mendelian genetics

General Instructions for Activity

  • Complete the following tasks by yourself or with your neighbor (no groups of 3!) :

    • Cut out the images and captions.

    • Glue them in the correct order on the construction paper.

    • Label the paper and as many steps as you can.

Key Terms and Concepts

Terminology

  • Zygote: The initial cell formed when a sperm cell fertilizes an egg cell.

  • Homo: Refers to the homozygous condition where an organism carries two identical alleles for a particular gene.

  • Hetero: Refers to the heterozygous condition where an organism carries two different alleles for a particular gene.

Mendelian Genetics

Introduction

  • Mendelian Genetics is a branch of genetics that studies the inheritance of traits based on the principles set forth by Gregor Mendel.

Gregor Mendel: The Father of Genetics

  • Identity:

    • Gregor Mendel was an Austrian monk who is known for founding the field of genetics through his work with pea plants from 1852 to 1866.

  • Contributions:

    • During the 1860s, Mendel theorized foundational aspects of genetics and inheritance even before the discovery of DNA, invention of the microscope, and observation of cells.

    • His work contradicted the prevailing belief that inheritance was simply a product of blending parental traits, as he proved this to be incorrect.

Mendel’s Research and Experiments

Pea Plant Studies

  • Scale of Study:

    • Mendel grew over 29,000 pea plants for his experiments.

    • Focused on how 7 different traits were inherited.

  • Traits Studied:

    • All traits distinguished as either one condition or the other (e.g., pure or distinct).

    • The specific traits included:

    1. Dimples vs. no dimples.

    2. Cleft chin vs. non-cleft chin.

    3. Widow's peak vs. straight hairline.

    4. Earlobe attachment (unattached vs. attached).

    5. Tongue rolling capability (can roll tongue vs. cannot roll tongue).

    6. Freckles vs. no freckles.

    7. Hand clasping preference (left thumb over right or vice versa).

    8. Bent pinkie vs. straight pinkie.

    9. Arm crossing preference.

    10. Ability to taste PTC (strong taster TT vs. non-taster tt).

Results From Mendel’s Experiments

  • Parental Generation (P):

    • Mendel began by mating/crossing true-breeding populations known as the parental generation.

  • F1 Generation Results:

    • The F1 generation showed only one of the parental traits (for instance, all purple flowers).

    • This observation led to the hypothesis of dominance.

  • F2 Generation Results:

    • When the F1 generation was self-fertilized, the F2 generation exhibited a 3:1 ratio (75% showing F1 phenotype and 25% showing the “hidden” phenotype).

    • Example: 3 purple flowers to 1 white flower.

Key Concepts in Genetics

Definitions

  • Gene:

    • Defined as a small segment of DNA located on a chromosome that codes for a specific protein or trait.

    • Each trait is determined by a pair of alleles, one from each parent.

  • Chromosome: Structures that carry genetic information.

  • Allele: Different versions of a gene.

    • Example: An allele for flower color may manifest differently in various plants.

  • Dominant Allele:

    • Always expressed when present, represented by a capital letter (e.g., P for purple).

  • Recessive Allele:

    • Can be masked by a dominant allele; only expresses in absence of a dominant allele, represented by a lowercase letter (e.g., p for white).

  • Genotype:

    • The genetic makeup of an organism regarding a specific trait, represented by two letters (PP, Pp, pP, or pp).

  • Phenotype:

    • The physical appearance of an organism based on its genotype (e.g., purple or white flowers).

  • Homozygous Genotype:

    • Individuals with two identical alleles for a trait (PP or pp).

  • Heterozygous Genotype:

    • Individuals with two different alleles for a trait (Pp).

Ratios from Mendel’s Crosses

  • Mendel deduced that:

    • The phenotypic ratio of the F2 generation derived from a monohybrid cross is approximately 3:1 (with variation based on traits).

    • The clarification on genotypes: PP, Pp, and pp yield specific phenotypes based on dominance.

Additional Principles

Laws of Inheritance

  • Law of Segregation:

    • During meiosis, the two alleles for a trait segregate into different gametes, ensuring each gamete receives one allele.

    • This principle was established during Metaphase 1 of meiosis.

  • Law of Independent Assortment:

    • States that each gene segregates independently from other genes during gamete formation.

    • Flower color does not affect pea color, with the notable exception for linked genes.

Examples and Illustrations

  • Dihybrid crosses demonstrate independent assortment using two traits.

    • Example Cross: PpYy x PpYy demonstrates traits are inherited independently.

  • Expected phenotypic ratios for dihybrid crosses often yield a 9:3:3:1 ratio, depending on the number of traits involved.

Review Questions and Conclusions

  • Key Questions to Explore:

    • What were Mendel's major conclusions based on his experimental data?

    • The impact of dominant and recessive alleles in phenotype expression.

    • Exploration of exceptions to Mendelian principles and implications for modern genetics.