Prepared by Madeleine Lacefield Tattoon, M.A.

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Kathleen Stassen Berger

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• Additive Heredity–an allele is a slight, normal variation of a particular gene

• some alleles are…–additive genes combine to make a phenotype

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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• More Complications–A small alteration in the sequence of base pairs or several extra repetitions in one triplet ma be inconsequential or may cascade to create a major problem

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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

• Each child’s genes elicit other people’s responses, and these responses shape development. In other words, a child’s environment is partly the result of his or her genes.

• 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

• Visual Acuity– In middle age, the elasticity of the lens decrease and the eyeball shape change again, so that many people become farsighted and need reading glasses

– Among the old, eye diseases, including cataracts, are common

– About 10 percent of people over age 90 are blind

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From Genotype to Phenotype

• Nearsightedness and Genes– If children have a vision problem it is most often myopia (nearsightedness)

– Nearsightedness is a symptom in more than 150 genetic syndromes

– Caused by physical trauma or illness, such as the rubella virus, or poor nutrition (such as vitamin A deficiency

– These factors cause “high” nearsightedness, so severe that it can lead to blindness

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From Genotype to Phenotype

• Culture and Cohort–genes are not the major cause of poor vision•historical and multicultural research finds that environment also influences nearsightedness–if diet is deficient of vitamin A

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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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From Genotype to Phenotype

• Practical Application–type 2 diabetes (adult-onset diabetes)•a chronic disease which the body does not produce enough insulin to adequately metabolize carbohydrate (glucose)… it typically developed in people aged 50 - 60…today it often appears in younger people

–begins when a person is vulnerable and has more body fat than is ideal

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From Genotype to Phenotype

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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

• Not Exactly 46 Chromosomes– a variable that most often correlates with chromosomal abnormalities is the age of the mother

– occur not only in the formation of gametes but also in their early duplication

– mosaic is having a condition (mosaicism) that involves having a mixture of cells, some normal and some with an odd number of chromosomes or a sense of missing genes

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Chromosomal and Genetic Abnormalities

• Down Syndrome– a condition in which a person has 47 chromosomes instead of the usual 46, with three rather than two chromosomes at the 21st position

– people with Down Syndrome typically have distinctive characteristics, including unusual facial features, heart abnormities, and language difficulties

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Chromosomal and Genetic Abnormalities

• Abnormalities of the 23rd Pair– humans have at least 44 autosomes and one X chromosome•an embryo cannot develop without an X chromosome

•an odd number of X chromosomes impairs cognition and psychosocial development and sexual maturation

•if a child has three sex chromosomes instead of two he/she may seem normal until puberty

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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

• Fragile X Syndrome– a genetic disorder in which part of the X chromosome seems to be attached to the rest of it by a very thin string of molecules

– the actual cause is too many repetitions of a particular part of a gene’s code

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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

• Who Should Get Counseling, and When? – genetic counseling

•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