Chp 2 Genetics .pdf

Chapter 2: Genetic Bases of Child Development Mechanisms of Heredity Heredity, Environment, and Development Children and Their Development, Cdn Ed. - Robert Kail  In the Beginning Genetics Basic Principles Genes & Heredity Conception t...

Chapter 2: Genetic Bases of Child Development Mechanisms of Heredity Heredity, Environment, and Development Children and Their Development, Cdn Ed. - Robert Kail  In the Beginning Genetics Basic Principles Genes & Heredity Conception to Birth Mutations and Chromosomal Aberrations Genetic Disorders  The Human Genome Genome refers to the complete set of genes that an organism possesses Human genome contains 20,000–25,000 genes on 23 pairs of chromosomes Human Genome Project (sequence vs. function)  The Biology of Heredity Gametes (sperm/egg cells) have 23 chromosomes each When combined, they provide 23 pairs of chromosomes The first 22 pairs of chromosomes are autosomes and the 23rd pair is sex chromosomes © 2009 Pearson Education Canada Inc. Autosome and Sex Chromosomes The Double Helix Structure & Composition of DNA Chromosomes consist of DNA Order of nucleotide bases form chemical compounds (genes) Genes are the template for the synthesis of amino acids peptides, & enzymes (protein) Strand of DNA Provides the Genetic code (genotype)  Mitosis refers to a process by which 2 identical cells are produced Meiosis refers to a process by which 4 cells are produced, with each containing only 23 chromosomes Figure 3.1 Mitosis and meiosis. Mitosis results in two cells identical with the parent and with each other. Meiosis results in four cells different from the parent cell and from each other. Adapted from Biology: Exploring Life (p.152) by G. D. Brum & L. K. McKane, 1989, New York: John Wiley & Sons. Adapted by permission of the authors. Meiosis Mutations/ Chromosomal Aberrations Mendel’s Studies: Principles of Heredity Mendel established that certain traits are transmitted from parent to offspring – Each trait is governed by two elements with one from each parent Principle of dominance: Some genes are always expressed (dominant); others are recessive – Phenotype: the expressed trait – Genotype: the underlying genes that govern the trait  Single Gene Inheritance Homozygous Alleles child’s parents have contributed similar genes for a trait (BB, bb) Heterozygous Alleles The parents have contributed different versions (Bb) The phenotype of the child is determined by dominance 1st generation w w P Pw Pw Pw Pw P 2nd generation P w P P P Pw w Pw ww Mendel’s Studies: Common Genetic Traits Dominant Recessive Brown eyes Blue, gray, or green eyes Normal hair Baldness (in men) Dark hair Blond hair Normal colour vision Colour blindness Freckles No freckles Dimples No dimples Free earlobes Attached earlobes Double-jointed Tight thumb ligaments thumbs  Principles of Genetic Transmission Polygenic inheritance occurs when traits are determined by a number of genes Incomplete dominance occurs when the dominant gene does not completely suppress the recessive gene (“Blends”) Codominance occurs when both genes are dominant and thus both are expressed Incomplete dominance CoDominance Codominance  Genetic Disorders 1. Inherited disorders Dominant/recessive alleles Usually recessive (HD exception) 2. Chromosomal abnormalities Extra, missing, or damaged chromosomes Genetic Disorders: Hereditary Disorders Mutations (genetic variations) can be adaptive or maladaptive Dominant disorders – Huntington’s chorea: a fatal syndrome in which the nervous system degenerates in adulthood (age 30-40) Gene Locus IT15 chromosome 4 Abnormal Repeats (> 40) of CAG (trinucleotide)  Recessive Alleles Metabolic Disturbances – Phenylketonuria (PKU): Inability to metabolize the amino acid phenylalanine due to lack of liver enzyme (toxic in CNS) (chrom12) – Tay-sachs disease: A defect of lipid metabolism due to the absence of the enzyme hexosominidase (chrom 15 HEXA gene) acidic fatty materials (gangliosides) build up  Recessive Alleles Non-metabolic disturbance Sickle-Cell Disease A Thymine base replaces an Adenine base in the DNA encoding the B- globin gene Incomplete Dominance Sickle Cell Trait: An Example of Incomplete Dominance © 2009 Pearson Education Canada Inc.  Sickle-Cell Anemia (SCA) Figure 3.5 Scanning electron micrographs of red blood cells from normal individuals (left) and individuals with sickle-cell anemia (right). (Bill Longcore/Photo Researchers, Inc.)  Chromosomal Disorders Fragile X (1/2000 M; 1/4000 F) expansion (>200 repeats) of the CGG codon (trinucleotide repeats) Sex chromosomal disorder results in a methylation of that portion of the DNA, effectively silencing the expression of the FMRP protein. appears 'fragile' under the microscope at that point; a phenomenon that gave the syndrome its name. Down’s Syndrome (trisomy 21) Autosomal Disorder extra chromosome in pair 21 Alzheimer’s disease  This picture shows the location of the FMR1 gene, which is the gene that causes Fragile X Syndrome. The gene is located in the "fragile" area of the x-chromosome. Evolution, Heredity, and Behaviour ( http://www.fraxa.org/html/about_prevalence.htm )  Disorders of the sex chromosomes Fragile X syndrome Caused by an abnormal gene (FMR1) on the X chromosome Absence of protein FMRP Dysmorphia: Elongated face Large ears Results in a variety of Prominent forehead and jaw physical and behavioural symptoms Developmental disabilities Hyperactivity Attention deficit Visual-motor coordination Aggresiveness http://www.fragilex.org/html/home.shtml Evolution, Heredity, and Behaviour We all have 46 chromosomes, therefore, egg and sperm have 23 chromosomes each. When they unite, you get 46 all together (23+23=46) People with Down syndrome have 47 chromosomes (an extra 21st). Now picture the 47 chromosomes reduced, or divided in half. You get one "half" with 23 chromosomes (or a typical cell) and one with 24 (with the extra 21st chromosome) The grid bellow, shows you what the chances are for two people with Down syndrome to have babies (assuming that both individuals are fertile and able to procreate) © 2009 Pearson Education Canada Inc.  Sex Chromosomal Disorders Turner’s Syndrome: 1/10,000 females (monosomy X/abnormal X) XO (but mosaic types) Small, stubby fingers, sterile low spatial skills Klinefelter syndrome: 1/1000 males XXY, lanky - rounded, sterile, language/learning, feminine traits Supermale (XYY): 1/1000 males  May be taller, big teeth, may be developmentally delayed, more aggressive??? Superfemale (XXX): 1/1000 females Taller, but usu no physical/medical symptoms; may show Low IQ, slow development, so rarely diagnosed Heredity, Environment, and Development Behavioral Genetics Paths From Genes to Behavior © 2022 Pearson Education Canada Inc.  Behavioral Genetics Methods Selective Breeding (Humans’ Best Friend) -can only occur if trait is heritable Family Studies - correlates genetic overlap with similarity - problem of environmental overlap (confound) Twin Studies - compares MZ to DZ twins - equal environments assumption - representativeness assumption Adoption Studies - teases out environmental effects  Family Studies Evolution, Heredity, and Behaviour  Twin Studies Age-Related Changes in Concordance for MZ and DZ Twins Concordance in IQ changes in (a) identical, or monozygotic (MZ), twins and (b) fraternal, or dizygotic (DZ), twins from 3 months to 6 years of age.The scales are different to accommodate different ranges of scores.The important point is that changes in performance are more similar for monozygotic twins. Adapted from “The Louisville Twin Study: Developmental Synchronies in Behavior” by R. S.Wilson, 1983, Child Development, 54, p. 301. Copyright © 1983 by The Society for Research in Child Development, Inc. Adapted by permission. Models of Gene-Environment Interaction The behavioural consequences of genetic instructions depend on the environment in which those instructions develop Gottesman’s limit-setting model: Range of ability is determined by genes – actual value of that ability is determined by the environment (reaction range)  Paths From Genes to Behavior Reaction range: – the same genotype can produce a range of phenotypes in reaction to environment Example – PKU genotype: those who eat normal diet will be intellectually challenged, but those who eat special diet will have normal intelligence © 2022 Pearson Education Canada Inc. The range of phenotype in response to the environment as a function of specific genotype  Range of Reaction Model Figure 3.9 The reaction range concept, showing the simultaneous influences of genes and environment. Adapted from “Developmental Genetics and Ontogenetic Psychology: Overdue Détente and Propositions from a Matchmaker” by I. I. Gottesman, 1974. In A. D. Pick (Ed.), Minnesota Symposia on Child Psychology, vol. 8, p. 60, University of Minnesota Press. Copyright © 1974 by the University of Minnesota. Adapted by permission.  Paths From Genes to Behavior Heredity and environment interact dynamically throughout development Our experiences are influenced by the timing of when our genes are expressed (‘turned on’) throughout our lifespan The timing of genetic expression can be influenced by our experiences © 2022 Pearson Education Canada Inc.  Paths From Genes to Behavior Environmental influences typically make children within a family different Two key types of environmental influences – Shared: environmental influences shared by siblings – Nonshared: environmental influences that differ Parents provide the child’s genes and environment, but the child also influences her own environment  Paths From Genes to Behavior Genotype-Environment Interaction (Differential response) - introverts versus extraverts Genotype-Environment Correlation (Scarr) (Differential exposure) - passive (no action of child) - reactive (differential response) - active (niche picking) The Relation Between Genes and Environment © 2022 Pearson Education Canada Inc.