Human Chromosomes

Key Concepts: Chromosomes and Genes

Human Chromosomes

Humans have 46 chromosomes in total, arranged in 23 pairs:

  • 22 pairs of autosomes

  • 1 pair of sex chromosomes (X and Y)

Each chromosome is composed of DNA and proteins (histones) that help package the DNA into a coiled structure. Chromosomes are most visible during cell division when the DNA is condensed.



Autosomes

  • Definition: Chromosomes that are not involved in determining an individual's sex.

  • Number: Humans have 22 pairs of autosomes (chromosomes 1–22).

  • Function: Autosomes contain genes for characteristics unrelated to sex, such as eye color, hair texture, and other traits.



Sex Chromosomes

  • Definition: The pair of chromosomes that determine the biological sex of an individual.

  • Number: One pair of sex chromosomes: XX in females and XY in males.

  • X Chromosome:

    • Larger than the Y chromosome.

    • Contains about 2,000 genes, most unrelated to sex.

  • Y Chromosome:

    • Much smaller than the X chromosome.

    • Contains fewer than 100 genes, with one key gene, SRY, which determines male sex development.



SRY Gene

  • Full Name: Sex-determining Region Y gene.

  • Function: It triggers the development of male characteristics by initiating the process of male differentiation in the embryo.

  • Impact: A person with the SRY gene will typically develop as male, while absence of the Y chromosome results in female development, making female the default sex.



Human Genes

  • Number of Genes: Humans have about 20,000 to 22,000 genes.

  • Gene Variation:

    • Genes can have multiple alleles, which are alternative forms of a gene.

    • Most genetic variation among people is due to differences in alleles and DNA bases within them.

  • Gene Splicing: A single gene can code for multiple proteins through processes like RNA splicing, increasing the diversity of proteins from the same genetic instructions.



Summary

  • 23 pairs of chromosomes: 22 autosomes and 1 pair of sex chromosomes (XX or XY).

  • X chromosome is much larger and has more genes than the Y chromosome.

  • SRY gene on the Y chromosome triggers male development.

  • Females are considered the "default sex" because absence of the Y chromosome leads to female development.

  • Human genome contains 20,000 to 22,000 genes, many with multiple alleles.



Review Questions

  1. Describe human chromosomes.
     Humans have 46 chromosomes in 23 pairs: 22 pairs of autosomes and 1 pair of sex chromosomes (XX for females and XY for males).

  2. Compare and contrast human autosomes and sex chromosomes.

    • Autosomes: 22 pairs, not involved in sex determination, contain genes for non-sex characteristics.

    • Sex Chromosomes: 1 pair, determine biological sex, with XX for females and XY for males.

  3. What is SRY?
     The SRY gene (Sex-determining Region Y) on the Y chromosome is responsible for initiating male development. It triggers the formation of male traits during embryo development.

  4. Why are females the "default sex" of the human species?
     In the absence of the Y chromosome (and thus the SRY gene), individuals will develop as female, making female the default sex in human development.