I have to draw a mating diagram to show a cross between two heterozygotes for lactose production. As both adults have the dominant allele L for persistant lactase production would I start the diagram with LL for each parent?
Yes, if you mean production. Lactose digestion genetics is quite a different matter, and more complicated.
Thankyou so much. For the next question I have to draw a similar diagram to show the cross between a person with lactose intolerance and one who is heterozygous. This would mean starting with one parent LL and one parent ll? The text book I've been given does not explain it very well.
I have completed the first diagram and I have to state the phenotypic and genotypic ratio of the offspring in the F1 generation. As the offspring will all be LL, how can I determine the ratio? I can work out the ratio for the lactose intolerant and heterozygous cross ok, but I'm not sure how to do this if all the offspring are the same!
Just realised where I was going wrong. Two heterozygotes crossed would start with Ll for each parent, giving a phenotypic ratio of 3:1 for the F1 offspring.
Correct.
Yes, that's correct. When you have two heterozygotes (Ll x Ll) for lactose production, you would start with Ll for each parent. This means that each parent has one dominant allele (L) and one recessive allele (l) for lactose production.
To determine the phenotypic and genotypic ratio of the offspring in the F1 generation, you need to consider the possible combinations of alleles from the parents. In this case, there are three possible genotypes: LL, Ll, and ll. However, since both parents are heterozygotes, LL is not a possible genotype in the F1 generation.
Therefore, the genotypic ratio for the F1 generation would be:
- 1 Ll (homozygous dominant)
- 2 ll (homozygous recessive)
Since the lactase production is determined by the dominant allele L, all the offspring in the F1 generation will have persistent lactase production. So the phenotypic ratio would be:
- 3 persistent lactase producers : 0 lactose intolerant individuals
I hope that clarifies your confusion. Let me know if you have any more questions!