Heeeelp!! Genetics!

hey guys..
the specification (edexcel) states:

explain the inheritance of two non-interacting unlinked genes;

understand autosomal linkage and recombinants in relation to meiosis

plz somebody help me!!!!!!!!!!!!!!!!! plzzzz

thanks in advance!!!

Reply 1

kohlstream

have you got the nelson book?

Reply 2

KP Nuts

The non-interacting unlinked genes bit is just a dyhibrid cross (its under that header in the NAS textbook).

Basically, if you get two organisms with homozygous alleles for two different characteristics - one organism with all dominant alleles, the other with all recessive alleles - and breed them you will get a 9:3:3:1 ratio of phenotypes in the second generation of offspring.

Its a lot easier to explain with an example. Okay, say we have some pea plants. Two unrelated characteristics (i.e. the alleles for which are NOT linked) could be their height (tall or dwarf) and the shape of their seeds (round or wrinkled).

The alleles are as follows;
T - tall (dominant) t - dwarf (recessive)
R - round seeds (dominant) r - wrinkled seeds (recessive)

We take one tall pea plant with round leaves (TTRR - remember they are homozygous) and one dwarf pea plant with wrinkled leaves (ttrr) and breed them.

Parents: TTRR x ttrr

F1: All TtRr

If we now breed these: TtRr x TtRr

The gametes present are: TR, Tr, tR, tr in equal ratios for both of the pea plants.

If you now draw a punnit square with these four gametes going along the top and down the side and work out the genotypes and finally phenotypes which are formed you will get;

9:3:3:1 - Tall and round : Tall and wrinkled : Dwarf and round : Dwarf and Wrinkled

The two middle phenotypes are new (i.e. they did not exist in their parents) and this is called recombination.

I hope this helps - im still trying to get my head around the second bit myself, but ill get back to ya when i understand it.