4.5 Mendelin Inheritance - Chromosomal Inheritance

Mendel's Laws of Inheritance

Overview of Gregor Mendel

• Gregor Johann Mendel (1822-1884) was an Austrian monk and a pioneer in genetic science.

• Mendel is referred to as the father of genetic science due to his foundational work in understanding heredity.

• In 1865, Mendel introduced his Laws of Inheritance, although his work initially received little attention.

• His groundbreaking discoveries came from extensive experiments with the garden pea (Pisum sativum).

• Despite the prominence of Darwin and Lamarck's theories during Mendel's time, his work was eventually recognized in 1900, solidifying his legacy in genetics.

Mendel's Experimental Approach

• Mendel's key methods in his experiments with peas allowed him to establish rules governing genetics that apply across many species.

• His research revealed fundamental principles of heredity that can be observed in various sexually reproducing organisms, including humans.

• The basic principles of heredity discovered in peas can extend to other plant and animal species as well.

• Mendel carefully selected pure-breeding pea plants for his initial crosses, meaning plants that consistently produced offspring with the same trait when self-pollinated over several generations. This ensured he was starting with a known genetic baseline. He then performed controlled crosses by manually transferring pollen between selected plants (cross-pollination or hybridization), and also allowed self-pollination in subsequent generations. This meticulous control over reproduction allowed him to track the inheritance of specific traits accurately.

Pea Plant Characteristics

• Mendel selected garden peas for several reasons:

1. Variety of Traits: Peas exhibit many physical traits, providing a rich ground for experimentation. Mendel focused on seven specific characteristics in his studies:

   • Seed shape (round or wrinkled)

   • Seed color (yellow or green)

   • Flower color (purple or white)

   • Pod shape (inflated or constricted)

   • Pod color (green or yellow)

   • Flower position (axial or terminal)

   • Plant height (tall or short)

2. Ease of Manipulation: Peas produce both male and female gametes, allowing them to self-fertilize and making hybridization easier. This controlled Mendel’s experiments effectively.

3. Controlled Fertilization: The large and easily manipulated flowers allowed Mendel to easily perform cross-fertilization to track traits over generations.

Mendel's Laws of Inheritance

Law of Segregation

• Mendel’s first law, Law of Segregation, is observed in a single-factor cross concerning plant height:

  • Mendel crossed tall plants (T) and dwarf plants (t).

  • F1 Generation: All offspring exhibited the dominant trait (tall).

  • F2 Generation: The offspring ratio revealed that 75% were tall and 25% were dwarf, demonstrating that traits can be dominant or recessive. The dwarf trait reappeared, indicating that alleles segregated during gamete formation. The reappearance of the dwarf trait in the F2 generation demonstrated that the alleles for height segregated (separated) during the formation of gametes in the F1 plants. Each gamete (sperm or egg cell) receives only one allele for each gene. This segregation occurs during meiosis, ensuring that offspring inherit one allele from each parent, leading to varied combinations.

Key Concepts of Trait Inheritance

• Traits can exist as dominant and recessive forms:

  • Dominant traits manifest in the phenotype even if only one allele is present (heterozygous).

  • Recessive traits require two alleles to be expressed (homozygous).

• Example: In pea plants, the dominant allele for flower color is purple, while the recessive allele is white.

• Terms to Understand:

  • Allele: A variant form of a gene responsible for a particular trait.

  • Genotype: The genetic makeup of an organism, specifically regarding alleles for a given trait.

  • Phenotype: The observable characteristics or traits of an organism, influenced by genotype.

  • Homozygous: An organism with two identical alleles for a trait (e.g., TT or tt).

  • Heterozygous: An organism with two different alleles for a trait (e.g., Tt).

Punnett Squares

• A Punnett Square is a graphical tool used to predict the genotypes and phenotypes of offspring from a genetic cross:

  • Predicted percentages can be calculated based on parental genotype combinations.

  • Example of a cross between a homozygous dominant (BB) and homozygous recessive (bb) results in a phenotypic ratio of 100% dominant phenotype.

  • For instance, in a monohybrid cross between two heterozygous parents (e.g., $Aa$), a Punnett square shows that the probability of an offspring being homozygous dominant ($AA$) is $1/4$, heterozygous ($Aa$) is $2/4$ ($1/2$), and homozygous recessive ($aa$) is $1/4$. This results in a genotypic ratio of $1:2:1$ and a phenotypic ratio of $3:1$ (assuming A is dominant).

Law of Independent Assortment

• The Law of Independent Assortment states that the inheritance of one trait will not affect the inheritance of another trait when considering dihybrid crosses (two traits simultaneously).

• Traits are distributed independently through the formation of gametes.

• This law is crucial for explaining genetic variation. It means that the allele a gamete receives for one gene (e.g., seed color) does not influence the allele received for another gene (e.g., seed shape), provided these genes are on different chromosomes or are far apart on the same chromosome. This independent shuffling of alleles during gamete formation leads to a greater diversity of possible genetic combinations in offspring than if traits were always inherited together.

Genetic Linkage

• Genetic linkage refers to the tendency of nearby genes on the same chromosome to be inherited together, leading to dependent assortment.

• Independent assortment means genes for separate traits are passed independently from one another, leading to a greater variability in traits.

Chromosome Theory of Inheritance

• The Chromosome Theory of Inheritance explains that genes are located on chromosomes and are passed from parents to offspring through gametes.

• Key components of the chromosome theory include:

1. Chromosomes carry genes for various traits.

2. Genes are inherited via chromosomes, contributing to the genetic makeup of the offspring.

3. Chromosomes come in pairs, one from each parent, creating homologous pairs that carry alleles for the same trait.

Sex Chromosomes

• Autosomes: Non-sex chromosomes, with humans having 22 pairs that do not determine sex.

• Sex Chromosomes: Systems of sex determination vary among organisms:

  • XX/XY System: Common in humans, where XX indicates females and XY indicates males.

  • ZW/ZZ System: Found in birds, where ZW indicates females and ZZ indicates males.

  • Haplodiploidy: Present in bees and ants, where males are haploid and females are diploid, based on whether the egg is fertilized.

  • Temperature-dependent sex determination: Occurs in certain reptiles, where incubation temperature influences the sex phenotype.

Morgan's Experiments with Fruit Flies

• Thomas Hunt Morgan used fruit flies to investigate the relationship between genetics and chromosomal inheritance.

• He discovered mutations such as white-eyed fruit flies, which helped establish the connection between sex chromosomes and inherited traits.

• His experiments demonstrated that genes could be located on the X chromosome, which directly influenced observable traits such as eye color.

• Morgan chose fruit flies (Drosophila melanogaster) as a model organism due to their rapid reproductive cycle, ease of breeding in large numbers, and distinct observable traits. His discovery of a white-eyed male fly, a mutation from the common red-eyed type, allowed him to meticulously trace its inheritance pattern. He found that the gene for eye color was located on the X chromosome, demonstrating sex-linked inheritance. This landmark work provided the first experimental evidence that genes reside on chromosomes and that specific traits are indeed linked to specific chromosomes, validating the Chromosome Theory of Inheritance.

Conclusion

• Mendel's foundational contributions laid the groundwork for our current understanding of genetics through the laws of inheritance, segregation, and independent assortment. The connection of these principles to chromosomal behavior has continued to shape genetic research and its implications across biology fields.

Contact Information

• Siamak Shirani Bidabadi

• Horticulture and Crop Science (Ph.D.)

• Email: sbidabad@asu.edu