WHAT WILL YOU KNOW? What is the relationship between genes and chromosomes? Is twinning genetic or can a person choose to have twins? How could a person have the gene for something that is never apparent? If both parents are alcoholics, will their children be alcoholics too? Why are some children born with Down syndrome?
What Genes Are Protein Contains sequence of chemicals consisting of long string of amino acids Consists of instructions for stringing together the right amino acids in the right order via pairs of four chemicals called bases
What Genes Are All living things are composed of cells that promote growth and sustain life according to instructions in their molecules of DNA DNA (deoxyribonucleic acid) Molecule that contains the chemical instructions for cells to manufacture various proteins Promotes growth and sustains life Chromosomes Molecules of DNA Consists of 46 chromosomes arranged in 23 pairs
What Genes Are Allele Variation of a gene or any of the possible forms in which a gene for a particular trait can occur Effects of variations vary greatly from causing lifethreatening conditions to having no detectable effect at all Genetic diversity Distinguishes each person Allows the human species to adapt to pressures of the environment
What Genes Are Methylation Processes additional DNA and RNA that enhances, transcribes, connects, and alters genes Genome Involves the full set of genes that are the instructions to make an individual member of a certain species
How Proteins Are Made
Variations Differences begin with genes Single nucleotide polymorphism (SNP) gene Two, three, or more versions May be inconsequential or destructive
Two Parents, Millions of Gametes Zygote Two gametes (sperm and ovum) combine and produce a new individual with 23 chromosomes from each parent. Gametes Reproductive cells (sperm and ova) Each gamete consists of 23 chromosomes. This is a computer illustration of a small segment of one gene. Even a small difference in one gene can cause major changes in a person s phenotype.
The Beginnings of Life Matching genes Genotype Organism s entire genetic inheritance, or genetic potential. Homozygous Two genes of one pair that are exactly the same in every letter of their code Heterozygous Two genes of one pair that differ in some way The Moment of Conception This ovum is about to become a zygote.
Uncertain Sex Every now and then, a baby is born with ambiguous genitals, meaning that the child's sex is not abundantly clear. When this happens, a quick analysis of the chromosomes is needed, to make sure there are exactly 46 and to see whether the 23rd pair is XY or XX. The karyotypes shown here indicate a normal baby boy (left) and girl (right).
Male or Female? Humans usually possess 46 chromosomes. 44 autosomes and 2 sex chromosomes Females: XX Males: XY Sex of offspring depends on whether the father's Y sperm or X sperm fertilizes the ovum.
Determining a Zygote s Sex
Too Many Boys? Is sex selection the parents right or a social wrong? Preference for boys in many areas of world My Strength, My Daughter That s the slogan these girls in New Delhi are shouting at a demonstration against abortion of female fetuses in India. Ways to prevent female birth Inactivating X sperm before conception In vitro fertilization (IVF) Aborting XX fetuses
New Cells, New Functions Duplication and division begins within hours after conception 23 pairs of chromosomes carrying all the genes duplicate, forming two complete sets of the genome Two cells duplicate and divide, becoming four, which themselves duplicate and divide, becoming eight, and so on Two sets move toward opposite sides of the zygote, and the single cell splits neatly down the middle into two cells, each containing the original genetic code
First Stages of the Germinal Period ANATOMICAL TRAVELOGUE/SCIENCE SOURCE (a) (b) (c) The original zygote as it divides into (a) two cells, (b) four cells, and (c) eight cells. Occasionally at this early stage, the cells separate completely, forming the beginning of monozygotic twins, quadruplets, or octuplets.
New Cells, New Functions At birth Zygote becomes 10 trillion cells Each cell carries complete copy of genetic instructions Stem cells Result from early duplication and division Are able to produce any other cell Placenta Allows early genetic testing
Siblings and Twins Monozygotic (identical) twins Originate from one zygote that splits apart very early in development Incomplete split results in conjoined twins. Same genotype but slight variations in phenotype are possible due to environmental influences. Dizygotic (fraternal) twins Result from fertilization of two separate ova by two separate sperm. Dizygotic twins have half their genes in common and occur twice as often as monozygotic twins. Incidence is genetic and varies by ethnicity and age.
Assisted Reproduction Assisted reproductive technology (ART) General term for the techniques designed to help infertile couples conceive and sustain a pregnancy in vitro fertilization (IVF) Involves fertilization that takes place outside a woman s body Involves mixing sperm with surgically removed ova from the woman s ovary and implanting zygote into a woman s uterus
Male/Female Differences One-third of all fertility problems originate in woman; another third from man; final third from unknown causes. Fertility decreases with age; faster decline for women Infertility: Failure to conceive a child after a year of trying. Subfertile: Less fertile than ideal; not sterile
From Genotype to Phenotype Phenotype Observable characteristics of an organism, including appearance, personality, intelligence, and all other traits Polygenic Trait influenced by many genes Multifactorial Trait affected by many factors, both genetic and environmental, that enhance, halt, shape, or alter the expression of genes, resulting in a phenotype that may differ markedly from the genotype.
From Genotype to Phenotype Epigenetic Environmental factors that affect genes and genetic expression enhancing, halting, shaping, or altering the expression of genes and resulting in a phenotype that may differ markedly from the genotype
Gene-Gene Interactions Human Genome Project Involves international effort to map the complete human genetic code Was essentially completed in 2001, though analysis is ongoing Some interesting findings Humans have far fewer genes than previously believed All living creatures share many genes
Gene-Gene Interactions Additive genes Genes that add something to some aspect of the phenotype Partly dependent on all inherited genes Example Height is affected by the contributions of about 180 genes
Gene-Gene Interactions Dominant-recessive heredity Dominant gene is far more influential than the recessive gene (non-additive). Dominant gene can completely control the phenotype with no noticeable effect of recessive gene. Effect of recessive genes can sometimes be noticed.
A Changeling? No. Many brown-eyed people carry a recessive gene for blue eyes. The only way to know for sure is if they have a blue-eyed parent (who gives one gene for blue eyes to every child) or if they have a blue-eyed child. Other recessive genes include red hair, Rh negative blood, and many genetic diseases.
More Complications Copy number variations Occur in abundance Involve genes with various repeats or detections of base pairs May be inconsequential, lethal, or something in between Parental imprinting Occurs when genes from either mother or father are affected May differ for XX versus XY embryos Syndrome examples: Prader-Willi syndrome and Angelman syndromes
Genotype and Phenotype Current consensus Genes affect every aspect of behavior Most environment influences on children raised in the same home are not shared Genes elicit responses that shape development Lifelong, people choose friends and environments that encourage their genetic predispositions
Genotype and Phenotype Alcoholism Genes create addictive pull Alcoholism is polygenic and culture is pivotal Is He Drunk? Risk Biological sex Gender Contexts Ethnicity
Nature and Nurture Connected: Alcoholism Genes can cause an overpowering addictive pull in some people. Inherited psychological traits affect alcoholism. Biological sex (XX or XY) and gender (cultural) also affect the risk of alcoholism. Environmental conditions can modify the genetic effects. Nature and nurture must combine to create an alcoholic.
Genotype and Phenotype Nearsightedness Age: Limited focus at birth; change in eyeball shape at puberty; myopia at middle age Genes: Alleles of Pax6 contribute to nearsighted Heritability: Heritability of 90% Other causes Environment Homework and lack of play
Chromosomal and Genetic Problems Why study conditions caused by extra chromosome or single destructive gene? They provide insight into the complexities of nature and nurture. Knowing their origins helps limit their effects. Information combats prejudice: Difference is not always deficit.
Chromosomal and Genetic Problems Not exactly 46 Some gametes have more or fewer than 23 chromosomes Some times only part of 23 rd chromosome missing Daniel has trisomy-21, and this photograph was taken at the only school in Chile where normal and special-needs children share classrooms. Down syndrome Called trisomy-21 because the person has three copies of chromosome 21 Involves around 300 distinct characteristics from third chromosome; unique individuals
Chromosomal and Genetic Problems Dominant disorders 7,000 known single-gene disorders are dominant Evident in phenotype Rare because people rarely live long enough to reproduce Exceptions Huntington disease: Fatal CNS disorder caused by genetic miscode of more than 35 repetitions of particular triplet Rare type of early onset (before age 60) Alzheimer disease
Chromosomal and Genetic Problems Recessive disorders Millions of different types, lethal conditions are rare Several types are sexlinked Fragile X syndrome Caused by more than 200 repetitions of one triplet on one gene Most common form of inherited form of inherited mental retardation Sickle-cell trait Offers some protection against malaria African carriers are more likely than non-carriers to survive Cystic fibrosis More common among people with northern European ancestors Carriers may have been protected against cholera
Genetic Testing for Psychological Disorders Genes are part of every psychological disorder Few scientists advocate genetic testing for schizophrenia In several U.S. states selling genetic tests is illegal Some testing is legally required
Genetic Testing for Psychological Disorders Genetic counseling Involves consultation and testing by trained experts Enable individuals to learn about their genetic heritage Helps couples clarify values and determine genetic risks Ethical rules Tests are confidential Decisions are made by the clients, not the counselors