Search results
ABSTRACT: The predominance of human biological research confirms that human life begins at conception—fertilization. At fertilization, the human being emerges as a whole, genetically distinct, individuated zygotic living human organism, a member of the species Homo sapiens, needing only the proper environment in order to grow and develop.
Figure 1. In humans, fertilization occurs soon after the oocyte leaves the ovary. Implantation occurs eight or nine days later. (credit: Ed Uthman) The gestation period is divided into three equal periods or trimesters.
The two main factors affecting population growth are the birth rate (b) and death rate (d). Population growth may also be affected by people coming into the population from somewhere else ( immigration , i ) or leaving the population for another area ( emigration , e ).
May 29, 2023 · The act or fact of coming into life, or of being born; generally applied to human beings; as, the birth of a son. 2. Lineage; extraction; descent; sometimes, high birth; noble extraction.
People also ask
What does birth mean?
Does human life begin at conception?
When does fertilization occur in humans?
What is developmental biology?
- Overview
- Ancient theories of pangenesis and blood in heredity
- Preformation and natural selection
Genetics is the study of heredity in general and of genes in particular. Genetics forms one of the central pillars of biology and overlaps with many other areas, such as agriculture, medicine, and biotechnology.
Is intelligence genetic?
Intelligence is a very complex human trait, the genetics of which has been a subject of controversy for some time. Even roughly measured via diverse cognitive tests, intelligence shows a strong contribution from the environment.
How is genetic testing done?
Genetic testing typically is issued only after a medical history, a physical examination, and the construction of a family pedigree documenting familial genetic diseases have been considered. The genetic tests themselves are carried out using chemical, radiological, histopathologic, and electrodiagnostic procedures. Genetic testing may involve cytogenetic analyses to investigate chromosomes, molecular assays to investigate genes and DNA, or biochemical assays to investigate enzymes, hormones, or amino acids.
genetics, study of heredity in general and of genes in particular. Genetics forms one of the central pillars of biology and overlaps with many other areas, such as agriculture, medicine, and biotechnology.
Although scientific evidence for patterns of genetic inheritance did not appear until Mendel’s work, history shows that humankind must have been interested in heredity long before the dawn of civilization. Curiosity must first have been based on human family resemblances, such as similarity in body structure, voice, gait, and gestures. Such notions were instrumental in the establishment of family and royal dynasties. Early nomadic tribes were interested in the qualities of the animals that they herded and domesticated and, undoubtedly, bred selectively. The first human settlements that practiced farming appear to have selected crop plants with favourable qualities. Ancient tomb paintings show racehorse breeding pedigrees containing clear depictions of the inheritance of several distinct physical traits in the horses. Despite this interest, the first recorded speculations on heredity did not exist until the time of the ancient Greeks; some aspects of their ideas are still considered relevant today.
Hippocrates (c. 460–c. 375 bce), known as the father of medicine, believed in the inheritance of acquired characteristics, and, to account for this, he devised the hypothesis known as pangenesis. He postulated that all organs of the body of a parent gave off invisible “seeds,” which were like miniaturized building components and were transmitted during sexual intercourse, reassembling themselves in the mother’s womb to form a baby.
Are you a student? Get Britannica Premium for only 24.95 - a 67% discount!
Learn More
Aristotle (384–322 bce) emphasized the importance of blood in heredity. He thought that the blood supplied generative material for building all parts of the adult body, and he reasoned that blood was the basis for passing on this generative power to the next generation. In fact, he believed that the male’s semen was purified blood and that a woman’s menstrual blood was her equivalent of semen. These male and female contributions united in the womb to produce a baby. The blood contained some type of hereditary essences, but he believed that the baby would develop under the influence of these essences, rather than being built from the essences themselves.
Aristotle’s ideas about the role of blood in procreation were probably the origin of the still prevalent notion that somehow the blood is involved in heredity. Today people still speak of certain traits as being “in the blood” and of “blood lines” and “blood ties.” The Greek model of inheritance, in which a teeming multitude of substances was invoked, differed from that of the Mendelian model. Mendel’s idea was that distinct differences between individuals are determined by differences in single yet powerful hereditary factors. These single hereditary factors were identified as genes. Copies of genes are transmitted through sperm and egg and guide the development of the offspring. Genes are also responsible for reproducing the distinct features of both parents that are visible in their children.
In the two millennia between the lives of Aristotle and Mendel, few new ideas were recorded on the nature of heredity. In the 17th and 18th centuries the idea of preformation was introduced. Scientists using the newly developed microscopes imagined that they could see miniature replicas of human beings inside sperm heads. French biologist Jean-Baptiste Lamarck invoked the idea of “the inheritance of acquired characters,” not as an explanation for heredity but as a model for evolution. He lived at a time when the fixity of species was taken for granted, yet he maintained that this fixity was only found in a constant environment. He enunciated the law of use and disuse, which states that when certain organs become specially developed as a result of some environmental need, then that state of development is hereditary and can be passed on to progeny. He believed that in this way, over many generations, giraffes could arise from deerlike animals that had to keep stretching their necks to reach high leaves on trees.
British naturalist Alfred Russel Wallace originally postulated the theory of evolution by natural selection. However, Charles Darwin’s observations during his circumnavigation of the globe aboard the HMS Beagle (1831–36) provided evidence for natural selection and his suggestion that humans and animals shared a common ancestry. Many scientists at the time believed in a hereditary mechanism that was a version of the ancient Greek idea of pangenesis, and Darwin’s ideas did not appear to fit with the theory of heredity that sprang from the experiments of Mendel.
May 3, 2024 · Birth, process of bringing forth a child from the uterus, or womb. The three stages of labor are dilatation, expulsion, and the placental stage. Learn more about these stages, the methods to relieve the pain of labor, and complications that can occur during the process in this article.
Mar 21, 2024 · Life span, the period of time between the birth and death of an organism. The maximum life span is a theoretical number whose exact value cannot be determined from existing knowledge about an organism; it is often given as a rough estimate based on the longest-lived organism of its species known to date.
