Chapter Three

1
Chapter Three

• Heredity and Environment

2
What Are Genes?

• Genes are made up of DNAthe 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

3
The Beginnings of Human Life

• Gametereproductive cell that directs process by
  which genetic information combined and
  transmitted
• Father gametessperm
• Mother gametesovum

4
Zygote and Genotype

• Male and female gametes fuse and become a zygote
• Zygote begins process of duplication and division
• two reproductive cells
• Genotypethe genetic information from the 46
  chromosomes
• set at human conception and endures through life

5
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

6
Sex Determination and Sex Ratio

• 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

7
Sex Determination and Sex Ratio

• Other factors include
• rarely, male sperm may only carry either X or Y
• sometimes a womans 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 the XX embryos in
  a spontaneous abortion, or miscarriage
• current sex ratio in United States is 52 males to
  48 females

8
Multiple Zygotes

• Monozygotic twinsidentical twins (or
  quadruplets) originate from one zygote
• share identical instructions
• possibility of cloning
• 1/3 of twins monozygotic
• Dizygotic twinsfrom 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
• Dizygotic twins
• women in late 30s 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

9
Additive Genes

• Additive genesone 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 persons
  phenotype
• when genes interact this way, all the involved
  genes contribute fairly equally

10
Dominant and Recessive Genes

• Nonadditive genesphenotype 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

11
Dominant and Recessive Genes

• X-linked geneslocated 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

12
From Genotype to Phenotype

• Every psychological characteristic is genetically
  influenced
• Every psychological characteristic and personal
  trait is affected by the environment
• Genotypegenetic potential
• Phenotypecombination of genetic potential and
  expression
• we are all carriers of the unexpressed genes
• we can pass them along through the sperm or ova

13
Behavior Genetics

• Behavior geneticsstudy of effects of genes on
  behavior
• personality patterns, psychological disorders,
  and intellectual abilities
• Senility Caused by Alzheizmers
• Most common and feared type of senility is
  Alzheimers disease
• amyloid B protein accumulates in the brain,
  leading to dysfunction and destruction of brain
  cells and disruption of the mind
• Can be geneticbut only when early-onset
• 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

14
Genetic Testing and Genetic Counseling

• 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
  subgroupespecially if closely related
• Women over 35 and men over 40

15
The Process of Genetic Counseling

• Counselor constructs couples family history
• charts patterns of health and illness over
  generations
• Some tests provide information before conception
• 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

16
A Basis for Decision

• Many want to know ahead of time
• Some do not
• There is a more knowledge of what is to comeor
  not
• 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