Chapter 3.docx

Chapter 3: Hereditary influences on development  Principles of hereditary transmission - Conception: the moment when an ovum released by a woman’s ovary and on its way to the uterus via the fallopian tube is fertilized by a man’s sperm to form a zygote  The genetic material  First development that occurs after conception is protective: when a sperm cell penetrates the lining of the ovum, a biochemical reaction repels other sperm prevent them form repeating fertilization  Phenotype: the ways in which a person’s genotype is expressed in observable or measurable characteristics  Genotype: the genes that a person inherits  Zygote: a single cell formed at conception from the union of sperm and ovum  Chromosome: a threadlike structure made up of genes; in humans there are 46 chromosomes in the nucleus of each body cells  Genes: hereditary blueprints for development that are transmitted unchanged from generation to generation (units that builds single protein) - Genes produce enzymes and other proteins that are necessary for creation and functioning of new cells, and regulate the timing of development - Internal and external environmental influence how genes function  Deoxyribonucleic acid (DNA): long, double –stranded molecules that make up chromosomes. It can duplicate itself= makes it possible for one-celled zygote to develop into a marvelously complex human being  Growth of the zygote and production of body cells  Mitosis: the process in which a cell duplicates its chromosomes and then divides into two genetically identical daughter cells. Step 1: original parent cells Step 2: each chromosome splits lengthwise, producing a duplicate Step 3: the duplicate sets of chromosomes move to opposite ends of the parent cell, which then begins to divide Step 4: the cell completes its division, producing two daughter cells that have identical sets of chromosomes  The germ (or sex) cells  Function: to produce gametes (sperm or ova)  Production of gametes through meiosis  Meiosis: the process in which a germ ell divides production gametes that each contain half of the parent cell’s original complement of chromosomes; in humans the products of meiosis contain 23 chromosomes. 1 Step 1: Each of the germ cell’s original chromosomes duplicates itself and the duplicate remains attached Step 2: Crossing-over takes place among adjacent chromosomes, thus creating new hereditary combinations (crossing-over: process in which genetic material is exchanged between pairs of chromosomes) Step 3: The original cell now divides to form 2 parent cells each of which has 23 duplicated chromosomes Step 4: Finally, each chromosome and its duplicate now split and segregate into separate gametes. Thus, each gamete has only half the chromosome of its parent cell.  Hereditary uniqueness  Independent assortment: the principle stating that each pair of chromosomes segregates indecently of all other chromosome pairs during meiosis  Multiple births  Monozygotic (identical) twins: twins who develop from a single zygote that later divides to form two genetically identical individuals (Occurs in about 2 of every 250 births around the world)  Dizygotic (or fraternal) twins: twins that result when a mother release two ova at roughly the same time and each is fertilized by a different sperm, producing two zygote that are genetically different (Occurs in about 1 of every 125 births) - Sex is determined by the 23 pair (sex chromosome) - Female 23 chromosome= XX - Male 23 chromosome= XY - Father determines the sex of their children, depending on whether the sperm that fertilizes the ova contains X or a Y chromosome  Male or female?  Autosomes: the 22 pairs of human chromosomes that are identical in males and females  X chromosome: the longer of the two sex chromosomes; normal females have two X chromosomes, whereas normal males have but one  Y chromosome: the shorter of the two sex chromosomes; normal males have one Y chromosome, whereas females have none  What do genes do?  Production of amino acids; forms enzymes and other proteins that are necessary for the formation and function of new cells  Genes regulate the production of pigment melanin in iris of the eye  Guide cell differentiation  Influence and are influenced by biochemical environment surrounding them during development 2  Some are responsible for regulating the pace and timing of development  Environmental factors clearly influence how genes function (combine with genetic influences to determine how a phenotype is translated into a particular phenotype (feel, look, thinks, behaves) Environment affects actions of genes at several levels: 1. Within the nucleus may affect the expression of genetic expression (Molecular) 2. The internal environment that surrounds the cell may affect the genes Expression (cellular) 3. External environment affects the expression of the genetic material (organism- environment, experience-expectant, experience-dependent)  How are genes expressed?  Single-gene inheritance patterns 1. Simple dominant recessive inheritance:  Alleles: alternative forms of a gene that can appear at a particular site on a chromosome (influence human characteristics by one pair of genes: one from mother one from father)  Simple dominant-recessive inheritance: a pattern of inheritance in which one allele dominates another so that only its phenotype is expressed  Dominant allele: a relatively powerful gene that is not expressed phenotypically when paired with a dominant allele  Recessive allele: a less powerful gene that is not expressed phenotypically when paired with a dominant allele  Homozygous: having inherited two allele for an attribute that are identical in their effects  Heterozygous: having inherited two alleles for an attribute that have different effects  Carrier: a heterozygous individual who displays no sign of a recessive allele in his or her own phenotype but can pass this gene to offspring (See figure 3.6 for an example, p.82) (Also see box 3.1 for a list of common dominant and recessive traits p.83) 2. Codominance:  Some genes are codominant: the phenotype they produce is a compromise between the two genes (E.g. human blood alleles A and B are equally expressive and neither dominates the other)  Codominance: condition in which two heterozygous but equally powerful alleles produce a phenotype in which both genes are fully and equally expressed  Other type of codominance happens when one of 2 heterozygous alleles is stronger than the other but fails to mask all its effects (e.g. sickle cell trait; problem because they cluster together, distribute less oxygen) - Consequence more severe for people who inherit two recessive sickle cell genes they develop: sickle cell anemia: a genetic blood disease that causes red blood cells to assume an unusual sickled shape and to become inefficient at distributing oxygen 3 3. Sex-linked inheritance:  Sex linked characteristics: an attribute determined by a recessive gene that appears on the X chromosome; more likely to characterize males (E.g. red/green color blindness, hemophilia) - Female cannot be color blond unless both of her X chromosomes contain a recessive gene for color blindness 4. Polygenic inheritance:  Polygenic traits: a characteristic that is influenced by the action of many genes rather than a single pair (E.g. height, weight, intelligence, skin color, temperament and susceptibility to cancer) (See figure 3.9 for an example)  Hereditary disorders Congenital defect: a problem that is present (though not necessarily apparent) at birth; such defects may stem from genetic and prenatal influences or from complications of the birth process (E.g. gene that produces Huntington’s disease is present from the moment of conception)  Chromosomal abnormalities - In meiosis, when 46 chromosomes are distributed into sperm or ova, its sometimes uneven (if germ cells are conceived the majority of these chromosomal abnormalities are lethal, fail to develop or will be aborted)  Abnormalities of the sex chromosomes  Many involves 23 pair (sex chromosomes)  Male born with extra X or Y = XXY or XYY  Female may survive if they inherit single X (XO) or even XXX, XXXX, XXXXX  Abnormalities of the autosome  Attributable to autosomes (22 pairs of chromosomes that are similar in male and female)  More common: abnormal sperm or ovum carrying an extra autosome combines with a normal gamete to form a zygote that has 47 chromosomes= trisomy)  Most frequent is the Down syndrost: a chromosomal abnormality (trisomy 21) caused by the presence of an extra 21 chromosome; people with this syndrome have distinctive physical appearance and are moderately to severely mentally retarded  Genetic abnormalities  Problem will not appear unless both parents carry harmful allele and child inherits this gene from each parent (exception are sex-linked defects that a mal child will display the recessive alleles for the traits appear on chromosome X that he inherited from the mother  See table 3.3 for description of major recessive hereditary diseases  Mutations: a change in the chemical structure or arrangement of one or more genes that has the effect of producing a new phenotype - Some are harmful and fatal, can include environment hazards 4  Predicting, detecting, and treating hereditary disorders  Predicting hereditary disorders  Genetic counseling: is a service designed to inform prospective parents about genetic diseases and to help them determine the likelihood that they would transmit such disorders to their children - Refers to the prediction of both chromosomal abnormalities and genetic abnor