Answer to Question #176021 in Genetics for John

Question

Using a worked example show how would you determine if a tall plant was

heterozygous or homozygous tall. Illustrate your answer with appropriate

crosses.

(d) Explain the concept of ‘multiple alleles’. Use the ABO blood grouping system to illustrate your answer. Produce a table to illustrate all possible phenotypes and their respective genotypes.

(It is not sufficient to copy and paste a table from a website)

Answer 1

a) Homozygous plants have two identical copies of a single gene. Heterozygous plants have two copies of a single gene, but those copies are alleles, or different versions of that gene. The second type is heterozygous. Plants that are heterozygous for a particular gene have one copy each of two different alleles, 'Tt'.

Example

The punnet square below shows a cross of heterozygous plants.

The "TT" and "Tt" crosses both have at least one "T" allele, so they are tall plants. However, the last cross "tt" doesn't have any "T" alleles and is short, because it is homozygous recessive.

b) Three or more alternative forms of a gene (alleles) that can occupy the same locus are called multiple alleles.ABO blood groups are the best examples for multiple allelism in human beings. The blood group A, B, AB and O types are characterised by the presence or absence of antigens on the surface of RBC.Blood type A person have antigen A on their RBCs and anti B antibodies in the plasma.Blood type B person have antigen B on their RBCs and anti A antibodies in the plasma.Blood type AB person have antigen A and B on the RBCs and no antibodies in the plasma.Blood type O person have no antigens on their RBCs and both anti-A and anti-B antibodies are present in the plasma.Bernstein discovered that these phenotypes were inherited by the interactions of three autosomal alleles of the genes named ‘I’ located on chromosome 9.I^A, I^B and I^O are the three alleles of the gene.The alleles I^A and I^B are responsible for the production of the respective antigen A and B. The alleles IO does not produce any antigen. The alleles I^A and I^B are dominant to the allele I^o, but co-dominant to each otherA child receives one of the three alleles from each parent, giving rise to six possible genotypes and four possible blood types.The genotypes are , I^AI^A, I^AI^O, I^BI^B, I^BI^O, I^AI^B, I^OI^O.The phenotypic expressions of I^AI^A, I^AI^O are A – type of bloodThe phenotypic expressions of I^BI^B, I^BI^O, are B- type of bloodThe phenotypic expression of I^AI^B is AB blood typeThe phenotypic expression of I^OI^O is O-type of blood.