# how do I find the number of bp in chromosome 3 by knowing number of bp in chromosome 1?

If i have a number of bp in chromosome 1 for example(298,295,559 bp) can I use this number to find the number of bp in chromosome 3.

• Chromosome of what?
– kmm
Sep 28 '15 at 15:12
• human chromosomes
– sola
Sep 28 '15 at 15:14
• Since chromosomes are numbered according to size, the only thing you can say is that #3 is less than 298,295,559 bp. Sep 28 '15 at 15:18
• ncbi.nlm.nih.gov/projects/genome/guide/human
– AMR
Sep 30 '15 at 5:38

You cannot. BP numbers vary between the chromosomes. Chromosomes are numbered by size, so you can always say that 2 is smaller than 1; however, there is no function to determine the exact size of one chromosome based on the exact size of another.

• Except for the point raised by MattDMo. At least we can tell that chromosome 3 is shorter than chromosome 1 as they are numbered based on their length (to the exception of sexual chromosomes) Sep 29 '15 at 0:16
• That is certainly true but knowing that two has fewer bp than one and twenty three has fewer than that doesn't really help us determine the number in either based on the number in one. Perhaps it points at a smaller stack of hay, but doesn't predict where the needle will be. Sep 29 '15 at 21:40
• I agree that It offers little information but it is sufficient to make one of your sentences wrong. You say There's no relation function for the chromosomal number to the number of base pairs it has., while there actually is such a relationship. Sep 29 '15 at 22:00

Chromosomes vary in length and there is no reason to expect a nice relationship in the relative length of chromosomes.

However, by definition chromosomes are numbered from the longest to the shortest (except for the concern of sexual chromosomes). The chromosome 1 is the longest, the chromosome 2 the second longest and the chromosome 8 is the 8th longest. Following @ping comment Also note that human chromosomes 21 and 22 are numbered the "wrong" way round in that 21 is actually smaller than 22; so even for autosomes using the numbering to determine size rankings doesn't necessarily work.

In consequence, if you know that the chromosome 1 is x bp (base pairs) long, then the only thing you can tell about chromosome 3 is that is has less than x base pairs. Note that x ≈ 250,000,000 base pairs in human.

• Also note that human chromosomes 21 and 22 are numbered the "wrong" way round in that 21 is actually smaller than 22; so even for autosomes using the numbering to determine size rankings doesn't necessarily work.
– ping
Sep 30 '15 at 20:35
• I didn't know that. Thank you, I added your comment in the answer Sep 30 '15 at 21:11