DNA, Chromosomes, traits and genetic crosses

Introduction to Teaching Experience

• Exciting yet stressful time before the first day of teaching.

• Desire to create an inviting classroom that excites students.

Classroom Pet: Spike the Snake

• Acquired a classroom pet, Spike, a Texas rat snake, given by a colleague's friend.

• Spike as a pet became a unique aspect of the science wing, drawing curiosity.

• Teaching Relevance: Integrated Spike into various biology topics (e.g., predation, cell division).

Understanding Heredity

• Heredity: How traits are passed from parents to offspring.

• Video content includes topics on:

  • Reproduction

  • Inheritance in pedigrees

  • Genetic problems using Punnett squares

  • Mendelian and non-Mendelian inheritance

• Importance of understanding DNA, chromosomes, genes, and traits in the context of heredity.

Traits and DNA

• Spike's traits: Patterns, size, and characteristics stem from his DNA.

• Environmental influences can modify traits (e.g., nourishment affecting size).

• DNA Presence: Found in the nuclei of nearly all body cells.

Asexual Reproduction in Snakes

• Many snake species can reproduce asexually, leading to offspring with DNA from one parent.

• DNA is essential for determining various traits and functions within cells.

  • Example: Risk for certain diseases can be coded in DNA.

Structure of DNA

• DNA: Stands for deoxyribonucleic acid, a nucleic acid type.

• Nucleic acids made of nucleotides consisting of:

  • Sugar: deoxyribose

  • Phosphate

  • Nitrogenous base (most crucial)

• Sugar-Phosphate Backbone: Made up of the sugar and phosphate parts of nucleotides.

DNA Bases

• Four types of nitrogenous bases:

  • A: Adenine

  • T: Thymine

  • C: Cytosine

  • G: Guanine

• Base Pairing Rules:

  • A pairs with T ("apples in the tree")

  • C pairs with G ("car in the garage")

Variability in DNA

• While certain DNA sequences are consistent within a species, sequences vary among individuals.

• Spike likely has a DNA sequence more similar to his parents than to a different species (e.g., rattlesnake).

Double-Helix Structure

• DNA exists as a double helix with two strands:

  • Nucleotides on one side pair with nucleotides on the opposite side.

  • Bases connected by hydrogen bonds forming base pairs.

Genes and Proteins

• Genes are portions of DNA coding for proteins that express traits.

• Example: Human eye color determined by multiple genes, which code for pigments.

• Proteins play roles in various bodily functions: transport, structure, enzymes, protection, etc.

Gene Expression and Regulation

• Not all genes are used to make proteins; non-coding DNA exists.

• Gene regulation allows cells to 'turn on' or 'turn off' portions of genes as needed.

Chromosomes

• When compacted, DNA forms chromosomes, facilitating cell division.

• For humans:

  • 46 chromosomes in most body cells

  • 23 chromosomes in sperm and egg cells (23 from each parent)

Recap of Key Concepts

• A chromosome consists of DNA sections coding for genes.

• Nucleotides and their sequences determine traits.

• Understanding these fundamentals crucial for grasping heredity concepts.

Conclusion

• Questions about dominant and recessive traits, alleles, and Punnett squares can deepen understanding.

• Encouragement to explore further in the heredity playlist.

Completing Mendelian Genetic Crosses

Monohybrid Crosses

• Definition: A genetic cross that examines the inheritance of a single trait.

• Steps to Perform a Monohybrid Cross:

  1. Identify Parental Genotypes: Determine the genotypes of the two parents (e.g., TT and tt for height).

  2. Create a Punnett Square: Draw a 2x2 grid.

  3. Fill in the Punnett Square: Combine the alleles from each parent.

  4. Analyze Results: Determine the phenotypic and genotypic ratios from the outcomes.

  5. Phenotypic Ratio: The ratio of the observable traits in the offspring, typically expressed as a ratio of dominant to recessive traits.

  6. Genotypic Ratio: The ratio of the genetic makeup of the offspring, indicating the combination of alleles present.

Dihybrid Crosses

• Definition: A genetic cross that examines the inheritance of two traits simultaneously.

• Steps to Perform a Dihybrid Cross:

  1. Identify Parental Genotypes: Determine the genotypes of the two parents for both traits (e.g., RrYy and RrYy).

  2. FOIL Method: Use the FOIL (First, Outside, Inside, Last) method to find all combinations of alleles (e.g., RY, Ry, rY, ry).

  3. Create a Punnett Square: Draw a 4x4 grid (16 squares total if both parents are heterozygous).

  4. Fill in the Punnett Square: Combine the allele combinations from each parent.

• Analyze Results: Determine phenotypic and genotypic ratios from the outcomes (9:3:3:1 typical ratio for two traits).

  Example:

Trait: Height in Pea Plants

Parental Genotypes: TT (tall) and tt (short)

Punnett Square:

• Results:

  • All offspring (100%) are Tt (tall)

  • Phenotypic Ratio: 100% tall

  • Genotypic Ratio: 100% Tt

This example illustrates the inheritance pattern for a single trait using a monohybrid cross.

Non-Mendelian Genetic Crosses

• IIncomplete Dominance
  Definition: A form of inheritance in which the phenotypes of the heterozygote are blended (e.g., red and white flowers producing pink).
  
  Example: In snapdragon flowers, crossing a red flower (RR) with a white flower (WW) produces offspring with pink flowers (RW).
  
  Punnett Square Steps: Similar to monohybrid crosses, but the resulting phenotype reflects this blending.

Codominance

• Definition: A genetic scenario where both alleles in a heterozygote are fully expressed (e.g., AB blood type).

• Punnett Square Steps: Similar to monohybrid crosses, but both alleles are expressed in the phenotypes.

• **Example of Codominance:** In the case of AB blood types, a person with genotype IAIB has both A and B antigen proteins expressed on the surface of their red blood cells. This results in the phenotypic expression of both blood types in the individual, illustrating codominance.

Multiple Alleles

• Definition: More than two alleles exist for a given trait (e.g., ABO blood groups).

• Punnett Square Steps: Include all possible allele combinations (IA, IB, i) when analyzing crosses.

• **Multiple Alleles** Definition: More than two alleles exist for a given trait (e.g., ABO blood groups). Example: In humans, the ABO blood group system includes the alleles IA (A type), IB (B type), and i (O type). An individual may possess different combinations such as: - IAIA (A blood type) - IAi (A blood type) - IBIB (B blood type) - IBi (B blood type) - IAIB (AB blood type) - ii (O blood type) Punnett Square Steps: Include all possible allele combinations (IA, IB, i) when analyzing crosses.

Sex-Linked Crosses

• Definition: Genetic crosses that involve genes located on the sex chromosomes, often X-linked (e.g., hemophilia).

• Punnett Square Steps:

  1. Identify Parental Genotypes: Include X-linked alleles (e.g., XHXh for a carrier female).

  2. Create a Punnett Square: Include X chromosome alleles and Y chromosome alleles for the male parent.

  3. Analyze Results: Determine the phenotypic ratios for male and female offspring separately, focusing on sex-linked traits.