The Developing Person Through the Life Span 8e

by Kathleen Stassen Berger

Chapter 3 – Heredity and Environment

PowerPoint Slides developed by Martin Wolfger and Michael JamesIvy Tech Community College-BloomingtonReviewed by Raquel HenryLone Star College, Kingwood

The Genetic Code

What Genes Are

DNA (deoxyribonucleic acid) • Molecule that contains the chemical instructions

for cells to manufacture various proteins• Promotes growth and sustains life

What Genes Are

Chromosomes:• Molecules of DNA• DNA consists of 46 chromosomes arranged in

23 pairs

What Genes Are

Genes• Specific sections of a chromosome carry

instructions via four chemicals.• Chemicals organized in four pairs (A-T, T-A,

C-G, G-C).• Each person has about 3 billion pairs.

What Genes AreAllele

• A variation that makes a gene different in some way from other genes for the same characteristics

• Many genes never vary; others have several possible alleles

Genome• The full set of genes that are the instructions to

make an individual member of a certain species

The Beginnings of Life

Gametes• Reproductive cells (sperm and ova)• Each consists of 23 chromosomes.

Zygote • Two gametes (sperm and ovum) combine

and produce a new individual with 23 chromosomes from each parent.

The Beginnings of Life

Matching Genes

Genes are passed down from generation to generation

Genotype• An organism’s genetic

inheritance, or genetic potential• Unique for each organism

Matching Genes

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. • Typically one allele has only a few base pairs

that differ from the other member of the pair.

Male or Female?

• Humans usually possess 46 chromosomes.– 44 autosomes and 2 sex chromosomes

termed the 23rd pair– Females: XX– Males: XY– Sex of offspring depends on whether the

father’s Y sperm or X sperm fertilizes the ovum.

Male and Female

The Beginnings of Life

• The zygote begins duplication and division then differentiation and specialization occur.

• Cells change from being stem cells, those from which any other specialized type of cell can form, to being only one kind of cell.

Twins

Monozygotic (identical) twins• Originate from one zygote that splits very early

in development• Same genotype

Dizygotic (fraternal) twins• Result from fertilization of two separate ova• by two separate sperm• Incidence is genetic and varies by ethnicity• and age.

Assisted ReproductionAssisted reproductive technology (ART)

• a general term for the techniques that help infertile couples conceive and sustain a pregnancy

In vitro fertilization (IVF) • takes place outside a woman’s body• involves mixing sperm with ova surgically removed

from the woman’s ovary • if a zygote is produced, it is inserted into a

woman’s uterus, where it may implant and develop into a baby.

From One Cell to Many

Phenotype • The observable characteristics of a person,

including appearance personality, intelligence, and all other traits

• Intimately connected with genotype

Almost every trait is: • polygenic (affected by many genes)• multifactorial (influenced by many factors)

Epigenetics

• Referring to environmental factors that affect genes and genetic expression

Gene-Gene Interactions

Human Genome Project• International effort to map the complete

human genetic code• Essentially completed in 2001; analysis is

ongoing• Found “only” about 20,000 genes in humans• Exact number is unknown

Additive Heredity

Additive genes• Genes that add something to some aspect of

the phenotype• Effects of additive genes add up to make the

phenotype• Example: Height is affected by the

contributions of about 100 genes

Dominant-Recessive Heredity

• Dominant-recessive pattern - Dominant gene is more influential than the recessive gene (non-additive).

– Dominant gene can completely control the phenotype with no noticeable effect of recessive gene.

– Genes for blood type B and Rh-positive blood are dominant.

• Carrier: a person whose genotype includes a gene that is not expressed in the phenotype– Unexpressed gene occurs in half of the

carrier’s gametes and is passed on to half of the carrier’s offspring

– Offspring can be carrier or express the gene in the phenotype (e.g. when unexpressed gene is inherited by both parents)

Dominant-Recessive Heredity

Dominant-Recessive Heredity

• X-linked: A gene carried on the x chromosome

Alcoholism

• Alcoholism probably has a genetic basis

– Genes can cause an overpowering addictive pull in some people

– Environmental conditions can modify the genetic effects

– Nature and nurture combine to create an alcoholic

Nearsightedness

• Termed myopia

• Low nearsightedness runs in families and is associated with minor variations in the Pax6 gene

• Environment also plays a role– Increase in nearsightedness

among East Asian schoolchildren

– Increased schoolwork may have caused nearsightedness in children with a Pax6 allelle

Heritability

• Statistical term that indicates what portion of the variation in a particular trait within a particular population is inherited.– Example: 90% of the height differences

among children of the same age is genetic.

• Environment can affect the expression of inherited genes.

Not Exactly 46

Down Syndrome (Trisomy-21)– Three copies of chromosome 21

– Specific facial characteristics (thick tongue, round face, slanted eyes)

– Hearing losses, heart abnormalities, muscle weakness, short stature

– Slow to develop language

– Accelerated aging (cataracts, dementia, certain forms of cancer common at age 40)

Gene Disordersa) Dominant-Gene Disorders• Half of the offspring of parents with a dominant

disorder will have the disorder.• Most dominant disorders begin in adulthood (fatal

dominant childhood conditions cannot be passed on).

• Many dominant disorders have relatively mild or variable symptoms.

• Tourette syndrome– Some who inherit the dominant gene exhibit

uncontrollable tics and explosive outbursts– Most have milder, barely noticeable symptoms

b) Recessive Disorders

• Fragile X syndrome – Most common form of inherited mental retardation– Additional symptoms include muscle weakness,

shyness, and poor social skills

• Sickle-cell trait– Offers some protection against malaria– African carriers are more likely than non-carriers to

survive

Gene Disorders

Genetic Problems

Genetic Counseling and Testing

Genetic Counseling• Consultation and testing by trained professionals• Enables prospective parents to learn about their

genetic heritage, including harmful conditions that may be passed on to their offspring

Ethical Guidelines• Test results are kept confidential• Decisions regarding sterilization, adoption, abortion,

or carrying a pregnancy to term are made by the clients

Genetic Counseling and Testing

Phenylketonuria (PKU)• Recessive condition• Results in inability to metabolize

phenylalanine (amino acid found in many foods)

• Buildup of phenylalanine causes brain damage, progressive mental retardation, and other symptoms

• Early testing and a special diet usually results in normal development