Human Growth and

Development

Chapter Three

Heredity and Environment

PowerPoints prepared by Cathie Robertson, Grossmont CollegeRevised by Jenni Fauchier, Metropolitan Community College

The Genetic Code

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

What Genes Are

• Genes are made up of DNA—the complex protein code of genetic information

• DNA directs the form and function of each body cell as it develops

• Each molecule of DNA is called a chromosome

• Chromosomes contain instructions to make all the proteins a living being needs

• The packet of instructions is called a genome

• Each person has 23 sets of chromosomes, or 46 chromosomes

• The human genome contains 30,000 genes

The Beginnings of Human Life

• Gamete—reproductive cell that directs process by which genetic information combined and transmitted

• Father gametes—sperm• Mother gametes—ovum

Zygote and Genotype

• Male and female gametes fuse and become a zygote

• Zygote begins process of duplication and division– two reproductive cells

• Genotype—the genetic information from the 46 chromosomes– set at human conception and endures

through life

Sex Determination and Sex Ratio

• Of 22 out of 23 pairs of human chromosome, the matching chromosomes are very closely matched– but not identical

•some genes come in slight, normal variations called alleles

• The 23rd pair is different– in females, it is designated XX– in males, it is designated XY

Sex Determination and Sex Ratio, cont.

• Females always contribute one X• Males will have 1/2 of the sperm

contributing an X and the other half contributing a Y

• Critical factor in determining the sex of a zygote is which sperm reaches the ovum first

Sex Determination and Sex Ratio, cont.

• 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

• Monozygotic twins—identical twins (or quadruplets) originate from one zygote– share identical instructions– possibility of cloning– 1/3 of twins monozygotic

Multiple Zygotes, cont.

• 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.

• Dizygotic twins– women in late 30’s are three times more

likely to have dizygotic twins•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)•50 percent of the time one twin is male

• The zygote contains a complete set of instructions to create a person

• Complex instructions on duplication, cell division, and differentiation

Duplication, Division, and Differentiation

• 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• by birth, your original zygote has

duplicated and divided into 10 trillion cells . . . by adulthood, it’s 100 trillion cells

• Every cell carries an exact copy of the complete genetic instructions inherited by the one-celled zygote

Differentiation

• Not just any cell found in the zygote can become a person

• At the 8-cell stage a third process, differentiation, occurs

• 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.

• 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

• Genotype—inheritance that can be observed or is expressed

Gene - Gene Interactions

• Multifactoral traits—inherited traits produced by interaction of genes and environment

• Polygenetic traits—inherited traits produced by gene interaction

• These are affected by on-off switching mechanisms, additive genes, and dominant-recessive genes

Additive Genes

• Additive genes—one of a number of genes affecting a specific trait– each additive gene contributes to the trait

•skin color and height are determined by them

•every additive gene has some impact on a person’s phenotype

– when genes interact this way, all the involved genes contribute fairly equally

• Nonadditive genes—phenotype shows one gene more influential than other genes

• 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

• X-linked genes—located on X chromosome

– if recessive gene is X-linked, that it is on the X chromosome is critical

– female has one X chromosome and one Y; males have only 2 X •females’ recessive genes can be counterbalanced

by Y chromosome, but males’ recessive genes can not because they have only the X chromosome

– if X-linked gene recessive, may cause color blindness, many allergies, several diseases, and learning disabilities

• 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.

• 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

• Since each gamete contains only 23 chromosomes, why is every conception genetically unique?– 8 million chromosomally different ova

x 8 million of the same = 64 trillion different possibilities of children from each couple

Mechanisms of Genetic Diversity

• Genetic diversity safeguards human health

• Minute differences can affect the ability to stave off certain diseases

• Genetic diversity maintains the species

Health Benefits of Genetic Diversity

From Genotype to Phenotype

• Every psychological characteristic is genetically influenced

• Every psychological characteristic and personal trait is affected by the environment

From Genotype to Phenotype, cont.

• Genotype—genetic potential• Phenotype—combination of

genetic potential and expression– we are all carriers of the

unexpressed genes•we can pass them along through

the sperm or ova

Behavior Genetics

• Behavior genetics—study of effects of genes on behavior– personality patterns, psychological

disorders, and intellectual abilities

• Most common and feared type of senility is Alzheimer’s disease– amyloid B protein accumulates in the brain,

leading to dysfunction and destruction of brain cells and disruption of the mind

• Can be genetic—but only when “early-onset”

Senility Caused by Alzheimer’s Disease

Senility Caused by Alzheimer’s Disease, cont.

• If “late-onset,” may be a combination of genes and environment– other predictors may include hypertension,

diabetes, high cholesterol, diet, exercise, not smoking, weight control, mental alertness, and physical health

Alcoholism

• 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.

• 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

• Culture counts too(whether alcohol is present in environment)

• 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

• 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

• 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.

• Three chromosomes at gene #21 (trisomy-21)

• Syndrome—a cluster of distinct characteristics that occur together in a given disorder

Down Syndrome

• Location of sex chromosome• Kleinfelters syndrome—XXY

– seemingly normal child has delayed puberty

• Fragile X syndrome– hanging on by a thread (mutated gene)– intensifies from generation to generation

Abnormalities of the 23rd Pair

• Individuals with a parent, sibling, or child with a serious genetic condition known to be dominant or recessive

• Couples with history of early spontaneous abortions, stillbirths, or infertility

• Couples from the same ethnic group or subgroup—especially if closely related

• Women over 35 and men over 40

Genetic Testing and Genetic Counseling

• Counselor constructs couples’ family history– charts patterns of health and illness

over generations

• Some tests provide information before conception

The Process of Genetic Counseling

The Process of Genetic Counseling, cont.

• Other tests are prenatal– alpha-fetoprotein assay– ultrasound (AKA sonogram)– amniocentesis – chorionic villi sampling– pre-implantation testing (used in in vitro

fertilization)– gamete selection; ova/and or sperm are

screened to select ones free of particular problems

• Many want to know ahead of time

• Some do not• There is a more knowledge of

what is to come—or not

A Basis for Decision

Alternatives

• If both partners are carriers of a serious condition or are at high risk because of age or family characteristics, they may turn to– in-vitro fertilization (IVF)– gamete intrafallopian transfer (GIFT)– zygote intrafallopian transfer (ZIF)– artificial insemination donor (AID)– postponement of pregnancy until promising

treatments are further developed