Question

Autosomal aneuploids that survive to birth invariably involve __________ chromosomes.

Answer 1

Trisomy of smaller chromosomes, such as chromosomes 13, 15, 18, 21, or 22, can result in offspring that survive to birth, with trisomy 21 leading to Down syndrome being the most common.

Step-by-step explanation:

Autosomal aneuploids that survive to birth invariably involve smaller chromosomes. When nondisjunction occurs during meiosis, it may result in gametes with an extra chromosome (trisomy) or a missing chromosome (monosomy). Offspring arising from these gametes will have 2n+1 or 2n-1 chromosomes, respectively.

Monosomic zygotes, which have 2n-1 chromosomes, typically do not survive to birth as they lack essential genes. However, trisomic zygotes, with 2n+1 chromosomes, can survive if the trisomy affects one of the smaller chromosomes, such as chromosomes 13, 15, 18, 21 or 22. The most common trisomy among viable births is trisomy 21, which leads to Down syndrome. This condition is characterized by physical and developmental challenges and is more likely to occur with increasing maternal age.

Answer 2

Autosomal aneuploids that survive to birth typically involve smaller chromosomes, particularly 13, 15, 18, 21, or 22, with trisomy 21 being the most common. The concept of 'gene dosage' explains why some aneuploidies are more viable than others. Maternal age is a significant factor in the incidence of trisomy 21, or Down Syndrome.

Step-by-step explanation:

Autosomal aneuploids that survive to birth invariably involve smaller chromosomes such as chromosomes 13, 15, 18, 21, or 22. These particular chromosomal conditions can lead to offspring that survive for various lengths of time beyond birth. In contrast, monosomic zygotes usually do not survive to birth due to the lack of essential genes.

Among these, trisomy 21, which results in Down Syndrome, is the most common survivable autosomal aneuploidy. It leads to distinct physical characteristics and developmental delays. Notably, the likelihood of giving birth to a child with Down Syndrome is correlated with maternal age, with older women having a higher probability of conceiving a child with this condition.

The difference in survivability between trisomies and monosomies underscores the concept of 'gene dosage'—the precise amount of gene product necessary for normal function. With trisomies, there is an excess of gene products that can be tolerated to some extent by the organism, while a lack of essential gene products in monosomies is usually not viable. It is also relevant that the size of the chromosome matters, with smaller chromosomes being more tolerable when duplicated.