Question

questions:

1. list the three principles of mendelian genetics and explain.
2. define genotype.
3. define phenotype.
4. what is a testcross?
5. why would you perform a testcross?
6. the h gene produces a protein that allows the a and b antigen to attach to the surface of a red blood cell. this is an example of __________.

Explanation:

1. Law of Dominance: In a cross between pure - breeding (homozygous) organisms for contrasting traits, the trait that appears in the F1 generation is dominant. The masked trait is recessive. For example, when pure - breeding tall (TT) and short (tt) pea plants are crossed, all F1 plants are tall (Tt) as the tall trait is dominant.

Law of Segregation: During gamete formation, the alleles (gene pairs) separate from each other so that each gamete carries only one allele for each gene. For instance, a Tt plant will produce two types of gametes, T and t, in equal proportions.
Law of Independent Assortment: The alleles of different genes assort independently of one another during gamete formation. This means that the inheritance of one trait does not influence the inheritance of another trait when the genes are on non - homologous chromosomes.

1. Genotype refers to the genetic constitution of an organism, the set of alleles it possesses for a particular gene or genes. For example, for a gene controlling plant height, the genotypes could be TT, Tt, or tt.
2. Phenotype is the observable physical or biochemical characteristics of an organism, which is the result of the interaction between its genotype and the environment. For example, tall or short plant height is a phenotype.
3. A testcross is a cross between an individual with an unknown genotype (usually dominant phenotype) and a homozygous recessive individual. For example, if a tall plant (unknown genotype) is crossed with a short (homozygous recessive, tt) plant.
4. A testcross is performed to determine the genotype of an individual with a dominant phenotype. If all offspring of the testcross have the dominant phenotype, the unknown individual is likely homozygous dominant. If the offspring show a 1:1 ratio of dominant to recessive phenotypes, the unknown individual is heterozygous.
5. This is an example of epistasis. Epistasis occurs when one gene masks or modifies the expression of another gene at a different locus. Here, the H gene product is necessary for the proper expression of the A and B antigen - related genes.

Answer:

1. Law of Dominance: Dominant trait appears in F1. Law of Segregation: Alleles separate during gamete formation. Law of Independent Assortment: Alleles of different genes assort independently.
2. The genetic constitution of an organism, set of alleles.
3. Observable physical or biochemical characteristics.
4. Cross between individual with unknown genotype and homozygous recessive individual.
5. To determine genotype of individual with dominant phenotype.
6. Epistasis