Cell Division and Mendelian Genetics: Key Concepts for Exam 4

Cell Division: Mitosis and Meiosis

Phases of Mitosis

Mitosis is the process by which a eukaryotic cell divides to produce two genetically identical daughter cells. The phases occur in a specific order:

• Prophase: Chromatin condenses into visible chromosomes; the mitotic spindle begins to form; the nuclear envelope starts to break down.

• Metaphase: Chromosomes align at the metaphase plate (cell equator).

• Anaphase: Sister chromatids separate and move toward opposite poles of the cell.

• Telophase: Chromosomes decondense; nuclear envelopes reform around each set of chromosomes.

• Cytokinesis: Division of the cytoplasm, resulting in two separate cells (not technically a phase of mitosis but closely associated).

Example: In human somatic cells (2n = 46), mitosis produces two cells each with 46 chromosomes.

Phases of Meiosis I

Meiosis I is the first division in meiosis, reducing the chromosome number by half and introducing genetic variation.

• Prophase I: Homologous chromosomes pair up (synapsis) and exchange genetic material (crossing over).

• Metaphase I: Homologous pairs align at the metaphase plate.

• Anaphase I: Homologous chromosomes separate and move to opposite poles.

• Telophase I and Cytokinesis: Two haploid cells form; chromosomes are still duplicated.

Example: In humans, meiosis I reduces the chromosome number from 46 to 23 per cell.

Phases of Meiosis II

Meiosis II resembles mitosis but separates sister chromatids in haploid cells.

• Prophase II: Chromosomes condense; new spindle forms in each haploid cell.

• Metaphase II: Chromosomes align at the metaphase plate.

• Anaphase II: Sister chromatids separate and move to opposite poles.

• Telophase II and Cytokinesis: Four haploid daughter cells are produced, each with a single set of chromosomes.

Example: In humans, meiosis II results in gametes with 23 chromosomes each.

Labeling Diagrams of Cell Division

• Be able to identify and label key structures (chromosomes, spindle fibers, centrioles, nuclear envelope) and distinguish between phases based on chromosome arrangement and cell structure.

Differences Between Mitosis and Meiosis

The following table summarizes the main differences:

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of daughter cells	2	4
Genetic composition	Identical to parent	Genetically unique
Chromosome number	Diploid (2n)	Haploid (n)
Role	Growth, repair, asexual reproduction	Sexual reproduction (gamete formation)

Mendelian Genetics

Punnett Squares: Monohybrid Cross

A monohybrid cross examines the inheritance of a single trait. A Punnett square predicts the genotypes and phenotypes of offspring.

• Genotypic ratio: The ratio of different genetic combinations (e.g., 1:2:1 for AA:Aa:aa).

• Phenotypic ratio: The ratio of observable traits (e.g., 3:1 for dominant:recessive).

Example: Crossing two heterozygotes (Aa x Aa):

• Genotypes: AA, Aa, Aa, aa

• Genotypic ratio: 1 AA : 2 Aa : 1 aa

• Phenotypic ratio (if A is dominant): 3 dominant : 1 recessive

Punnett Squares: Dihybrid Cross

A dihybrid cross examines the inheritance of two traits simultaneously. The classic example is crossing two individuals heterozygous for both traits (AaBb x AaBb).

• Genotypic ratio: More complex, with 9 possible genotypes.

• Phenotypic ratio: For unlinked genes, the classic ratio is 9:3:3:1 (dominant for both : dominant for one, recessive for other : vice versa : recessive for both).

Example: Crossing AaBb x AaBb:

• Phenotypic ratio: 9 (both dominant) : 3 (A dominant, b recessive) : 3 (a recessive, B dominant) : 1 (both recessive)

Sex-Linked Traits

Sex-linked traits are controlled by genes located on sex chromosomes (usually the X chromosome in humans). Inheritance patterns differ between males (XY) and females (XX).

• X-linked recessive traits: More common in males, as they have only one X chromosome (e.g., color blindness, hemophilia).

• X-linked dominant traits: Affected males pass the trait to all daughters but no sons.

Example: A carrier female (XAXa) and a normal male (XAY) can produce sons with a 50% chance of being affected.

Sex-Linked vs. Autosomal Traits

• Sex-linked traits: Genes located on sex chromosomes (X or Y); inheritance patterns depend on the sex of the offspring.

• Autosomal traits: Genes located on non-sex chromosomes (autosomes); inheritance patterns are the same for both sexes.

Example: Cystic fibrosis is autosomal recessive; red-green color blindness is X-linked recessive.

Additional info: Understanding these concepts is essential for interpreting genetic crosses, predicting inheritance patterns, and analyzing pedigrees in genetics.