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

AlleleVariation 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 life-threatening conditions to having no detectable effect at all

Genetic diversityDistinguishes 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

ZygoteTwo 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 genesGenotype

• 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

Ways to prevent female birth

• Inactivating X sperm before conception

• In vitro fertilization (IVF)

• Aborting XX fetuses

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.

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 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

Two cells duplicate and divide, becoming four, which

themselves duplicate and divide, becoming eight, and so

on

First Stages of the Germinal Period

(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.

ANATOMICAL TRAVELOGUE/SCIENCE SOURCE

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

Risk

Biological sex

Gender

Contexts

EthnicityIs He Drunk?

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 46Some gametes have more or fewer than 23 chromosomes

Some times only part of 23rd

chromosome missing

Down syndrome

Called trisomy-21 because the person has three copies of chromosome 21

Involves around 300 distinct characteristics from third chromosome; unique individuals

Daniel has trisomy-21, and this

photograph was taken at the only

school in Chile where normal and

special-needs children share

classrooms.

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 sex-linked

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