Chapter 3 HEREDITARY INFLUENCES ON DEVELOPMENT

Chapter 3 HEREDITARY INFLUENCES ON DEVELOPMENT

PRINCIPLES OF HEREDITARY TRANSMISSION • Development begins at conception • Sperm cell penetrates ovum • Zygote is formed • 46 chromosomes (23 from each parent) • Genes, stretches of DNA • Provides biological basis for development

PRINCIPLES OF HEREDITARY TRANSMISSION • Growth of Zygote, Production of Body Cells • Zygote replicates through mitosis • Each division duplicates chromosomes • Each new cell contains the 46 we inherited at conception

Figure 3.1 Mitosis: the way that cells replicate themselves.

PRINCIPLES OF HEREDITARY TRANSMISSION • The Germ Cells (produce sperm and ova) • Production of Gametes through Meiosis • Duplication of 46 chromosomes • Crossing-over: adjacent chromosomes break and exchange segments of genes • Pairs of duplicated chromosomes segregate into 2 new cells • Cells divide, 23 single chromosomes

Figure 3.2 Diagram of the meiosis of a male germ cell.

PRINCIPLES OF HEREDITARY TRANSMISSION • The Germ Cells • Hereditary Uniqueness • Independent assortment – each chromosome pair segregates independently, resulting in genetic uniqueness

PRINCIPLES OF HEREDITARY TRANSMISSION • Multiple Births • Monozygotic twins: single zygote divides, are genetically identical • Dizygotic (fraternal) twins: 2 ova released and fertilized by different sperm, are as genetically similar as any sibling pair

Figure 3.3 Identical, or monozygotic, twins (left) develop from a single zygote. Because they have inherited identical sets of genes, they look alike, are the same sex, and share all other inherited characteristics. Fraternal, or dizygotic, twins (right) have no more genes in common than siblings born at different times. Consequently, they may not look alike (as we see in this photo) and may not even be the same sex.

PRINCIPLES OF HEREDITARY TRANSMISSION • Male or Female • Karyotypes – chromosomal portraits • 22 pairs (autosomes) are similar in males and females • 23rd pair are the sex chromosomes • Males – X and Y, Females – 2 X’s • Ova contain X’s, sperm an X or a Y • Males determine sex of children

Figure 3.4 These karoytypes of a male (left) and a female (right) have been arranged so that the chromosomes could be displayed in pairs. Note that the twenty-third pair of chromosomes for the male consists of one elongated X chromosome and a Y chromosome that is noticeably smaller, whereas the twenty-third pair for the female consists of two X chromosomes.

PRINCIPLES OF HEREDITARY TRANSMISSION • What Do Genes Do? • Produce enzymes and proteins necessary for creation and functioning of cells • Guide cell differentiation • Regulate the pace/timing of development • Environmental factors (internal and external) influence how genes function

Table 3.1 Different Levels of Gene-Environment Interaction That Influence Genetic Expression

PRINCIPLES OF HEREDITARY TRANSMISSION • How are Genes Expressed? • Single-Gene Inheritance Patterns • Simple Dominant-Recessive Inheritance • 1 pair of genes (alleles), 1 from each parent • Either dominant or recessive • Homozygous – same alleles • Heterozygous – different alleles

Figure 3.5 Possible genotypes (and phenotypes) resulting from a mating of two heterozygotes for normal vision.

PRINCIPLES OF HEREDITARY TRANSMISSION • How are Genes Expressed? • Codominance • Phenotype is a compromise between the dominant and recessive alleles • Sex-Linked Inheritance • Genes located on sex chromosomes • Most from recessive genes found only on X chromosomes (common in males)

Figure 3.6 Normal (round) and “sickled” (elongated) red blood cells from a person with sickle-cell anemia.

Figure 3.7 Sex-linked inheritance of red/green color blindness. In the example here, the mother can distinguish reds from greens but is a carrier because one of her X chromosomes contains a color-blind allele. Notice that her sons have a 50 percent chance of inheriting the color-blind allele and being color-blind, whereas none of her daughters would display the trait. A girl can be color-blind only if her father is color blind and her mother is at least a carrier of the color-blind gene.

PRINCIPLES OF HEREDITARY TRANSMISSION • How are Genes Expressed? • Polygenic Inheritance • Characteristics influenced by many pairs of alleles • Most complex human attributes are polygenic

Figure 3.8 Single-gene and multiple gene distributions for traits with additive gene effects. (a) A single gene with two alleles yields three genotypes and three phenotypes. (b) Two genes, each with two alleles, yield nine genotypes and 5 phenotypes. (c) Three genes, each with two alleles, yield twenty-seven genotypes and seven phenotypes. (d) Normal bell-shaped curve of continuous variation.

HEREDITARY DISORDERS • Congenital defects – present at birth (5%) • Chromosomal Abnormalities – too many or too few • Sex Chromosome Abnormalities • Abnormalities of the Autosome • Down syndrome most common – trisomy-21 (extra 21st chromosome)

Figure 3.9 Sources of Congenital Defects

Table 3.2 Four Common Sex Chromosome Abnormalities

Table 3.2 Four Common Sex Chromosome Abnormalities (continued)

HEREDITARY DISORDERS • Genetic Abnormalities • Many passed to children by parents who are carriers of recessive alleles • Some are caused by dominant alleles • Some result from mutations – changes in structure of one or more genes • Spontaneous • Environmental hazards

Table 3.3 Brief Description of Major Recessive Hereditary Diseases

HEREDITARY DISORDERS • Predicting Hereditary Disorders • Genetic counseling – both chromosomal and genetic abnormalities • Obtain a pedigree – family history • DNA from parents’ blood • Consider options based on risk

HEREDITARY DISORDERS • Detecting Hereditary Disorders • Amniocentesis – withdrawal of a sample of amniotic fluid, tests fetal cells within fluid • Risk of miscarriage higher than risk of birth defect in women younger than 35 • Conducted 11th/14th week of pregnancy • Results 2 to 3 weeks later

Figure 3.11 In amniocentesis, a needle is inserted through the abdominal wall into the uterus. Fluid is withdrawn and fetal cells are cultured, a process that takes about 3 weeks.

HEREDITARY DISORDERS • Detecting Hereditary Disorders • Chorionic villus sampling – collects cells from chorion, • Conducted 8th/9th week of pregnancy • Results in 24 hours • Risk of miscarriage 1 in 50 • Ultrasound – sound waves provide outline of fetus – useful after 14th week, safe

Figure 3.12 Chorionic villus sampling can be performed much earlier in pregnancy, and results are available within 24 hours. Two approaches to obtaining a sample of chorionic villi are shown here: inserting a thin tube through the vagina into the uterus or a needle through the abdominal wall. In either of these methods, ultrasound is used for guidance. ADAPTED FROM MOORE & PERSAUD, 1993.

Figure 3.13 Photo of 3-D ultrasound of fetus.

HEREDITARY DISORDERS • Treating Hereditary Disorders • Special diets for metabolic disorders • Fetal surgery, hormone therapy • Gene replacement therapy – relieves symptoms, doesn’t cure disorder • Germline gene therapy – replace harmful genes early in embryonic stage to cure defect; not yet used in humans

HEREDITARY INFLUENCES ON BEHAVIOR • Behavioral genetics - study of how genes and environment influence behavior • Methods of studying hereditary influences • Selective breeding – animal studies • Family studies – examining kinship • Twin studies – identical vs. fraternal • Adoption studies – children similar to biological or adoptive parents?

HEREDITARY INFLUENCES ON BEHAVIOR • Contribution of Genes and Environment • Concordance rates - % of pairs of people who both display a trait if one member has it • Gene Influences • Heritability coefficient = (r identical – r fraternal) X 2

Figure 3.15 Concordance rates for identical and fraternal twins for several behavioral dimensions. FROM PLOMIN ET. AL, 1994.

Figure 3.16 Concordance rates for identical and fraternal twins for several behavioral dimensions. FROM PLOMIN ET AL., 1994.

Table 3.4 Average Correlation Coefficients for Intelligence-Test Scores from Family Studies Involving Persons at Four Levels of Kinship

HEREDITARY INFLUENCES ON BEHAVIOR • Contribution of Genes and Environment • Nonshared Environmental Influences = 1-r(identical twins reared together) • Shared Environmental Influences = 1 – (H + NSE)

HEREDITARY INFLUENCES ON BEHAVIOR • Myths about Heritability Estimates • Cannot tell us if we have inherited a trait • Differences among individuals due to differences in inherited genes • Only apply to populations under particular environmental circumstances • Clearly heritable traits CAN be modified by environmental influences

HEREDITARY INFLUENCES ON BEHAVIOR • Hereditary Influences on Intellectual Performance • As children age • Genes contribute more • Nonshared environment increases • Shared environment decreases

Figure 3.17 Changes in the correlation between the IQ scores of identical and fraternal twins over childhood. DATA FROM WILSON, 1983.

HEREDITARY INFLUENCES ON BEHAVIOR • Hereditary Contributions to Personality • Introversion/extraversion and empathetic concern are both genetically influenced • Moderate heritability (+.40) • Nonshared environmental influences are also important • Shared environmental influences are relevant for religious & social values

Table 3.5 Personality Resemblances among Family Members at Three Levels of Kinship

HEREDITARY INFLUENCES ON BEHAVIOR • Hereditary Contributions to Behavior Disorders and Mental Illness • Schizophrenia, alcoholism, criminality, depression, hyperactivity, bipolar disorder, neurotic disorders – all genetically influenced • Inherit a predisposition, not the disorder

THEORIES OF HEREDITARY AND ENVIRONMENTAL INTERACTIONS • The Canalization Principle • Multiple pathways individuals may develop • Nature and nurture combine to determine pathway • Either genes or environment may limit the extent the other can influence development

THEORIES OF HEREDITARY AND ENVIRONMENTAL INTERACTIONS • The Range-of-Reaction Principle • Genotype sets a range of possible outcomes • Environment largely influences where within the range an attribute will fall

Figure 3.18 Hypothetical reaction ranges for the intellectual performances of three children in restricted, average, and intellectually-enriching environments. ADAPTED FROM GOTTESMAN, 1963.

THEORIES OF HEREDITARY AND ENVIRONMENTAL INTERACTIONS • Genotype-Environment Correlations • Passive – home environment is influenced by parents genotypes • Evocative – genetically influenced attributes affects behavior of others toward the child • Active – environments children seek will be compatible with genetic predispositions

Chapter 3 HEREDITARY INFLUENCES ON DEVELOPMENT

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