Mendelian Genetics

Gregor Mendel:
- Conducted seminal research that led to the understanding of inheritance through the study of plants, particularly peas.
- Made discoveries surrounding meiosis, which contributed to the chromosomal theory of inheritance:
- Genes are located on chromosomes.
- Transmission of chromosomes during meiosis results in observable patterns of inheritance.
- Initially explored selective breeding of traits, confronting the prevailing theories of blending inheritance versus the inheritance of acquired characters.

Study of Peas:
- Mendel performed studies on pea plants focusing on self-fertilization versus cross-fertilization.
- Worked with pure lines and hybrids.
Monohybrid Crosses:
- Examined the inheritance of a single trait.
- Challenged the blending hypothesis with his new concept of particulate inheritance, proposing that traits are inherited as distinct units (alleles).

Allele: Alternate versions of a gene; even minor differences in DNA can lead to differences in protein structure/function.
- Example: Variants in a gene may affect hair texture.
Genotype: The actual genetic makeup of an organism, often in reference to specific alleles for one or more genes.
Phenotype: Observable traits and functions, including physical attributes (hair texture, height) and other characteristics such as blood type and disease susceptibility.
- Differences in genotype result in variations in phenotype.
  - Examples of Genotypes: AA, Bb, AABb.

Each chromosome consists of multiple genes:
- Each individual inherits pairs of chromosomes, leading to two variants (alleles) of every gene.
Genotype Possibilities:
- Homozygous: Both alleles of a gene are identical.
- Heterozygous: Two different alleles present for a specific gene.
Important note: Multiple alleles may exist for many genes, yet an individual can inherit only two alleles for each gene due to chromosome pairings.

Dominant Allele: When in a heterozygous condition, it dictates the phenotype of the organism.
Recessive Allele: Only expressed in the phenotype when present in a homozygous recessive genotype.
- Convention: Typically, uppercase letters denote dominant alleles; lowercase denotes recessive alleles.

Genotype to Phenotype Correlation:
- Dominant allele: unattached earlobes (E)
- Recessive allele: attached earlobes (e)
- Genotype EE results in a phenotype of unattached earlobes.
- Genotype ee results in attached earlobes.
- Genotype Ee would result in the phenotype of unattached earlobes, due to the dominant allele.

Genes have variations known as alleles.
Generally, individuals can inherit two alleles (homozygous or heterozygous) due to chromosome pairings.
Genotypes influence phenotypic outcomes; the presence of alleles determines whether traits are exhibited as dominant or recessive.
Mendel’s monohybrid crosses revealed fundamental genetic principles, leading to the identification of dominant and recessive traits.
His findings, combined with the discovery of meiosis, established the chromosomal theory of inheritance, asserting:
- Genes reside on chromosomes.
- Inheritance patterns relate to chromosomal behavior during meiosis.

Based on the principle of segregation: Each parent has an equal probability of passing on either allele they possess.
This segregation occurs as homologous chromosomes separate during gamete formation.
Probabilities derived from each parent's genetic material can be applied to assess fertilization outcomes across any pairing of genotypes.

Test Cross: Technique used to determine the genotype of an individual with an unknown genotype by crossing with a homozygous recessive individual.
- Example: Yellow pea (Y, dominant) crossed with green pea (yy, homozygous recessive).
- If the yellow pea is either Yy or YY, the resulting offspring will clarify its genotype based on the phenotypes observed.

Progressed from single trait analysis (monohybrid cross) to examining two traits simultaneously (dihybrid cross) by applying the principle of independent assortment.
- This allows for complex calculations of allele combinations using forked-line diagrams, particularly when considering more than two traits.

Two traits: Polydactyly (dominant, extra digits) and widow's peak (dominant hairline shape).
- Construct Punnett square to analyze offspring probabilities when one parent possesses typical traits (ppww) and the other is heterozygous for both (PpWw).

Certain genes are located exclusively on sex chromosomes (X or Y), known as sex-linked genes.
- Examples include forms of hemophilia and red-green colorblindness, which are caused by recessive alleles on the X chromosome.
Modified Punnett Squares must be utilized for sex-linked traits:
- Important consideration: Males possess only one X chromosome, leading to an increased likelihood of recessive traits exhibiting in their phenotype.

Mendelian principles of inheritance — segregation and independent assortment — are intrinsically linked to meiosis.
- This chromosomal behavior explains the inheritance patterns and sex-linked traits.

Punnett Squares: Predict offspring genotype probabilities based on given parent genotype combinations.
Principle of Segregation: Each gamete has a 50% chance of carrying either allele from a parent due to meiosis separation of homologous chromosomes.
Offspring genotype outcomes are independent from one another; one child's genotype doesn't affect another's.
Test Cross: Crucial for identifying unknown genotypes through a homozygous recessive partner.
Sex-linked Traits: Require special consideration due to their unique inheritance on sex chromosomes, notably influencing male phenotypes more than females.
Types of Crosses: Monohybrid for single traits, dihybrid for combinations of traits; Punnett Squares must correspond to whether genes are autosomal or sex-linked, ensuring correct representation of all X and Y chromosomes.