What does the statement, “Alleles of different genes are inherited independently of each other.” mean?
ETA: If you start reading this answer and then think, but this is not the original question, then please scroll down for a bit. I realised that and have answered the question.
First, a disclaimer. I am a biology student myself. Therefore, if someone sees something I have said that is wrong, please don’t hesitate to correct me. I am not one to get offended by what others have to say.
Now, for your question.
I assume you are aware of the fact that each cell has a total of 46 chromosomes in them. Those consist of 22 pairs of identical chromosomes and 1 pair of sex chromosomes, which in humans are X and Y and are not identical.
There is one human disease I am aware of which is a mendelian disease and rather well-known which I am going to use for example purposes here. It is Cystic Fibrosis, located on the long arm of chromosome 7.
Here is an illustrated version of Chr. 7, with CFTR, the gene responsible for Cystic Fibrosis also located. (I am not sure if these are all the genes on Chr. 7, but I think it unlikely; there might be more.)
Keep in mind each cell has two copies, identical in almost every aspect.
Cystic Fibrosis is inherited in an autosomal recessive manner. This means that CFTR is located on an autosome (non-sex chromosome) and is caused by a recessive allele, meaning two copies of it must be needed to cause Cystic Fibrosis in someone. For the purposes of this answer, I am going to call the recessive Cystic Fibrosis causing allele cf and the wild type, normal allele +.
Now assume a person(Person 1) who has one copy of Chr. 7 with the cf allele and another with the + allele. He is a carrier of Cystic Fibrosis, meaning while not affected by it directly, his children can be affected if they receive his cf allele and another cf allele from their mom. Therefore for him to have cf/cf children, or children affected by Cystic Fibrosis, his spouse also has to be cf/+ and both him and her have to give down their cf allele.
Now Person 1 has a genotype of cf/+. His diploid cells have two copies of Chr. 7, one with the cf allele and one with the + allele. His gametes have only one copy of Chr. 7 though. Half his gametes will have the cf copy of Chr. 7, and the other half of his gametes will have the + copy of Chr. 7. Therefore his offspring get there other copy of Chr. 7 from their other parent.
Now, after coming this far, I realized this may have not been your question after all. Was your question about different genes altogether instead of alleles of the same gene?
If so, here is a brief answer:
If genes are on different chromosomes altogether, they will be inherited completely independently. For instance, a gene X located on chromosome 1 will have nothing to do whatsoever with a gene Y located on chromosome 2. (Note: these are invented genes, not based on any real human genes.)
However, if both gene X and Y are located on chromosome 1, but are a fair distance from each other, they are unlikely to get inherited as a unit due to cross-over that happens during gamete production.
Above is a old illustration describing crossovers. This illustrates that even if both genes are located on the same chromosome, the alleles are not being inherited together as a unit, but are being swapped to the other copy of the same chromosome, making it get inherited with the other allele of the other gene. Yes, I know this is becoming messy so time to name things.
Place allele 1 of Gene X and allele 2 of Gene Y on the black copy of chromosome in the above illustration, making sure to keep them far from each other (opposite ends will do.) Now place allele 2 of Gene X and allele 1 of gene Y on the white copy on the same location as you did on the black copy. That’s important.
Now after the crossover, allele 1 of both Gene X and Gene Y would have come on the same chromosome, leaving allele 2 of both on the other chromosome. So, inherited independently.
Now move both Gene X and Y closer to each other, quite close actually. Now they are likely to go together, with a crossover happening in the gap between them extremely unlikely. Those two are not linked genes, and the alleles are quite likely to travel with the allele of the other gene on the same chromosome. So, allele 1 of Gene X is going to go very frequently with allele 2 of Gene Y, and vice versa, instead of allele 1 of both ending up together. In the chance of that happening, it is called a linkage break.
Hope that helps!
Link to Quora: