Robertsonian Translocations & Correlation to Down Syndrome 

yeah exactly dr thank u

Hey man, it should be 3 because that's how you get Down's syndrome in the first place hence why it is called trisomy 21. The 3 chromosomes is because the child has 2 chromosome 21 from both mother and father and another extra one which is the translocated chromosome 14/21(which is considered as chromosome 21, hence a total of 3 chromosome 21) from the carrier parent, and chromosome 14.(this is one of the 6 possible combinations as explained in the video). Take note that even though the parent (who is a carrier) has 45 chromosomes they do not manifest the phenotype or have Down syndrome because they do not posses the 3 chromosome 21 as the child. Instead they have only 1 chromosome 21+ 1 chromosome 14, but they also have one translocated chromosome 14/21 due to the Robertsonian translocation happening during meiosis . (Compare this to the normal 2 chromosome 21 and 2 chromosome 14 to understand this clearer) So when the translocation carrier’s gamete which is 21+14+14/21 fuses with normal gamete of 21+14, one of the combination is 21,14/21,21,14. (note that the 14/21 is considered as the chromosome 21) which means the zygote has 3 chromosome 21. Hence they manifest trisomy 21 or down syndrome, and yet have 46 chromosomes. Remember that Robertsonian translocation is what occurs in the carrier parents(hence they have 45 chromosomes), whereas the child is the one possessing trisomy 21 because of the fertilization between the carrier parent(who posses this translocation) and the normal parent. Sorry this takes so long, cause I just understood this and I wanted to put it into words so I ended up babbling here. xD

@@izzahfarhana7434 thank you so much really, sorry took so long to respond didn't see the msg. But now i understand it clearly so thank u!!

@@anna-mariakhalil5588 hi! no problem i’m really glad you understand it now :D

@@izzahfarhana7434 its amazing dr thank u

@@izzahfarhana7434 dr please could i know way to ask u about ganetic i have alot of unclear point a ,all respect