A boy with Down syndrome (trisomy 21) has 46 chromosomes. His ...

Question

A boy with Down syndrome (trisomy 21) has 46 chromosomes. His parents and his two older sisters have a normal phenotype, but each has 45 chromosomes.

Explain how this is possible.

Accepted Answer

Hi, everyone. Welcome back. Let's look at our next question. It says which of the following types of translocation involves the fusion of two non homologous chromosomes. And our answer choices are a unbalanced translocation. B, reciprocal balance translocation, C Robertsonian translocation or D none of the above. Well, we're talking about fusion of two non homologous chromosomes. Uh The best answer choice here is choice c Robertsonian translocation, which actually another name for it is chromosomal fusion and you have a portion of one chromosome becoming fused to a second non homologous chromosome. That second chromosome is usually acro is acrocentric or telocentric. And when the piece fuses onto the second chromosome, there's a loss of the centromere and the P arm of that second chromosome. And that actually fuses with the little bit to the short arm of the other chromosome. So you usually have two acrocentric or telocentric chromosomes fusing together and the two little small bits fused together but are generally lost. So you have that fusion of two homologous nonn homologous chromosomes into a single new chromosome happening in Robertsonian translocation. Since we have a correct answer, we can now eliminate choice D none of the above. So we'll cross that out choice, a unbalanced translocation. Um This one's a little bit tricky because Robertsonian translocation is essentially a form of unbalanced translocation. Um But it's not as correct an answer here while we're talking about the fusion of two non homologous chromosomes. Uh unbalanced just means we have a piece of a chromosome of chromosome becoming attached and the fragment attaching to a non homologous chromosome with no reciprocal event. Um There's no loss of centromere and pr and then fusion into fusion of the small parts. So it's kind of subtly different from that Robertsonian translocation. But again, if you're thinking of fusion of two parts into a new chromosome, uh choice C is the better answer. So that's why we wouldn't choose choice. A and then finally choice B, reciprocal balance translocation as that term balanced indicates there you have two pieces of chromosomes becoming detached and then attaching to a non homologous chromosome. But in that case, you're swapping those two pieces. So I'll put swapping pieces. So there's no loss or duplication of genetic material, you're just two pieces become detached and switch with each other and then reattach to the non homologous chromosomes. So they're in the genes are in different locations on different chromosomes, but you haven't lost anything and you haven't duplicated anything but it's not the fusion of two chromosomes into a new one. So not our correct answer. So once again, which of the following types of translocation involve the fusion of two non homologous chromosomes. Choice c Robertsonian translocation. See you in the next video.

A boy with Down syndrome (trisomy 21) has 46 chromosomes. His parents and his two older sisters have a normal phenotype, but each has 45 chromosomes.
Explain how this is possible.

Key Concepts

Chromosomal Abnormalities

Nondisjunction

Phenotype vs. Genotype

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