homozygous individuals.

• Codominance: Both alleles are expressed (e.g., AB blood type).

Case Study: ABO Blood Types

The ABO blood group system is determined by three alleles (A, B, O) of a single gene encoding a glycosyltransferase enzyme.

Genotype (alleles)	Protein status	Phenotype (blood type)
AA or AO	Only A version produced	Type A
BB or BO	Only B version produced	Type B
AB	Both A and B version produced	Type AB
OO	No protein produced	Type O

• A is dominant to O; B is dominant to O; A and B are codominant.

DNA Mutations and Their Consequences

Types of Mutations

• Nucleotide substitution: One nucleotide is replaced by another.

• Nucleotide insertion: Addition of one or more nucleotides.

• Nucleotide deletion: Removal of one or more nucleotides.

• Frameshift mutation: Insertions or deletions that alter the reading frame of a gene.

• Chromosomal rearrangement: Large-scale changes in chromosome structure.

Forward and Back Mutations

• Forward mutation: Alters the function of a gene.

• Back mutation: Reverses the effect of a forward mutation (possible for insertions, not deletions).

Misconceptions About Mutations

• Most mutations are neutral, not all are bad.

• Not all mutations give rise to superpowers; most are neutral or loss-of-function.

• Gain-of-function mutations occur naturally and are not always beneficial.

Additional info:

• COVID-19 variants illustrate rapid mutation rates in viruses due to low replication fidelity, leading to dominant variants through natural selection.