Alleles are different forms of the same gene that differ in the sequence. In diploid cells, there are two copies of most genes. The number of copies of the gene is the gene dosage. The two copies may be identical (homozygous) or may consist of two different alleles (heterozygous).

Gene dosage can cause problems when it is incorrect, such as in trisomy 21 (Down syndrome).

Because there are 2 X chromosomes in females but only one in males, females inactivate one of the 2 X chromosomes to achieve the correct gene dosage for X-linked genes (X-inactivation).

Alleles are different forms of the same gene that differ in the sequence. In diploid cells, there are two copies of most genes. The number of copies of the gene is the gene dosage. The two copies may be identical (homozygous) or may consist of two different alleles (heterozygous).

Gene dosage can cause problems when it is incorrect, such as in trisomy 21 (Down syndrome).

Because there are 2 X chromosomes in females but only one in males, females inactivate one of the 2 X chromosomes to achieve the correct gene dosage for X-linked genes (X-inactivation).

Dr. Shapiro, thanks for your answer. Now I understood why every papper about transgenic use the term "gene dosage", because there is only one kind of allele. If the genotype of the transgenic locus is hemizygous it means that it has one transgenic allele and the other is null. On the other hand, if I increase the gene dosage or one copy of the same transgenic allele in the same locus I will have a homozygous genotype for the transgenic locus. Is it right? Thanks!

Dr. Shapiro is correct. I would add that "gene dosage" is also used to describe the number of copies of a gene across multiple loci (see "copy number variation").

I think your question was: if you start with a single copy of a transgene in the genome and increase the gene dosage to 2 copies by adding a second copy of the same gene at the same locus in the genome, would you then have a homozygous genotype? The answer is yes.

It would be next to impossible to add a transgene to the same site in two homologous chromosomes to obtain a homozygous genotype. It would be much easier to self the hemizygote to obtain a homozygote.

As others have expressed above alleles are the different forms of genes for equivalent traits. So, these are different options out of them only one will be present in the single copy of the genome while, gene dosage is the number of multiple copies of a particular gene present in a single genome.

Alleles are the part of gene.In a gene different alleles are present which are different in sequences.but it is important to study it to understand the whole gene..

Ok we'll start from gene

Gene is the basic unit of heredity and is that part of DNA which codes for functional RNA and protein (which further forms enzyme) .

​

So, not every part of DNA is considered as gene ( chromosome is condensed form of DNA, single chromosomes can contain several types of Gene)

Alleles: I'll explain you allele with an example, because sometimes bookish language is hard to understand Eg- T is a gene, which controls the height of a pea plant, but this does not explain us why some plants are tall, meanwhile others are dwarf what is the reason behind this? Here comes the answer, " gene which codes for a pair of contrasting traits are known as alleles that is, they are slightly different forms of the same gene, that's why some plants are tall while others are dwarf " 'T' (allele) is used for tall trait on the other hand 't' (allele) is used for dwarf trait. Allele are always present in pairs ( well the reason is so obvious, because chromosomes are always paired) Tall plant - TT Tall plant -Tt Dwarf plant - tt Now, don't get all confused why Tt is tall and tt is dwarf because there are some laws which we have to follow , one of them is " law of dominance " according to which " In a dissimilar pair of factors (Genes) one member of the pair dominates ( dominant) other hand (recessive) " ​when identical alleles are present (TT)in pair then we call this condition as homozygous, while heterozygous (Tt) when contrasting pair is present

Good luck.

( here am not going to explain you about crossing, because your question is all about the difference between gene and an allele)

So, basically An allele is a variant form of a gene. Some genes have a variety of different forms, which are located at the same position, or genetic locus, on a chromosome. Humans are called diploid organisms because they have two alleles ,one at each genetic locus, with one allele inherited from each parent.

A gene is a unit of hereditary information. Except in some viruses, genes are made up of DNA, a complex molecule that codes genetic information for the transmission of inherited traits. Alleles are also genetic sequences, and they too code for the transmission of traits. So, what it is the difference between a gene and an allele?

The short answer is that an allele is a variant form of a gene. Explained in greater detail, each gene resides at a specific locus (location on a chromosome) in two copies, one copy of the gene inherited from each parent. The copies, however, are not necessarily the same. When the copies of a gene differ from each other, they are known as alleles. A given gene may have multiple different alleles, though only two alleles are present at the gene’s locus in any individual.

Alleles can sometimes result in different phenotypes (observable traits), with certain alleles being dominant (overriding the traits of other alleles) or, in some cases, multiple alleles acting in a codominant fashion. An example of the latter is the human ABO blood group system, in which persons with type AB blood have one allele for A and one for B (persons with neither allele are type O). An example of dominant allele expression is flower color in pea plants. A plant with purple flowers actually has a genotype (genetic makeup) consisting of a gene with a dominant P and a recessive p allele.

reference /https://www.britannica.com/story/whats-the-difference-between-a-gene-and-an-allele

regards

Follow this link

https://www.britannica.com/story/whats-the-difference-between-a-gene-and-an-allele

regards

Good luck with that.

" Is it correct if I say that the increment of allele "A" or increase of the gene dosage in a hemizygous genotype will confer a homozygous genotype with 20 grams of protein expression?  " this is not correct if there is a competition for a factor necessary to expression

The effect of gene dosage might not be 'additive'. See this paper:

Article Gene dosage effect of the wheat Wxalleles and their interact...

From Abstrct:

"........When Wx-B1a and Wx-A1a were combined, increase in amylase content was not in proportion to increase in gene dosage. The enhanced amylase synthesis was shown by 2-gene and 3-gene interaction, indicating that not only type of the three Wx genes and its dose but the interaction among them have significant roles in determining the amylose content............"

By the way, gene dosage effect were observed and reported in transgenic plants. People have seen that homozygous transgenic plants with gus gene have higher expression level of GUS protein than that from hemizygous or heterozygous transgenic plants.

And also as Jack Erron Haggard mentioned that " 'gene dosage' is also used to describe the number of copies of a gene across multiple loci." In transgenesis, insertion of 2 or more identical transgene can exist in the genome. Howwever, most of these suffer gene silencing. In this case, gene dosage is increased but the transgene expression ablility is lost.

Yuan-Yeu, could you please give the reference(s) for your interesting proposal " People have seen that homozygous transgenic plants with gus gene have higher expression level of GUS protein than that from hemizygous or heterozygous transgenic plants "

Dear Herve,

I will try to find out the paper. I think that it is from one of Dr. Vibha Srivastava's papers in rice. She is also a RG member.

Dear Herve,

Please see p.908 of the attached paper, the yellow highlighted part:

".....GUS activity in T1 population may vary depending on the allelic dosage of GUS gene (Chawla et al., 2006; Srivastava et al., 2004)..........". And, read further with the two listed papers.

Thank you Yuan-Yeu to complete informations.

You are welcome, Herve.

For the homozygous gus gene, we all know that in some cases, 2 copies of transgene insertions in a genome can lead to 'gene-silencing', but what about the homozygous status of a transgene (also 2 copies)? Have any reports about the homozygous status of a transgene leading to transgene silencing? If not, what is the mechnism to prevent homozygous transgene silencing??

Hi Herve Levesque,

1. By the way, the paper I listed above (Nandy et al. 2012) shows "homozygous gus transgenic rice lines' have higher gus expression level than that from "heterozygous gus transgenic lines". [from T1 generation]

2. In anothere pape (see below)r, it demonstrates that "homozygous" lines have higher gus expression level than "HEMIzygous (or primary transgenic)" lines in gus transgenic rice.

Paper: Theor Appl Genet. 2002 Mar;104(4):553-561. The relationship between homozygous and hemizygous transgene expression levels over generations in populations of transgenic rice plants. James VA, Avart C, Worland B, Snape JW, Vain P.

Article The relationship between homozygous and hemizygous transgene...

Sorry Yuan-Yeu for my answer that could have been misinterpreted, I would like to say "thank you to have completed informations"!!! My english is not very well. Herve

Thanks, Herve. No worry. I understood your earlier post. I just wanted to add some more info regarding expression level between 'hemizygous", "heterozygous" and "homozygous" gus transgenic lines in rice.