Written and collected by Zia H Shah MD

Introduction

Genetic diseases encompass a wide range of conditions caused by abnormalities in the genome. These include chromosomal abnormalities (errors in chromosome number or structure), single-gene (Mendelian) disorders caused by mutations in one gene, and mitochondrial disorders due to mutations in mitochondrial DNA. Scientists have identified well over 6,000 distinct genetic disorders en.wikipedia.org, and new ones continue to be described as our understanding grows. Collectively, such conditions are not exceedingly rare – it is estimated that around 1 in 50 people is affected by some known single-gene disorder, and about 1 in 263 people has a chromosomal disorder en.wikipedia.org. These numbers underscore the global burden of genetic diseases and highlight that genetic variation (including deleterious mutations) is an intrinsic aspect of the human condition. In this article, we provide an overview of common genetic diseases and defects across these categories, including their worldwide prevalence. We then discuss how genetic variability (including disease-causing mutations) arises and persists through evolutionary mechanisms such as mutation, natural selection, and genetic drift. Finally, we examine how evidence from genetics, genomics, and fossils bears on the question of human origins, using these findings to scientifically critique the traditional creationist doctrine of Adam and Eve – the idea that all humans descended from a single original pair created de novo. The evidence from population genetics, comparative genomics, and paleoanthropology will be integrated to assess the plausibility of a bottleneck of two ancestors in light of modern science.

Chromosomal Abnormalities: Examples and Prevalence

Chromosomal abnormalities involve the gain, loss, or alteration of whole chromosomes or large chromosome segments. These errors often occur during meiosis (the formation of eggs and sperm) and can lead to developmental disorders. A well-known example is Down syndrome, or trisomy 21, in which individuals have an extra copy of chromosome 21. Down syndrome is the most common chromosomal condition associated with intellectual disability. It occurs in approximately 1 out of every 800 births worldwide, although the exact incidence can vary with maternal age and between populations. This translates to hundreds of thousands of new Down syndrome cases globally each year; one recent study estimated about 1.58 million people worldwide were living with Down syndrome in 2020. Affected individuals have characteristic facial features and are prone to various health issues (congenital heart defects, leukemia, thyroid disorders, etc.), but many lead fulfilling lives with proper medical care.

Another example is Turner syndrome, a condition in which females have one of their two X chromosomes partially or completely missing (denoted 45,X). Turner syndrome affects development and is associated with short stature, ovarian failure (infertility), and certain physical features (e.g. webbed neck, cardiac defects). This chromosomal disorder occurs in about 1 in 2,000 female births worldwide. However, it is thought to be much more common in conception, with many 45,X embryos not surviving to term (contributing to miscarriages). Other chromosomal abnormalities include Klinefelter syndrome (males with an extra X chromosome, XXY), trisomy 18 (Edwards syndrome), trisomy 13 (Patau syndrome), and various partial deletions or duplications of chromosomes. While individually rare, as a group chromosomal disorders affect on the order of 0.4% of newborns (around 1 in 263) en.wikipedia.org. Many such conditions result in developmental disabilities or health problems, and some are incompatible with life. The prevalence of chromosomal disorders worldwide underscores the role of chromosome segregation errors in human reproduction – a natural, if unfortunate, outcome of the imperfection of meiotic cell division.

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