Mendel’s Laws and Genetics: Understanding Heredity
Mendel’s Laws and Genetics
Introduction
In the second half of the nineteenth century, Gregor Mendel discovered the basic principles governing the transmission of hereditary characters. For his experiments, he chose the pea plant due to its easy cultivation, rapid growth, and easily observable characteristics. Pea plants can self-pollinate and are easy to artificially impregnate, making them ideal for study. Mendel selected seven characteristics and utilized pure lines, studying the descent along several generations and analyzing the resultant data quantitatively.
Mendel’s Experiments
1st Group of Experiments: Monohybrid Crosses
Mendel began by studying the inheritance of a single character among the parental (P) generation and its descendants (F1). He artificially fertilized two pure lines that differed only in seed color (yellow/green). The resulting hybrid offspring (F1) all had yellow seeds. He called this the first filial generation. The green seed characteristic disappeared in this generation. Mendel repeated the experiment with the other selected characteristics and found that in each case, only one of the two parental characteristics appeared in the F1 generation. He called the expressed characteristic dominant (A) and the other recessive (a).
2nd Group of Experiments: Dihybrid Crosses
Mendel allowed the F1 hybrids obtained from the previous crosses to self-fertilize. He observed that for every four seeds produced in each plant, three were yellow and one was green. He repeated the experiment with other characteristics and obtained similar results (3:1 ratio). He proposed that each character is determined by two hereditary factors, one from each parent.
3rd Group of Experiments: Trihybrid Crosses
After verifying the results of crossing pure lines that differed in only one character, Mendel crossed two pure lines of peas for two characteristics (smooth yellow seeds and wrinkled green seeds). The result in the F1 generation was uniform: all descendants had smooth yellow seeds. He found that the results were consistent with his first group of experiments. After allowing the F1 hybrids to self-fertilize, he obtained a second filial generation (F2) with all possible combinations of characteristics in a 9:3:3:1 ratio (9 yellow smooth, 3 yellow wrinkled, 3 green smooth, 1 green wrinkled). He concluded that each factor is inherited independently.
Genetics
Genetics is the science responsible for studying the mechanisms of inheritance and the laws governing them.
Key Concepts in Genetics
- Genes: Genes are hereditary factors, fragments of DNA that contain the necessary information for a specific hereditary character. They are always located in the same place (locus) on chromosomes.
- Locus: The fixed position a gene occupies on a chromosome.
- Alleles: Different alternatives that a gene may have for a specific character. They are located at the same locus on homologous chromosomes. When gametes are formed, each pair of homologous chromosomes separates, so each gamete has only one allele for each character.
- Homozygous: When an individual’s alleles for a gene are identical (AA, aa, BB).
- Heterozygous: When an individual’s alleles for a gene are different (Aa, Bb).
- Dominant Allele: The allele that is expressed in the phenotype.
- Recessive Allele: The allele that is not expressed in the phenotype when a dominant allele is present.
- Genotype: The set of genes that an individual has inherited from its parents (e.g., Aa).
- Phenotype: The set of observable characteristics that an organism expresses (e.g., eye color).
Mendel’s Laws
1st Law (Law of Segregation):
When crossing a homozygous dominant individual (AA) with a homozygous recessive individual (aa), the F1 offspring will be heterozygous (Aa) and express the dominant phenotype. When two pure lines differing in one character are crossed, the offspring are phenotypically and genotypically uniform.
2nd Law (Law of Independent Assortment):
When F1 individuals (Aa) are self-fertilized, the alleles segregate independently during gamete formation, resulting in a 3:1 phenotypic ratio in the F2 generation (AA, Aa, Aa, aa).
3rd Law (Law of Dominance):
In a cross between homozygous parents differing in two characters (e.g., seed color and shape), the F1 generation will be heterozygous for both characters (AaBb) and express the dominant phenotype for both. The F2 generation will exhibit a 9:3:3:1 phenotypic ratio, demonstrating that alleles for different traits are inherited independently.
Other Inheritance Patterns
Intermediate Inheritance:
When both alleles are expressed, resulting in a hybrid or heterozygous individual with a phenotype that is intermediate between the two parents (e.g., red flower (RR) + white flower (WW) = pink flower (RW)).
Codominance:
Both alleles are expressed simultaneously in the heterozygote, resulting in a phenotype that displays characteristics of both parents (e.g., blood type AB).
Examples of Genetic Traits
- Albinism: A homozygous recessive condition characterized by a lack of melanin.
- Blood Groups:
- Group A: IAIA, IAi
- Group B: IBIB, IBi
- Group O: ii
- Group AB: IAIB
- Color Blindness: A recessive condition located on the X chromosome, resulting in sex-linked inheritance. Women must be homozygous for the recessive allele to express the trait. (Carrier female: XDXd, Affected female: XdXd, Affected male: XdY)
- Hemophilia: A recessive condition located on the X chromosome, primarily affecting males. (Carrier female: XHXh, Affected female: XhXh, Affected male: XhY)