Mendelian Genetics

Mendelian Genetics

Overview of Mendelian Genetics

  • A particulate mechanism for the inheritance of traits.

  • Gregor Johann Mendel (1822-1884) - recognized as the "Father of Genetics".

  • Monk in Brno (formerly in Austria, now in the Czech Republic).

    • Monastery served as a scholarly, cultural center for the region.

    • Well-educated individual with a strong foundation in mathematics and statistics.

    • Conducted experiments with garden peas (Pisum sativum) from 1556 to 1864.

Mendel's Publication and Pea Selection

  • Published his findings in a paper in 1866.

  • Chose the garden pea for specific reasons:

    • Adapted to local environment and soil conditions.

    • Resistant to various diseases.

    • Short growing season and capable of surviving through the winter.

    • Abundant offspring production.

    • Easy to control breeding methods.

    • Exhibited several visible characteristics, each trait having two versions, known as phenotypes.

Mendel's Methodology

  • Method of experimentation included:

    • Removing flower parts to create flowers that were either male or female.

    • Hand-pollination from male flowers to female flowers, followed by covering the female flowers with bags.

    • Maintaining comprehensive records due to his training in mathematics and statistics.

    • Tracking the offspring of his matings up to the grandchild (F2) generation.

Mendel's Terminology

  • True-breeding: A plant whose offspring consistently display the same traits as the parent.

  • Cross: The act of mating a "male" plant with a "female" plant.

  • Hybridization: The process of crossing two different true-breeding plants.

  • P Generation: The original male and female used for crossbreeding.

    • Offspring of this generation are referred to as the F1 (first filial) generation.

Generational Terms

  • F1 Generation: Direct offspring from the P generation, sometimes referred to as "kids".

  • F2 Generation: Offspring of the F1 generation, commonly referred to as "grandkids".

Dominant and Recessive Traits

  • Dominant Trait/Allele: An allele that is expressed (visible) if at least one copy is present.

  • Recessive Trait/Allele: An allele that is expressed (visible) only when two copies are present.

Monohybrid Cross Analysis

  • Example of the pea plants:

    • For every green pea observed, there were approximately three yellow peas.

    • Visualization:

    • P Plant 1: Yellow Peas

    • P Plant 2: Green Peas

    • In the F1 generation, all the offspring produced were yellow peas, demonstrating a dominant trait.

  • Phenotypic Ratio: Observed trait expression ratio of offspring, such as yellow to green, expressed as 3:1 ratio.

  • Genotypic Ratio: Comprised of the genetic makeup of the phenotype.

Mendelian Model Explained in Modern Terms

  1. Organisms possess alternate versions of individual genes, known as alleles.

    • Example: Pea color gene can have yellow or green alleles.

  2. A diploid (2n) organism carries two alleles for each gene, inherited from each biological parent.

  3. If the alleles are different, the dominant allele determines the organism's phenotype.

Law of Segregation

  • During gamete formation, the two alleles for each gene segregate into different gametes (sex cells).

  • This segregation links to the process of meiosis, particularly the separation of homologous chromosomes during anaphase I.

Punnett Square Analysis

  • Alleles and Dominance:

    • Y represents yellow peas (dominant), while y denotes green peas (recessive).

    • Example cross: (Yy×Yy)(Yy \times Yy) yields offspring with a phenotypic ratio of 3 yellow: 1 green.

  • Phenotypic Ratio Resulting from the Fertilization of Gametes:

    • Yellow Peas (YY or Yy) vs Green Peas (yy).

  • Calculated ratios for genotypes include:

    • YY: 1

    • Yy: 2

    • yy: 1

  • Resulting phenotype ratio is yellow and green, expressed as 3:1.

Test Cross Explanation

  • To determine the genotype of a yellow pea phenotype; it can either be homozygous dominant (YY) or heterozygous (Yy).

  • Conducted by crossing the yellow pea with a homozygous recessive pea plant (yy).

    • If cross yields all yellow peas then the genotype of the yellow plant must be homozygous dominant (YY).

    • If offspring results in a 1:1 ratio of yellow to green, then the yellow pea's genotype is heterozygous (Yy).

Dihybrid Cross Overview

  • Involves two genes; for example:

    • Seed color with yellow (Y) or green (y).

    • Seed shape with round (R) or wrinkled (r).

  • Cross of:

    • P Generation: Yellow & wrinkled (YYrr) x Green & round (yyRR).

  • Resulting in F1 generation:

    • All dihybrids are YyRr (yellow & round).

  • Law of Independent Assortment:

    • In dihybrid crosses, alleles for two genes assort independently if not located on the same chromosome.

    • Example outcomes:

    • Phenotypic ratio in F2: 9 yellow & round : 3 yellow & wrinkled : 3 green & round : 1 green & wrinkled.

Mechanism of Independent Assortment

  • Linked to the random alignment of homologous chromosome pairs during metaphase I of meiosis.

  • Clarification that allele relationships are not always strictly dominant/recessive.

Incomplete Dominance and Codominance

  • Incomplete Dominance:

    • Example: Flower color in snapdragons.

    • CR = red, CW = white.

    • Genotypes: CRCR (red), CRCW (pink), CWCW (white).

    • The heterozygous phenotype is an intermediate between the two homozygous phenotypes.

  • Codominance:

    • Example: Human blood types.

    • IAIB = Blood type AB, I^A = Blood type A, I^B = Blood type B.

    • ii = Blood type O, where both alleles express equally in the phenotype.