Homologous Chromosomes - Definition, Characteristics, Pairing in Meiosis, Nondisjunction and Significance

What are Homologous Chromosomes?

Homologous chromosomes are pairs that share similar length, shape, centromere position and genetic content.

• Chromosomes are genetic material in nucleus made of DNA and proteins.
• Carry genes at same loci - same set of genes.
• One homolog from each parent - creates unique combination.
• Humans have 46 chromosomes - 22 pairs autosomes + 1 pair sex chromosomes.
• Female XX homologous, Male XY non-homologous.

Characteristics

• Similar gene sequence, gene loci, centromere position, length.
• Same genes but different alleles.
• Found in diploid organisms.
• Exchange via crossing over in meiosis.
• Leads to haploid gametes and genetic diversity.

Pairing During Meiosis

Pairing promotes recombination and proper segregation.

• Sister chromatids: DNA replication in S phase creates identical copies.
• Meiosis I - homologs exchange via crossing over.
• Meiosis II - forms four haploid gametes.
• Synapsis in prophase I - pairing helped by synaptonemal complex.
• Chiasmata - X-shaped structures during crossing over.

Nondisjunction

Nondisjunction - chromosomes fail to separate, causing abnormal chromosome number.

• Can occur in mitosis or meiosis.
• Example: Down syndrome - extra chromosome 21.

Significance

• Sexual reproduction - genetically different offspring.
• Creates larger gene pool and adaptation.
• Useful in DNA double-strand break repair.
• Important for diagnosing genetic disorders.