Unit 3: DNA and GeneticsUnit 3: DNA and Genetics

Module 9: Human GeneticsModule 9: Human Genetics

I. How can you study human heredity?I. How can you study human heredity?

1. 1. Population samplingPopulation sampling determines how determines how often a trait appears in a often a trait appears in a small, randomly small, randomly selected groupselected group. This percentage is then . This percentage is then applied to the entire populationapplied to the entire population to predict to predict the number of individuals with that trait.the number of individuals with that trait.

2. 2. PedigreesPedigrees graphically record the graphically record the inheritance of a single trait over several inheritance of a single trait over several generationsgenerations. Typically, the occurrence of . Typically, the occurrence of the trait is determined based on the trait is determined based on family/historical documents, interviews, family/historical documents, interviews, photographs, and medical records. photographs, and medical records.

a.a. Specific shapes are used to represent Specific shapes are used to represent individuals in a pedigree:individuals in a pedigree:

IndividualIndividual With TraitWith Trait Without TraitWithout Trait

FemaleFemale

MaleMale

b.b. Connecting lines are used to indicate Connecting lines are used to indicate relationships among individuals within relationships among individuals within the family.the family.

P1P1

parentalparental

F1F1

first filialfirst filial

F2F2

Second filialSecond filial

c. Pedigrees c. Pedigrees demonstrate the pattern of demonstrate the pattern of inheritanceinheritance (dominant/recessive, sex-linked) (dominant/recessive, sex-linked) of the single trait.of the single trait.

d. Pedigrees can be interpreted to d. Pedigrees can be interpreted to determine the determine the presence of carrierspresence of carriers (individuals who do not express the trait but (individuals who do not express the trait but may pass the gene on to offspring).may pass the gene on to offspring).

Example: The two parents (P1 generation) Example: The two parents (P1 generation) must have been carriers (Bb) for a recessive must have been carriers (Bb) for a recessive trait. Neither showed the trait, but they had trait. Neither showed the trait, but they had a child with the trait (bb).a child with the trait (bb).

Practice PedigreePractice PedigreeType O blood is recessive to Type A and B Type O blood is recessive to Type A and B blood. Tom had type B blood and married blood. Tom had type B blood and married Shana who had type A blood. Together, Shana who had type A blood. Together, they had 2 children: Cherith (Type O) and they had 2 children: Cherith (Type O) and Bryan (Type AB). Bryan married Ali (Type Bryan (Type AB). Bryan married Ali (Type O) and they had 2 children: Christian O) and they had 2 children: Christian (Type A) and Jon (who could not donate (Type A) and Jon (who could not donate blood to Christian). Ali had an affair with blood to Christian). Ali had an affair with Trent, who was homozygous for blood type Trent, who was homozygous for blood type A. Ali and Trent had a child with Type A A. Ali and Trent had a child with Type A blood.blood.

II.II. How do you get a genetic disease?How do you get a genetic disease?A. A. Gene disorders are inherited as a Gene disorders are inherited as a single gene on a chromosomesingle gene on a chromosome. Most . Most gene disorders are gene disorders are recessiverecessive. Thus, in . Thus, in order to express the disorder, the order to express the disorder, the individual must be homozygous individual must be homozygous recessive. Science hypothesizes that recessive. Science hypothesizes that gene disorders arose from mutationsgene disorders arose from mutations that disabled specific proteins, or that disabled specific proteins, or increase production harmfully.increase production harmfully.

1. 1. AutosomalAutosomal genetic genetic diseases occur when the gene defect diseases occur when the gene defect is on one of the is on one of the first 22 pairsfirst 22 pairs of of chromosomes (called the autosomal chromosomes (called the autosomal chromosomes).chromosomes).

A. A. Huntington’sHuntington’s disease is inherited as an disease is inherited as an autosomal dominant autosomal dominant gene. Huntington’s disease gene. Huntington’s disease breaks down certain areas of the brainbreaks down certain areas of the brain. In . In addition to being dominant, Huntington’s is also addition to being dominant, Huntington’s is also unique because symptoms begin appearing in the unique because symptoms begin appearing in the person’s late forties.person’s late forties.

B. B. Sickle-cell anemiaSickle-cell anemia is inherited as a is inherited as a codominant autosomal genecodominant autosomal gene. Sickle-cell anemia . Sickle-cell anemia is leads to is leads to misshapen red blood cells misshapen red blood cells which lead which lead to poor circulation and pain. Sickle cell is unique to poor circulation and pain. Sickle cell is unique because because heterozygous individuals are not heterozygous individuals are not affected by sickle-cell AND are able to resist affected by sickle-cell AND are able to resist malaria malaria (which is handy in certain areas of the (which is handy in certain areas of the world). Currently, sickle-cell is primarily in world). Currently, sickle-cell is primarily in African populations.African populations.

c.c. Cystic fibrosisCystic fibrosis is inherited as a is inherited as a recessiverecessive autosomal gene. Cystic fibrosis leads to autosomal gene. Cystic fibrosis leads to increased mucus productionincreased mucus production in the lungs in the lungs and digestive tract, which may be fatal. and digestive tract, which may be fatal. Currently, this disease is primarily in Currently, this disease is primarily in Caucasian populations. Caucasian populations.

d.d. Tay-Sach’sTay-Sach’s is inherited as a is inherited as a recessiverecessive autosomal gene. Tay-Sach’s autosomal gene. Tay-Sach’s degeneratesdegenerates (breaks down) the (breaks down) the central nervous systemcentral nervous system leading to premature death. Currently, Tay-leading to premature death. Currently, Tay-Sach’s is primarily in Jewish and Sach’s is primarily in Jewish and Pennsylvania Dutch populations.Pennsylvania Dutch populations.

Alyssa Gold

                                                                 

May 30, 1997 - March 17, 2001

e.e. Phenylketonuria (Phenylketonuria (PKUPKU) is inherited as ) is inherited as recessiverecessive autosomal gene. PKU leads to the autosomal gene. PKU leads to the inability to break down the amino acid inability to break down the amino acid phenylalaninephenylalanine when ingested. The when ingested. The phenylalanine builds up in the brain and phenylalanine builds up in the brain and leads to decreased mental function. PKU is leads to decreased mental function. PKU is unique because, if detected early, it unique because, if detected early, it can be can be entirely controlled by dietentirely controlled by diet. Individuals can . Individuals can simply not consume products containing simply not consume products containing phenylalanine (such as milk and diet sodas). phenylalanine (such as milk and diet sodas). However, any damage done before However, any damage done before detection is irreversible. In hospitals, detection is irreversible. In hospitals, children are tested at birth.children are tested at birth.

2.2. Sex-linked genetic diseases occur Sex-linked genetic diseases occur when the gene defect is on the last when the gene defect is on the last pair (23rd) of chromosomespair (23rd) of chromosomes (called the (called the sex chromosomes). Because males sex chromosomes). Because males inherit only a single X chromosome inherit only a single X chromosome (they are XY) and the X carries the (they are XY) and the X carries the majority of sex-linked genes, majority of sex-linked genes, males males are MORE LIKELY to express sex-linked are MORE LIKELY to express sex-linked disorders and cannot be carriers of disorders and cannot be carriers of these traitsthese traits. .

a.a. HemophiliaHemophilia is inherited as a is inherited as a recessive recessive sex-linked genesex-linked gene. Hemophilia leads to . Hemophilia leads to low production of blood clotting low production of blood clotting factorsfactors which leads to excessive which leads to excessive bruising and bleeding. bruising and bleeding.

b.b. Red-green color blindnessRed-green color blindness in inherited in inherited

as a as a recessive sex-linked generecessive sex-linked gene. People . People with red-green color blindness are with red-green color blindness are unable to distinguish red from green unable to distinguish red from green colorscolors (both colors often appear a (both colors often appear a muddy brown).muddy brown).

B.B. Chromosomal disorders are inherited due to Chromosomal disorders are inherited due to problems with an entire chromosomeproblems with an entire chromosome (which (which may contain hundreds of genes!) Thus, an may contain hundreds of genes!) Thus, an individual with even one chromosomal individual with even one chromosomal defect will most likely express the disorder. defect will most likely express the disorder. Science hypothesizes that Science hypothesizes that chromosomal chromosomal disorders arise from mistakes in meiosisdisorders arise from mistakes in meiosis during gamete formation. For example, a during gamete formation. For example, a sperm cell may receive 22 instead of 23 sperm cell may receive 22 instead of 23 chromosomes. This chromosomes. This incorrect distribution of incorrect distribution of chromosomes is called nondisjunction. chromosomes is called nondisjunction. Nondisjunction may lead to aneuploidy - an Nondisjunction may lead to aneuploidy - an incorrect number of chromosomes in a incorrect number of chromosomes in a fertilized zygote.fertilized zygote.

1.1. An An autosomal chromosome autosomal chromosome aneuoploidyaneuoploidy refers to having one extra refers to having one extra autosome. For example, autosome. For example, Trisomy 21Trisomy 21 (three #21 chromosomes), leads to (three #21 chromosomes), leads to Down’s SyndromeDown’s Syndrome. Characteristics of . Characteristics of Down’s Syndrome include Down’s Syndrome include some level some level of mental retardation, heart defects, of mental retardation, heart defects, flat facial features, and an enlarged flat facial features, and an enlarged tonguetongue..

2.2. A A sex chromosome aneuploidysex chromosome aneuploidy refers to refers to having one extra or one too few sex having one extra or one too few sex chromosomes. chromosomes.

a.a. Turner’sTurner’s Syndrome is the result of inheriting Syndrome is the result of inheriting a single X chromosome (a single X chromosome (genotype XOgenotype XO). ). These individuals are These individuals are female but lack female but lack secondary sex characteristics, are infertile, secondary sex characteristics, are infertile, and have some lack of mental functionand have some lack of mental function..

b.b. Klinefelter’s Klinefelter’s Syndrome is the result of Syndrome is the result of inheriting an extra X chromosome in males inheriting an extra X chromosome in males ((genotype XXYgenotype XXY). These individuals are ). These individuals are male male but lack secondary sex characteristics, are but lack secondary sex characteristics, are infertile, and have some lack of mental infertile, and have some lack of mental functionfunction..

III.III. Can we tell if a baby has a genetic disease?Can we tell if a baby has a genetic disease?

A.A. A A genetic counselorgenetic counselor can help prospective can help prospective parents parents determine the likelihood of passing determine the likelihood of passing some harmful genetic traits to their some harmful genetic traits to their offspringoffspring and may suggest further testing and may suggest further testing procedures. Counselors may also procedures. Counselors may also interpret interpret diagnostic proceduresdiagnostic procedures done by the doctor done by the doctor for parents.for parents.

1. 1. Sonograms use sound waves to Sonograms use sound waves to produce an image of the developing fetusproduce an image of the developing fetus. . This may be used to detect This may be used to detect physical physical abnormalitiesabnormalities (such as cleft palate). (such as cleft palate).

2.2. Blood testsBlood tests of the pregnant mother may of the pregnant mother may screen for certain proteins to assess the risk screen for certain proteins to assess the risk level of certain genetic disorderslevel of certain genetic disorders (such as (such as Down’s Syndrome).Down’s Syndrome).

3.3. Amniocentesis removes amniotic fluid Amniocentesis removes amniotic fluid containing fetal cellscontaining fetal cells. The cells are then . The cells are then cultured until mitosis occurs and the cultured until mitosis occurs and the chromosomes are visible. A chromosomes are visible. A karyotypekaryotype ( (a a picture of the chromosomespicture of the chromosomes) is made using ) is made using the visible chromosomes. The karyotype the visible chromosomes. The karyotype allows doctors to allows doctors to detect chromosomal detect chromosomal abnormalities but does NOT detect gene abnormalities but does NOT detect gene abnormalitiesabnormalities..

4.4. Chorionic villi samplingChorionic villi sampling ( (CVSCVS) ) removes removes actual tissue from the placentaactual tissue from the placenta (which (which is composed on embryonic cells) in is composed on embryonic cells) in order to order to create a karyotypecreate a karyotype. This may . This may be done earlier in the pregnancy, but be done earlier in the pregnancy, but is far more invasive and thus riskier.is far more invasive and thus riskier.

B.B. The The Human Genome ProjectHuman Genome Project has has allowed science to develop certain allowed science to develop certain genetic markers. genetic markers. A genetic marker A genetic marker detects the presence of certain gene detects the presence of certain gene variations on the chromosomesvariations on the chromosomes. . These genes may either be a direct These genes may either be a direct cause of a disorder or may simply cause of a disorder or may simply indicate a predisposition for a trait. indicate a predisposition for a trait. Doctors or genetic counselors may use Doctors or genetic counselors may use genetic markers to screen parents and genetic markers to screen parents and determine if the parents may be determine if the parents may be carriers for genetic disorderscarriers for genetic disorders..

IV.IV. Can you prevent and/or treat genetic Can you prevent and/or treat genetic disorders?disorders?A. Currently, there is no “cure” for genetic A. Currently, there is no “cure” for genetic disorders because the disorder stems from disorders because the disorder stems from your DNA. However, the your DNA. However, the symptoms of symptoms of genetic disorders can be treatedgenetic disorders can be treated and and experimental trials for replacing defective experimental trials for replacing defective genes are underway. genes are underway. Gene therapies are Gene therapies are being developed using information from the being developed using information from the Human Genome ProjectHuman Genome Project. These therapies . These therapies seek to seek to use engineered cell invaders (such use engineered cell invaders (such as a virus) in order to actually replace the as a virus) in order to actually replace the defective gene in target cells with a defective gene in target cells with a functioning gene.functioning gene.

B.B. Environmental factorsEnvironmental factors may play a large role in the may play a large role in the expression or progression of certain genetic problems. expression or progression of certain genetic problems. Environmental factors that interact with genes can be Environmental factors that interact with genes can be controlled to help prevent the eventual expression of controlled to help prevent the eventual expression of known genetic predispositions. known genetic predispositions.

1.1. Appropriate diet Appropriate diet can stop the progression of PKU. can stop the progression of PKU. Diet may also limit the risk for genetic Diet may also limit the risk for genetic predispositions such as heart disease, alcoholism, predispositions such as heart disease, alcoholism, and certain cancers.and certain cancers.

2.2. Environmental toxins such as UV radiation and Environmental toxins such as UV radiation and tobacco products can directly change our genestobacco products can directly change our genes. . Harmful behaviors (such as smoking) and positive Harmful behaviors (such as smoking) and positive behaviors (such as using sun screen) increase or behaviors (such as using sun screen) increase or reduce the likelihood of genetic mutations from reduce the likelihood of genetic mutations from these toxins. The mutations may lead to cancers i f these toxins. The mutations may lead to cancers i f protective genes are disrupted.protective genes are disrupted.