Inheritance of character (A)

Mendal's first law
The alleles of a gene exist in pairs but when gametes are formed, the member of each pair will segregate(separate) and pass into different gametes. Thus each gamete contains only one allele of each gene.

duel with one unlinked pair of allele (monohybrid inheritance)

Mendal's second law
During meiosis, the two alleles of a gene pair segregate independently. Each member of an allelic pair may assort randomly.

duel with two unlinked pair of allele (dihybrid inheritance)

TtRr x TtRr -->
dominant+dominant :dominant+recessive :recessive+dominant :r+r =
9:3:3:1

Test cross
cross one with homozygous recessive

Back cross
Cross one with its parents.

Gene Interaction
A. Incomplete Dominance
No dominance, intermediate character shown
e.g. Red + White = Pink

B. Codominance
Both dominant, both character shown
e.g. Red + White =Partly Red +Partly White

Multiple allele
Character controlled by more than 2 allele forms in the same locus of a given pair of homologous chromosome.
e.g. human ABO blood group.

Sex Linkage
Usually X-linked
rule

1. male can only pass X to daughter
2. female can pass X to both son and daughter

Linkage
Gene located in the same chromosome move as a unit in nuclear division

With four possible character, maximum of 3 combination can be obtained
e.g. (T link r/ t link R + itself) or (T link r/t link R + T link R/ t link r)
Which does not follow mendal's law.

Crossing over
During synapsis, crossing over may occur between adjacent homologous chromatid at chiasmata.
It is a process of exchange of genetic materials between homologous chromosomes which give rise to new combinations.

e.g. (T link R/ t link r) may produce 3 combination instead of 2.

The closer the proximity of genes the stronger the linkage
The farther apart the genes the more frequent the crossing over.

Importance of linkage and crossing over

• Crossing over causes reshuffling of genes which results in new combination. This lead to greater variation.
• Crossing over is a major source of observable genetic variation within population.
• *Crossing over has Evolutionary significance

*It permits rearrangement of the various alleles between members of homologous chromosome pairs. If a new favourable alleles appear on a particular chromosome, and here is already another favourable alleles appear on a particular chromosome, and there is already another favourable allele at a different locus on the homologous chromosome, crossing over would produce some chromosomes that carry both favourable alleles. Offspring that receive this combination will be favoured by natural selection and will perpetuate the combination. At the same time, offspring receiving the less favourable combination will be selected against, and the frequency of these alleles in the population will diminish .

• discovery of linkage and crossing over led to the mapping of chromosome which promote the study of structure of gene and chromosome