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Mendelian Genetics Overview

Definition and Purpose

• Fournier Square: A predictive tool used to determine the possible genetic combinations expressed by a particular trait or gene through a grid structure.
• P Generation: The parental generation that produces gametes, transitioning from a diploid to haploid state during reproduction.

Alleles and Traits

• Dominant Trait: Exhibited when at least one dominant allele is present.
• Recessive Trait: Must have two recessive alleles to be expressed phenotypically.
• Fur Color Example:
  • Black fur (dominant) can have alleles: $BB$ (homozygous dominant) OR $Bb$ (heterozygous).
  • Brown fur (recessive) has the allele: $bb$ (homozygous recessive).

Genotypes and Phenotypes

• For a black fur male:
  • Possible Genotypes: $BB$ or $Bb$
  • Brown fur phenotype can only be $bb$.
• When black fur is crossed with black fur:
  • Generations:
  • F1 Generation: All offspring display black fur.
  • F2 Generation: Result of a cross between two heterozygous black fur individuals results is:
    • Phenotype ratio: 3 black: 1 brown (3:1)
    • Genotype breakdown: 1 homozygous dominant ($BB$): 2 heterozygous ($Bb$): 1 homozygous recessive ($bb$)

Test Crosses

• Purpose: To determine whether an organism expressing a dominant phenotype is homozygous or heterozygous by crossing it with a homozygous recessive.
• Example: Crossing $BB$ with a $bb$ will show if all offspring are black (indicating the initial black was homozygous) or if there's a mix indicating heterozygous status.

Mendelian Principles

• Segregation: The separation ofalleles during gamete formation. This ensures variety in each offspring. For instance:
  • Crossing individuals results in various combinations based on chromosome segregation during meiosis.

Independent Assortment and Dihybrid Crosses

• Independent Assortment: States traits segregate independently; traits do not influence one another.
• Example of dihybrid cross using guinea pigs:
  • Traits examined: fur color and hair length.
  • Outcomes of parental generations: dominant (black, $B$) and recessive (brown, $b$) colors and short (dominant, $S$) vs. long hair (recessive, $s$).
• Resulting phenotype and genotype ratios:
  • Phenotypic ratio: 9 black short: 3 black long: 3 brown short: 1 brown long (9:3:3:1).

Genetic Recombination and Crossing Over

• Explains that during meiosis, genes can be recombined, which leads to variety in offspring.
• Chromosomal Theory of Inheritance: Genes are segments of chromosomes.
• Thomas Morgan's experiments:
  • Found certain traits are linked, meaning they do not assort independently and are inherited together due to their proximity on chromosomes.

Sex Chromosomes and Hemizygosity

• Sex Chromosomes: X and Y chromosomes differ in males (XY) compared to females (XX).
• Males are hemizygous, meaning they have one allele for traits on the X chromosome resulting in direct expression of recessive traits owning to lack of a second allele to mask them.
• Example of color blindness as an X-linked trait:
  • Color blindness is a recessive trait on the X chromosome. Affected males (X^eY) have normal vision females (X^E X^e) will pass the gene with a chance for male offspring to be color blind or normal.

Exceptions to Mendel's Rules

• Codominance and Incomplete Dominance:
  • Codominance: Phenotypes of both alleles are expressed equally (e.g., AB blood group).
  • Incomplete Dominance: A blend of both phenotypes appears (e.g., red and white flowers produce pink offspring).