Mendel's Law of Segregation
Mendel's Law of Segregation essentially has four parts.
- Alternative versions of genes account for variations in inherited characters. This is the concept of alleles. Alleles are different versions of genes that impart the same characteristic. Each human has a gene that controls height, but there are variations among these genes in accordance with the specific height the gene "codes" for.
- For each character, an organism inherits two genes, one from each parent. This means that when somatic cells are produced from two gametes, one allele comes from the mother, one from the father. These alleles may be the same (true-breeding organisms, e.g. ww and rr in Fig. 3), or different (hybrids, e.g. wr in Fig. 3).
- If the two alleles differ, then one, the dominant allele, is fully expressed in the organism's appearance; the other, the segregate during gamete production. This is the last part of Mendel's generalization. The two alleles of the organism are separated into different gametes, ensuring variation.
Figure 3 : The color alleles of Mirabilis jalapa are not dominant or recessive.
(1) Parental generation. (2) F1 generation. (3) F2 generation. The "red" and "white" allele together make a "pink" phenotype, resulting in a 1:2:1 ratio of red:pink:white in the F2 generation.
During his experiments, Mendel encountered some traits that did not follow the laws he had encountered. These traits did not appear independently, but always together with at least one other trait. Mendel could not explain what happened and chose not to mention it in his work. Today, we know that these traits are close together on the same chromosome.
The parts of the previous version I didn't merge. Someone please have another look.
Mendel's First Law: Each adult pea plant has two genes - a gene pair - for each characteristic. The two memebers of each gene pair separate (segregate) randomly into the eggs or sperm of the plant, so that each egg or sperm contains only one member of each gene pair. The offspring therefore inherits one randomly selected gene from each parent for each characteristic.
The first law of Mendelian Genetics was easily illustrated due to the phenomenon of dominance. Certain characteristics, such as yellow seeds, were found to be "dominant" over other "recessive" characteristics, in this case over green seeds. A yellow-seeded plant crossed with a green-seeded plant produced offspring that were entirely yellow-seeded. However, when these yellow-seeded offspring were crossed with the original green-seeded parent strain (a procedure known as back-crossing), half of the plants in the second offspring generation bore yellow seeds and half bore green seeds. The following diagram illustrates these crossesdominant yellow characteristic and a lower-case y to indicate the recessive green characteristic. These two variants are called alleles of the gene.
YY X yy Parental generation (P)
|
V
Yy First generation of offspring (F1)
All seeds are yellow (Y allele is dominant)
Yy X yy Second cross, F1 with green P
|
V
Yy and yy Second generation of offspring (F2),
with an equal proportion of Yy and yy
Mendel's Second Law: During the formation of sperm and egg, the segregation of alleles for one gene is independent of the segregation of alleles for another gene. This law was slightly more complex to demonstrate, requiring the statistical analysis of offspring of plants that differed in two separate characteristics.
typing hands getting tired, put this demo in later
