Understanding Genetic Theory: Mendel’s Laws and Inheritance
Understanding Genetic Theory
The initial genetic theory suggested that offspring characteristics were a blend of their parents’ traits. The goal was to create individuals with the combined advantages of each parent through artificial selection, repeated until pure traits were achieved.
Mendel’s Experiments with Peas
Mendel, interested in understanding how traits are transmitted between generations, began experimenting with peas in 1856. He focused on crosses involving pure lines, each with a single distinct trait.
Mendel’s First Law: The Law of Uniformity
In one experiment, Mendel crossed wrinkled peas with smooth peas. The resulting F1 generation all exhibited a smooth and uniform appearance, confirming the Law of Uniformity. This law states that when two pure lines are crossed, all offspring will be identical in their traits.
Mendel’s Second Law: The Law of Segregation
When the F1 plants were crossed, the F2 generation showed a 3:1 ratio of smooth to wrinkled peas. This indicated that genetic information is not simply blended but rather exists as discrete units. Mendel proposed that each trait is determined by a hereditary factor, with each individual possessing two factors for each trait, one inherited from each parent. Today, we call these factors genes. The combination of genes is the genotype, and the observable characteristics are the phenotype. These genes can be dominant or recessive. Individuals can be homozygous (pure) or heterozygous (hybrid). This led to the Law of Segregation, which states that the two hereditary factors do not mix but segregate during gamete formation, ensuring that each gamete carries only one factor for each trait.
Mendel’s Third Law: The Law of Independent Assortment
Mendel also examined how two different traits are inherited together, such as texture and color. This led to the Law of Independent Assortment, which states that hereditary factors for different traits are not linked and maintain independence through generations. However, this is not always the case, as genes located close together on the same chromosome tend to be inherited together, a phenomenon known as gene linkage. The closer the genes, the lower the frequency of recombination during meiosis.
Sex Determination and Inheritance
The concept of gender involves the sex cells capable of originating new individuals when two different kinds unite. Organisms that produce two types of gametes are called hermaphrodites. Sex can be defined by sex chromosomes. In diploid species, the chromosomes are the same in males and females (autosomes), except for the heterochromosomes, which differ between sexes (X and Y). The XX is omogametic and XY is heterogametic.
Sexuality is the ability of organisms to exchange genetic material. Reproduction is the generation of new individuals, which can be sexual or asexual. In asexual reproduction, offspring are identical to the parents. In sexual reproduction, offspring are different from the parents due to the mixing of genetic material.
Sex-Linked Inheritance
Sex-linked inheritance involves genes located on the X and Y chromosomes. A segment of the X chromosome has a counterpart on the Y chromosome, allowing for pairing during meiosis. Genes in this region control the same traits. The differential regions of the X and Y chromosomes contain genes that determine sex-specific traits. Genes on the X chromosome are called ginandrics, and genes on the Y chromosome are called holandrics.
Examples of Sex-Linked Traits
- Daltonism (Color Blindness): The inability to distinguish between green and red, governed by recessive genes on the X chromosome.
- Hemophilia: A hereditary condition characterized by impaired blood clotting, also linked to the X chromosome.
Sex-Influenced Traits
Sex-influenced traits are autosomal traits that are expressed differently in males and females due to hormonal influences. An example is hereditary baldness.
Other Inheritance Patterns
- Codominance: Two alleles have equal dominance, and both traits are expressed in the phenotype.
- Dihybrids: Individuals heterozygous for two pairs of genes.
- Pilihibrids: Individuals heterozygous for multiple genes.
- Lethal Alleles: Gene pairs with information deficits that can lead to death.
- Retroencreuamen (Test Cross): Crossing an individual with an unknown genotype with a homozygous recessive individual to determine the genotype.
Gene symbols are typically represented in italics. Dominant alleles are capitalized, and recessive alleles are lowercase.
ABO Blood Groups
ABO blood groups are determined by the presence or absence of A and B antigens on red blood cells. Individuals with type O blood have neither A nor B antigens.
Probability in Genetics
The concept of probability is crucial in genetics. It is defined as the ratio between favorable outcomes and possible outcomes. The probability of independent events occurring together is calculated by multiplying their individual probabilities. The probability of mutually exclusive events occurring is calculated by adding their individual probabilities.