Meiosis, Mendelian Genetics, and Human Blood Groups Study of Blood Types of Blood Groups

Original Cell State: Meiosis begins with reproductive or sexual cells located in germline tissue (e.g., ovaries, testes, or pine cones).
Meiosis I: Division involves the separation of homologous chromosomes, resulting in haploid cells that are still replicated (resemblant of "X's").
Meiosis II: Results in four distinct haploid gametes, such as sperm, egg cells, or pollen grains.
Genetic Variation: Genetic distinctness in gametes is achieved through crossing over, recombination, and independent assortment.
Fertilization: Fusing two haploid gametes creates a diploid zygote, which then undergoes mitosis for growth.
Reduction Division: A term for meiosis referring to the reduction from diploid to haploid chromosome counts.

Gregor Mendel: Known as the "father of heredity," Mendel published his work in 1863 based on experiments with the garden variety pea plant.
The Theory of Heredity: Mendel proposed that traits are passed via "factors" (now called genes) inherited from each parent.
Gene Count: While early estimates varied, humans have approximately $23,000$ genes.

Gene: A portion of DNA (typically hundreds to thousands of base pairs) that codes for a specific trait.
Allele: Different versions of a gene (e.g., purple vs. white flowers).
Genotype: The specific combination of alleles, usually represented by letters (e.g., $AA$ , $Aa$ , $aa$ ).
Phenotype: The physical expression or observable trait of the organism.
Homozygous: Possessing two identical alleles for a gene.
Heterozygous: Possessing two different alleles; the dominant allele typically masks the recessive one.
True Breeding: Organisms that produce offspring identical to themselves over multiple generations due to homozygous genotypes.

Reginald C. Punnett: Creator of the Punnett square, used to predict the percentage chance of offspring genotypes.
Monohybrid Cross: A study of one gene. Crossing two heterozygous individuals results in a $3:1$ phenotype ratio (dominant to recessive) and a $1:2:1$ genotype ratio ( $1$ homozygous dominant, $2$ heterozygous, $1$ homozygous recessive).
Dihybrid Cross: A study of two genes ( $16$ squares). Gamete combinations are determined using the FOIL method (First, Outside, Inside, Last).
Test Cross: Breeding an individual of unknown genotype with a homozygous recessive individual to determine the unknown's alleles.

Genetics of Blood: Controlled by a single gene ( $I$ for Immunoglobulin) with three possible alleles: $I^A$ , $I^B$ , and $i$ .
Allele Interactions: $I^A$ and $I^B$ are codominant, while $i$ is recessive.
Phenotypes: * Type A: Genotypes $I^A I^A$ or $I^A i$ . * Type B: Genotypes $I^B I^B$ or $I^B i$ . * Type AB: Genotype $I^A I^B$ . * Type O: Genotype $ii$ .
Rh Factor: A separate gene determining positive ( $+$ ) or negative ( $-$ ) status, originally discovered in Rhesus monkeys.
Compatibility: Type $O^-$ is the universal donor; Type $AB^+$ is the universal recipient.

Question: Where are germline tissue cells located? * Response: They are associated with reproductive tissue, such as ovaries in females, testes in males, or structures like pine cones in plants.
Question: How can a family with parents of dominant features have children with varied eye colors (e.g., blue, green, and brown)? * Response: If the parents are heterozygous, there is a $25\%$ chance for different combinations. Genetic scrambling during meiosis (crossing over) also increases variation.
Question: Can a Type O father and a Type B heterozygous mother have a Type O child? * Response: Yes. If the mother is heterozygous ( $I^B i$ ) and the father is Type O ( $ii$ ), there is a $50\%$ chance the child will inherit a recessive allele from each parent ( $ii$ ).