Codominance, Genetic Traits, and Chromosomal Disorders
Codominance and Genetic Inheritance
Codominance
Codominance refers to the relationship between two alleles for the same gene, where both alleles are expressed in the phenotype. The characteristics determined by both codominant alleles appear as a mixture.
Dominance and Recessiveness
This describes the relationship between two alleles where the dominant allele expresses its information, while the recessive allele remains hidden. For the recessive trait to manifest, the individual must be homozygous recessive (having two copies of the recessive allele).
Other Key Genetic Terms
- Gene: A hereditary factor that controls a character. It is a piece of DNA with the information for at least one protein.
- Locus: The location that a gene occupies on a chromosome.
- Alleles: Different forms of a gene for a particular character.
- Genotype: The genetic constitution of an organism; the specific alleles an individual has for a character (e.g., AA, Aa, or aa).
- Phenotype: The outward, observable expression of the genotype.
- Mutation: A change in the genetic material, either in genes or chromosomes.
- Deletion: A type of mutation caused by the loss of a fragment of a chromosome. It can cause syndromes like Cri-du-chat, Prader-Willi, and Angelman.
- Aneuploidy: A condition caused by having an abnormal number of chromosomes (either more or fewer).
- Syndrome: A group of symptoms.
- Nondisjunction: A phenomenon where errors occur during gamete formation in meiosis, because chromosomes fail to separate during anaphase (e.g., Down syndrome, Turner syndrome).
- Family Trees: Diagrams showing the inheritance of a particular character in a family across several generations.
Genetic Diseases
Genetic diseases are hereditary conditions transmitted from parents to offspring. They are caused by mutations and can be classified as follows:
Gene Mutations (Genic)
These are changes in the chemical makeup of genes.
Autosomal Genes
- Albinism (recessive): Lack of melanin production, resulting in clear skin, white hair, and pinkish eyes.
- Sickle Cell Anemia (recessive): Red blood cells have a sickle shape, causing them to break and clump, hindering blood flow.
- Huntington’s Chorea (dominant): Characterized by exaggerated movements, grimaces, sudden difficulty speaking and swallowing, and dementia.
Sex-Linked Genes (on Sex Chromosomes)
- Hemophilia (recessive): Poor blood clotting, causing internal or external bleeding.
- Colorblindness (recessive): A genetic defect involving the inability to distinguish between red and green colors.
Genomic Mutations
Changes in the number of chromosomes.
Changes in Sex Chromosomes (Heterosomes)
- Turner Syndrome (XO) (45 chromosomes): Sexually immature females, underdeveloped breasts, folds of skin on the neck, and mild mental disorders.
- Triple X Syndrome (XXX) (47 chromosomes): Childlike appearance, underdeveloped sexual characteristics.
- Klinefelter Syndrome (XXY) (47 chromosomes): Sterile males with thin body structure.
- XYY Syndrome (“Double Y”) (47 chromosomes): Tall stature, intellectual disability, and aggressive behavior disorders.
Changes in Autosomes (Trisomies)
- Down Syndrome (Trisomy 21): Slanted eyes, intellectual disabilities, and rough skin.
- Patau Syndrome (Trisomy 13): Cardiac conditions, cleft lip, and extra fingers.
- Edwards Syndrome (Trisomy 18): Malformations, webbed fingers, small head, and receding chin.
Chromosomal Disorders
These disorders occur due to gains in chromosomes or loss of a chromosomal fragment.
- Cri-du-chat Syndrome (Cat Cry Syndrome): Loss of a fragment of chromosome 5. Small head, intellectual disability, a cry resembling a cat’s meow, and underdeveloped larynx.
- Fragile X Syndrome: Multiple repeats of a small fragment at one end of the X chromosome, causing intellectual disability.