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Conceiving new life Conceiving new life Dr. Ali D. Abbas Instructor, Fundamentals of Nursing Department, College of Nursing, University of Baghdad [email protected] LEARNING OBJECTIVES After mastering the contents of this lecture, the student should be able to: 1. Explain the fertilization. 2. Identify the mechanisms of heredity. 3. Identify the patterns of genetic transmission. 4. Describe influences of heredity and environment. 5. Identify some characteristics influenced by heredity and environment. TERMINOLOGIES Animalculists Heritability Fertilization Heterozygous Genetic Homozygous Genotypes Phenotypes ٠ Conceiving new life CONTENTS 1. Introduction 2 2. How fertilization takes place 2 3. Mechanisms of heredity 4 Genetic code 4 What determines sex? 6 Patterns of genetic transmission 7 4. Influences of heredity and environment 8 Studying heredity and environment 8 Some characteristics influenced by heredity and environment 9 5. References 10 ١ Conceiving new life Introduction The animalculists (so named because the male sperm were then called animalcules) claimed that fully formed "little people" were contained in the heads of sperm, ready to grow when deposited in the nurturing environment of the womb. The ovists, inspired by the influential work of the English physician William Harvey, held an opposite but equally incorrect view: that a female's ovaries contained tiny, already formed humans whose growth was activated by the male's sperm. Finally, in the late eighteenth century, the German-born anatomist Kaspar Friedrich Wolff demonstrated that embryos are not preformed in either parent and that both contribute equally to the formation of a new being. Although scientists have now found a way to clone (make a genetic copy of) a human embryo, and this has value for therapeutic research purposes, ethical and religious concerns about the dignity of individual human life make it unlikely—at least in the near future—that cloning will become a common means of reproduction. How fertilization takes place Fertilization, or conception, is the process by which sperm and ovum the male and female gametes, or sex cells combine to create a single cell called a zygote, which then duplicates itself again and again by cell division to become a baby. At birth, a girl has all the ova (plural of ovum) she will ever have about 400,000. These immature ova are in her two ovaries (see Fig.1), each ovum in its ٢ Conceiving new life own small sac, or follicle. In a sexually mature woman, ovulation rupture of a mature follicle in either ovary and expulsion of its ovum occurs about once every 28 days until menopause. The ovum is swept along through the fallopian tube by tiny hair cells, called cilia, toward the uterus, or womb. Fertilization normally occurs during the brief time the ovum is passing through the fallopian tube. Fig. (1) Female reproductive system Sperm are produced in the testicles (testes), or reproductive glands, of a mature male (refer to Fig.2) at a rate of several hundred million a day and are ejaculated in the semen at sexual climax. They enter the vagina and try to swim through the cervix (the opening of the uterus) and into the fallopian tubes, but only a tiny fraction make it that far. Fig. (2) Male reproductive system ٣ Conceiving new life Fertilization is most likely if intercourse occurs on the day of ovulation or during the five days before. If fertilization does not occur, the ovum and any sperm cells in the woman's body die. The sperm are absorbed by the woman's white blood cells, and the ovum passes through the uterus and exits through the vagina. Mechanisms of heredity The science of genetics is the study of heredity, the inborn factors, inherited from the biological parents that affect development. When ovum and sperm unite, they endow the baby to be with a genetic makeup that influences a wide range of characteristics from color of eyes and hair to health, intellect, and personality. Genetic code The basis of heredity is a chemical called deoxyribonucleic acid (DNA), which contains all the inherited material passed from biological parents to children. DNA carries the biochemical instructions that direct the formation of each cell in the body and tell the cells how to make the proteins that enable them to carry out specific body functions. The structure of DNA resembles a long, spiraling ladder made of four chemical units called bases (see Fig. 3). The complete sequence of genes in the human body constitutes the human genome. The genome specifies the order in which genes are expressed, or activated. ٤ Conceiving new life Fig. (3) DNA The genetic code Every cell in the normal human body except the sex cells has 23 pairs of chromosomes 46 in all. Through a type of cell division called meiosis, each sex cell, or gamete (sperm or ovum), ends up with only 23 chromosomes one from each pair. Thus, when sperm and ovum fuse at conception, they produce a zygote with 46 chromosomes, 23 from the father and 23 from the mother. At conception, then, the single celled zygote has all the biological information needed to guide its development into a human baby. This happens through mitosis, a process by which the cells divide in half over and over again. When a cell divides, the DNA spirals replicate themselves, so that each newly formed cell has the same DNA structure as all the others. ٥ Conceiving new life What determines sex? At the moment of conception, the 23 chromosomes from the sperm and the 23 from the mother's ovum form 23 pairs. Twentytwo pairs are autosomes, chromosomes that are not related to sexual expression. The twenty-third pair is sex chromosomes one from the father and one from the mother that govern the baby's sex. Sex chromosomes are either X chromosomes or Y chromosomes. The sex chromosome of every ovum is an X chromosome, but the sperm may contain either an X or a Y chromosome. The Y chromosome contains the gene for maleness, called the SRY gene. When an ovum (X) is fertilized by an Xcarrying sperm, the zygote formed is XX, a female. When an ovum (X) is fertilized by a Y-carrying sperm, the resulting zygote is XY, a male (see Fig.4). Fig. (4) ٦ Conceiving new life Patterns of genetic transmission Dominant and Recessive Inheritance: Can you curl your tongue? If so, you inherited this ability through dominant inheritance. If your parents can curl their tongues but you cannot, recessive inheritance occurred. How do these two types of inheritance work? Genes that can produce alternative expressions of a characteristic (such as ability or inability to curl the tongue) are called alleles. Every person receives a pair of alleles for a given characteristic, one from each parent. When both alleles are the same, the person is homozygous for the characteristic; when they are different, the person is heterozygous. In dominant inheritance, when a person is heterozygous for a particular trait, the dominant allele governs. In other words, when an offspring receives two contradictory alleles for a trait, only one of them, the dominant one, will be expressed. Recessive inheritance, the expression of a recessive trait, occurs only when a person receives two recessive alleles, one from each parent. Multifactorial transmission, a combination of genetic and environmental factors, plays a role in the expression of most traits. Genotypes and Phenotypes: Multifactorial Transmission If you can curl your tongue, that ability is part of your phenotype, the observable characteristics through which your genotype, or underlying genetic makeup, is expressed. Except for monozygotic twins, no two people have the same genotype. The phenotype is the product of the genotype and any relevant environmental influences. The difference between genotype and ٧ Conceiving new life phenotype helps explain why a clone can never be an exact duplicate of another human being. Influences of heredity and environment While certain rare physical disorders are virtually 100 % inherited, phenotypes for most complex normal traits, such as those having to do with health, intelligence, and personality are subject to a complex array of hereditary and environmental forces. Studying heredity and environment One approach to the study of heredity and environment is quantitative: it seeks to measure how much heredity and environment influence particular traits. This is the traditional goal of the science of behavioral genetics. Measuring Heritability: Heritability is a statistical estimate of how great contribution heredity makes toward individual differences in a specific observed trait at a certain time within a given population. Heritability does not refer to the relative influence of heredity and environment in a particular individual; those influences may be virtually impossible to separate. Nor does heritability tell us how traits develop. It merely indicates the statistical extent to which genes contribute to a trait. Effects of the Prenatal Environment: One largely neglected environmental influence is what happens in the womb. Two newer types of twin studies co-twin control and chorion control studies allow researchers to look at the nature and timing of nongenetic influences in utero. ٨ Conceiving new life Some characteristics influenced by heredity and environment 1. Physical and physiological traits. Not only do monozygotic twins generally look alike, they are also more concordant than dizygotic twins in their risk for such medical disorders as hypertension (high blood pressure), heart disease, stroke, rheumatoid arthritis, peptic ulcers, and epilepsy. Life span, too, seems to be influenced by the genes. Obesity is a multifactorial condition. 2. Intelligence and school achievement. Heredity exerts a strong influence on general intelligence and also on specific abilities. Still, experience counts, too; an enriched or impoverished environment can substantially affect the development and expression of innate ability. 3. Personality. Certain aspects of personality appear to be inherited, at least in part. Analyses of five major groupings of traits extraversion, neuroticism (a group of traits involving anxiety),conscientiousness, agreeableness, and openness to experience suggest a heritability of about 40 percent. Setting aside variances attributable to measurement error brings heritability closer to 66 percent for these trait groupings. 4. Psychopathology. There is evidence for a strong hereditary influence on such conditions as schizophrenia, autism, alcoholism, and depression. All tend to run in families and to show greater concordance between monozygotic twins than between dizygotic twins. However, heredity alone does not produce such disorders; an inherited tendency can be triggered by environmental factors. For ٩ Conceiving new life example, researchers have linked a gene or genes on chromosome 1 to vulnerability to alcoholism or depression, or both, depending on circumstances. References Diane, P.; Sally, O., and Ruth, F.: Human Development, 9th ed., McGraw Hill Company, United States, 2004, P.P. 54-99. Papalia, D.; Olds, S., and Feldman, R.: Human Development, 9th ed., McGraw Hill Company, United States, 2008, P.P. 63-72. ١٠