Notes on 6.4: Homologous chromosomes, autosomes, sex chromosomes, and sex determination

6.4.1 Homologous chromosomes

  • A pair of chromosomes that carry the same gene loci (same positions on the chromosome) and therefore the same genes at the same loci. The plural of locus is loci.
  • Chromosomes occur in pairs in many organisms, including humans. In a human cell with 4646 chromosomes, there are 2323 pairs.
  • In humans: one chromosome of each pair is inherited from the father and one from the mother.
  • In females all 46 chromosomes can be matched up with another chromosome (i.e., all are homologous pairs in the sense of matching partners), whereas in males, 44 chromosomes can be matched with a partner, and the remaining pair is the sex chromosomes (X and Y).
  • Matched pairs are called homologous chromosomes. They carry the same genes at the same loci, but the alleles (the forms of those genes) may differ because each chromosome comes from a different parent.
  • Chromosomes that do not share the same gene loci are non-homologous.
  • To help assign chromosomes to their homologous partner, chromosomes can be stained with Giemsa. Staining reveals banding patterns:
    • Dark bands correspond to regions with a higher concentration of adenine (A) and thymine (T).
    • Light bands correspond to regions with higher cytosine (C) and guanine (G).
  • An ideogram is a stylised representation of a haploid set of chromosomes arranged by decreasing size and showing their banding patterns (useful for identifying chromosomes and their relative sizes).
  • Figure references (conceptual):
    • 6.17a: Homologous chromosomes with similar banding patterns.
    • 6.17b: Non-homologous chromosomes with different banding patterns.
  • Important nuance: Even when homologous chromosomes share the same gene loci, the alleles may differ because each chromosome was inherited from a different parent.
  • Summary concepts:
    • Gene loci (plural: loci): the position of a gene on a chromosome.
    • Allele: a variant form of a gene.
    • Homologous chromosomes share the same loci and generally the same genes, but alleles may differ.
    • Every organism has a mechanism to pair homologous chromosomes during meiosis, balancing parental contributions.

6.4.2 Autosomes and sex chromosomes

  • Chromosomes can be divided into two groups depending on whether they determine sex: autosomes and sex chromosomes (also called allosomes).
  • Autosomes: do not determine sex; numbered pairs 1–22 in humans.
  • Sex chromosomes determine sex; in mammals the typical arrangement is X and Y.
  • Humans have a total of 4646 chromosomes arranged into 2323 pairs: 2222 autosomal pairs + 1 sex chromosome pair.
  • Distinguishing autosomes:
    • Relative size of the chromosome.
    • Position of the centromere (centromere appears as a constriction; it can be near the middle or toward an end).
    • Banding patterns produced by staining.
  • Size-based numbering in humans: autosomes are numbered 11 (largest) to 2222 (smallest). The larger the chromosome, the more DNA and typically more genes it carries.
  • Homologous autosomes: the two chromosomes with the same number (e.g., the two number-5 chromosomes) are homologous.
  • Non-homologous autosomes: different numbers (e.g., a number-5 chromosome and a number-14 chromosome).
  • Sex chromosomes (allosomes): a pair that differs between males and females. In humans, typical sex chromosome set is:
    • Males: 46,XY46, XY
    • Females: 46,XX46, XX
  • Centromere: the constricted region where sister chromatids are held together.
  • Figure concept: an ideogram (Figure 6.18) shows the human chromosomes with banding patterns aiding identification.
  • Key terms:
    • Autosome: one of a pair of homologous chromosomes identical in appearance in both males and females.
    • Sex chromosome: a pair that differs between sexes (X and Y in mammals).
    • Allosome: another term for sex chromosome.
  • Summary: autosomes are the 22 paired chromosomes common to both sexes; the sex chromosome pair (XX or XY in humans) determines biological sex.

6.4.3 Sex determination

  • Mammals use the XX/XY system for sex determination.
  • The X chromosome in humans contains roughly ${800}$ genes, while the Y chromosome contains roughly ${50}$ genes.
  • The Y chromosome carries the SRY gene at the top (short arm region not shown here) that codes for a protein controlling the development of male characteristics.
  • Therefore, if an embryo contains a Y chromosome (i.e., has a Y chromosome from the father), it develops as male (XY). If there is no Y chromosome (i.e., XX), it develops as female.
  • During meiosis in males, the X and Y chromosomes pair up during metaphase before being separated into sperm cells. This produces a 50% chance that a sperm carries an X chromosome and a 50% chance it carries a Y chromosome:
    • This can be described as: when gametes are formed, half contain an X and half contain a Y.
    • The fertilized zygote then determines sex based on whether the sperm contributed an X or a Y chromosome.
  • Practical implication: the paternal contribution (the sex chromosome in the sperm) determines the offspring's sex in humans.
  • Note: A similar XX/XY pattern with exceptions applies to many other mammals.

Birds and reptiles: ZW/ZZ system

  • In birds, the sex chromosome system is reversed relative to mammals: the male has two similar sex chromosomes (ZZ) and the female has one Z and one W (ZW).
  • Consequently, in birds and many reptiles, the sex of the offspring is determined by the female’s sex chromosome (the mother).
  • This ZW/ZZ system is also observed in some reptiles (snakes, monitor lizards) and in some amphibians.
  • Example: tiger snakes (Nothos spp.) can have males with ZZ and females with ZW.

Reptiles: environmental sex determination (ESD)

  • In some reptile species, sex is determined by the incubation temperature of the eggs rather than genetic factors.
  • Example: green turtles (Chelonia mydas) exhibit ESD.
  • Reproductive biology:
    • Female turtles typically lay about 110110 eggs, buried in sandy beaches.
    • After laying, females return to the sea.
    • The sex of hatchlings depends on incubation temperature:
    • Temperatures above 31opextoextC31^ op ext{o} ext{C} produce predominantly males.
    • Temperatures below 27opextoextC27^ op ext{o} ext{C} produce predominantly females.
    • Temperatures around 29opextoextC29^ op ext{o} ext{C} yield around equal numbers of both sexes.
  • Environmental sex determination highlights a non-genetic mechanism for sex ratio in some species, with potential implications for population dynamics under climate change.

Sample Problem 3: Explaining male sex chromosomes

  • Question: In humans, if the sex chromosomes possessed by an individual are XY, the person is biologically male. Explain why.
  • Answer outline:
    1) The Y chromosome carries the SRY gene (a Y-linked gene).
    2) The SRY gene codes for a protein that initiates the development of male sexual characteristics.
    3) Therefore, the presence of a Y chromosome (and SRY) leads to male development; lack of Y (i.e., XX) leads to female development.
  • Key points to include in a full answer:
    • The Y chromosome contains relatively few genes (~50) compared with the X chromosome (~800).
    • SRY is located on the Y chromosome and acts as a switch to trigger male differentiation during embryonic development.
    • The combination XY undergoes meiosis in males to produce sperm carrying either X or Y, which, upon fertilization, determines the offspring’s sex.
  • Context note: This explanation reflects the standard XX/XY system in mammals and is consistent with foundational genetics concepts discussed in earlier lectures (e.g., gene loci, alleles, meiosis).

Connections to broader concepts and real-world relevance

  • Sex chromosome composition (XX/XY vs ZW/ZZ) illustrates diversity in sex determination across taxa and highlights how chromosomal composition influences development.
  • The existence of autosomes (22 pairs in humans) versus sex chromosomes underlines how most genetic information is carried in autosomes, while sex chromosomes carry key sex-determining genes.
  • Giemsa banding and ideograms show how cytogenetic techniques help identify chromosomes and track structural variation, which is foundational for understanding karyotypes and genetic diseases.
  • Environmental sex determination (ESD) demonstrates that not all sex determination is genetic; ecological and environmental factors can shape population dynamics, with potential implications for conservation in the face of climate change.
  • Ethical and practical implications: understanding sex determination is crucial for assisted reproduction, understanding sex-linked disorders, and informing conservation strategies for species with temperature-dependent sex ratios.
  • Foundational principles reinforced: inheritance from two parents, segregation of alleles during meiosis, and the role of specific genes (e.g., SRY) in developmental pathways.

Key terms and definitions (glossary)

  • Homologous chromosomes: paired chromosomes that carry the same genes at the same loci.
  • Locus (plural: loci): the position of a gene on a chromosome.
  • Allele: a variant form of a gene.
  • Autosomes: the non-sex chromosomes; in humans, the pairs 11–2222.
  • Sex chromosomes (allosomes): chromosomes that determine sex (XX/XY in humans; ZZ/ZW in birds; various systems in other groups).
  • Centromere: the constricted region where sister chromatids are held together.
  • Ideogram: a stylised representation of a haploid set of chromosomes arranged by decreasing size.
  • Giemsa staining: a method producing characteristic dark (AT-rich) and light (CG-rich) bands on chromosomes.
  • SRY: a gene on the Y chromosome that codes for a protein driving male development.
  • Environmental sex determination (ESD): sex determined by environmental factors such as incubation temperature in some reptiles.
  • Haploid: having one set of chromosomes (gametes).
  • Diploid: having two sets of chromosomes (somatic cells).
  • XX/XY system: the mammalian sex-determination system (females XX, males XY).
  • ZW/ZZ system: sex-determination system in birds/reptiles where females are ZW and males are ZZ.
  • 46 chromosomes in humans: the typical diploid number for somatic cells.
  • 23 pairs: the total number of chromosome pairs in humans.
  • 50 genes on Y vs ~800 on X: qualitative difference in gene content contributing to sex-specific development.
  • Percentage probabilities:
    • Sperm carrying X or Y with equal probability 0.50.5 each.
    • Consequently, a 50% chance of XX (female) or XY (male) offspring in humans with standard fertilization.
  • Temperature thresholds for ESD in green turtles: >31^ op ext{o}C (males), <27^ op ext{o}C (females), around 29opextoC29^ op ext{o}C (mixed).