Kathleen Stassen Berger

Prepared by Madeleine Lacefield Tattoon, M.A.

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Part I

Heredity and Environment

Chapter Three

The Genetic Code

From One Cell to Many

From Genotype to Phenotype

Chromosomal and Genetic

Abnormalities

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The Genetic Code• “Genes play a leading role in the drama of human development, yet they rarely take center stage. Genes are pervasive and powerful, but they are also hidden and elusive.”

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What Genes Are• DNA (deoxyribonucleic acid)

– Molecule that contains the chemical instructions for cells to manufacture various proteins.

– Chromosome• a molecule of DNA that contains the instructions to make proteins

• Humans have 46 chromosomes (23 pairs), and about 25,000 genes.

– Genome• the code for making a human being

• Every person has a slightly different code, but the human genome is 99.5% the same for any 2 people.

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What Genes Are• Genes are as section of chromosomes and the basic unit for the transmission of heredity, consisting of a string of chemicals that code for the manufacture of certain proteins.

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The Beginnings of Life“…development begins at conception…each human reproductive

cell or gamete, contains 23 chromosomes, half of that person’s 46…”

gameteA reproductive cell; that is, a sperm or ovum that can produce a new individual if it combines with a gamete from the other sex to make a zygote

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The Beginnings of Life• Matching Genes

–conception occurs in the usual way•zygote–the single cell formed from the fusing of two gametes, a sperm and an ovum

•genotype–An organism’s entire genetic inheritance, or genetic potential

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The Beginnings of Life

• Male or Female?

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From One Cell to Many– phenotype

• the observable characteristic of a person, including appearance, personality, intelligence, and all other traits

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From One Cell to Many• New Cells, New Functions

– Gene-Gene Interactions• occurs through cell differentiation, gene-gene (polygenic), and gene-environment interaction

– Multifactorial • refers to a trait that is affected by many factors, both genetic and environmental

– The Human Genome Project is an international effort to map the entire human genome• researchers have found that humans have only about 25,000 genes, 99% of which are present in the genomes of other creatures as well

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From One Cell to Many• Dominant-Recessive Heredity

– the interaction of a pair of alleles in such a way that the phenotype reveals the influence of one allele (the dominant gene) more than that of the other (the recessive gene)

– a special case of the dominant-recessive pattern occurs with genes that are x-linked, located on the x chromosome

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From One Cell to Many• Twins, Clones, Assisted Reproduction (ART)

– dizygotic (fraternal) twins • result from two sperm penetrating two ova, and share 50% of their genes

– monozygotic (identical) twins • originate from one zygote, and share 100% genes

– a clone • originates from a live organism

– ART• general term for the technique designed to help infertile couples conceive and then sustain a pregnancy

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From One Cell to Many

•Assisted Reproduction (ART)– general term for the technique designed to help infertile couples conceive and then sustain a pregnancy

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From Genotype to Phenotype

• Scientist in many nations have studied thousands of twins, both monozygotic and dizygotic, raised together in the same home and raised separately in different homes

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From Genotype to Phenotype

• Genes affect every aspect of human behavior, including social and cognitive behavior

• Most environmental influences on children raised in the same home are not shared

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From Genotype to Phenotype

• Children, adolescents, and especially adults choose environments that are compatible with their genes (called niche-picking), and thus genetic influences in adulthood

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From Genotype to Phenotype

• Carrier– a person whose genotype includes a gene that is not expressed in the phenotype…such an unexpressed gene occurs in half of the carrier’s gametes and thus is passed on to half of the carrier’s children, who will most likely be carriers, too…

– Generally, only when the gene is inherited from both parents does the characteristic appear in the phenotype.

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From Genotype to Phenotype

• Addiction–…inherited biochemistry making people vulnerable to various addition…

–…any one can abuse drugs or alcohol…but genes create an addictive pull that can be overpowering, extremely weak, or somewhere in between…

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From Genotype to Phenotype

• Visual Acuity– New borns cannot focus more than 2 feet away– Children see better each year until about age 8– Many adolescents become nearsighted when eyeball shape changes

– Vision is more likely to improve than to worsen until age 40

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From Genotype to Phenotype

• Practical Application–developmental application of nature-nurture interaction•family history of genetic problems

•someone inherited a problem–alcoholism in the genes–lack of outdoor play

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Chromosomal and Genetic Abnormalities

• abnormalities caused by identifiable problems…those with an extra chromosome or a single gene– study of these problems is relevant to the study of development…•providing insight into the complexities of nature and nurture

•knowing their origins helps limit these effects

•information combats the prejudice that surrounds such problems

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Chromosomal and Genetic Abnormalities

• Hemophelia

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Chromosomal and Genetic Abnormalities

• Dominant-Gene Disorders– everyone carries genes or alleles that could produce serous diseases or handicaps in the next generation

– 7,000 single-gene disorders•their dominant effects are apparent in the phenotype

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Chromosomal and Genetic Abnormalities• Recessive-Gene Disorder

– most recessive disorders are not X-linked

– double recessive patterns are lethal…one recessive gene is protective

– sometimes a person who carried a lethal gene has many descendants who marry each other… the genetic disease then becomes common in that group

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Chromosomal and Genetic Abnormalities

• Genetic Counseling and Testing– consultation and testing by trained experts that enable individuals to learn about their genetic heritage, including harmful conditions that they might pass along to any children they may conceive

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Chromosomal and Genetic Abnormalities

• Is knowledge Always Power?– Genetic counselors, scientist, and the general public usually favor testing•having some information is better than having none

– high risk individuals (who might hear bad news) do not always want to know•the truth might jeopardize their marriage, their insurance coverage, or their chance of parenthood

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Chromosomal and Genetic Abnormalities

• Coping with Uncertainty– much is uncertain in genetic testing and counseling

– those who learn that they have a harmful dominant gene have new information, as well as new uncertainties

– interaction of genes and the environment makes development overt the life span unpredictable, even if the genes are known