Are controlled by multiple genes one of the major

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are controlled by multiple genes. One of the major goals of the new field of behavioral genetics, which studies the interplay of heredity and the environment, is to identify and study these polygenic traits. Another new and related field of research, known as epigenetics, studies how non-genetic factors can dramatically affect how (and if) inherited genes are expressed throughout our lives (Brody et al., 2016; Iakoubov et al., 2015; Wallack
& Thornburg, 2016). Unlike simple genetic transmission, which is based on changes in the DNA sequence, changes in gene expression can have other causes, such as age, environment, lifestyle, or disease. (The term “epi” means “above” or “outside of.”) In other words, nurture can shape nature! Epigenetic factors can switch genes “ON” or “OFF.” For example, epigenetic factors like malnutrition or childhood abuse can prevent a child from reaching his or her full potential genetic height or maximum genetic intelligence (Denholm et al., 2013; Venable & Raine, 2016). The good news is that with environmental changes even identical twins are not destined to develop the same diseases. There is an activity on prenatal development available in the Resources Tab. Three Stages of Prenatal Development Now that we’ve discussed the general principles of how our genes and our environment interact to form us as unique individuals, let’s go back to the moment of your conception. At that point in time, you were a single cell barely 1/175 of an inch in diameter—smaller than the period at the end of this sentence. This new cell, called a zygote, then began a process of rapid cell division that resulted in a multimillion-celled infant (you) some nine months later. The vast changes that occur during the nine months of a full-term pregnancy are usually divided into three stages: the germinal period, embryonic period, and fetal period ( Process Diagram 9.1 ). Prenatal growth and growth during the first few years after birth are both proximodistal (near to far), which means that the innermost parts of the body develop before the outermost parts. Thus, a fetus’s arms develop before its hands and fingers. Development at this stage also proceeds cephalocaudally (head to tail)—a fetus’s head is disproportionately large compared with the lower part of its body. Can you see how these two terms—proximodistal and cephalocaudal—help explain why an infant can lift its head before it can lift its arms and lift its arms before lifting its legs? STOP! This Process Diagram contains essential information NOT found elsewhere in the text, which is likely to appear on quizzes and exams. Be sure to study it CAREFULLY! PROCESS DIAGRAM 9.1Prenatal Development
0 0 Hazards to Prenatal Development As you recall, human development begins with the genes we inherit from our biological parents, and epigenetic factors, like age, lifestyle, and diseases, can dramatically affect how (and if) these inherited genes are expressed. For example, during pregnancy the placenta connects the fetus to the mother’s uterus and serves as the link for delivery of food and excretion of wastes.

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