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Medical Genetics
 

Medical Genetics

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  • You should recall that mitosis is a version of the cell cycle that produces daughter cells with 46 chromosomes in humans. Mitosis is used for growth and repair.
  • During mitosis DNA is duplicated. This DNA is used to form doubled chromosomes. Each chromosome has two identical chromatids. These doubled chromosomes will soon be split into single chromosomes.
  • The doubled chromosomes line up in a single row during metaphase of mitosis.
  • The doubled chromosomes are pulled apart. Each chromatid now becomes a single chromosome.
  • The meiosis version of the cell cycle produces cells with half the chromosome number of body cells. Meiosis is used just to produce the egg and sperm cells. These gametes will have 23 chromosomes.
  • During metaphase of meiosis the chromosomes line up in a double row.
  • The chromosomes separate. Each daughter cell receives doubled chromosomes. These cells will have twice the DNA they need.
  • A second meiotic division is needed to separate the doubled chromosomes into single chromosomes.
  • An important difference between mitosis and meiosis is the way the chromosomes line up during metaphase. They are in a single file in mitosis and a double file in meiosis.
  • Homologous chromosomes are paired together during metaphase. The two number one chromosomes are paired together, the two number two chromosomes are paired together and so on for all 23 pairs. *This arrangement will ensure that each daughter cell will receive ONE of each chromosome pair. *It also insures that daughter cells will receive ONE allele for most traits. *New combinations of alleles result from the random arrangement of the chromosomes in metaphase of meiosis. For example, the red chromosomes represent the chromosomes a person inherited from their mother. The blue chromosomes represent those inherited from a person’s father. When the person produces gametes in meiosis he or she will pass on chromosomes from their parents—the grandparents of the offspring. The offspring will receive one number one chromosome. It could be a copy of chromosome one from their grandfather or grandmother. The odds of the chromosomes lining in up in metaphase the same way two times are one in 70 million, million times!
  • A gene is a unit of heredity that controls the development of one trait. Genes are made of DNA. There are about 30,000 genes in man.
  • An allele is a member of a paired gene. Most genes are made of paired alleles. A person gets one allele from each parent. A single letter represents an allele.
  • For example, the letter D is used to represent the allele for dwarfism. A small d is the allele for normal height. If a person has DD or Dd they will be inherit dwarfism. If a person has dd they will grow to a normal height. Click on the link to hear the dwarf band. (It works best in Internet Explorer)
  • Alleles can be dominant or recessive. Dominant alleles are expressed over recessive alleles. Recessive alleles are not expressed in the presence of a dominant allele. Recessive alleles are only expressed if both recessive alleles are present.
  • Homozygous is having both alleles alike. AA or aa are homozygous.
  • Heterozygous is when the alleles are different. Aa is homozygous.
  • Genotype refers to the genetic makeup. Genotypes are represented by alleles. DD and Dd are genotypes for dwarfism.
  • The term phenotype refers to a trait. A person’s genotype determines their phenotype. Dwarfism is a phenotype.
  • Codominant is when two different alleles are both dominant. For example, A is the allele for type A blood. B is the allele for type B blood. If a person inherits both the A and the B allele they will have type AB blood.
  • A karyotype is a picture of chromosomes from an individual.
  • Homologous chromosomes are chromosomes of the same pair. Karyotypes have homologous chromosomes paired together.
  • A mutation is a change in a gene or chromosome. It will cause an abnormal trait.
  • A mutagen is an agent that causes mutations. There are numerous mutagens in cigarette smoke. Pesticides are mutagens. X-rays, ultraviolet light and nuclear radiation are also mutagens. Most mutagens are also carcinogens or cancer causing agents.
  • Homologous chromosomes line up in a double file in metaphase I of meiosis as discussed before.
  • The homologous pairs separate in the first meiotic division. *Two daughter cells are produced. These cells have doubled chromosomes and must be divided again.
  • The second meiotic division produces four gametes with single chromosomes.
  • Gametes unite during fertilization. *Chromosomes from both gametes make up the zygote formed.
  • Sometimes one pair of chromosomes fails to separate during meiosis. This is called nondisjunction. How do you think this will affect the offspring?
  • A trisomy will result after fertilization. *The zygote will have three chromosomes for the chromosome pair that did not separate during meiosis.
  • Sex chromosomes are the X and y chromosomes. They determine the sex of an individual.
  • Males have Xy. Sperm cells with either have an X or a y. Females have XX. The egg will have one X chromosome.
  • The autosomes are all the chromosomes except the sex chromosomes. Chromosomes 1 through 22 are autosomes.
  • X-linked traits have their alleles on the X chromosome. Females with two X chromosomes will have two alleles for an X-linked trait. Males only have one allele for an X-linked trait.
  • Here is the karyotype for a normal male. There are two copies of each of the 22 autosomes and the X and y chromosomes.
  • The female karyotype also has two copies of the 22 autosomes plus two X chromosomes.
  • This is a karyotype for a person with an abnormal number of chromosomes. The have three copies of chromosome 21 because of nondisjunction in of the parents.
  • People with Down syndrome have a large tongue which often causes them have an open mouth. Their face is flat and the eyes are slanted. The palm of the hand has a single crease. Some individuals with Down syndrome are retarded but others are not.
  • This slide shows the correlation of Down syndrome with the age of the mother. Paternal age is also a factor but it is not as strongly correlated as the age of the mother.
  • Trisomy 18 is also known as Edward syndrome.
  • Characteristics of Edward syndrome include heart defects, displaced liver and low-set ears. The hands have an unusual shape as shown on this slide. These individuals have severe retardation. Most of them abort before they are born. Those few who are born have a lifespan of less than one year.
  • Trisomy 13 or Patau syndrome is illustrated here.
  • A cleft lip and palate are present in this condition. If you count the fingers on this baby you will find there are six digits. Polydactylism refers to extra digits. Defects of the heart, brain and kidneys are also present in this condition. Most individuals with this condition abort. Those babies that do survive live less than one month. The baby in this picture was stillborn.
  • Klinefelter syndrome results from an abnormal number of sex chromosomes. People with Klinefelter syndrome have a y chromosome so they are males. Having an extra X will cause them to have some feminine traits.
  • They may exhibit slight breast development. Small testes and sterility is another characteristic of Klinefelter syndrome. They often have lower intelligence but are not retarded. Click on the link for the Klinefelter website to read about people with this condition.
  • Having only one X chromosome causes Turner syndrome. These individuals are females.
  • Girls with Turner syndrome are short and do not go through puberty. The reason they do not is because they produce little estrogen. It takes two X chromosomes to produce a normal amount of estrogen. Most individuals with Turner syndrome are sterile. An unusual characteristic is extra skin on the neck. The girl in this picture is 18, but has not gone through puberty.
  • An abnormal number of chromosomes can be detected before birth by fetal testing. One method is amniocentesis. In this procedure amniotic fluid is withdrawn from the uterus. Some of the fetal cells will be present in amniotic fluid. The amniotic fluid is cultured for several weeks to increase the number of fetal cells present. These cells are then used to make a karyotype of the baby to check for chromosome number. Another type of fetal testing is chorionic villus sampling. Chorionic villi of the placenta are sampled. A karyotype can be prepared the next day with this method since the cells are much more dense.
  • Sickle cell anemia is a disease in which the red blood cells have a sickle shape. Sickle cells do carry as much oxygen and often plug up small blood vessels. The lack of oxygen causes anemia. When the oxygen supply is reduced to a part of the body pain will result. A reduced supply of oxygen to the brain can cause a stroke. Leg ulcers are the result of poor circulation. Sickle cells have a shorter life span than normal RBCs. When they break down bilirubin is released. Excess bilirubin can cause jaundice and gall stones. The spleen, kidneys and lungs are damaged during sickle cell anemia.
  • Sickle cell anemia is a disease in which the red blood cells have a sickle shape. Sickle cells do carry as much oxygen and often plug up small blood vessels. The lack of oxygen causes anemia. When the oxygen supply is reduced to a part of the body pain will result. A reduced supply of oxygen to the brain can cause a stroke. Leg ulcers are the result of poor circulation. Sickle cells have a shorter life span than normal RBCs. When they break down bilirubin is released. Excess bilirubin can cause jaundice and gall stones. The spleen, kidneys and lungs are damaged during sickle cell anemia.
  • Hemophilia is a blood clotting disease. It is caused by a recessive allele on the X chromosome. Hemophilia is an X-linked recessive trait. It is more common in males than females.
  • Albinism is another recessive condition. Albinos are unable to produce pigment in their skin, hair and eyes.
  • Normal people are able to synthesize the pigment melanin from amino acids. This process requires an enzyme. *The A allele has the directions to make this enzyme. *The a allele has a mistake in the directions and cannot make the enzyme. *The AA genotype has two copies of the directions for making pigment. Aa has only one good copy of the directions for making pigment, but only one copy is needed. The aa genotype cannot make the enzyme.
  • PKU disease results from an excess of phenylalanine. Too much phenylalanine can cause retardation in children. Click on the link to read Molly’s story about having PKU disease.
  • People without PKU disease produce an enzyme that converts excess phenylalanine to tyrosine. *The P allele has the directions for making this enzyme. *The p allele cannot make the enzyme. The genotypes PP and Pp are normal because they can produce the enzyme. The genotype pp, is unable to make the enzyme to breakdown excess phenylalanine. This individual will have PKU disease.
  • A man and a woman are both carriers for albinism. What is the chance their children will inherit albinism?
  • The man will produce sperm cells by meiosis. During this process the alleles will separate into different sperm cells. He can make two different kinds of sperm cells. One kind will have the A allele and the other the a allele. The same is true of the woman when she produces an egg.  
  • One way to determine every combination of the man’s sperm cells and woman’s eggs is to align them on a square. It is called a Punnent square. *Each box of the square will receive an allele from one sperm and one egg. *AA will go in the box in the upper left. *The next square will receive the A allele from the sperm and the a allele from the egg. *Filling in each box will give every possible combination of eggs and sperms from a given couple.*
  • There is a lot of information we can derive from the Punnent square. The possible genotypes of the couple’s offspring are 1AA, 2Aa and 1aa out of four. Phenotypes are 3 normal and 1 albino out of 4. The probability of albinism is 1 out of 4 or 25%.
  • Can you solve this problem for PKU disease?
  • Set up the Punnent square as shown in this slide. *The letters in the boxes are these
  • This slide illustrates the possible genotypes, phenotypes and probability for PKU in the couple’s offspring.
  • Can you solve this problem for sickle cell anemia?
  • The Punnent square will look like this. *The genotypes of the offspring are in the Punnent square.
  • Genotypes, phenotypes and probability are shown on this slide.
  • Try this problem for dwarfism. See if you can work it before advancing to the next slide.
  • The Punnent square will look like this. *Here is the completed Punnent square.
  • Genotypes, phenotypes and the probability of dwarfism for this problem are illustrated on this slide.
  • X-linked recessive traits have their alleles on the X chromosome. The inheritance pattern is different in males than females.
  • If a woman has one h allele she will not have hemophilia because a dominant H will enable her to produce clotting factors. A woman would have to have two h alleles to inherit hemophilia. However, a male only needs one h allele to have hemophilia.
  • Be sure you write the alleles for hemophilia on the X chromosome to solve this problem.
  • It is always a good idea to write down every possible genotype and their corresponding phenotypes before you try to solve a genetics problem. Here are the genotypes for this problem. ****
  • Note that all the females are carriers and that none of males inherit hemophilia from their father.
  • See if you can work this problem before advancing to the next slide.
  • Your Punnent square should look like this. *
  • The chance for hemophilia in a girl for this problem is 0%. One of the two boxes for boys has hemophilia. Therefore the change of hemophilia is 50% if the child is a boy.
  • This concludes the presentation on medical genetics.

Medical Genetics Medical Genetics Presentation Transcript

  • Medical Genetics www.freelivedoctor.com
  • Mitosis
    • Produces daughter cells with 46 chromosomes
    • Used in growth and repair
    www.freelivedoctor.com
  • Mitosis
    • DNA is duplicated
    • Doubled chromosomes form from duplicated DNA
    • Each cms has 2 identical chromatids
    www.freelivedoctor.com Chromatid Chromatid
  • Chromosomes line up in a single row. Mitosis Metaphase www.freelivedoctor.com
  • Chromosomes separate Each chromatid becomes a single chromosome www.freelivedoctor.com
  • Meiosis
    • Reduce the chromosome number to half that of body cells
    • Produce gametes
      • Egg
      • Sperm
    www.freelivedoctor.com
  • Chromosomes line up in a double row. Meiosis Metaphase www.freelivedoctor.com
  • Chromosomes separate Each each daughter cell gets doubled chromosomes www.freelivedoctor.com
  • Doubled Chromosomes Separate in Second Meiotic Division www.freelivedoctor.com
  • www.freelivedoctor.com Mitosis Metaphase Meiosis Metaphase
  • Double Filed Chromosomes
    • Daughter cells receive ONE of each cms pair
    • Daughter cells receive ONE allele for most traits
    • New combinations of alleles possible
    www.freelivedoctor.com Cms 1 Cms 1 Cms 2 Cms 2
  • Gene
    • A unit of heredity that controls the development of one trait
    • Made of DNA
    www.freelivedoctor.com
  • Allele
    • Member of a paired gene
      • One allele comes from each parent
    • Represented by a single letter
    www.freelivedoctor.com
  • Examples of Alleles Dwarfism = D Normal height = d DD = Dwarfism Dd = Dwarfism dd = Normal height www.freelivedoctor.com Dwarf Band
  • Dominant & Recessive Alleles
    • Dominant alleles are expressed
    • Recessive alleles are not expressed in the presence of a dominant allele
      • Recessive alleles are only expressed if both recessive alleles are present
    www.freelivedoctor.com
  • Homozygous
    • Both alleles alike
    • AA or aa
    www.freelivedoctor.com
  • Heterozygous
    • Alleles are different
    • Aa
    www.freelivedoctor.com
  • Genotype
    • Genetic make up
    • Represented by alleles
    • DD & Dd are genotypes for dwarfism
    www.freelivedoctor.com
  • Phenotype
    • A trait
    • Genotype determines the phenotype
    • Dwarfism is a phenotype
    www.freelivedoctor.com
  • Codominant
    • Two different alleles are both dominant
    • A = allele for type A blood
    • B = allele for type B blood
    • AB = results in type AB blood
    www.freelivedoctor.com
  • Karyotype
    • Picture of chromosomes from an individual
    www.freelivedoctor.com
  • Homologous Chromosomes
    • Chromosomes of the same pair
    • Karyotypes are usually arranged with homologous chromosomes paired together
    www.freelivedoctor.com
  • Mutation
    • Change in a gene or chromosome
    • Causes an abnormal trait
    www.freelivedoctor.com
  • Mutagen Agent that causes mutations www.freelivedoctor.com Cigarette smoke Pesticides X-rays Ulatraviolet light Nuclear radiation
  • Homologous chromosomes line up in a double file in metaphase I of meiosis www.freelivedoctor.com
  • Homologous Pairs Separate www.freelivedoctor.com
  • Four Gametes With Single Chromosomes www.freelivedoctor.com
  • Fertilization www.freelivedoctor.com
  • Nondisjunction www.freelivedoctor.com
  • Trisomy www.freelivedoctor.com
  • Sex Chromosomes www.freelivedoctor.com
  • Sex Chromosomes
    • Male have Xy
      • Male gametes have either X or y
    • Females have XX
      • Female gametes have X
    www.freelivedoctor.com
  • Autosomes Chromosomes 1-22 www.freelivedoctor.com
  • X-Linked Traits
    • Alleles are on the X chromosome
    • Females have two alleles
    • Males have one allele
      • Only one X chromosome
    www.freelivedoctor.com
  • Normal Male www.freelivedoctor.com
  • Normal Female www.freelivedoctor.com
  • Trisomy 21 Down Syndrome www.freelivedoctor.com
  • Down Syndrome
    • Large tongue
    • Flat face
    • Slanted eyes
    • Single crease across palm
    • Mental retardation
      • Some are not
    www.freelivedoctor.com
  • Maternal Age & Down Syndrome www.freelivedoctor.com
  • Trisomy 18 Edward Syndrome www.freelivedoctor.com
  • Edward Syndrome
    • Heart defects
    • Displaced liver
    • Low-set ears
    • Abnormal hands
    • Severe retardation
    • 98% abort
    • Lifespan < 1 year
    www.freelivedoctor.com
  • Trisomy 13 Patau Syndrome www.freelivedoctor.com
  • Patau Syndrome
    • Cleft lip and palate
    • Extra fingers & toes
      • polydactylism
    • Defects
      • Heart
      • Brain
      • Kidneys
    • Most abort
    • Live span < 1 month
    www.freelivedoctor.com
  • Klinefelter Syndrome www.freelivedoctor.com
  • Klinefelter Syndrome
    • Breast development
    • Small testes
    • Sterile
    • Low intelligence
      • Not retarded
    www.freelivedoctor.com Klinefelter Website
  • Turner Syndrome www.freelivedoctor.com
  • Turner Syndrome
    • Short
    • Not go through pruberty
    • Produce little estrogen
    • Sterile
    • Extra skin on neck
    www.freelivedoctor.com
  • Fetal Testing www.freelivedoctor.com
  • Sickle Cell Anemia
    • RBCs sickle shaped
    • Anemia
    • Pain
    • Stroke
    • Leg ulcers
    • Jaundice
    • Gall stones
    • Spleen, kidneys & lungs
    www.freelivedoctor.com
  • Sickle Cell Anemia
    • Recessive allele, s codes for hemoglobin S
      • Long rod-like molecules
      • Stretches RBC into sickle shape
    • Homozygous recessive, ss have sickle cell anemia
    • Heterozygous, Ss are carriers
    www.freelivedoctor.com
  • Hemophilia
    • Blood clotting impaired
    • Recessive allele, h
    • carried on X cms
    • X-linked recessive trait
    • More common in males
    www.freelivedoctor.com
  • Albinism
    • Lack of pigment
      • Skin
      • Hair
      • Eyes
    www.freelivedoctor.com
  • Amino Acids Melanin Pigment Enzyme www.freelivedoctor.com A a AA = Normal pigmentation Aa = Normal pigmentation aa = Albino
  • PKU Disease
    • Phenylalanine excess
    • Mental retardation if untreated
    www.freelivedoctor.com Molly’s Story
  • Phenylalanine Tyrosine Enzyme www.freelivedoctor.com P p PP = Normal Pp = Normal pp = PKU
  • A man & woman are both carriers (heterozygous) for albinism. What is the chance their children will inherit albinism? www.freelivedoctor.com
  • Man = Aa Woman = Aa www.freelivedoctor.com AA = Normal pigmentation Aa = Normal pigmentation (carrier) aa = Abino A a a A
  • AA Aa Aa aa www.freelivedoctor.com A a a A
  • Genotypes 1 AA, 2Aa, 1aa Phenotypes 3 Normal 1 Albino Probability 25% for albinism www.freelivedoctor.com AA Aa Aa aa
  • A man & woman are both carriers (heterozygous) for PKU disease. What is the chance their children will inherit PKU disease? www.freelivedoctor.com
  • p p P P www.freelivedoctor.com PP P p P p pp PP = Normal Pp = Normal (carrier) pp = PKU disease
  • Genotypes 1 PP, 2Pp, 1pp Phenotypes 3 Normal 1 PKU disease Probability 25% for PKU disease www.freelivedoctor.com PP Pp Pp pp
  • A man with sickle cell anemia marries a woman who is a carrier. What is the chance their children will inherit sickle cell anemia? www.freelivedoctor.com
  • s s s S www.freelivedoctor.com Ss Ss ss ss SS = Normal Ss = Normal (carrier) ss = Sickle Cell
  • Genotypes 2 Ss, 2ss Phenotypes 2 Normal (carriers) 2 Sickle cell Probability 50% for Sickle cell www.freelivedoctor.com Ss Ss ss ss
  • A man with heterozygous dwarfism marries a woman who has normal height. What is the chance their children will inherit dwarfism? Dwarfism is dominant. www.freelivedoctor.com
  • d d D d www.freelivedoctor.com Dd dd Dd dd DD = Dwarf Dd = Dwarf dd = Normal
  • Genotypes 2 Dd, 2dd Phenotypes 2 Normal 2 Dwarfs Probability 50% for Dwarfism www.freelivedoctor.com Dd dd Dd dd
  • X-linked Recessive Traits
    • Alleles are on the X chromosome
    • Inheritance pattern different in males and females
    www.freelivedoctor.com
  • www.freelivedoctor.com X H X H = Normal Female X H X h = Normal Female (Carrier) X h X h = Hemophilic Female X H y = Normal Male X h y = Hemophiliac Male
  • A man with hemophilia marries a normal woman who is not a carrier. What is the chance their children will inherit hemophilia? Hemophilia is X-linked recessive. www.freelivedoctor.com
  • y X H X h X H X h X H X H X h X H y X H y www.freelivedoctor.com X h X H = Normal Female X H X h = Normal Female (Carrier) X h X h = Hemophilic Female X H y = Normal Male X h y = Hemophiliac Male
  • Genotypes 2 X H X h , 2 X H y Phenotypes 2 Carrier Females 2 Normal Males Probability O% for Hemophilia www.freelivedoctor.com y X H X h X H X h X H X H X h X H y X H y
  • A normal man marries a normal woman who is a carrier for hemophilia. What is the chance their children will inherit hemophilia? www.freelivedoctor.com
  • y X h X H X H www.freelivedoctor.com X h X H = Normal Female X H X h = Normal Female (Carrier) X h X h = Hemophilic Female X H y = Normal Male X h y = Hemophiliac Male X H X H X H X h X H y X h y
  • Genotypes X H X H , X H X h, X H y, X h y Phenotypes 2 Normal Females 1 Normal Males 1 Male Hemophiliac Probability 50% for Male Hemophilic 0% for Female Hemophilic y X h X H X H X H X H X H X h X H y X h y www.freelivedoctor.com
  • The End www.freelivedoctor.com