Inheritance of Genes

Key Concepts in Genetics

Genetics is the scientific study of heredity and variation. Heredity is the transmission of traits from one generation to the next, while variation refers to the differences in appearance that offspring show from parents and siblings. Genes, the units of heredity, are made up of DNA segments and are located on chromosomes at specific positions called loci (singular: locus).

• Gene: A segment of DNA that codes for a specific trait.

• Locus: The specific location of a gene on a chromosome.

• Allele: Alternative versions of a gene found at the same locus.

• Homologous chromosomes (homologs): Chromosome pairs (one from each parent) that are similar in length, centromere position, and gene content, but may carry different alleles.

• Gamete: A reproductive cell (sperm or egg) that carries half the genetic material of the organism (haploid).

• Somatic cell: Any body cell other than a gamete; these are diploid.

• Clone: A group of genetically identical individuals from the same parent, produced by asexual reproduction.

Example: In humans, the gene for eye color is found at the same locus on both homologous chromosomes, but one may carry the allele for brown eyes and the other for blue eyes.

Chromosomes and Karyotypes

Types of Chromosomes

• Autosomes: The 22 pairs of chromosomes in humans that do not determine sex.

• Sex chromosomes: The X and Y chromosomes that determine biological sex (XX = female, XY = male).

• Karyotype: An ordered display of the pairs of chromosomes from a cell, used to detect chromosomal abnormalities.

Example: A human karyotype shows 46 chromosomes arranged in 23 pairs, including one pair of sex chromosomes.

Sexual Life Cycles

Diploid and Haploid Stages

Sexual life cycles alternate between diploid (2n) and haploid (n) stages. Diploid cells have two sets of chromosomes, while haploid cells have one set.

• Diploid (2n): Cells with two sets of chromosomes (e.g., somatic cells in humans, 2n = 46).

• Haploid (n): Cells with one set of chromosomes (e.g., gametes in humans, n = 23).

• Fertilization: The union of two gametes, restoring the diploid number.

• Zygote: The fertilized egg, which is diploid.

• Life cycle: The sequence of stages from one generation to the next.

Example: In humans, meiosis produces haploid gametes; fertilization restores the diploid state in the zygote.

Human Life Cycle

• Meiosis occurs in the gonads to produce gametes (sperm and eggs).

• Fertilization produces a diploid zygote, which divides by mitosis to form a multicellular organism.

• Mitosis occurs during growth and development; meiosis only occurs to produce gametes.

Why reduction is necessary: Meiosis reduces chromosome number by half to prevent chromosome doubling in each generation.

Plant Life Cycle: Alternation of Generations

• Sporophyte: The diploid multicellular stage that produces haploid spores by meiosis.

• Gametophyte: The haploid multicellular stage that produces gametes by mitosis.

• Fertilization of gametes forms a diploid sporophyte, completing the cycle.

Meiosis

Overview and Stages

Meiosis is a type of cell division that reduces the chromosome number by half, producing four haploid cells from one diploid cell. It consists of two consecutive divisions: meiosis I and meiosis II.

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.

• Meiosis II: Sister chromatids separate, similar to mitosis.

Meiosis I Stages

• Prophase I: Homologous chromosomes pair up (synapsis) and exchange genetic material (crossing over). The synaptonemal complex forms, holding homologs together.

• 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, each with duplicated chromosomes.

Meiosis II Stages

• Prophase II: Chromosomes condense 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.

Key Terms in Meiosis

• Tetrad: A group of four chromatids formed by the pairing of two homologous chromosomes during prophase I.

• Crossing-over: The exchange of genetic material between non-sister chromatids, resulting in recombinant chromosomes.

• Synaptonemal complex: A protein structure that forms between homologous chromosomes during synapsis.

• Recombinant chromosomes: Chromosomes that have exchanged genetic material due to crossing over.

Comparison: Mitosis vs. Meiosis

Genetic Variation in Sexual Life Cycles

Sources of Genetic Variation

• Crossing-over: Produces recombinant chromosomes by exchanging DNA between homologous chromosomes during prophase I.

• Independent assortment: Homologous chromosomes are randomly distributed to daughter cells during metaphase I. The number of possible combinations is , where n is the haploid number.

• Random fertilization: Any sperm can fertilize any egg, greatly increasing genetic diversity.

Importance: Genetic diversity increases the adaptability and survival of populations in changing environments.

Alterations of Chromosome Number and Structure

Chromosome Number Abnormalities

• Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers.

• Euploidy: The normal number of chromosomes for a species.

• Aneuploidy: An abnormal number of chromosomes (e.g., monosomy, trisomy).

• Monosomy: Only one copy of a particular chromosome (2n-1).

• Trisomy: Three copies of a particular chromosome (2n+1).

Examples of Chromosomal Disorders

Note: The presence of a Y chromosome typically determines male sex, but exceptions exist (e.g., Swyer syndrome).

Chromosome Structure Abnormalities

• Deletion: Loss of a chromosome segment.

• Duplication: Repetition of a chromosome segment.

• Translocation: A segment from one chromosome is transferred to a non-homologous chromosome.

• Inversion: A chromosome segment is reversed end to end.

Summary Table: Key Terms and Definitions

Additional info: For more detailed mechanisms and diagrams, refer to textbook figures (e.g., Figures 13.3, 13.5, 13.6, 13.7, 13.8, 13.9, 13.10, 13.11, 13.12, 15.13, 15.15) as referenced in the lecture outline.