Glossary of Mendels Laws,Homozygote,Heterozygote,Back Cross,Test Cross,Monhybrid Cross,Dominance,Phenotype,Genetype

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	Glossary of Mendel's Laws - Alleles Alternative forms of the same gene. Alleles for a trait are located at corresponding positions on homologous chromosomes. For example, there is a gene for hair texture (whether hair is curly or straight). One form of the hair texture gene codes for curly hair. A different code for the same gene makes hair straight. So the gene for hair texture exists as two alleles-one curly code, and one straight code. We could use a C for the curly allele, and a c for the straight allele. A persons genotype with respect to hair texture has three possibilities: CC, Cc, or cc.
Thus, homozygous means having two of the same allele in the genotype (CC or cc). Heterozygous means one of each allele in the genotype (Cc). Allelic pair: The combination of two alleles which comprise the gene pair. Homozygote: An individual which contains only one allele at the allelic pair; for example DD is homozygous dominant and dd is homozygous recessive; pure lines are homozygous for the gene or character of interest.
Heterozygote: An individual that contains one of each member of the gene pair; for example the Dd heterozygote. Back cross The cross of an F 1 hybrid to one of the homozygous parents; for pea plant height the cross would be Dd x DD or Dd x dd. Test cross The cross of any individual to a homozygous recessive parent and is used to determine if the individual is homozygous dominant or heterozygous. Monohybrid cross A cross between parents that differ at a single gene pair (usually AA x aa). Monohybrid The offspring of two parents that are homozygous for alternate alleles of a gene pair. A monohybrid cross is not the cross of two monohybrids. Monohybrids are good for describing the relationship between alleles.
When an allele is homozygous, it will show its phenotype. It is the phenotype of the heterozygote which permits us to determine the relationship of the alleles. Dominance The ability of one allele to express its phenotype at the expense of an alternate allele. Generally the dominant allele will make a gene product that the recessive cannot; therefore the dominant allele will express itself whenever it is present. Phenotype Physical appearance of a particular trait. Literally means the form that is shown outward.
	Dominant The allele that expresses itself at the expense of an alternate allele; the phenotype that is expressed in the F 1 generation from the cross of two pure lines. Recessive An allele whose expression is suppressed in the presence of a dominant allele; the phenotype that disappears in the F 1 generation from the cross of two pure lines and reappears in the F 2 generation. Genotype The genes present in the DNA of an organism. We will use a pair of letters (Tt, YY, ss, etc.) to represent genotypes for one particular trait. There are always two letters in the genotype because (as a result of sexual reproduction) one code for the trait comes from female parent and the other comes from male parent, so every offspring gets two codes (two letters).

But there are three possible genotypes two big letters (like TI), one of each (Tt), or two lowercase letters (tt). Each possible combination has a term for it. When we have two capital or two lowercase letters in the genotype (TI or tt) it is called homozygous (homo means the same).

Sometimes the term pure is used instead of homozygous. When the genotype is made up of one capital letter and one lowercase letter (Tt) it is called heterozygous.