Inheritance and Chromosomes

Genes and Heredity

The transmission of traits from one generation to the next is known as inheritance or heredity. Offspring inherit genes from their parents, but are not identical to either parent or their siblings, resulting in both similarity and variation. The scientific study of heredity and variation is called genetics.

• Genes are units of heredity, composed of DNA segments.

• Genes are passed to offspring via gametes (sperm and eggs).

• Most DNA is organized into chromosomes.

• Humans have 46 chromosomes in somatic cells (all cells except gametes).

• A gene's specific location on a chromosome is its locus.

Asexual vs. Sexual Reproduction

Organisms reproduce either asexually or sexually, affecting genetic diversity.

• Asexual reproduction: One parent passes all genes to offspring; offspring are genetically identical (clones).

• Sexual reproduction: Two parents contribute genes, resulting in offspring with unique gene combinations.

Chromosome Sets and Human Life Cycle

Chromosome Sets in Human Cells

Human somatic cells contain 23 pairs of chromosomes, displayed in a karyotype. Each pair consists of homologous chromosomes (homologs), which have the same length, centromere position, and staining pattern, and carry genes for the same traits.

• Sex chromosomes (X and Y) determine sex: females (XX), males (XY).

• The other 22 pairs are autosomes.

• Each homologous pair includes one chromosome from each parent.

• A diploid cell (2n) has two sets of chromosomes; for humans, 2n = 46.

• After DNA synthesis, each chromosome is replicated as two sister chromatids.

Gametes and Haploid Cells

Gametes (sperm and egg) contain a single set of chromosomes and are haploid (n). For humans, n = 23. Each gamete has 22 autosomes and one sex chromosome (X or Y).

Human Life Cycle

The human life cycle alternates between haploid and diploid stages. Fertilization unites gametes, forming a diploid zygote that develops into an adult via mitosis. Gametes are produced by meiosis in the ovaries and testes.

• Meiosis reduces chromosome number in gametes.

• Fertilization restores diploid chromosome number.

Sexual Life Cycles in Eukaryotes

Types of Sexual Life Cycles

All sexually reproducing organisms alternate meiosis and fertilization, but the timing varies:

• Animals: Gametes are the only haploid cells; fertilization produces a diploid zygote, which grows by mitosis.

• Plants and some algae: Exhibit alternation of generations with both diploid (sporophyte) and haploid (gametophyte) multicellular stages.

• Fungi and some protists: Only the zygote is diploid; meiosis produces haploid cells that grow into multicellular haploid organisms.

Meiosis: Reducing Chromosome Number

Overview of Meiosis

Meiosis is a specialized cell division that reduces chromosome number from diploid to haploid, ensuring genetic diversity in offspring. It consists of two consecutive divisions: meiosis I and meiosis II, resulting in four haploid daughter cells.

• Chromosomes duplicate before meiosis.

• Sister chromatids are held together by cohesins.

• Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids.

Phases of Meiosis I

• Prophase I: Homologous chromosomes pair and undergo crossing over at chiasmata.

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

• Anaphase I: Homologs separate to opposite poles; sister chromatids remain attached.

• Telophase I and Cytokinesis: Two haploid cells form; chromosomes still consist of sister chromatids.

Phases of Meiosis II

• Prophase II: Spindle apparatus forms; chromosomes move toward metaphase plate.

• Metaphase II: Chromosomes align at metaphase plate; sister chromatids are no longer identical due to crossing over.

• Anaphase II: Sister chromatids separate to opposite poles.

• Telophase II and Cytokinesis: Four genetically distinct haploid cells are produced.

Genetic Variation and Evolution

Sources of Genetic Variation

Genetic variation is essential for evolution and is produced by several mechanisms during sexual reproduction:

• Mutations: Create new alleles and are the original source of genetic diversity.

• Independent assortment: Homologous chromosomes randomly orient during metaphase I, producing many possible combinations.

• Crossing over: Homologous chromosomes exchange genetic material during prophase I, creating recombinant chromosomes.

• Random fertilization: Any sperm can fuse with any egg, resulting in a vast number of possible zygote combinations.

Evolutionary Significance

Genetic variation within populations enables natural selection to favor traits best suited to the environment. Sexual reproduction and mechanisms like meiosis and fertilization continually shuffle alleles, increasing diversity.

• Asexually reproducing organisms may also increase diversity by incorporating foreign DNA.

Comparison of Mitosis and Meiosis

Key Differences

• Mitosis: Produces two genetically identical diploid cells; conserves chromosome number.

• Meiosis: Produces four genetically distinct haploid cells; reduces chromosome number.

• Three events unique to meiosis I: synapsis and crossing over, alignment of homologous pairs, and separation of homologs.

Summary Table: Mitosis vs. Meiosis

Key Terms and Concepts

• Homologous chromosomes: Chromosome pairs with the same genes.

• Diploid (2n): Two sets of chromosomes.

• Haploid (n): One set of chromosomes.

• Gametes: Reproductive cells (sperm, egg).

• Zygote: Fertilized egg.

• Crossing over: Exchange of genetic material between homologs.

• Independent assortment: Random distribution of homologs.

• Mutation: Change in DNA sequence.

Equations

• Number of possible chromosome combinations due to independent assortment: where is the haploid number.

Example: For humans, , so possible combinations.

Additional info: The notes expand on the original content by providing definitions, examples, and a comparison table for mitosis and meiosis, ensuring completeness and academic quality for exam preparation.