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Biology and Behavior How Children Develop (3rd ed.) Siegler, DeLoache & Eisenberg Chapter 3 Class 4: genetic inf luences Biological Foundations of Development: Genetic Influences How similar in appearance are you and your sibs to your parents? Some of us look like one parent, some both, some neither. • GENOTYPE: Set of genetic traits a person inherits • PHENOTYPE: Set of traits a person actually displays. Results from genotype and life experiences Class 4: genetic inf luences Fundamental Relations 1. Parents’ genetic contribution to the child’s genotype 2. Contributions of the child’s genotype to his or her own phenotype 3. Contribution of the child’s environment to his or her own phenotype 4. Influence of the child’s phenotype on his or her environment Class 4: genetic inf luences Parents’ Genotype Child’s Genotype Child’s Environment Child’s Phenotype Biological Foundations of Development: Genetic Influences Genetic Foundations Mendel: Able to predict color of pea plant offspring. Proposed genes: factors controlling physical traits • Watson and Crick (1953) introduced their model of the structure of DNA • Scientists are attempting to figure out the function of the 25,0000 genes that make up the human genome Class 4: genetic inf luences Genetic Code • We all have trillions of cells. • Each cell has a nucleus; and each nucleus has chromosomes. • Chromosomes (46 in humans) carry genetic information • They are paired…except for 23rd in male (XY). • Chromosomes made up of deoxyriboneucleic acid – DNA. Looks like a twisted ladder – a gene is segment of this ladder. • Genes instruct production of protein – crucial for chemical reactions that trigger development DNA can duplicate itself – mitosis. This allows a single celled ovum to become a many-celled adult Class 4: genetic inf luences The Sex Cells Individuals are created when two gametes (sex cells) – sperm/ovum - combine. • Gametes have 23 chromosomes, which divide by meiosis. Meiosis involves gene shuffling – crossing over – where genes exchange chromosomes. • Meiosis leads to variability in offspring. Why useful? Class 4: genetic inf luences Multiple Offspring • No variability occurs when a duplicating zygote splits into two. These are identical, or monozygotic, twins (4/1000). • Influencing factors: • When two ova are released and both are fertilized – fraternal, or dizygotic, twins. Influencing factors: Ethnicity (8/1000 whites, 14/1000 blacks), Age (rises to 35), less for women on poor diet. Class 4: genetic inf luences Boy or Girl? • Every pair of chromosomes can be distinguished. 22 match (autosomes): Longest = 1, shortest = 22. The 23rd consists of sex chromosomes M=X Y F=X X • Does an X or Y bearing sperm fertilize the ovum? A gene on the Y chromosome encodes the protein that triggers the formation of the testes, which subsequently produce testosterone, which in turn molds maleness. Class 4: genetic inf luences Patterns of Genetic Inheritance Two+ genes (alleles) occur at same place of chromosome: one from Mom and one from Dad. Alike alleles: child displays inherited trait (child is homozygous). Different alleles, relationships between alleles determine trait (heterozygous). Dominant-Recessive (D-R) Relationships The allele that affects characteristic is The allele that does not is Dominant alleles lead to certain dominant characteristics. Two recessive alleles lead to a recessive characteristic. Class 4: genetic inf luences Class 4: genetic inf luences Dominant Recessive Dark hair Curly hair Farsightedness Type A blood Double jointedness Disorders and defects are often product of recessive alleles. Class 4: genetic inf luences Polygenic Inheritance When traits are governed by more than one gene Applies to most traits and behaviors of interest to behavioral scientists Genetic Transmission of Diseases and Disorders: cancer, heart disease, asthma, psychiatric disorders, behavior disorders Class 4: genetic inf luences Phenylketonuria (PKU) is a common defect (1 in 8000) If both parents have recessive alleles = 1 in 4 children. PKU is lack of enzymes that neutralizes phenylalanine (proteins) – found in milk, eggs, fish. Causes lethargy by 5 months, retardedness by 1 year. But, can be treated: •Development is slowed but deficit disappears by school years (Mazzocco et al., 1994). Class 4: genetic inf luences Sickle cell anemia Affects only Black Americans. Two recessive alleles cause round blood cells to be sickle shaped. Cells clog blood vessels: causes pain, swelling. • Death is common before 20, few live after 40. Also caused by oxygen deprivation, recessive allele asserts itself. Why Black Americans? Class 4: genetic inf luences Mutation Where do harmful genes come from? Mutation: a sudden permanent change in a segment of DNA. May affect 1 or 2 genes, or many. Some occur by chance, some from environmental agents. Example: Class 4: genetic inf luences Ionizing radiation before birth leads to miscarriage/children with defects. X-Linked Inheritance Males and females just as likely to inherit recessive disorders. BUT, when harmful allele is carried in the X chromosome, X-linked inheritance occurs. • Females’ dominant X suppresses recessive X. • Males more likely affected: their sex chromosomes do not match – Y is shorter and often cannot override the X. Class 4: genetic inf luences Inheritance of Hemophilia, a Sex-Linked Disorder Carrier Mother X X X Normal Father Y From Seifert/Hoffnung, Child and Adolescent Development, 5/e, Figure 3.8, p. 70. Used by permission of Houghton Mifflin Company. Class 4: genetic inf luences XX Normal Daughter (25%) XX Carrier Daughter (25%) XY Normal Son (25%) XY Hemophilic Son (25%) Examples of X-linked inheritance Red-Green Color Blindness : Affects males twice as often as females (Cohen, 1984) Miscarriage: infant and childhood death greater for males. Learning disabilities: behavior disorders: more common in boys. BUT: because Y-bearing sperm are lighter and quicker they reach ovum more quickly. Thus 120/100 boys to girl (and more boys conceived). Class 4: genetic inf luences Genetic Imprinting Over 1,000 traits follow D-R inheritance. But: Genetic imprinting: • alleles chemically marked so that the mother’s or father’s pair is activated. For example: 1. Diabetes comes from father 2. Asthma and hay fever come from mother Class 4: genetic inf luences Chromosomal Abnormalities In addition to recessive alleles, chromosomal abnormalities cause serious developmental problems. Often a chromosome pair does not separate properly. More serious than problem from single gene – involves many. Class 4: genetic inf luences Down Syndrome (1 in 800; 1/1900 at 20; 1/130 at 39) Mostly from 21st pair of chromosomes fail to separate. Individual has 3 of this chromosome rather than 2. • Children have distinct physical features: 1. 2. Short, stocky, flattened face, almond shape eyes. Often have heart defects, eye cataracts. Death: 14% by 1 year, 21% by 10, others by middle adulthood. Class 4: genetic inf luences FREQUENCY OF DOWN SYNDROME (PER 1000) Figure 3.8 Relationship Between Maternal Age and the Incidence of Down Syndrome 100 90 80 70 60 50 40 30 20 10 0 15 Class 4: genetic inf luences 20 25 30 35 40 MATERNAL AGE (YEARS) 45 50 Behavioral: 1. mental retardation, 2. speech problems, 3. slow motor development – All become worse with age. Smile less readily, no eye contact, explore less. BUT: Early intervention helps social and emotional skills – not cognitive. So, it is genetic but environmental factors affect it. Class 4: genetic inf luences The Family Study The mainstay of modern behavioral genetics research A trait of interest is measured among groups of people who vary in genetic relatedness Correlations for traits measured for individuals who Are genetically more similar Share the same environment Class 4: genetic inf luences Types of Family Studies Twin-Study Designs: Correlations for pairs of monozygotic twins on a trait of interest are compared to those of dizygotic twins Adoption Studies: Researchers examine whether adopted children are more like their biological or their adopted relatives Class 4: genetic inf luences Minnesota Study of Twins Reared Apart Located and studied twin siblings who have not met since they were infants The team of investigators were struck by the similarities they found in traits like IQ, reaction to stress, and traditionalism These similarities may be influenced by selective placement and similarities in fostering environments as well as by genetic factors Class 4: genetic inf luences Reproductive Choices Genetic Counseling Helps assess chance of hereditary disorders. High risk: 1. previous miscarriages, 2. genetic problems in family. There is an interview, family tree is established – pedigree. Blood tests, genetic analyses allow prediction of harmful carrier. Should parents “take a chance”? Or adopt? Class 4: genetic inf luences Prenatal Diagnosis and Fetal Medicine Methods of detection are available for those who are high risk. Ultrasound: high-frequency sound waves give picture of fetus – gross physical defect can be seen. Amniocentesis; most common: needle inserted to abdominal wall. Uterus fluid is obtained. Cells examined. • But, can cause miscarriages or injury to fetus. Class 4: genetic inf luences