Topic Genetic Basis for Variation: Q) What is reciprocal cross?

It refers to a cross where the same genetic characteristic (e.g. eye colour) is used but the sexes of the parents are reversed. Assuming a red-eyed female fly is crossed with a white-eyed male fly. The reciprocal cross is now the cross between the red-eyed male and the white-eyed female.

This cross is good for testing if the genetic characteristic (or gene) is found on the X chromosome (i.e. X-linked) or not (i.e. autosomal gene). If it is the former, then the phenotypic ratio of the offspring in the two crosses are different. If the gene is autosomal, the phenotypic ratio is the same.

Topic Genetic Basis for Variation: Q) Explain what is meant by the Mendel’s Law of Independent Assortment , and how does this give rise to the typical 9 :3 :3 :1 ratio from a cross between two double heterozygotes (i.e. AaBb x AaBb) ? What is the assumption you need to make with regards to the location of the two genes ?

Mendel’s Law of independent assortment states that members of any gene pair segregate from one another independently of the members of the other gene pairs. This independent assortment is related to meiosis since the different pairs of homologous chromosomes can arrange independently of each other on the metaphase plate during metaphase I. As the result, while each gamete will only contain one allele for each gene locus, the alleles of different gene loci are assorted randomly with each other in the gametes.

For two independently assorting pairs of alleles, A/a and B/b, the total no. of different combination is 4. Hence, there are 4 diff possible gametes in equal proportion.

Proportion of gametes of one parent :         1     :      1      :     1     :     1 

When two heterozygotes interbreed, there will be a total of 16 genotypic combinations but only 4 phenotypic classes (i.e. 9A_B_ :3aaB_  :3A_bb  :1aabb).

The assumption – the two genes are found on different chromosomes ! If they are found on the same chromosome (i.e. both genes are linked), then the proportion of the 4 types of gametes will not be equal – more parental gametes and less recombinant gametes (formed by crossing over).

P.S. Unfortunately, I cannot insert the circles representing the gametes I drew using Word Doc :(   but you can refer to N09/P2/4 worked solution 

Topic Genetic Basis for Variation: Explain the terms ’true breeding plants’ and ‘test cross’.

Testcross – refers to breeding an organism of unknown genotype with a homozygous recessive individual. The phenotypic ratio of the offspring reveals the unknown genotype of the organism.

True breeding plants – refers to plants that produce offspring of the same variety (i.e. same heritable phenotype) when they self-pollinate. This is because its genotype is homozygous.

True breeding organisms can also be called pure-bred.  In the pure-bred Siamese cat, their ancestors were inbred until all the genes that produce the physical characteristics and temperament associated with the Siamese breed are homozygous.

Source: http://en.wikipedia.org/wiki/File:Siam_lilacpoint.jpg

How about reciprocal cross? Look out for it soon!

Topic Genetic Basis for Variation: Q) What is the difference between the term gene and allele ?

Ans :  

A gene is a sequence of nucleotides coding for a particular polypeptide (FYI : while this is the ‘conventional’ definition, note that there are also rRNA and tRNA genes that code for the specific RNA)

An allele is the alternative form of a gene. It means that mutation results in slightly different DNA sequences which in turn gives rise to slightly different protein variants. This can result in different phenotypes (e.g. your blood group depends on 2 alleles coding for specific antigens on the surface or rbc).

Source: http://en.wikipedia.org/wiki/File:ABO_blood_type.svg

When we talk about a gene, it is actually about the group of all possible DNA sequences (in different individuals) that code for the particular protein, and each of the different DNA sequences will be considered an allele of this gene.

Since we are diploid organisms, it means that we have two copies (or 2 allleles) of every gene in our genome. These two alleles occupy corresponding location in a pair of homologous chromosome. If both alleles are identical, the organism is homozygous for that gene. If they are different, the organism is heterozygous.

Note: Even though the organism's genotype for the gene may be heterozygous, only the dominant allele is expressed to form the phenotype (i.e. the recessive allele is masked). That is why in the standard Mendelian dihybrid cross of two double heterozygotes (e.g. AaBb x AaBb), there are only 4 phenotypic classes (9A_B_ : 3aaB_ : 3A_bb : 1aabb) even though there are 16 genotypic combinations! More on this in our future post...