Genetics PDF - Unidad IV

# UNIDAD IV GENÉTICA ## 4.1 HERENCIA BIOLÓGICA - It is the set of hereditary characteristics that parents transmit to their offspring. ## 4.2 GENÉTICA HUMANA - It is a branch of genetics that studies the hereditary characteristics, physical and mental, normal and pathological of human beings. #...

# UNIDAD IV GENÉTICA ## 4.1 HERENCIA BIOLÓGICA - It is the set of hereditary characteristics that parents transmit to their offspring. ## 4.2 GENÉTICA HUMANA - It is a branch of genetics that studies the hereditary characteristics, physical and mental, normal and pathological of human beings. ## 4.3 BASIC CONCEPTS - **Gene:** It is a small segment of DNA. It is the smallest unit of inheritance. - **Genome:** It is the set of genes that make up an organism. - **Locus:** It is the specific site that a gene occupies on the chromosome. - **Alleles:** They are genes that occupy the same locus and cause different characteristics. - **Dominant gene:** It is the one that establishes that a characteristic prevails over another. It is represented by uppercase letters. - **Recessive gene:** It is the gene whose characteristic does not manifest in the presence of the dominant. It is represented by lowercase letters. - **Homozygous genes:** It is when the two genes in the pair are exactly the same for a characteristic. For example: SS (dominant) and bb (recessive). - **Heterozygous genes:** It is when the two genes in the pair are different for a characteristic. That is, a dominant gene and a recessive gene, for example, Sb. - **Genotype:** It is the set of genes that determines the phenotype. They are the internal, invisible characteristics that an individual inherits from his parents. - **Phenotype:** It is the set of external characteristics that an individual presents, for example, hair color and eye color, etc. The phenotype is determined by the genetic makeup plus the influence of the environment. - **Chromosome:** It is the condensed DNA filament, visible in the nucleus of cells during mitosis. Its number is constant for each animal or plant species, for example, the human species has 46 chromosomes, the horse 66, the rat 42, the dove 80, etc. Of the 46 human chromosomes, 44 are autosomes and 2 sex chromosomes. In men, 44 + XY and in women, 44 + XX. - **Karyotype:** It is the complete set of chromosome pairs of a cell, in terms of shape, size and number characteristic of each species. - **Hybrid:** They are the individuals resulting from the crossing of individuals of different species. - **Monohybridism:** It is the crossing between individuals for which a single characteristic is considered, conditioned by a pair of genes. - **Dihybridism:** It is the crossing in which two different characteristics are considered, located on the same chromosome or not, each one of them conditioned by a pair of genes. Therefore there are 4 genes in play. - **Polyhybrid:** It refers to individuals whose parents differ in two or more pairs of characteristics. - **Filial 1 (F1):** Refers to the first generation. - **Filial 2 (F2):** Refers to the second generation. ## 4.4 MENDEL'S LAWS - **Juan Gregorio Mendel (1822-1884)**, an Augustinian monk at the *Brünn* monastery (Austria), born in Austria. He carried out several experiments and studies on hybridization, after eight years of research his observations and conclusions laid the foundation for Genetics. Mendel, without knowing the existence of chromosomes or genes, established 3 laws of what is now known as "Mendel's Laws". ### 4.4.1 First Law: "Law of Dominance or Uniformity" "In the crossing of two pure variants (two homozygous organisms), which differ from each other in one pair of alternative characters (monohybridism), all the first generation of hybrids are all the same." ### 4.4.2 Second Law: "Constant Proportions" >"In the second generation there is segregation of the characters with the formation of 25% of homozygous dominant individuals; 50% of hybrids, whose phenotype is the same as that of the first generation and 25% of homozygous recessive." - "In the second generation there is segregation of the characters with the formation of 25% of homozygous dominant individuals, 50% of hybrids, whose phenotype is the same as that of the first generation and 25% of homozygous recessive." - This is, the character of one of the parents that had been hidden in the first generation, reappears in the second generation at a proportion of 25%. ### 4.4.3 Third Law: "Independent combinations" -"In crossbreeding of 2 or more pairs of independent characteristics, each pair of genes separates at random, inheriting independently of the other pair". ## 4.5 HYBRIDIZATION - It is the crossing of individuals of different species. Hybrids are characterized by either having characteristics that are intermediate between both parents, or having characteristics halfway between both parents, or by resembling one parent more in some characteristics and the other more in others; new characteristics often appear in hybrids. - Examples of hybrids are crosses made between animals of different breeds or between plants of different species. Man carries out hybridization in animals and plants, with an objective: genetic improvement. ### 4.5.1 Monohybridism - Crossing of individuals with only one different characteristic, conditioned by one pair of genes. - **Example 1**: Establish up to the second generation or filial 2 (F2), the crossing of red flowers BB with white flowers bb, both lines are homozygous dominant. - **P**: BBxb - **B**: Red color (dominant characteristic) - **b**: White color (recessive characteristic) - **Gametes**: B, B, b, b - **F1 (filial 1)**= - **100% Bb (genotype proportion)** - **100% Red flowers (phenotype proportion)** - **Autofecundation F2** = **F1xF1 Bb x Bb** - **F2 (filial 2)** = - **BB (25%), Bb (50%), bb (25%) (genotype proportion), 1:2:1** - **75% Red flowers and 25% White flowers. Proportion phenotype 3:1** - **Example 2**: Establish the crossing of tall plants and short plants, both are pure lines, the tall characteristic is dominant. ### 4.5.2 Dihybridism - Crossing of individuals with two characteristics, each one of them conditioned by a pair of genes. - **Example:** Mendel's experiment where the crossing between two types of plants with seeds from pure races is observed, one race with yellow and smooth seeds (VVRR) and the other with green and wrinkled seeds (vvrr). - **Characteristics**: the color and shape of the seeds. - **Homozygous pure plant:** yellow smooth seeds (VVRR) - **Recessive homozygous plant:** green and wrinkled seeds (vvrr) - **P**: WRR x vvrr - **Gametes V, R, v, r** - **F1**: VvRr yellow and smooth - **F2**: - The hybrid in the F1 (VvRr) originates 4 types of gametes, which when uniting generate 16 possible combinations. Therefore, 4 types of seeds are obtained in the F2 in a proportion 9:3:3:1. According to this result, which confirms Mendel's law of independent combinations, each pair of alleles that controls one characteristic is transmitted independently of any other pair of alleles that controls another characteristic.