Biology 239: Chromosomes and Heredity

• Chromosomes are studied in Biology 239, referencing textbook sections 3.1 and 4.1.
• This unit defines key terms such as diploid, haploid, somatic, and trisomy.
• The unit also includes how to read and interpret a karyotype.

Chromosomes and Reproduction

• In 1667, studies first indicated that semen contains spermatozoa.
• Studies in 1854 on frogs and sea urchins demonstrated the union of male and female gametes, showing equal contribution.
• The observation of threadlike structures in the nuclei of salamander cells during cell division occurred in 1879.
• Advances in microscopy and differential staining occurred in 1880 that included staining chromosomes (chroma = color).
• It was proposed in the early 1900s that each egg or sperm contains only one of each chromosome of a pair.
• The chromosomal theory of inheritance links chromosomes to Mendelian principles of segregation and independent assortment.

Chromosomal Theory of Inheritance

• Hereditary information resides on genes, which are located on chromosomes.
• Both egg and sperm contribute equally to the genetics of offspring through their nuclei.
• The word "gene" was coined in 1909 to describe Mendel's "units of inheritance."
• In 1901, X and Y chromosomes were discovered that determine sex in grasshoppers.
• It was later confirmed that other species have separate sex chromosomes.
• One sex has two of the same sex chromosome (e.g., XX), and the other has one of each (e.g., XY).

Exceptions to Chromosome Rules

• Platypuses have complex sex chromosome arrangements with females being (XXXXX)x2 and males being XXXXXYYYYY.
• Sex determination in most turtles, alligators, and crocodiles is temperature-dependent during egg development.
• In most arachnids, females are XX, and males are XO (where O signifies nothing).
• Clownfish are all born male; when two males pair, they fight, and the winner becomes female.

Chromosomes and Sex

• In most mammals, males are XY (heterogametic), and females are XX (homogametic).
• In birds, butterflies, and some reptiles, males are ZZ, and females are ZW.
• In humans, the Y chromosome determines male sex.
• In Drosophila, sex is determined by the ratio of X chromosomes to autosomes.
• XX or XXY results in a 1:1 ratio and a female.
• XY or XO results in a 1:2 ratio and a male.

Mitosis and Meiosis

• Mitosis is a nuclear division resulting in 2 daughter cells with identical chromosome numbers to the parent cell, creating clones.
• Meiosis is a nuclear division that halves the number of chromosomes in each egg and sperm compared to somatic cells, creating unique cells.
• In Drosophila melanogaster, there are 4 chromosome pairs.
  • 2n = 2 complete sets.
  • n = 1 complete set (i.e., 4 chromosomes).
• The number of chromosomes in a normal gamete is represented by (n).

Chromosome Types

• Haploid cells have one copy of each chromosome.
• Diploid cells have two copies of each chromosome.
• Homologous chromosomes are members of a chromosome pair that contain the same genes, vary in alleles, and pair at meiosis.
• Non-homologous chromosomes contain different genes and do not pair at meiosis.
• Sister chromatids are identical alleles for all genes.
• A chromatid is one copy of a newly copied chromosome joined to its other copy by a single centromere.

Karyotypes

• A karyotype is the general appearance of the complete set of chromosomes, including size, number, and shape.
• It is made by taking pictures of metaphase chromosomes, cutting them out, and arranging them.
• Karyotypes are useful for studying somatic cells, not gametes.
• Karyotyping can reveal abnormalities in chromosome number such as trisomy 18.
• Trisomy 18 is a genetic disorder with onset before birth, causing thin, frail babies with feeding difficulties and heart malformations, often leading to death in utero or within a year.
• Trisomy 21 (Down's syndrome) results in developmental delays, mild to moderate intellectual disability, and characteristic physical features.
• Most other trisomies except for sex chromosome trisomies such as XXX, XXY, and XYY are not viable.