Sex Determination and Genetic Mutations

Sex Determination and Genetic Inheritance

Offspring formed from the union of a sperm and an egg develop as females, while unfertilized eggs develop as males.

Environmental Sex Determination

In this case, the sex is not determined by genes or chromosomes but by environmental factors:

• In some reptiles, like crocodiles and turtles, the temperature of the eggs during a certain period of development is the deciding factor in determining sex.
• Some young fish are male and may become female as they get older.

Sex-Linked Inheritance

Sex-linkage appears when genes are located on sex chromosomes. Since males and females have different sex chromosomes, sex-linked genes show different phenotypic ratios depending on the sex.

X-Linked Inheritance

X-linked inheritance appears when the gene is located on the X chromosome.

• Women can be either homozygous or heterozygous. In the latter case, they carry the recessive allele without being affected; they are called carriers.
• Men have only one X chromosome; thus, they cannot be carriers.

Y-Linked Inheritance

Y-linked inheritance appears when genes are located on the Y chromosome.

• As a consequence, these genes are never present in women.
• Very few genes are located here. The gene responsible for hairy ears (auricular hypertrichosis) is one of these.

Genetic Mutations

A mutation is an alteration in the genetic material. Theoretically, the process of passing down genetic information through generations is supposed to be perfect, but from time to time, a miscode is produced. In this way, mutations appear, constituting the raw material that feeds natural selection and evolution.

Types of Mutations by Genetic Alteration

• Genic Mutations: The alteration affects a specific gene.
• Chromosomal Mutations: The alteration affects the structure of chromosomes.
  • Deletion: Loss of a segment.
  • Duplication: A segment is repeated.
  • Inversion: A segment is reversed.
  • Translocation: Two segments are exchanged. It can be either intrachromosomal or interchromosomal.
• Genomic Mutations: The alteration affects the number of chromosomes.
  • Euploidy: Affecting the number of chromosomal sets.
    • Monoploidy
    • Polyploidy
  • Aneuploidy: Affecting the number of individual chromosomes.
    • Monosomy (2n-1): Only one homologous chromosome.
    • Trisomy (2n+1): Three homologous chromosomes.
    • Tetrasomy (2n+2): Four homologous chromosomes.

Origin of Mutations

Mutations can be either spontaneous or induced. In the latter case, they are produced by a mutagen. There are several types of mutagens:

• Physical Mutagens: Ionizing radiations, like X-rays or gamma rays.
• Chemical Mutagens: Certain chemicals, like nitrous acid.
• Biological Mutagens: Certain viruses increase the risk of developing cancer.