Chapter 3 Heredity and Environment Dr. M. Davis-Brantley

The Genetic Code n Development that is dynamic, ongoing, interactional, and unique; just four chemicals are the basic building blocks of the genetic code

What Are Genes? n Genes are made up of DNA—the complex protein code of genetic information n DNA directs the form and function of each body cell as it develops

What Are Genes, cont. n Each molecule of DNA is called a chromosome n Chromosomes contain instructions to make all the proteins a living being needs n The packet of instructions is called a genome n Each person has 23 sets of chromosomes, or 46 chromosomes n The human genome contains 30,000 genes

The Beginnings of Human Life n Gamete—reproductive cell that directs process by which genetic information combined and transmitted n Father gametes—sperm n Mother gametes—ovum

Zygote and Genotype n Male and female gametes fuse and become a zygote n Zygote begins process of duplication and division immediate –two reproductive cells have now become one n Genotype—the genetic information from the 46 chromosomes –set at human conception and endures through life

Sex Determination and Sex Ratio n 23 rd pair is the chromosome pair that determines the zygote’s sex n Female the 23 rd pair is composed of XX chromosomes –Females are composed of XX so they will only produce XX n Male the 23 rd pair is composed of XY chromosomes –Males are composed of XY so they can offer either X or Y –Therefore, critical factor in determination of sex is which sperm penetrates the ovum first

Sex Determination and Sex Ratio, cont. n Females always contribute one X n Males will have 1/2 of the sperm contributing an X and the other half contributing a Y n Critical factor in determining the sex of a zygote is which sperm reaches the ovum first

Sex Determination and Sex Ratio, cont. n Other factors include –rarely, male sperm may only carry either X or Y –sometimes a woman’s uterus either unusually alkaline or acid, giving either an X or Y sperm an advantage –in a stressful pregnancy XY embryos are more likely to be expelled than are XX embryos in a spontaneous abortion, or miscarriage –current sex ratio in United States is 52 males to 48 females

Multiple Zygotes n Monozygotic twins—identical twins (or quadruplets) originate from one zygote –share identical instructions –possibility of cloning –1/3 of twins monozygotic –Video--Black/White Twins Black/White TwinsBlack/White Twins

Multiple Zygotes, cont. n Dizygotic twins—from two separate zygotes –Dizygotic births occur once in every 60 births, and occur as frequently as 1 in 6 pregnancies, but usually only 1 twin develops past embryo stage

Multiple Zygotes, cont. n Dizygotic twins –women in late 30’s are three times more likely to have dizygotic twins n as menopause approaches, ovulation becomes irregular with some cycles producing no ovas and others producing multiple ovas –share no more genes than other offspring (about 50 percent) n 50 percent of the time one twin is male

n The zygote contains a complete set of instructions to create a person n Complex instructions on duplication, cell division, and differentiation Duplication, Division, and Differentiation

Gene - Gene Interactions n Multifactoral traits—inherited traits produced by interaction of genes and environment n Polygenetic traits—inherited traits produced by gene interaction n These are affected by on-off switching mechanisms, additive genes, and dominant-recessive genes

Additive Genes n Additive genes—one of a number of genes affecting a specific trait –each additive gene contributes to the trait n skin color and height are determined by them n every additive gene has some impact on a person’s phenotype –when genes interact this way, all the involved genes contribute fairly equally

n Nonadditive genes—phenotype shows one gene more influential than other genes n This is also referred to as the dominant-recessive pattern –gene showing the most influence is referred to as dominant –gene showing the least influence is referred to as recessive Dominant and Recessive Genes

From Genotype to Phenotype, cont. n Genotype—genetic potential n Phenotype—combination of genetic potential and expression –we are all carriers of the unexpressed genes n we can pass them along through the sperm or ova

Behavior Genetics n Behavior genetics—study of effects of genes on behavior

n A gamete with more than or less than 23 chromosomes creates a zygote with chromosomal abnormalities –most likely variable that creates chromosomal abnormalities is mother’s age (over 35) –father’s age (over 40) also a variable Chromosomal Abnormalities

n Most zygotes with chromosomal abnormalities never come to term –spontaneous abortion occurs in about one-half of all fetus with chromosomal abnormalities Chromosomal Abnormalities, cont.

n Three chromosomes at gene #21 (trisomy-21) n Syndrome—a cluster of distinct characteristics that occur together in a given disorder Down Syndrome

n Location of sex chromosome n Kleinfelters syndrome—XXY –seemingly normal child has delayed puberty n Turner’s Syndrome n Fragile X syndrome –hanging on by a thread (mutated gene) –intensifies from generation to generation Abnormalities of the 23rd Pair

Don’t worry about the following slides for the test. They are for additional information only and to help you understand the concepts better.

Duplication and Division n Zygote begins duplication and division within hours after conception –the 23 pairs of chromosomes duplicate, forming two complete sets of the genetic code for that person (zygote) –these two pair sets move toward the opposite sides of the zygote and the single cell in the zygote splits down the middle –the zygote’s outer membrane surrounds two cells, each containing a complete set of the original genetic code –these two cells then duplicate and divide to become four, then eight, and so on

Duplication and Division, cont. n by birth, your original zygote has duplicated and divided into 10 trillion cells... by adulthood, it’s 100 trillion cells n Every cell carries an exact copy of the complete genetic instructions inherited by the one-celled zygote

Differentiation n Not just any cell found in the zygote can become a person n At the 8-cell stage a third process, differentiation, occurs n Cells begin to specialize –they take different forms –they reproduce at different rates, depending on where in the growing mass they are located

Differentiation, cont. n Certain genes affect differentiation by switching other genes on and others off so that the other genes produce the right proteins at the right times—on-off switching mechanisms n Genotype—inheritance that can be observed or is expressed

n Genes direct the creation of 20 amino acids that produce thousands of proteins forming the body’s structure and directing biochemical functions –proteins of each body cell are continually affected by other proteins, nutrients, and toxins that influence the cell functioning More Complications

More Complications, cont. n genetic imprinting—tendency of certain genes to be expressed differently when inherited from mother than from father (tagging) –some of the genes which influence height, insulin production, and several forms of mental retardation affect a child differently depending on which parent they came from

From Genotype to Phenotype n Every psychological characteristic is genetically influenced n Every psychological characteristic and personal trait is affected by the environment

Behavior Genetics n Behavior genetics—study of effects of genes on behavior –personality patterns, psychological disorders, and intellectual abilities

Alcoholism n Inherited biochemistry makes some people highly susceptible to alcohol addiction –addictive pull can be overpowering, or weak, or something in the middle –may explain ethnic variations

Alcoholism, cont. n Not simply a biochemical reaction—it is psychological and physical, and biological; thus alcoholism is polygenetic, with alcoholics inheriting a combination of biochemistry- affecting and temperament-affecting genes n Culture counts too(whether alcohol is present in environment)

n We now give attention to these because we can recognize –disruptions of normal development –origins of genetic and chromosomal abnormalities –misinformation and prejudice add to problems of people with these abnormalities Chromosomal and Genetic Abnormalities