Biological Foundations 2024 PDF

Biological Foundations Section C Fall 2024 GENETIC FOUNDATIONS ENVIRONMENTAL CONTEXTS FOR DEVELOPMENT Think about physical and behavioural similarities between you and your parents Did you show combined features of both parents, or resembles just on...

Biological Foundations Section C Fall 2024 GENETIC FOUNDATIONS ENVIRONMENTAL CONTEXTS FOR DEVELOPMENT Think about physical and behavioural similarities between you and your parents Did you show combined features of both parents, or resembles just one parent, or not like either parent?  The directly observable characteristics are called phenotypes They depend in part on the individual’s genotype—the complex blend of genetic information that determines our species and influences all our unique characteristics Phenotypes are also affected by each person’s lifelong history of experiences  What are genes, and how are they transmitted from one generation to the next? GENETIC Describe various patterns of gene–gene interaction FOUNDATIONS Describe major chromosomal abnormalities, and explain how they occur Chromosomes Within each of the trillions of cells in the human body* is a nucleus, Contains rod- like structures called chromosomes, which store and transmit genetic information Human chromosomes come in 23 matching pairs Each member of a pair corresponds to the other in size, shape, and genetic functions One chromosome is inherited from the mother and one from the father The Genetic Code Chromosomes are made up of a chemical substance called deoxyribonucleic acid, or DNA DNA is a long, double-stranded molecule that looks like a twisted ladder Each rung of the ladder consists of a specific pair of chemical substances called bases, joined together between the two sides. It is this sequence of base pairs that provides genetic instructions A gene is a segment of DNA along the length of the chromosome. Genes can be of different lengths— perhaps 100 to several thousand ladder rungs long  An estimated 21,000 protein-coding genes, which directly affect our body’s characteristics, lie along the human chromosomes They send instructions for making a rich assortment of proteins to the cytoplasm, the area surrounding the cell nucleus Proteins, which trigger chemical reactions throughout the body, are the biological foundation on which our characteristics are built An additional 18,000 regulator genes modify the instructions given by protein- coding genes, greatly complicating their genetic impact (Pennisi, 2012)  So even at this microscopic level, biological events of profound developmental significance are the result of both genetic and nongenetic forces. The Sex Cells New individuals are created when two special cells called gametes, or sex cells—the sperm and ovum—combine A gamete contains only 23 chromosomes, half as many as a regular body cell Gametes are formed through a cell division process called meiosis, which halves the number of chromosomes normally present in body cells When sperm and ovum unite at conception, the resulting cell, called a zygote, will again have 46 chromosomes  Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. This process is required to produce egg and sperm cells for sexual reproduction In meiosis, the chromosomes pair up and exchange segments, so that genes from one are replaced by genes from another Then chance determines which member of each pair will gather with others and end up in the same gamete  These events make the likelihood extremely low—about 1 in 700 trillion—that non- twin siblings will be genetically identical (Gould & Keeton, 1996) The genetic variability produced by meiosis is adaptive: It increases the chances that at least some members of a species will cope with ever- changing environments and will survive Boy or Girl? 22 of the 23 pairs of chromosomes are matching pairs, called autosomes (meaning not sex chromosomes). The twenty- third pair consists of sex chromosomes In females, this pair is called XX; in males, it is called XY. The X is a relatively long chromosome, whereas the Y is short and carries little genetic material  When gametes form in males, the X and Y chromosomes separate into different sperm cells The gametes that form in females all carry an X chromosome Therefore, the sex of the new organism is determined by whether an X-bearing or a Y- bearing sperm fertilizes the ovum Which pair of siblings are twins?  Fraternal, or dizygotic, twins, the most common type of multiple offspring, resulting from the release and fertilization of two ova Sometimes a zygote that has started to duplicate separates into two clusters of cells that develop into two individuals. These are called identical, or monozygotic, twins because they have the same genetic makeup The frequency of identical twins is the same around the world— about 1 in every 350 to 400 births (Kulkarni et al., 2013) Patterns of Gene–Gene Interactions Image taken from www.cnn.com  Chromosomes come in matching pairs. Two forms of each gene occur at the same place on the chromosomes, one inherited from the mother and one from the father Each form of a gene is called an allele. If the alleles from both parents are alike, the child is homozygous and will display the inherited trait If the alleles differ, then the child is heterozygous, and relationships between the alleles influence the phenotype  Dominant–Recessive Pattern In many heterozygous pairings, dominant–recessive inheritance occurs: Only one allele affects the child’s characteristics It is called dominant; the second allele, which has no effect, is called recessive  The allele for brown eye is dominant (we can represent it with a capital D), whereas the one for blue eye is recessive (symbolized by a lowercase b) A child who inherits a homozygous pair of dominant alleles (DD) and a child who inherits a heterozygous pair (Db) will both be brown eyed, even though their genotypes differ Blue eye can result only from having two recessive alleles (bb)  Still, heterozygous individuals with just one recessive allele (Db) can pass that trait to their children Therefore, they are called carriers of the trait  Recessive Disorder Recessive disorder is phenylketonuria, or PKU, affects the way the body breaks down proteins contained in many foods Infants born with two recessive alleles lack an enzyme that converts one of the basic amino acids that make up proteins (phenylalanine) into a by-product essential for body functioning (tyrosine) Without this enzyme, phenylalanine quickly builds to toxic levels that damage the central nervous system. By 1 year, infants with PKU suffer from permanent intellectual disability Only rarely are serious diseases due to dominant alleles. Think about why this is so?  Children who inherit the dominant allele always develop the disorder They seldom live long enough to reproduce, so the harmful dominant allele is eliminated from the family’s heredity in a single generation  Some dominant disorders, however, do persist One is Huntington’s disease, a condition in which the central nervous system degenerates Its symptoms usually do not appear until age 35 or later, after the person has passed the dominant allele to his or her children. Incomplete- Dominance Pattern In some heterozygous circumstances, the dominant– recessive relationship does not hold completely Incomplete dominance, a pattern of inheritance in which both alleles are expressed in the phenotype, resulting in a combined trait, or one that is intermediate between the two  The sickle cell trait, a heterozygous condition present in many black Africans, provides an example. Sickle cell anemia occurs in full form when a child inherits two recessive genes They cause the usually round red blood cells to become sickle (crescent-moon) shaped, especially under low- oxygen conditions The sickled cells clog the blood vessels and block the flow of blood, causing intense pain, swelling, and tissue damage Heterozygous individuals are protected However, when they experience oxygen from the disease under most deprivation—for example, at high circumstances altitudes or after intense physical exercise—the single recessive allele asserts itself, and a temporary, mild form of the illness occurs The sickle cell allele is common among black Africans for a special reason For example, only 8 percent of African Americans are carriers, compared with 20 percent of black Africans (Centers for Disease Control and Prevention, 2015m) X-Linked Pattern But when a harmful allele is carried on the X chromosome, X- linked inheritance applies Males are more likely to be affected  Because their sex chromosomes do not match In females, any recessive allele on one X chromosome has a good chance of being suppressed by a dominant allele on the other X But the Y chromosome is only about one-third as long and therefore lacks many corresponding genes to override those on the X A well-known example is hemophilia, a disorder in which the blood fails to clot normally Genomic Imprinting In genomic imprinting, alleles are imprinted, or chemically marked through regulatory processes within the genome, in such a way that one pair member (either the mother’s or the father’s) is activated, regardless of its makeup (Hirasawa & Feil, 2010) The imprint is often temporary; it may be erased in the next generation, and it may not occur in all individuals The number of genes subjected to genomic imprinting is believed to be small—less than 1 percent  Still, these genes have a significant impact on brain development and physical health, as disruptions in imprinting reveal For example, imprinting is involved in several childhood cancers and in Prader-Willi syndrome, a disorder with symptoms of intellectual disability and severe obesity  Genomic imprinting can operate on the sex chromosomes, as fragile X syndrome—the most common inherited cause of intellectual disability An abnormal repetition of a sequence of DNA bases occurs on the X chromosome, damaging a particular gene cognitive impairments attention deficits and high anxiety symptoms of autism Mutation How are harmful genes created in the first place? The answer is mutation, a sudden but permanent change in a segment of DNA A mutation may affect only one or two genes, or it may involve many genes spontaneously, by chance hazardous environmental agents Although nonionizing forms of radiation— electromagnetic waves and microwaves—have no demonstrated impact on DNA, ionizing (high- energy) radiation is an established cause of mutation Polygenic Inheritance Characteristics that vary on a continuum among people, such as height, weight, intelligence, and personality These traits are due to polygenic inheritance, in which many genes affect the characteristic in question Polygenic inheritance is complex, and much about it is still unknown Chromosomal Abnormalities Most chromosomal defects result from mistakes during meiosis, when the ovum and sperm are formed A chromosome pair does not separate properly, or part of a chromosome breaks off Produce many physical and mental symptoms  Down Syndrome The most common chromosomal disorder, occurring in 1 out of every 700 live births, is Down syndrome In 95 percent of cases, it results from a failure of the twenty-first pair of chromosomes to separate during meiosis, so the new individual receives three of these chromosomes rather than the normal two Or, an extra broken piece of a twenty-first chromosome is attached to another chromosome (called translocation pattern)  The consequences of Down syndrome include intellectual disability memory and speech problems limited vocabulary slow motor development The disorder is also associated with distinct physical features a short, stocky build a flattened face a protruding tongue almond-shaped eyes Also, may have, eye cataracts, hearing loss, and heart and intestinal defects (U.S. Department of Health and Human Services, 2015) Abnormalities of the Sex Chromosomes In contrast, sex chromosome disorders often are not recognized until adolescence when, in some deviations, puberty is delayed The most common problems involve the presence of an extra chromosome (either X or Y) or the absence of one X in females  Research has discredited a variety of myths about individuals with sex chromosome disorders For example, males with XYY syndrome are not necessarily more aggressive and antisocial than XY males (Stochholm et al., 2012) And most children with sex chromosome disorders do not suffer from intellectual disability  Verbal difficulties—for example, with reading and vocabulary—are common among girls with triple X syndrome and boys with Klinefelter syndrome, both of whom inherit an extra X chromosome In contrast, girls with Turner syndrome, who are missing an X, have trouble with spatial relationships (Otter et al., 2013; Ross et al., 2012; Temple & Shephard, 2012) Brain-imaging evidence confirms that adding to or subtracting from the usual number of X chromosomes alters the development of certain brain structures, yielding particular intellectual deficits (Bryant et al., 2012; Hong et al., 2014)