Pattern of inheritance seen in simple genetics
experiments, where the two animals (or two plants) being crossed are genetically identical except for one gene, which is heterozygous. In other words, inheritance is the passing of characteristics from parents to offspring, while monohybrid inheritance is inheritance for a single characteristic.
Characteristics that can be passed on in this way are determined by genes
. Genes may exist in different forms known as alleles
. Since chromosomes in animals and plants are found as matching pairs in the nucleus
of each cell, there will always be two genes for a characteristic in a cell. If the same allele is present twice, the organism is said to be homozygous for this characteristic. If, however, one chromosome contains one allele and the other chromosome a contrasting allele, the organism is said to be heterozygous for that characteristic. In a heterozygous organism the appearance of the organism (phenotype) may be determined by one allele and not the other. The allele that determines the phenotype is said to be dominantly expressed (see dominance
). The expression of the other allele is described as being recessive.
The gene in question may code for some obvious external features such as seed colour, with one parent having green seeds and the other having yellow seeds. The offspring are monohybrids, that is, hybrids for one gene only, having received one copy of the gene from each parent. Known as the F1 generation, they are all identical, and usually resemble one parent, whose version of the gene (the dominant allele) masks the effect of the other version (the recessive allele). Although the characteristic coded for by the recessive allele (for example, green seeds) completely disappears in this generation, it can reappear in offspring of the next generation if they have two recessive alleles. On average, this will occur in one out of four offspring from a cross between two of the monohybrids. The next generation (called F2) shows a 3:1 ratio for the characteristic in question, 75% being like the original parent with the recessive allele. Austrian biologist Gregor Mendel
first carried out experiments of this type (crossing varieties of artificially bred plants, such as peas) and they revealed the principles of genetics. The same basic mechanism underlies all inheritance, but in most plants and animals there are so many genetic differences interacting to produce the external appearance that such simple, clear-cut patterns of inheritance are not evident.
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