Chapter 2. Heredity, Prenatal Development, and Birth PDF

Chapter 2 HEREDITY, PRENATAL DEVELOPMENT, AND BIRTH Learning Objectives: Heredity Define genes Distinguish between mitosis and meiosis, genotype and phenotype, homozygous and heterozygous, and dominant and recessive Describe some genetic disorders, due to a gene defect, and chromosomal; disorders D...

Chapter 2 HEREDITY, PRENATAL DEVELOPMENT, AND BIRTH Learning Objectives: Heredity Define genes Distinguish between mitosis and meiosis, genotype and phenotype, homozygous and heterozygous, and dominant and recessive Describe some genetic disorders, due to a gene defect, and chromosomal; disorders Define polygenic and incomplete dominance Describe the function of genetic counseling and why individuals may seek genetic counseling Define behavioral genetics, describe genotype-environment correlations and genotype-environmental interactions, and define epigenetics HEREDITY: GENES The basic building block of the nature perspective is the gene. Proteins are responsible for influencing the structure and functions of the cells. Normal human cells contain 46 chromosomes (23 pairs; one from each parent) in the nucleus of the cells. HEREDITY: MITOSIS AND MEIOSIS Most cells of the body are created by a process called mitosis. Mitosis is the cell’s nucleus making an exact copy of all the chromosomes and splitting into two new cells. The cells used in sexual reproduction, the gametes (sperm or ova), are formed by meiosis. Each sperm and egg possesses only 23 chromosomes, which are combined to produce the normal 46 (chromosomes).  HEREDITY: MITOSIS AND MEIOSIS Given the amount of genes present and the unpredictability of the meiosis process, the likelihood of having offspring that are genetically identical (not twins) is one of the trillions (Gould & Keeton, 1997). HEREDITY: MITOSIS VS. MEIOSIS 23 pairs of chromosomes are created at conception, and 22 pairs are similar in length. These are called autosomes. The remaining pair, or sex chromosomes, may differ in length. If a child receives the combination of XY the child will be genetically male. If a child receives the combination XX the child will be genetically female. HEREDITY: GENOTYPES AND PHENOTYPES If you look in the mirror. What do you see, your genotype or phenotype? Give one example. HEREDITY: GENOTYPES AND PHENOTYPES The word genotype refers to the total of all the genes a person inherits The word phenotype refers to the features that are expressed. GENOTYPES: HOMOZYGOUS AND HETEROZYGOUS Genes are inherited in pairs on the chromosomes, we may receive either the same version of a gene from our mother or father, that is, be homozygous for that characteristic the gene influences. If we receive a different version of the gene from each parent, that is referred to as heterozygous. GENOTYPES: HOMOZYGOUS AND HETEROZYGOUS In the homozygous situation, we will display that characteristic. In the heterozygous condition, it becomes clear that not all genes are created equal. PHENOTYPES Some genes are dominant, meaning they express themselves in the phenotype even when paired with a different version of the gene, while their silent partner is recessive Recessive genes express themselves only when paired with a similar version gene. PHENOTYPES Genetics refers to different versions of a gene as alleles. Some recessive traits include red hair, facial dimples, curly hair, normal vision, and dark hair. Most characteristics do not result from a single gene; they are polygenic, meaning they result from several genes. The dominant and recessive patterns described above are usually not that simple either. the dominant gene does not completely suppress the recessive gene; this is called incomplete dominance. To inherit the disorder a person must receive the recessive gene from both parents. Those who have inherited only one recessive gene are called carriers and should be unaffected by this recessive trait. HEREDITY: MONOZYGOTIC AND DIZYGOTIC TWINS HEREDITY: MONOZYGOTIC AND DIZYGOTIC TWINS Monozygotic or identical twins occur when a fertilized egg splits apart in the first two weeks of development. The result is the creation of two separate, but genetically identical offspring. They possess the same genotype and often the same phenotype. 1/3 of twins are monozygotic. HEREDITY: MONOZYGOTIC AND DIZYGOTIC TWINS Dizygotic or Fraternal twins are when two eggs or ova are released and fertilized by two separate sperm. Two individuals share the same amount of genetic material as would any two children from the same mother and father. They posses a different genotype and phenotype  HEREDITY: GENETIC DISORDERS Most of the known genetic disorders are dominant gene-linked; however, the vast majority of dominant gene-linked disorders are not serious or debilitating. Some genetic disorders are sex-linked; the defective gene is found on the X chromosome. Males have only one X chromosome so are at greater risk for sex-linked disorders due to a recessive gene such as hemophilia, color-blindness, and baldness. For females to be affected by the genetic defects, they need to inherit the recessive gene on both X-chromosomes, but if the defective gene is dominant, females can be equally at risk. HEREDITY: CHROMOSOMAL ABNORMALITIES A chromosomal abnormality occurs when a child inherits too many or two few chromosomes. Most common cause of chromosomal abnormalities is the age of the mother. Some gametes do not divide evenly when they are forming. Therefore, some cells have more than 46 chromosomes. Most of these zygote fail to develop and are spontaneously aborted by the mother’s body. HEREDITY: CHROMOSOMAL ABNORMALITIES TRISOMY 21 OR DOWN SYNDROME It occurs when there are three rather than two 21st chromosomes. A person with Down syndrome typically exhibits an intellectual disability and possesses certain physical features, such as short fingers and toes, folds of skin over the eyes, and a protruding tongue. HEREDITY: CHROMOSOMAL ABNORMALITIES TRISOMY 13 OR PATAU SYNDROME Caused by a chromosomal abnormality, in which some or all of the cells of the body contain extra genetic material from chromosome 13 HEREDITY: CHROMOSOMAL ABNORMALITIES TRISOMY 18 OR EDWARDS SYNDROME A genetic disorder caused by the presence of a third copy of all or part of chromosomes 18 Many parts of the body are affected. Babies are often born small and have heart defects HEREDITY: CHROMOSOMAL DISORDERS SEX-LINKED CHROMOSOMAL DISORDERS Turner Syndrome - is caused when all or part of one of the X chromosomes if lost before or soon after conception due to random events. (In Females) Klinefelter Syndrome - is caused when an extra X chromosomes is present in the cells of a male due to random events.(In Males) HEREDITY: CHROMOSOMAL DISORDERS HEREDITY: CHROMOSOMAL DISORDERS HEREDITY: GENETIC COUNSELING A service that assists individuals identify, test for, and explain potential genetic conditions that could adversely affect themselves or their offspring is referred to as genetic counseling (CDC,2015b) Family history of a genetic condition. Membership in a certain ethnic group with a higher risk of a genetic condition. Information regarding the results of genetic testing, including blood tests, amniocentesis, or ultra sounds. Learning about the chances of having a baby with a genetic condition if the parents are older, have had several miscarriages, have offspring with birth defects, experience infertility, or have a medical conditions. HEREDITY:BEHAVIORAL GENETICS is the scientific study of the interplay between the genetic and environmental contributions to behavior. This bidirectional interplay suggests that the environment can affect the expression of genes just as genetic predispositions can impact a person’s potentials. Genotype-Environmental Correlations refers to the processes by which genetic factors contribute to variations in the environment (Plomin et al., 2013) 3 Types of Genotype-environment correlations: Passive genotype-environment correlation Evoactive genotype-environment correlation Active genotype-environment correlation HEREDITY:BEHAVIORAL GENETICS Genotype-Environmental Correlations Passive genotype-environment correlation occurs when children passively inherit the genes and the environments their family provides. Evoactive genotype-environment correlation refers to how the social environment reacts to individuals based on their inherited characteristics. Active genotype-environment correlation occurs when individuals seek out environments that support their genetic tendencies. This is also referred to as niche picking. HEREDITY:BEHAVIORAL GENETICS Genotype-Environmental Interactions When different genetic makeups react differently to environmental conditions. Gens might affect traits or health only under certain environmental influences. this helps explain why people with the same genes might show different traits or disease risks. Epigenetics Study of how gene activity is regulated by chemical changes to DNA or histones, without altering the DNA sequence itself. These chnges can be influenced by the environment and lifestyle and can sometimes be passed to future generations. it explains how genes can be turned on or off, affecting traits an health. Learning Objectives: Prenatal Development Describe the changes that occur in the three periods of prenatal development Describe what occurs during prenatal brain development Define teratogens and describe the factors that influence their effects List and describe the effects of several common teratogens Explain maternal and paternal factors that affect the developing fetus Explain the types of prenatal assessment Describe both the minor and major complications of pregnancy PRENATAL DEVELOPMENT:THE GERMINAL PERIOD The germinal period (about 14 days in length) lasts from conception to implantation of the fertilized egg in the lining of the uterus (See Figure 2.5). The cell containing the combined genetic information from both parents, is referred to as a zygote. The blastocyst consists of both an inner and outer group of cells. The inner group of cells, or embryonic disk will become the embryo, while the outer group of cells, or trophoblast, becomes the support system which nourishes the developing organism. PRENATAL DEVELOPMENT:THE GERMINAL PERIOD This stage ends when the blastocyst fully implants into the uterine wall (U.S. National Library of Medicine, 2015a). Approximately 50-75% of blastocysts do not implant in the uterine wall (Betts et al., 2019). Mitosis is a fragile process and fewer than one half of all zygotes survive beyond the first two weeks (Hall, 2004). PRENATAL DEVELOPMENT:THE EMBRYONIC PERIOD Upon implantation this multi-cellular organism is called an embryo. Blood vessels grow forming the placenta. The placenta is a structure connected to the uterus that provides nourishment and oxygen from the mother to the developing embryo via the umbilical cord. Growth during prenatal development occurs in two major directions: from head to tail called cephalocaudal development and from the midline outward referred to as proximodistal development. The embryo is approximately 1 inch in length and weighs about 8 grams at the end of eight weeks (Betts et al., 2019). PRENATAL DEVELOPMENT:THE FETAL PERIOD From the ninth week until birth, the organism is referred to as a fetus. The fetus is about 3 inches long and weighs about 28 grams. The first chance of survival outside the womb, known as the age of viability is reached at about 24 weeks (Morgan, Goldenberg, & Schulkin, 2008). At 24 weeks the fetus can feel pain (Royal College of Obstetricians and Gynecologists, 1997). The fetus gains about 5 pounds and 7 inches during this last trimester of pregnancy, and during the 8th month a layer of fat develops under the skin. PRENATAL DEVELOPMENT: PRENATAL BRAIN DEVELOPMENT Prenatal brain development begins in the third gestational week with the differentiation of stem cells, which are capable of producing all the different cells that make up the brain (Stiles & Jernigan, 2010). The location of these stem cells in the embryo is referred to as the neural plate. By the end of the third week, two ridges appear along the neural plate first forming the neural groove and then the neural tube. The open region in the center of the neural tube forms the brain’s ventricles and spinal canal. By the end of the embryonic period, or week eight, the neural tube has further differentiated into the forebrain, midbrain, and hindbrain. Brain development during the fetal period involves neuron production, migration, and differentiation. PRENATAL DEVELOPMENT: PRENATAL BRAIN DEVELOPMENT Neurogenesis, or the formation of neurons, is largely completed after five months of gestation. Neural migration is mostly completed in the cerebral cortex by 24 weeks (Poduri & Volpe, 2018). Once in position, neurons begin to produce dendrites and axons that begin to form the neural networks responsible for information processing. Regions of the brain that contain the cell bodies are referred to as the gray matter because they look gray in appearance. The axons that form the neural pathways make up the white matter because they are covered in myelin, a fatty substance that is white in appearance. Myelin aids in both the insulation and efficiency of neural transmission. TERATOGENS The study of factors that contribute to birth defects is called teratology. Teratogens are environmental factors that can contribute to birth defects. Factors influencing prenatal risks: Timing of exposure: Damage may be greater if certain substances are introduced during a critical time of development. Amount of exposure: Some substances are not harmful unless the amount reaches critical levels. Number of teratogens: Fetuses exposed to greater numbers of teratogens are in greater risk. Genetics: A mother’s genetic make-up can influence teratogenic effects. Being male or female: Males are known to experience more damage from teratogens than females. TERATOGENS ALCOHOL: Alcohol consumption, particularly during the second month of prenatal development but at any point during pregnancy, may lead to neurocognitive and behavioral difficulties that can last a lifetime. FASD or Fetal Alcohol Spectrum Disorders: an umbrella term for the range of effects that can occur due to alcohol consumption during pregnancy. Cognitively, these children have poor judgment, poor impulse control, higher rates of ADHD, learning issues, and lower IQ scores.  TERATOGENS TOBACCO: Another widely used teratogen is tobacco as more than 7% of pregnant women smoked in 2016. In 2010 representing 52% of live births, When comparing the ages of women showed that among women with recent who smoked: live births: Women