Human Genetics and Heredity: From Reproduction to Genetic Disorders

Posted on Feb 2, 2025 in Biology

Reproduction

Through breeding, new individuals arise that are similar to their progenitors. This ensures the survival of the species.

Asexual Reproduction

Involves a single progenitor and leads to exact copies of itself. The offspring contain the same precise genetic information as the progenitor.

Sexual Reproduction

Involves two progenitors, which contribute specialized cells called gametes that contain hereditary characters. These gametes originate from the two progenitors.

  • Female gamete: Egg, oosphere
  • Male gamete: Spermatozoid, antherozoid

Individuals and Sex

Gametes are formed in specialized organs called gonads or gametangia.

  • Gonads: Testes and ovaries
  • Unisexual or dioecious: Individuals with either male or female gonads.
  • Monoecious or hermaphroditic: Individuals with both types of gonads.

Genetics

Genetics is the science that studies the mechanisms of heredity and the laws by which they are governed.

  • Locus: The fixed position occupied by a gene on a chromosome.
  • Allele: The various alternatives that a gene may present.
  • Homozygote or purebred: An individual who has two identical alleles for a character.
  • Heterozygote: An individual who has different alleles for a character. (Monohybrid (Aa), Dihybrid (AaBb))
  • Genotype: The set of genes of an organism.
  • Phenotype: The set of characters that an organism manifests.

Interpretation of Mendel’s Experiments

  • First Law, Crossing Homozygous for a Single Character: When two inbred lines that differ in one character are crossed, the offspring is uniform (first filial generation, F1).

  • Second Law, Crossing of Hybrids for a Single Character: When crossing the first-generation hybrids, the alleles are separated and distributed to gametes independently (segregation of characters in the F2).

  • Third Law, Inheritance of Two Characters: Different alleles are inherited independently of each other and combine at random in the offspring.

Types of Inheritance

  • Dominant Inheritance: Occurs when the dominant allele prevents the expression of the recessive phenotype.

  • Intermediate Inheritance: When both alleles are expressed, the heterozygote has a new phenotype with intermediate characteristics of both parents.

  • Codominant Inheritance: Occurs when heterozygotes have features of both progenitors because both alleles are expressed.

Pedigrees can study the transmission of a particular character through several generations of individuals related to each other.

  • Dominant Inheritance: Familial hypercholesterolemia is a disease characterized by excess cholesterol in the blood, which is deposited in plaques in the heart arteries, potentially causing heart attacks. It depends on dominant alleles.

  • Recessive Inheritance: Albinism is characterized by the absence or reduction of pigmentation in the skin, hair, and eyes due to the absence of melanin. It depends on a recessive allele.

Blood group inheritance depends on three alleles: A, B, and 0. Alleles A and B are codominant, and both are dominant over 0.

Sex Determination in Humans

In humans, sex determination depends on sex chromosomes or heterochromosomes. Humans have 46 chromosomes in their somatic cells, of which 44 are autosomes, and two are sex chromosomes. Women are XX (homogametic), and men are XY (heterogametic). Each egg has 22 autosomes and one X chromosome. Sperm cells can carry either an X or a Y chromosome. Therefore, there is a 50% chance of having a girl or a boy.

Alterations in Chromosome Number

  • Monosomy: The loss of a chromosome. Turner syndrome is caused by losing one of the sex chromosomes (X0). Individuals with this condition are phenotypically female.

  • Trisomy: The presence of an extra chromosome in a homologous pair. Down syndrome is an example.

Sex-Linked Inheritance

This is the mode of inheritance determined by genes located on sex chromosomes.

  • Homologous Segment: A matching segment in both X and Y chromosomes with genes for the same characters.

  • Differential Segment: This segment has no corresponding segment in the other chromosome. It is found in the differential segment of the X chromosome.

Prenatal Diagnosis

Prenatal diagnosis is a set of techniques to determine the existence of certain congenital anomalies in the fetus.

  • Ultrasonography
  • Amniocentesis: Extraction of amniotic fluid containing fetal cells.
  • Chorionic villus sampling: Biopsy of the chorionic villi.
  • Cordocentesis: Umbilical cord blood sampling by puncture.