Question

What evidence indicates that humans with aneuploid karyotypes occur at conception but are usually inviable?

Short answer

Question: Provide evidence that indicates aneuploid karyotypes occur in humans at conception and usually result in inviability. Answer: Evidence that aneuploid karyotypes occur in humans at conception is found in studies of human pre-implantation embryos, where up to 70% or more of them are aneuploid. The inviability of aneuploid karyotypes is demonstrated by the high percentage of embryos not implanting in the uterus or spontaneously aborting during early pregnancy. Some exceptions like Down syndrome, Patau syndrome, and Edwards syndrome may produce viable offspring, but these are associated with serious health challenges and reduced life expectancy.

Step-by-step solution

Define Aneuploid Karyotypes

Aneuploid karyotypes are the result of an abnormal number of chromosomes in a cell. In humans, the normal number of chromosomes is 46 (23 pairs), which is called a diploid set. So an aneuploid karyotype is a condition where there is either an extra chromosome or a missing chromosome.

Occurrence at Conception

Aneuploidy occurs during the formation of eggs and sperm in a process called meiosis. Errors during meiosis can lead to aneuploidy, as chromosomes may fail to separate properly, resulting in gametes with too many or too few chromosomes. If an aneuploid gamete is fertilized, this leads to an aneuploid zygote. Thus, aneuploid karyotypes can be identified at conception, as they result from errors in meiosis that produce gametes with an abnormal number of chromosomes.

Evidence of Aneuploidy from Early Human Development

Studies conducted on human pre-implantation embryos have found that a significant percentage of these embryos (up to 70% or more) are aneuploid. This indicates that aneuploid karyotypes are relatively common in early human development, supporting the notion that they occur at conception.

Inviability of Aneuploid Karyotypes

The vast majority of aneuploid embryos fail to develop normally and do not implant in the uterus. As a result, they are typically lost during early stages of pregnancy. The high percentage of embryos not implanting in the uterus or spontaneously aborting during early pregnancy is partly due to aneuploid karyotypes. This provides evidence that aneuploid karyotypes are generally inviable, as they lead to failure in embryonic development.

Survival Exceptions

There are a few aneuploid conditions that may result in viable offspring. These include Down syndrome (trisomy 21), Patau syndrome (trisomy 13), and Edwards syndrome (trisomy 18). However, these conditions are associated with various degrees of physical and developmental disabilities and reduced life expectancy. This highlights that even though some aneuploid karyotypes may result in viable offspring, they usually come with serious health challenges. In conclusion, evidence from early human development studies and the high rate of spontaneous abortions demonstrate that aneuploid karyotypes occur at conception and are generally inviable. Some exceptions like Down syndrome, Patau syndrome, and Edwards syndrome may produce viable offspring, despite the associated health challenges.