seek assistance from a genetic counselor to determine the risk for genetic disorders. This information can help you decide among reproductive options.”
1 Identify three ways that genetic disorders are passed. Why does Dr. Rasheed advise genetic testing?
2 What are some of the reproductive options available to Zennia? What are some of the advantages and disadvantages of each option?
3 Which option do you suggest? Why?
4 Suppose that Zennia became pregnant. What types of prenatal screening tests might Dr. Rasheed prescribe? Discuss some of the advantages and disadvantages of each.
5 What advice would you give Zennia? Why?
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Key Terms
Chromosome 35
Gene 35
Deoxyribonucleic acid (DNA) 35
Mitosis 36
Meiosis 36
Gamete 36
Zygote 36
Dizygotic (DZ) twins 36
Monozygotic (MZ) twins 37
Genotype 37
Allele 38
Dominant–recessive inheritance 38
Incomplete dominance 39
Sickle cell trait 39
Polygenic inheritance 40
Genomic imprinting 40
Phenylketonuria (PKU) 41
Hemophilia 43
Fragile X syndrome 43
Down syndrome 43
Mutation 44
Genetic counseling 46
Artificial insemination 47
In vitro fertilization 47
Surrogacy 47
Adoption 48
Ultrasound 50
Fetal MRI 50
Amniocentesis 50
Chorionic villus sampling (CVS) 51
Noninvasive prenatal testing (NIPT) 52
Phenotype 53
Behavioral genetics 53
Heritability 53
Gene–environment interaction 54
Range of reaction 54
Canalization 54
Gene–environment correlation 55
Niche-picking 57
Summary
2.1 Discuss the genetic foundations of development.The human body is composed of trillions of units called cells, each with a nucleus containing 23 matching pairs of chromosomes, which contain genes, composed of stretches of deoxyribonucleic acid (DNA) that carry the plan for creating all of the traits that organisms carry. Some genes are passed through dominant–recessive inheritance, in which some genes are dominant and will always be expressed regardless of the gene they are paired with. Other genes are recessive and will only be expressed if paired with another recessive gene. When a person is heterozygous for a particular trait, the dominant gene is expressed and the person remains a carrier of the recessive gene. Some traits are carried on the X chromosome. Some traits are passed through incomplete dominance in which both genes influence the characteristic. Polygenic traits are the result of interactions among many genes. Some traits are determined by genomic imprinting, determined by whether it is inherited from the mother or the father.
2.2 Identify examples of genetic disorders and chromosomal abnormalities.Genetic disorders carried through dominant–recessive inheritance include phenylketonuria (PKU), a recessive disorder, and Huntington disease, carried by a dominant allele. Some recessive genetic disorders, like the gene for hemophilia, are carried on the X chromosome. Males are more likely to be affected by X-linked genetic disorders, such as hemophilia. Fragile X syndrome is an example of a dominant–recessive disorder carried on the X chromosome. Because the gene is dominant, it must appear on only one X chromosome to be displayed. Other X-linked genetic disorders include Klinefelter syndrome, Jacob’s syndrome, triple X syndrome, and Turner syndrome. Some disorders, such as trisomy 21, known as Down syndrome, are the result of chromosomal abnormalities. Others result from mutations, genetic abnormalities that may occur randomly or as a result of exposure to toxins.
2.3 Examine the choices available to prospective parents in having healthy children.Genetic counseling is a medical specialty that helps prospective parents determine the likelihood that their children will inherit genetic defects and chromosomal abnormalities. Single women, gay and lesbian couples, and individuals at risk for bearing children with genetic or chromosomal abnormalities may seek alternative methods of conception such as artificial insemination, in vitro fertilization, and surrogacy. Others consider adopting a child. Prenatal diagnosis is recommended when genetic testing has determined a risk for genetic abnormalities. Some prenatal tests, such as ultrasound, are conducted routinely. Advances in genetics and in medicine have led to therapies that can be administered prenatally to reduce the effects of many genetic abnormalities.
2.4 Summarize the interaction of heredity and environment, including behavioral genetics and the epigenetic framework.Behavioral genetics is the field of study that examines how genes and experience combine to influence the diversity of human traits, abilities, and behaviors. Heritability research examines the contributions of the genotype in determining phenotypes but also provides information on the role of experience through three types of studies: selective breeding studies, family studies, and adoption studies. Genetics contributes to many traits, such as intellectual ability, sociability, anxiety, agreeableness, activity level, obesity, and susceptibility to various illnesses. Passive, evocative, and active gene–environment correlations illustrate how traits often are supported by both our genes and environment. Reaction range refers to the idea that there is a wide range of potential expressions of a genetic trait, depending on environmental opportunities and constraints. Some traits illustrate canalization and require extreme changes in the environment to alter their course. The epigenetic framework is a model for understanding the dynamic ongoing interactions between heredity and environment whereby the epigenome’s instructions to turn genes on and off throughout development are influenced by the environment.
Review Questions
2.1 What genes are shared by twins?What are four types of genetic inheritance?
2.2 Give an example of the following:a dominant–recessive disorderan X-linked disordera chromosomal abnormalityWhat is imprinting?What is a mutation?
2.3 What is genetic counseling?What are the differences between artificial insemination, in vitro fertilization, surrogacy, and adoption?What are the five common methods of prenatal diagnosis?
2.4 What is behavioral genetics?What are three types of gene—environment correlations?What is the range of reaction?What is the epigenetic framework?