Introduction to Genetics

• Biology is introduced for the 3rd year of general media.

Ideas Beforehand

• Since ancient times, humans have been curious on how characteristics are transmitted from one individual to another.
• When living beings reproduce sexually, descendants have differing traits from each other, even siblings and in relation to their parents; but traits also exist that resemble one or both parents.

Genetics

• Genetics is a branch of biology related to the study of genes, genetic variation, and heredity in organisms.
• Genetic inheritance is the transmission of characteristics from parents to their descendants (children).

Mendelian Inheritance

• Though heredity had been observed for millennia, Gregor Mendel, a 19th-century scientist and Augustinian friar, was the first to study genetics scientifically.
• Mendel proposed a hypothesis to express how characteristics are inherited, and is considered the father of genetics.
• Mendel worked with 34 varieties of the pea Pisum sativum, which met the following conditions:
  • Displayed contrasting differential characters.
  • Were easy to protect from external pollen influence during flowering.
  • Produced fertile hybrids.

Mendel's Crossbreeding Method

• Crossings between different varieties of peas were easily achieved at will.
• Self-pollination could be avoided or prevented by castrating a plant's flowers (removing the anthers).

Mendel's Experiments

• Mendel observed repeating patterns in the inheritance of characteristics under study.
• For instance, when crossing a purebred purple flower with a purebred white flower, the first generation offspring all had purple flowers
• However, in the second generation (grandchildren), the white color reappeared in 25% of the descent.

Mendel's Laws

• There are three main laws
• Factors in pairs: Genetic traits are controlled by pairs of factors present in each organism.
• Dominance/recessivity: When two different factors are present for a given trait, one will dominate over the other, and the expressed trait is known as the phenotype.
• Segregation: During gamete formation, paired factors separate randomly so that each gamete receives one or the other with equal probability.
• Law of Independent Transmission In the formation of gametes, the pairs of factors that segregate separate independently of one another

Genetic Inheritance

• The "factors" that Mendel proposed are now known as genes.
• Parental traits must be encoded in genes to be inherited by their offspring.
• Genotype: The set of genes is known as the "genotype".
• Phenotype: Observable characteristics are known as the "phenotype".
• Genotype refers to the genetic makeup of an individual.
• Phenotype refers to the physical appearance of a trait.

Genotype Characteristics

• The genotype is a set of genes characterizing species, whether vegetal or animal.
• The genotype has a DNA format.
• Humans receive it through inheritance from their parents.
• The genes responsible for transmitting hereditary traits are distributed within chromosomes inside the cell's nucleus.
• In prokaryotic cells that lack a nucleus, the chromosome is found in the cytoplasm within a region called the nucleoid.

Alleles

• A portion of DNA that codes for a given trait is calle a Gene.
• Alleles are alternative forms in which a gene is presented
• When an individual possesses two identical alleles for a given trait, they are homozygous.
• When an individual possesses two different alleles for a given trait, they are heterozygous
• A dominant allele is expressed both in homozygous and heterozygous conditions.
• A recessive allele can only manifest in the absence of a dominant allele.
• For a recessive allele to be observed in the phenotype, the organism must have two copies of it (one from each parent).

Chromosomes

• Chromosomes are structures carrying genes and composed of DNA
• Locus: the position that occupies the gene in the chromosome

Parts of a Chromosome

• A chromosome consists of two arms separated by a centromere.
• The ends of the arms are called telomeres.
• The arms are divided into two chromatids each.

Characteristics of Chromosomes

• Chromosomes are highly organized structures inside cells.
• Chromosomes are composed of DNA and other proteins.
• Chromosomes contain most of an individual's genetic information.
• Chromosomes have a defined, X-shaped form, which is observable during meiosis and mitosis.
• Each chromosome presents a characteristic shape and size and typically exists in identical pairs for all individuals of the same species.
• Human species have 23 pairs (46 chromosomes)
• Depending on amount of chromosome, cells can be Diploid (2n) or Haploid (1n)

Types of Chromosome

• Chromosomes are classified according to their function.
• Somatic chromosomes, also known as autosomes, confer non-sexual characteristics to an individual.
• Sexual chromosomes determine the individual's sexual characteristics, differentiating them by biological sex.
• The human species has 23 pairs of chromosomes, with 22 pairs being autosomal (non-sex) type and one pair determining sexual characteristics.
• Males have an XY pair of sex chromosomes, while females have an XX pair.

Types of Cells Types According to Chromosomal Load

• Haploid Cells: Cells with only half of the genetic information (n).
  • Correspond to sex cells, also called GAMETES: eggs in women and sperm in men
• Diploid Cells: Cells with a double set of chromosomes (2n).
  • All cells that form the body (except sex cells) have two sets of chromosomes.