By analyzing the results of various crosses, Mendel concluded that each alternative form of a trait is specified by alternative forms of a gene. To follow the inheritance of genes from parent to child, Mendel first needed to be sure which genes each parent carried.
Since pea plants are naturally self-fertilizing, "pure-bred" strains were readily available. Each strain contained only one form of the gene that determined a trait. Pure-bred plants with yellow seeds only produced offspring with yellow seeds. Pure-bred plants with green seeds only produced offspring with green seeds. From the results of further experiments, Mendel reasoned that pure-bred plants must have two copies of the same gene for each trait. Funded by The Josiah Macy, Jr.
All rights reserved. Most genes exist in more than one form that, when expressed, result in different characteristics. Genes may often exist in more than one form, and these forms are termed alleles of the gene. An allele is one of at least two alternative forms of a particular gene. Alleles provide the genetic instructions for products that, although similar in type, are visibly different phenotypically different.
The term allele is derived the Greek term alleon used to describe a difference in morphology or form. At the genetic level, alleles contain differing base sequences in their nucleic acid e.
As a form of a gene, an allele carries the instructions for a particular variation of the gene's protein product. Although underlying genetic molecular complexities sometimes blur the differences in expression; nature also provides simple examples of alleles. For example, a single gene may control the flower color of some plants.
Some of these mutations produce alterations in gene products that are fatal, and these mutations are extinguished. However, other mutations in germ cells sperm and eggs can be passed along from generation to generation, and they provide the basis for the many variations in phenotype that make each of us unique. Over time, mutations have created variants of genes that are responsible for differences in the color of our hair, our eyes, and our skin. Mutations influence our intelligence, our height, our weight, our personalities, our blood pressure, our cholesterol levels, and how fast we can run.
Mutations have introduced gene variants that encode for slightly different proteins, which in turn, influence all aspects of our phenotype. It is important to emphasize an individual's phenotype is not solely the result of their genome; instead, phenotype is the result of the interaction between and individual's genome and their environment from the time of conception until death.
This has resulted in alternate forms of many genes. In other words, a given gene can have multiple alleles i. Some genes have just a few alleles, but others have many. Recall also that chromosomes come in pairs. Humans have 22 pairs of autosomal chromosomes with the same gene in both members of a given pair and one pair of sex chromosomes, which are designated XX in females and XY in males.
The X and Y chromosomes are physically different from one another in that the Y chromosome is much shorter, and the Y chromosome only has about nine gene loci that match those on the X chromosome. This means that, except for the genes on an XY pair of chromosomes, we have two copies of each gene - one from each of our parents.
The alleles that we receive from each parent might be the same homozygous or they might differ heterozygous.
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