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Chapter 3. Heredity and Environment Dr. M. Davis-Brantley. The Genetic Code. Development that is dynamic, ongoing, interactional, and unique; just four chemicals are the basic building blocks of the genetic code. What Are Genes?.

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chapter 3

Chapter 3

Heredity and Environment

Dr. M. Davis-Brantley

the genetic code
The Genetic Code

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

what are genes
What Are Genes?
  • 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
what are genes cont
What Are Genes, cont.
  • 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
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
Zygote and Genotype
  • Male and female gametes fuse and become a zygote
  • Zygote begins process of duplication and division immediate
    • two reproductive cells have now become one
  • Genotype—the genetic information from the 46 chromosomes
    • set at human conception and endures through life
sex determination and sex ratio
Sex Determination and Sex Ratio
  • 23rd pair is the chromosome pair that determines the zygote’s sex
  • Female the 23rd pair is composed of XX chromosomes
    • Females are composed of XX so they will only produce XX
  • Male the 23rd 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
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 cont1
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
Multiple Zygotes

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

multiple zygotes cont
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 cont1
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
duplication division and differentiation
The zygote contains a complete set of instructions to create a person

Complex instructions on duplication, cell division, and differentiation

Duplication, Division, and Differentiation
gene gene interactions
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

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

dominant and recessive genes
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
from genotype to phenotype cont
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

Behavior genetics—study of effects of genes on behavior

chromosomal 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
chromosomal abnormalities cont
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.
down syndrome
Three chromosomes at gene #21 (trisomy-21)

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

Down Syndrome
abnormalities of the 23rd pair
Location of sex chromosome

Kleinfelters syndrome—XXY

seemingly normal child has delayed puberty

Turner’s Syndrome

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
Duplication and Division

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
Duplication and Division, cont.
  • 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

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
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
more complications
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
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
from genotype to phenotype
From Genotype to Phenotype

Every psychological characteristic is genetically influenced

Every psychological characteristic and personal trait is affected by the environment

behavior genetics1
Behavior Genetics

Behavior genetics—study of effects of genes on behavior

personality patterns, psychological disorders, and intellectual abilities


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
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)

chromosomal and genetic abnormalities
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