Plz help i cant figure this out! 10 pts BEST ANSWER?

lethal genes do not necessarily have to be recessive. Some genes produce a dominant phenotypic effect in heterozygotes but are llethal in the homozygous dominant condition. For example, the gene for creeper in chickens is dominant, producing fully viable, but abnormal, heteerozygotes, which have shortened, crooked legs. In the homozygous condition, the creeper gene results in the death of the embryo on about the forth day of incubation. A gene such as CREEPER is an incompletely dominant lethal gene.

In a cross between two creepers, three-quarters of the eggs hatched. What fraction of the chicks should be creepers? What fraction should be normal? What phenotypic ratio would you expect from a cross between a creeper and a normal chicken?



I have no clue how to do this cuz i dont no what lethal and non lethal means??:S :S..i have figured out the rest of my h/w but i just cant get this one..help plz! and thx!



10 pts BEST ANSWER

Plz help i cant figure this out! 10 pts BEST ANSWER?
Lethal means deadly (ie: death of the organism because of the gene phenotype). Non-lethal means that the organism survives.



Let's represent the creeper gene by the letter c. Your problem states that the gene is dominant lethal, so only individuals with the CC phenotype will die. The individuals which are heterozygous (Cc) will survive but will have shortened crooked legs.



To find out what phenotypic ratio you would get out of a cross between two creeper chickens (Cc x Cc) do a Punnett square analysis :





---------C----------c----------



C------CC--------Cc---------



c-------Cc---------cc--------





So 50% of the offspring will exhibit a creeper phenotype, 25% will exhibit a normal phenotype, and 25% will have the lethal CC geneotype and will die.



To find out what phenotypic ratio you would get out of a cross between a creeper chicken (Cc) and a normal chicken (cc), do the same thing as above :





---------C----------c----------



c------Cc---------cc---------



c------Cc---------cc--------





So 50% of the offspring will have the creeper phenotype and 50% will have a normal phenotype.



Hope this helps
Reply:Lethal means deadly, non-lethal means not deadly.



A chick that is homozygous for the creeper gene will die, and a chick that is heterozygous will have crooked legs, while a chick that is homozygous for the wild type gene will be normal.



In the first question, two creepers are crossed. These must both be heterozygote because they are not dead, and not normal. In the cross, you would expect 1/2 heterozygous, 1/4 to be homozygous wild type and 1/4 to be homozygous creeper.



The 1/2 of the eggs will be heterozygous, but 1/4 of the eggs were homozygous mutants so they didn't hatch. This means that of the living chicks, 2/3 will be creepers and 1/3 will be normal.



If you cross a creeper and a normal chicken, that is a heterozygote with a homozygous wildtype. In this cross, none of the chicks can be homozygous for the creeper gene, because one parent doesn't have it. So all of the eggs will hatch, and 1/2 will be creepers and 1/2 will be normal.



Hope this helps! Email me if you get stuck.



Ruth