Mendel’s Laws and Chromosome Role in Inheritance
Mendel’s Laws
Mendel’s experiments began by crossing two purebred individuals with contrasting traits. He observed several characters. For instance, the smooth seed character dominates over the wrinkled seed character. When crossing a purebred smooth seed plant with a purebred wrinkled seed plant, the resulting parental generation (P) produced only smooth seeds. Self-pollination of the F1 generation resulted in both smooth and wrinkled seeds, with a ratio of approximately three smooth seeds for every one wrinkled seed. These results led Mendel to propose that characteristics were inherited. He theorized that each individual carried two factors for each trait, one inherited from each parent, even if not visibly expressed in the F1 generation. These factors would then segregate and be expressed in the F2 generation.
Mendel’s First Law: Law of Uniformity
When two purebred individuals with contrasting traits are crossed, all individuals in the F1 generation are genotypically and phenotypically the same (e.g., 100% smooth seeds). In Mendel’s experiment, the smooth seed parent (LL) produced gametes with the L allele, while the wrinkled seed parent (ll) produced gametes with the l allele. The F1 generation was therefore Ll.
Mendel’s Second Law: Law of Segregation
Individuals in the F2 generation, resulting from the self-crossing of F1 hybrids, are phenotypically different due to the separation of the factors (alleles) responsible for these traits. These factors, initially together in the hybrid, segregate during gamete formation and are allocated among the gametes. In the example, the F1 generation (Ll) produces gametes with either L or l alleles, resulting in F2 individuals with genotypes LL, Ll, Ll, and ll, and a phenotypic ratio of 3 smooth to 1 wrinkled.
Backcrossing
To distinguish between purebred and hybrid individuals with the same phenotype, backcrossing is performed. This involves crossing the individual in question with a purebred individual possessing the recessive trait. This allows for the identification of heterozygotes.
Mendel’s Third Law: Law of Independent Assortment
Mendel also studied the inheritance of two characters simultaneously. He crossed purebred plants with yellow smooth seeds and green wrinkled seeds. The F1 generation was 100% yellow and smooth. Self-pollination of the F1 plants resulted in four phenotypes: yellow smooth, yellow wrinkled, green smooth, and green wrinkled, in a 9:3:3:1 ratio. This demonstrates that different traits are inherited independently of each other, as the factors responsible for these traits are passed to offspring separately and combined in all possible ways.
Gene Linkage
Mendel’s third law does not always hold true because genes located on the same chromosome tend to be inherited together, a phenomenon known as linkage. However, crossing over during meiosis can lead to recombination, where linked genes are separated. The frequency of recombination is related to the distance between genes on the chromosome.
Nuclear Division and Chromosomes
During nuclear division, the nuclear envelope and nucleoli disappear, and chromosomes become visible. Chromosomes are composed of DNA and histones. The number of chromosomes is constant for all individuals of the same species but varies between species. Humans have 46 chromosomes.
Chromosome Structure
During interphase, DNA replication occurs, resulting in duplicated chromatin fibers joined at the centromere. Each chromosome then consists of two identical sister chromatids. Chromosome arms extend from either side of the centromere. Secondary constrictions may also be present, and if located near the end of the chromosome, the resulting segment is called a satellite. The distal part of the chromosome is called a telomere. A protein disk called the kinetochore surrounds the centromere and serves as a microtubule organizing center during chromosome movement. A secondary constriction may also contain the nucleolar organizer region, which encodes ribosomal RNA.
Role of Chromosomes
Chromosomes facilitate the distribution of genetic information contained in DNA during cell division. DNA in its chromosomal form cannot be transcribed.
Chromosome Number
Each species has a specific number of chromosomes, which is constant for all cells of that species. Most species are diploid, meaning they have two sets of chromosomes, one from each parent. Gametes, however, are haploid. The complete set of chromosomes arranged in pairs is called a karyotype.