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Genetics and Prenatal Development

•The Beginning of Life

•Conception

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The Human Cell

• The human body is comprised of over 200 different kinds of cells which are the smallest self-contained structures– Cell membrane: the outside layer of the cell– Cytoplasm: is comprised of specialized structures– Mitochondria: are the powerhouses that process

nutrients and provide the cell’s energy – Endoplasmic reticulum, Golgi apparatus, and

ribosomes: produce proteins– Neucleus: The inner part of the cell

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The Nucleus

• Chromosomes

• Genes

• Deoxyribonucleic acid (DNA)

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Chromosomes

• Rod shaped structures found in the center of the nucleus of every cell in the body.

• Each sperm and each ovum contains 23 chromosomes.

• The chromosomes contain the DNA and genes.• The fertilized egg (zygote) and all the body cells

that develop from it (except the sperm cells and the ova) contain 46 chromosomes.

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Chromosomes

• 22 of the pairs are called autosomes and are numbered from largest to smallest.

• The autosomes are not involved in determining sex.

• The 23rd pair are the sex chromosomes: – XX in females– XY in males

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KaryotypeA photograph of a cell’s chromosomes

arranged in pairs according to size

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A Portion of a DNA Molecule

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DNADeoxyribonucleic Acid

Nucleotides are the building blocks of DNA

They contain 4 nitrogen-carbon-hydrogen basis that bond to form specific pairs:

adenine can only pair with thymine

cytosine can only pair with guanine

The combination of base pairs cannot vary

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DNAWhat Can Vary:

1- Which side of the ladder each base comes from

2- The order in which the base pairs occur along the ladder

3- The overall number of base pairs

These variations account for differences between species.

All organisms use just these 4 bases, but with different numbers and arrangements

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DNA

There are 3.12 billion base pairs in human DNA

The DNA in each normal human being is about 99.9% the same as every other normal human being

Only .1% accounts for the biological contribution to all our individual differences in physical and psychological characteristics

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DNASingle Nucleotide Polymorphisms

(SNPs)• A large portion of the .1% individual

difference takes the form of single nucleotide polymorphisms.

• SNPs (snips) are nucleotide variations that occur on average about every 1,250 base pairs

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DNA

• They determine the nature of each cell in the body and how it will function.

• At each level of the spiral or rungs of the ladder are particular chemical pairs. The arrangement of these pairs along the DNA molecule determines which kind of proteins will be formed in the cell.

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Genes

• The basic unit of genetic information

• They determine the nature and the function of the cell.

• The human genes (about 120,000) are referred to as the human genome.

• A genome is the full set of genes in each cell of an organism.

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Proteins

Proteins are molecules that perform an array of crucial functions in the human body:Enzymes: break down and altar

biochemicalsHemoglobin: binds with oxygen allowing it to be transported to cells throughout the body

Collagen: in bones and connective tissues

Hormones: regulate physical growth

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Genes and Protein Synthesis

• A nuclear enzyme attaches to a segment of DNA causing nucleotide bonds to separate.

• Transcription occurs resulting in messenger RNA (mRNA).

• Transfer RNA (tRNA) initiates translation into amino acid.

• Ribosomes move along the RNA bonding amino acids into polypeptide chains which make proteins.

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1- Transcription: the transfer of information from an DNA molecule into an RNA (ribonucleic acid) molecule.

2- Messenger RNA (mRNA): a type of RNA synthesized from DNA; attaches to ribosomes to specify the sequence of amino acids that form proteins.

3- Translation: the transfer of information from an RNA molecule into a Polypeptide, in which language of the nucleic acids is translated into that of amino acids.

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Celera GenomicsThe Human Genome Project

• In June 26, 2000, they both made an announcement that the “correct alphabetical order of the 3.12 billion letters” of the human genome had been mapped.

• It will be many years before the incredibly complex functions of the genome in making and maintaining a living human being are fully understood.

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Scientific Breakthroughs

Greater insights into disease will be achieved

Cures may be found

Incurable diseases may be prevented

There will be new insights into the evolutionary origins of humans

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Controversial Issues

Will it be ethical for parents to have their children screened prior to birth and decide not to have a child with a genome that is merely undesirable ?

What about employers not hiring people with bad genomes and insurance companies refusing to insure them?

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Cell Division and Reproduction

When the cell is ready to divide and reproduce:

the DNA staircase unwinds and the two long chains separate

each chain attracts new biochemical material from the cell to synthesize a new and complementary chain

Ultimately a new cell is formed

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Terms• Gametes:

Sex cells (ovum or sperm)

• Diploid cells:

Cells having 2 copies of each chromosome

• Haploid gametes:

Gametes having 1 copy of each chromosome

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Meiosis• Meiosis takes place in the testicles and ovaries.• A diploid cell (having 2 copies of each

chromosome) undergoes a special form of cell division to create haploid gametes (having 1 copy of each chromosome).

• An egg and a sperm fuse together to form a new diploid cell called zygote (a process called fertilization)

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Mitosis• In the first step of mitosis, all chromosomes are

copied, so that instead of 2 copies, the cell briefly has 4 copies of each chromosome.

• Shortly afterwards, the cell divides in half, resulting in two cells each has a complete copy of the genetic information.

• These cells grow larger and eventually undergo mitosis.

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• Mitosis: each cell divides and duplicates itself exactly

• Meiosis: How reproductive cells (ova and sperm) are produced

• Results in gametes, cells that contain only 23 chromosomes

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• In Males

Meiosis takes place in the testes and involves 2 rounds of division

Results in 4 fertile sperm cells

By puberty, males begin producing many thousands of sperm cells on an ongoing basis, and they continue to do so through out their life span

• In Females

Meiosis begins in the ovaries before birth and partly completes all of the roughly 400,000 ova a woman will ever have.

It occurs in a two-stage process..

Results in one relatively big ovum and 2 small polar bodies that aren’t capable of being fertilized

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Mutation

A mutation is an alteration in the DNA that typically occurs during mitosis and meiosis.

In most cases mutation is maladaptive and the new cell simply dies or repairs and eliminates the mutation

A small number of of mutations are viable – the cell survives

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Mutation

In mitotic cell division, if a viable mutation occurs early in development, it will then be passed along to all cells replicated.

In meiotic cell division, mutation only affects the ensuing gametes and stops there, Unless a mutated gamete happens to be involved in producing offspring – in which case the mutation can be passed along to the next generation and beyond.

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Gregor Mendel (1800s)

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Genotype

The genetic makeup of a given individual

Recessive Gene

The gene pair that determines a trait in an individual only if the other member of that pair is also recessive

Phenotype

The traits that are expressed in the individual

Dominant Gene

One gene of a gene pair that will cause a particular trait to be expressed

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AllelesA pair of genes, found on corresponding chromosomes, that affect the same trait

The child might inherit an allele for brown eyes (B) from the father and an allele for blue eyes (b) from the mother

The child’s genotype for eye color would be Bb.What actual eye color will the child display?The allele for brown eyes is dominant (B). The allele for blue eyes is recessive (b).The dominant trait will be expressed as the

phenotype

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Homozygous

Referring to the arrangement in which the two alleles for a simple dominant- recessive trait are the same.

Homozygous Individual

(Eye Color)

Could be BB or bb

Heterozygous

Referring to the arrangement in which the two alleles for a simple dominant-recessive trait differ.

Heterozygous Individual

(Eye Color)

Could be Bb, or bB

The chance for having blue eyes is 25%

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Incomplete Dominance

Where people with a single recessive gene for a trait show some of the trait along with other normal manifestations.

Example:

Sickle-cell anemia

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Sickle-Cell Anemia

• Occurs at its highest rate in individuals of black African ancestry.

• People with a single recessive gene for the trait have a marked percentage of abnormal “sickle-shaped” red blood cells that interfere with oxygen transport throughout the body.

• They also have normal (dominant) red blood cells as well.

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Sickle-Cell Anemia

• The sickle cells are resistant to malarial infection, so those individuals with the trail would have survived long enough to have children in areas of the world where mosquito-borne malaria is highly prevalent.

• Sickle-cell carriers experience pain in the joints, blood clotting, swelling and infections under conditions of oxygen shortage.

• It occurs when a person inherits both recessive alleles

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Codominance

Where neither the dominant nor recessive allele is dominant and the resulting phenotype is a blend of the two.

Example:

If an individual gets an allele for each blood types A and B, the result is type AB blood type

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Polygenic Inheritance

The overall system of interactions among genes and gene pairs

More complex traits do not result from the alleles of a single gene pair, but rather from a combination of many gene pairs

In determining height, several gene pairs combine to create people with taller or shorter phenotype.

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Inherited Disorders

• Sex-Linked Disorders– Genetic Disorders– Chromosomal Disorders

• Autosomal Disorders– Genetic Disorders– Chromosomal Disorders

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Sex-Linked Disorders

• Involve the sex chromosome # 23

• Occur via dominant-recessive patterns

• A recessive gene on the X chromosome is more likely to be expressed as the phenotype males because the Y chromosome has no allele that might contract the gene.

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Genetic Sex-Linked Disorders

1- Color BlindnessGenetic X-linked recessive disorder. Occurs in 1 of

10 males2- Hemophilia A and B Recessive disorders that affect 1 of 5,000 males.

These interfere with normal blood clotting and occur at different loci (the position on a chromosome occupied by a particular gene) on the X chromosome.

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Chromosomal Sex-Linked Disorders

1- Fragile X Syndrome

Occurs in about 1 of 1,200 males and 1 of 2,500 females. Results from a breakage of the tip of an X chromosome.

2- Klinefelter Syndrome (XXY, XXXY, XXXXY)

Occurs in about 1 of 1,000 males. It is caused by an extra X chromosome

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Chromosomal Sex-linked Disorders

3- Superfemale Syndrome (XXX, XXXX, XXXXX)

Occurs in about 1 of 1,000 females. Women appear normal, but tend to score slightly below average in intelligence.

4- Supermale Syndrome (XYY, XYYY, XYYYY)

Occurs in about 1 0f 1,000 males. The men tend to be taller than average, with a greater incidence of acne and minor skeletal abnormalities.

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Chromosomal Sex-linked Disorders

5- Turner’s Syndrome (XO)

Occurs in about 1 of 10,000 females. One of the X chromosomes is either missing or inactive. These women have immature female appearance, do not develop secondary sex characteristics, and lack internal reproductive organs.

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Autosomal Disorders

• Disorders involving the other 22 pairs of chromosomes.

• Can result from an extra chromosome or defective genes

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Genetic Autosomnal Disorders

1- Angelman’s syndromeOccurs in about 1 of 10,000 to 15,000 people. It is

determined by a set of mutated genes on chromosome 15.

2- Cystic FibrosisA recessive disorder that occurs in about 1 of 2,5000

people of white European ancestry. Related to a mutated gene on chromosome 17. Characterized by excessive secretion of the mucus in the body.

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Genetic Autosomal Disorders

3- Huntington Disease

A dominant disorder that occurs in about 1 of 10,000 people. A dominant gene on chromosome 4 is responsible.

It causes degeneration of neurons producing dementia, and random jerking movements.

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Genetic Autosomal Disorders

4- Phenylketonuria (PKU)A recessive disorder that occurs in about 1 of

10,000 people. A defective gene on chromosome 12 is responsible

5- Prader Willi SyndromeA recessive disorder that occurs in 1 of

10,000 to 15,000 people. It is determined by a set of mutated genes on chromosome 15.

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Genetic Autosomnal Disorders

6- Sickle-Cell Anemia

Occurs in about 1 of 12 U.S. blacks. The defective gene on chromosome 11 is responsible.

7- Tay-sachs Disease

A recessive disorder that occurs in about 1 of 5,000 people of European Ashkenazi Jewish ancestry. Defective gene on chromosome 15 is responsible.

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Chromosomal Autosomal Disorders

Down Syndrome

Occurs in about 1 in 1,000 live births. An extra chromosome is attached to the 21st pair. Risk increases with maternal age. Pregnancies of women over age 35 accounts for 20% of Down syndrome birth

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Genetics and EnvironmentSandra Scarr

• Active genotype-environment effects

• Passive genotype-environment effects

• Evocative genotype-environment effects

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Stop and Discuss• Gametes

• Zygote

• Monozygotic twins

• Dizygotic twins

• Diploid cells

• Haploid gametes

• Dominant/recessive genes

• Chromosomes

• Genes

• DNA

• Meiosis

• Mitosis

• Allele

• Phenotype/genotype

• Homozygous/hetrozygous

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Genetic Counseling• Can help couples obtain valuable

information about the parents’ genetic makeup .

• It can help potential parents to evaluate genetic risk factors in childbearing and enable them to make intelligent decisions.

• It includes analysis of parental medical records and family histories to construct a family pedigree.

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Family History Parental Conditions

Neonatal deaths

Malformations

Mental retardation

Congenital anomalies (e.g. club feet)

Diseases that run in families

Inability to thrive

Genetic or chromosomal abnormality

Infertility

Mother/father’s age

Stillbirths

Ethnic background

Exposure to toxic agents

Cancer

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Detecting Birth Defects

Amniocentesis

Chronic Villus Sampling

Ultrasound Sonography

Maternal Blood Test

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Genetic Engineering

• Alteration of Human Genes

• 1- Gene Therapy

• 2- Germ-line Genetic Alterations

Germ-line Genetic Intervention

• 3- Genetic Enhancement

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1- Gene Therapy• Genetic alteration of somatic cells to treat

disease.• Researchers inject genes that are targeted to

treat a particular disease in to a patient’s blood stream.

• When the genes arrive at the site of the defective genes, they produce chemicals that can treat the problem.

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2-Germ-line Genetic Alteration

• Can correct problems for unborn individuals and future generations.

• It targets the genes in the reproductive cells – the egg and the sperm that combine the DNA to conceive a new human.

• Scientist might detect defective cells soon after conception, removing them from the mother and placing them in a test-tube culture.

• Gene therapy could be employed to correct the defects in the cells.

• The result could be cloning. Parents could some day customize their children.

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3-Genetic Enhancement

• Non therapeutic genetic alteration

• An attempt to enhance an already healthy genetic makeup by inserting a gene for improvement (e.g. height, intelligence, eye color)

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What Do You Think?

Genetic Engineering

Gene TherapyGerm-Line Genetic

AlterationsGenetic enhancement

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Cloning

• Producing genetic replicas of the organism

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Stop and Discuss

• In the light of scriptural truth, how ethical are these issues?

• 1-Amniocentesis

• 2-Chronic villus sampling

• 3-Gene therapy

• 4-Germ-line genetic alteration

• 5-Genetic enhancement

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Stages of Prenatal Period1- Germinal Stage

(fertilization to 2 weeks)Blastocyst

Cell Division

Specialized Cells

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2- Embryonic Stage(2 weeks to 8 weeks)

• 1-Ectoderm (outer layer)Skin, teeth, hair, sense organs, brain, spinal

cord• 2-Endoderm (inner layer)Digestive system, pancreas, respiratory

system• 3-Mesoderm (in between both)Muscles, bones, blood, circulatory system

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3- Fetal Stage(8 weeks to birth)

Fetus

Increases in size

Proportions similar to adults

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Environmental Influences

1- Habituation

2- Classical Conditioning

3- Operant Conditioning

4- Social Learning

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Sociocultural InfluencesPaul Blates (87, 88)

• Normative Age-Graded InfluenceThe biological and social changes (e.g. aging,

entering school, marriage)• Normative History-Graded InfluenceHistorical events (e.g. wars, depression)• Nonnormative InfluencesIndividual environmental factors (e.g.

divorce, unemployment)

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Prenatal Care

• Diet

• The father’s involvement

• Age of mother

• Illness of mother

• Drug use

• Alcohol

• Teratogens