Principles of Genetics and Speciation

Introduction to DNA and Chromosomes

• DNA is defined as the genetic material of a complex molecule whose structures direct protein synthesis.

• Structure: It possesses a double helix configuration, frequently compared to a ladder.

• Composition:     * DNA features a Sugar and Phosphate Backbone.     * It is composed of four amino acids that form base pairs.

• Base Amino Acid pairs:     * Adenine     * Guanine     * Thymine     * Cytosine

• Pairing Rules:     * Adenine pairs with Thymine ($A-T$).     * Cytosine pairs with Guanine ($G-G$).

• Chromosomes:     * Located within the cell nucleus.     * These are genetic units upon which genes and/or alleles are located.     * Humans possess $46$ chromosomes, which are organized into $23$ pairs.     * The specific combination of these chromosomes during conception is unique and allows for the appearance of unique traits in organisms.

Genes, Alleles, and Genetic Terms

• Gene: A unit that determines wholly or partially a biological trait. Genes occur in pairs (e.g., the gene for brown hair, skin color, or a widow’s peak).

• Alleles: Defined as biochemically different or alternate forms of a gene.     * Example (Blood Type): There are genes for blood type; every individual is either type $A$, $B$, $AB$, or $O$. The alleles representing these types are $A$, $B$, and $O$.     * Inherited Combinations: An individual receives one allele from each parent, resulting in options such as $AA$, $AO$, $AB$, $OO$, $BB$, or $BO$.

• Heterozygous: Having mixed alleles for the same gene.     * Notation: Denoted with one capital letter and one lower-case letter (e.g., $Bb$).

• Homozygous: Having similar alleles for the same gene.     * Notation: Denoted with either two upper-case letters ($BB$) or two lower-case letters ($bb$) depending on dominance.

• Allele Dominance:     * Capital Letter = Dominant allele.     * Lower-case Letter = Recessive allele.

Genetics of Blood Type

• Blood types utilize three alleles to produce four phenotypes and six genotypes.

• Genotype and Phenotype Correlation:

  Genotypes	Phenotypes
  $AA$, $AO$	$A$
  $BB$, $BO$	$B$
  $AB$	$AB$
  $OO$	$O$
  Principles of Blood Type Dominance:
  * $O$ is recessive to both $A$ and $B$.
  * $A$ and $B$ are codominant.
  * Rh factor proteins (positive or negative) are a separate factor.
  * $O-$ is the universal donor.
  * $AB$ is the universal recipient.
  Phenotypes, Genotypes, and Dominance Patterns

  • Phenotype: The physical representation of an allele or gene.     * Example (Handedness): If $R$ is right-handed and $r$ is left-handed, the physical trait of being right-handed is the phenotype.

  • Genotype: The actual biological or genetic representation of an allele or gene (e.g., $RR$ or $Rr$).

  • Dominant: The allele or gene that has the capacity to mask another.     * Example: The speaker is right-handed but possesses a recessive left-handed allele. They did not know they carried the left-handed trait until their child was born left-handed.

  • Recessive: The allele or gene that is not represented when a dominant trait is present.     * Note: To be represented physically, there must be two copies of the recessive allele ($rr$). The speaker's daughter has the genotype $rr$ and is left-handed.

  • Codominant: A state where two alleles or genes are equally dominant.     * In blood type $AB$, neither $A$ nor $B$ masks the other; both are expressed.

  Mendelian Genetics and Mendel’s Experiments

  • Genetics was initially understood through basic or "simple" inheritance (traits you either have or do not have).

  • Gregor Mendel: Discovered basic genetic mechanisms by using pea plants and following traits through generations.

  • Experimental Plant Traits:     * Color: Yellow vs. Green.     * Texture: Smooth vs. Wrinkled.     * Height: Tall vs. Short.     * (Mendel also followed flower color, though not focused on here).

  • The Crossbreeding Process:     * Mendel crossbred pure strains of tall and short plants.     * The offspring of the first generation ($F_1$) were all tall.     * When $F_1$ plants were interbred, short plants reappeared in the second generation ($F_2$).     * The resulting ratio was one short plant for every three tall plants ($\frac{1}{4}$ ratio for the recessive trait).

  • Generational Notation: $P$ (Parent) $\rightarrow F_1 \rightarrow F_2 \rightarrow F_3$\dots denotes each filial generation.

  Mendel’s Laws of Genetics

  • Law of Segregation: The principle that alleles for genes for a particular trait will retain their separate identities through generations.

  • Law of Independent Assortment: The principle that some traits are inherited independently of one another.     * Mendel bred pure smooth yellow pea plants with wrinkled green pea plants.     * The $F_1$ generation was entirely yellow and smooth.     * The $F_2$ generation produced some smooth green and wrinkled yellow plants, proving independent inheritance.

  • Punnett Squares: Used to predict outcomes. When breeding a tall breed ($T$) with a short breed ($t$):     * Bring down the top trait and combine it with the trait to the left ($T$ and $t$ yields $Tt$).

  Mendelian Inheritance in Humans

  • Multiple human traits are transmitted in Mendelian fashion, including:     * Mid-digital hair     * Tongue rolling     * Widow's peak     * Earlobe attachment     * Hitchhiker's thumb     * Relative finger length

  • Dominant Traits List: Widow's peak, Hitchhiker's thumb, Dimples, Tongue rolling, Mid-digital hair, Unattached ear lobes.

  • Recessive Traits: Defined as the absence or opposite of the listed dominant traits.

  • Complexity Note: In reality, many of these traits are not as clear-cut as traditionally taught and may actually be controlled by more than one gene.

  Cellular Reproduction: Mitosis and Meiosis

  • Mitosis:     * Ordinary cell division occurring rapidly for zygote growth.     * Consists of a cell cloning itself to produce two daughter cells bearing the same number of chromosomes as the parent.     * Process: An original cell with $46$ chromosomes (or $23$ pairs) divides to produce two daughter cells, each with $46$ chromosomes ($23$ pairs).

  • Meiosis:     * Specialized cell reproduction producing sex cells (gametes).     * Instead of two daughter cells, it produces four.     * Each daughter cell carries $\frac{1}{2}$ ($23$ chromosomes) of the genetic material of the original cell.     * Phase 1: The parent cell with $46$ chromosomes divides to result in two daughter cells with $23$ chromosomes each.     * Phase 2: Those daughter cells divide again to produce four total cells with $23$ chromosomes each.

  • Reproductive Logic: Sex cells have only $23$ chromosomes so they can pair and unite with another sex cell to produce a zygote with $46$ total chromosomes.

  • Genetic Variety: Due to independent assortment, a parent's genotype may be assorted in $8,000,000$ different ways to produce one offspring.

  Genetic Recombination and Inheritance Patterns

  • Two primary methods allow for unique offspring:     * Crossing-Over: The exchange of DNA between two different chromosomes during meiosis.     * Segregation: The random separation of chromosomes during meiosis.

  • Important Clarifications:     * Dominant does not mean "most common."     * Recessive traits are not completely inactive when a dominant allele is present.

  • Chromosome Types:     * Autosomal Dominant/Recessive: Linked to any chromosome except $X$ and $Y$.     * Sex-linked traits: Linked to the $X$ and $Y$ chromosomes.

  Sex-Linked and Complex Traits

  • Sex-Linked Traits:     * Most are $X$-linked.     * They appear more frequently in males because there is no second chromosome present that can block the trait.     * Females are typically carriers and only represent the trait physically if they are homozygous for it.     * Examples: Red-Green Color Blindness, Hemophilia, Muscular dystrophy.

  • Polygenic Traits: Traits influenced by genes at two or more loci.     * Skin Color: Governed by $6$ loci and at least $12$ alleles.     * Eye Color: Governed by over $15$ genes.

  • Pleiotropy: The opposite of polygenic inheritance, where one single gene affects several different traits.     * Example: Phenylketonuria ($PKU$), an autosomal recessive disorder.     * Mechanism: Individuals homozygous for the $PKU$ allele do not produce phenylketonurase ($\text{the enzyme}$). This enzyme is necessary to convert the amino acid phenylalanine into the amino acid tyrosine.

  Species and Speciation

  • Species: A group of organisms that can breed successfully and produce fertile offspring.

  • Speciation: The process of changing from one organism to another. It occurs if a subgroup of organisms finds itself in a new environment, leading to genetic changes that prevent interbreeding.     * Allopatric Speciation: Possible divergence of closely related species due to reproductive isolation.     * Isolation Factors: Geographical barriers, changes in body structure, or breeding at different times throughout the year.

  • Evolutionary Time Theories:     * Gradualism: The idea that new species evolve from others over a very long period of time.     * Punctuated Equilibrium: The idea that species remain stable for a long duration but may diverge quickly when changes occur.