.Genes and Alleles Genes contain information about a specific characteristic or trait and can either be dominant or
recessive.
Genes are ound on chromosomes and each gene has a designated place on every chromosome,called a locus. Not all copies of a gene are identical and alternative forms of a gene, called alleles, lead to the alternative form of a trait. Alleles are away of identifying the two members of a gene pair which produce opposite contrasting phenotypes. An allele of a gene is it's partner gene, for example bis an allele of B and vice versa. When the alleles are identical, the individual is homozygous for that trait. While if the pair is made of two different alleles, the individual is
heterozygous. A homozygous pair of can be either dominant (AA, BB) or
recessive (aa, bb). Heterozygous pairs are made up of one dominant and one recessive allele (Aa, Bb). In heterozygous individuals only one allele, the dominant, gains expression while the other allele, the recessive, is hidden but still present. Capital letters represent dominant genes and lower case letters, recessive genes. The word genotype is was created to identify genes of an individual and phenotype for the expression of the trait and genes. Phenotype and genotype are terms used to describe the difference between the visible expression of the trait vs. the actual gene make up. An individual which expresses a dominant trait may carry a recessive allele, but the recessive expression is hidden by it's dominant partner. In Mendel's garden pea experiment, he also crossed plants that had two different pairs of alleles. He made a dihybrid cross of round, yellow seeds vs. wrinkled green seeds. The genes for round and yello ware dominant over their alleles of wrinkled and green. In the F1 generation all seeds were round and yellow as expected. Out of 556 plants in the F2 generation, 315 were round and yellow, 108 were round and green, 101 were wrinkled and yellow and 35 were wrinkled and green. When brought down to its lowest terms, it comes to a ratio of nearly 9:3:3:1. Mendel observed that the results were the same as the product of two monohybrid crosses. This lead to the law of probability which states that the chance of two or more independent events occurring together is the product of the chances of their separate occurrences. For example, crosses between parents that differ in three traits, called trihybrid crosses, are a combination of three monohybrid crosses together. Mendel also noticed that the pairs of alleles separated and behaved independently with respect to the other pair. Mendel then wrote the law of independent assortment which observes that independent combinations of different pairs of alleles may occur. Mendel believed that a single gene was responsible for one single trait all by itself. We now know that many genes have control over the production of traits. Individuals inherit genes from their parents, not traits. Another important fact is that genes behave asseparate units and traits are the product of complex gene interaction. Recessive Genes Recessive genes can only be expressed in homozygous (aa) individuals. There are more heterozygous (Aa) carriers than homozygous (aa) carriers who actually express the trait. All three genotypes (AA, aa, Aa) are possible through out anypopulation. Even in carriers that are not phenotypically expressed (Aa), the recessive allele can be identified in a cross.The three criteria for identifying recessive genes: The first appearance of the recessive trait within a family usually is in the children of the unaffected parents. 25% of the children will be express trait. Both males and females can express the trait unless it is a recessive sex linked gene.