Genetic Inheritance Krt

Embed Size (px)

Citation preview

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 1


    Mendels Legacy Genetics is everywhere

    these days and it willcontinue as a dominantforce in biology andsociety for decades to

    come. Wouldnt it be nice if

    people understood itbetter?

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 2


    1. Introduction

    2. Basic Genetics &Inheritance

    Gene Chromosome Loci Allele Features Gametes Zygote Diploid

    Dominant & Recessive Homozygous Heterozygous

    3. Review

    4. Inheritance Principle of Inheritance

    A Punnett square

    5. Genetic Problems6. Genetic engineering

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 3


    Basic DefinitionGenetics study of inheritance of characteristicsGenome complete set of genetic instructions

    Genomicsfield in which thebody is studiedin terms of


  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 4

    Why is this important? It is important, forexamples ,for Paint Horse breeders tounderstand genetic inheritance so they can breedfor certain coat patterns, possibly raising thevalue of their horses.

    What does a gene do? Genes transfer thegenetic information from a parent to its offspring,determining the appearance of the offspringincluding the coat color and pattern.

    What is geneticinheritance?

    It is the genes (genetic codes)parents pass on to theiroffspring (=keturunan).

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 5


  • 8/3/2019 Genetic Inheritance Krt


  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 7

    What are Genes?

    Physically, genes are linked together like a strand ofpearls to form a chromosome.

    Genes determine color,size, and make-up forevery living species.

    Each equine speciesreceives half of theirgenes from their sire(father) and half from theirdam (mother).

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 8

    What are Chromosomes?

    Chromosomes are long pieces ofDNA with supporting proteins, andmade up of genes.

    Every Paint Horse has 64chromosomes.

    Each chromosome was initiallycreated through fertilization,where genetic information fromthe sire (father) was united withgenetic information from the dam(mother).

    Loci and Alleles are also found onchromosomes.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 9


    The instructions that tell cellswhat we look like are carried inthese. There are 23 pairs ofthem in a normal human cell.


    These are the units which

    make up chromosomes thatare responsible forinheritance of specificcharacteristics



  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 10

    Human Chromosomes

    We have 46chromosomes, or 23pairs.

    44 of them are calledautosomesand arenumbered 1 through 22.Chromosome 1 is thelongest, 22 is the

    shortest. The other 2 chromosomes are the sex chromosomes:the X chromosome and the Y chromosome.

    Males have and X and a Y;XY

    Females have 2 Xs; XX.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 11 11

    Chromosomes and Genes Come inPairs

    23 pairs of chromosomes pairs 1-22 are autosomes pair 23 are sex chromosomes


  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 12

    What are Loci?

    Loci The different points or markers along a


    Loci are like street signs in a busy city,allowing us to find our way along acomplex chromosome.

    At a specific locus, scientists canlocate certain genes. These genes

    appear at that locus for everyindividual of that species.

    At one locus there can be two AllelesLoci are numbered. Thisdepicts a chromosome half.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 13

    What are Alleles?

    Alleles: The different versions of genes or alternativeversions of genes that account for variations in inheritedcharacteristics (variant forms of the same gene). Twoalleles are found at each loci on a chromosome.

    For each character, an organisminherits 2 alleles, one from eachparent

    If the two alleles differ, then one, thedominant allele, is fully expressedin the organisms appearance; theother, the recessive allele, has nonoticeable effect on the organismsappearance

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 14

    Genes, Alleles, and Chromosomes

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 15

    For exampleConsider youreye color

    You have a loci on a chromosome for eye color. Atthat loci there are two allelesone from yourmother and one from your father.

    The dominant of the two alleles is the eye color

    that you display. This is yourphenotype(the eyecolor seen).

    The genetic classification (what we cant see butwhat is on the loci) is called thegenotype.

    The alleles for each character segregate(separate) during gamete production (meiosis).

    Mendels Law of Segregation

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 16

    Mendelian genetics


    (heritable feature, i.e., fur color)


    (variant for a character, i.e.,

    brown) True-bred

    (all offspring of same variety)


    (crossing of 2 different true-breds)

    P generation (parents)

    F1 generation (first filialgeneration)

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 17

    A Mendelian Genetic Primer

    Genes come in pairs that separate in theformation of gametes.

    The members of the pair may be identical(homozygous) or non-identical(heterozygous).

    Each form of a particular gene is an allele. Only two alleles of a given gene are possible in

    an individual although many alleles of a geneare possible within a population.

    One allele is dominant over another (or soMendel believed).

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 18


    things like eye colour,

    skin colour and haircolour which are

    controlled by genes.

    GAMETESSperm and egg cells are both

    this type of cell. Contain halfthe amount of DNA of normaldiploid cells

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 19


    When a sperm and egg cellfuse together, they producethis.


    We use this word todescribe cells which

    contain the full complementof genetic material. Inhumans this would be 46chromosomes (23 pairs)

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 20

    What is Dominant or Recessive Inheritance?

    Simple Dominance explains how genes are expressed.1. Dominant allele masks the phenotype of the

    recessive allele

    2. Recessive allele is expressed only if in a doubledose (homozygous)3. In Simple Dominance, two genes (one received from

    each parent) are passed on to the offspring. Theparents genotype determines the genotypicpossibilities of the offspring.

    4. In Simple Dominance, one gene is dominant over theother. The characteristic for which this gene codes isphysically displayed.

    5. Scientists identify this dominant gene with acapital letter.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 21

    What is Simple Dominance?

    6. The recessive form of the gene is submissiveto the dominant form and will not be physicallydisplayed. However, it will always be found in

    the genotype and could be passed on to thisindividuals offspring.

    7. Scientists identify this recessive gene witha lower-case letter.

    8. Because two genes are passed to an offspring,several pairing possibilities can occur.Homozygous or heterozygous pairing canoccur in the offsprings genotype.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 22

    What is Homozygous?

    Homozygous1. Homo means same.

    2. Homozygous identical alleles3. A homozygous genotype is two of the

    same alleles (two dominant or tworecessive) at one locus.

    4. Consider the Black or Sorrel (=merah bata)base coat colors. Ecodes for Black ande codes for Sorrel. Ehas simpledominance over e.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 23

    What is Homozygous Pairing?


    4. A homozygous dominant pair for Black basecoat isEE. If one dominant Egene is in the

    genotype, the Black coat is expressed. This isan example of simple dominance becauseonly one dominant Eresults in a Black coat.

    5. A homozygous recessive pair for a Sorrelcoat is ee. Becauseeis recessive, the only

    way a Sorrel coat will be expressed is if youhave a homozygous recessive genotype ofee.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 24

    What is Heterozygous?

    Heterozygous Hetero means different or other. heterozygous different alleles A heterozygous genotype is two different alleles at

    one locus. One allele of the pair is dominant, while the other is

    recessive. For example, what color would a foal with the

    genotype Eebe? Remember, Ehas simple

    dominance over e, and Ecodes for Black where ecodes for Sorrel.

    The coat color will be Black becauseEisdominant.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 25


  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 26

    Lets Review

    1. What are genes?

    Answer:Genes are tiny units of inheritance foundin DNA that code for the make-up of an individual.

    2. What makes up a chromosome?

    Answer:Genes make up chromosomes, loci arefound on chromosomes and alleles are found atloci.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 27

    Lets Review

    3. What is the title given to specific parts along achromosome where two alleles are found?


    4. Why are there two alleles at each loci?

    Answer:Two alleles are at each loci because theoffspring receives one from its mother and one fromits father.

    5. What is genotype?Answer:The genotype is the genetic material thatwe cant see expressed on the outside of anindividual, such as a recessive allele.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 28

    Lets Review

    6. What is phenotype?

    Answer:The phenotype is the physicalappearance of an individual, a characteristic thatwe see, such as that determined by a dominant

    allele.7. In simple dominance, what allele will be


    Answer:The dominant allele is expressed insimple dominance.

    8. If I have a heterozygous pair of alleles, whatgenotype do I have?

    Answer: I have one dominant allele and onerecessive allele.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 29



  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 30

    Mendels Principles of Inheritance

    Inherited traits are transmitted by genes which occurin alternate forms called alleles

    Principle of Dominance - when 2 forms of the same

    gene are present, the dominant allele is expressed Principle of Segregation - in meiosis two alleles

    separate so that each gamete receives only one formof the gene

    Principle of Independent Assortment - each trait isinherited independent of other traits (chance)

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 31

    Gregor Mendel

    Gregor Mendel (1822-1884)

    Belief at the time wasthat the traits of parentswere fused or blended,

    no real idea of how that


  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 32

    Gregor Mendel

    Mendel studied the garden pea

    1. The pea was a good choice because there were alarge number of true breeding varieties, shortgeneration time, easy to grow and perfect flowers


    Mendel observed variations in the garden pea as tothe height, flower color, seed coat color, and seedshape. In each case there were distinct contrastingforms.

    3. Mendel bred his plants over many generations and

    counted the variations in each successive generation.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 33

    Mendels Model Organism

    The Garden Pea

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 34

    Gregor Mendel

    4. He focused on 1 or 2 traits in each experiment. Aftercollecting all the data he analyzed the results andderived his conclusions.

    5. He crossed plants with different traits, and learned that

    the offspring showed the dominant trait. Most often butthat expressions of the recessive traits showed up in aconsistent ratio. It was Mendels records that showed

    him the way.

    6. Mendel found that inheritance of traits was not dueto blending but instead the passing of specifictraits, or units of inheritance, what we now cal ledgenes

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 35

    Gregor Mendel

    7. Mendel crossed a tall growing variety of pea with ashort growing variety. If the fusing theory was correct

    you should get an intermediate height plant

    8. But instead they resembled the tall parent, but it thisgeneration were allowed to breed there would bethrowbacks to the short variety at a ratio of 3:1 (3 tallfor every one short)

    9. No matter what trait he selected for the second

    generation always contained individuals of both traitsalways on a 3:1 ratio.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 36

    Gregor Mendel

    10. Mendel predicted the discovery of genes (specificunits of inheritance) but also the existence of Alleles,the pairing of two alternate forms of the gene.

    11. He deduced that during the formation of gametes(haplod reproductive cells), the two alleles separatefrom one another so that each gamete has only one ofthe alleles.

    12. During sexual reproduction, the allelic pairs are

    restored when the sperm and egg combine.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 37

    Gregor Mendel

    13. Today we know a gene is a portionof DNA strandevery gene contains

    the information necessary tomanufacture a special polypeptide orprotein.

    14. Genes are the unit of DNA whichcontains the information code for aspecific amino acid.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 38

    The Reality of Round and Wrinkled TwoAlternative Traits of the Seed Shape Character

    Note that each ofseed is a newindividual of a

    different generation seeds are not ofthe same generationas the plant thatbears them.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 39

    Mendels Monohybrid Cross P to F1

    A Punnett square,something well cover in a


  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 40

    Mendel Continued crosses to the F2 (the grandchildren)

    What was learned?The green trait was not lost oraltered, even though itdisappeared in the F1.

    One trait is dominant to theother in its expression.The reappearance of therecessive trait in of the F2suggests genes come in pairsthat separate in the formationof sex cells.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 41

    Monohybrid Crosses and the Principle ofSegregation

    A cross between individualsdiffering in single character is amonohybrid cross.

    The analysis of monohybridcrosses allowed Mendel to deducethe Principle of Segregation ....

    Genes come in pairs that separatein the formation of sex cells (andthese sex cells unite randomly atfertilization).

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 42

    Principle of Segregation

    The principle of segregation is explained by the behavior ofhomologous chromosomes at meiosis.


  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 43

    IV b. A Punnett Square

    In a Punnettsquare for amonohybrid

    cross, thePrinciple ofSegregation isapplied.

    A Punnett Squareis a Handy Way ofAnalyzingCrosses

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 44

    Different Genotypes Can Produce theSame Phenotype

    1 2 3

    1 2

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 45

    Consistency isGood


    investigated by


    No matter what thecharacter, Mendelobserved a 3:1 ratio of

    characters in the F2.

    Monohybrid Crosses Yielded Consistent Results

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 46

    Monohybrid Crosses Yielded Consistent Results

    Therefore, the Principle of Segregation indeed is ageneral principle of genetics.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 47

    What Works for Peas Also Works for Humans

    An albino woman

    In the cross Aax Aa,where A is a dominantallele for wild type

    (standard) pigmentationand ais a recessive allelefor no pigmentation(albinism), of offspringwill be wild type and willbe albino.

    Wh did M d l l d th t th

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 48

    Why did Mendel conclude that theinheritance of one trait is independentof another?

    Because its the only way to explain

    the pattern of inheritance.



  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 49

    The PunnettSquare for aDihybrid Cross

    Note that were

    simultaneouslyapplying the

    Principles ofSegregations andIndependentAssortment.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 50

    The Independent Alignment ofDifferent Pairs of HomologousChromosomes At Meiosis

    Accounts for the Principle ofIndependent Assortment

    The alignment of one pair

    of homologs isindependent of any other.

    Principle of Independent Assortment:

    The assortment of one pair of genesinto gametes is independent of theassortment of another pair of genes.

    Wh k f

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 51

    Consider a crossbetween parentsheterozygous for bothdeafness andalbinism.This is the same9:3:3:1 ratio seen for

    Mendels cross

    involving pea color

    and shape.

    What works forpeas also worksfor humans

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 52

    Some Alleles are relatedthrough incompleteDominance

    Dominance relationships maydiffer, but the Principle of

    Segregation remains thesame.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 53

    Pleiotropy When One Allele InfluencesMany Traits

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 54

    Pleiotropy inAction Anemia,


    weakness,impairedgrowth,liver andspleen



    (phenotypes)associated withthe sickle cellallele.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 55

    Polygenic Inheritance When a Single Traitis Influenced by Many Genes

    Height is apolygenic trait

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 56

    Multiple Alleles

    Many genes are presentin 3 or more versions(alleles) this is known

    as multiple alleles.The human ABO bloodgroup is determined bythree alleles (IA, IB, and io)

    of a single gene.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 57

    CodominanceThe human ABO blood groupillustrates another geneticphenomenon codominance.

    Codominance occurs whenthe phenotype associated with

    each allele is expressed in theheterozygote.

    The AB



    IB) is an

    example of


  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 58


  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 59

    Human Chromosomes

    Chromosome 1 isthe longest, 22 isthe shortest.

    The other 2chromosomes arethe sexchromosomes: theX chromosome and

    the Y chromosome.

    We have 46 chromosomes (23 pairs), and 44 of them arecalled autosomes and are numbered 1 through 22

    Males have and X and a Y; females have 2Xs: XY vs. XX.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 60

    1. Chromosomal Disorders

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 61

    Chromosomal Disorders

    Polyploidy extra set of chromosomes

    most embryos die

    Aneuploidy missing a chromosome or having an extrachromosome

    results from nondisjunction

    trisomy is the condition of having an extra

    chromosome monosomy is the condition of missing a


    Euploid is a normal chromosome number

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 62

    Causes of Aneuploidy

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 63

    Prenatal Tests

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 64

    Prenatal Tests

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 65

    2. Colorblindness

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 66


    We have 3 colorreceptors in the retinas ofour eyes. They respond

    best to red, green, andblue light.

    Each receptor is made bya gene. The bluereceptor is on anautosome, while the redand green receptors areon the X chromosome(sex-linked).

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 67


    Most colorblind people aremales, who have mutated,inactive versions of either thered or the green (sometimes

    both) color receptors. Most females with a mutant

    receptor gene areheterozygous: the normal

    version of the receptor genesgives them normal colorvision.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 68

    Inheritance of Colorblindness

    A heterozygous female has normalcolor vision. Sons get their only Xfrom their mother. So, of the sonsof a heterozygous mother arecolorblind, and are normal.

    A colorblind male will give his

    X to his daughters only. If themother is homozygousnormal, all of the children willbe normal.

    However, the daughters will heterozygous carriers of the trait,and of their sons will be colorblind.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 69

    3. Sex Determination

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 70

    Sex Determination

    The basic rule: if the Y chromosome is present, the person is

    male. If absent, the person is female.

    In meiosis, the X and Y chromosomes separate and go

    into different sperm cells: the sperm carry the X and the other half carry

    the Y All eggs have one of the mothers X chromosomes, so

    when they are fertilized

    of the zygotes are XX (female), and are XY (male) The Y chromosome has the main sex-determining gene

    on it, called SRY.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 71

    Sex Determination

    About 4 weeks after fertilization, an embryothat contains the SRY gene developstestes, the primary male sex organ

    The testes secrete the hormonetestosterone that signals the other cells ofthe embryo to develop in the male pattern.

    If the embryo does not have the SRY gene,it develops ovaries instead, which secreteestrogen and causes development in thefemale pattern.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 72

    Sex Determination

    An egg contributes anX chromosome

    A sperm contributeseither an X or a Ychromosome

    A gene on the Y chromosome (SRY)determines sex

    Sex Chromosomes and Their Genes

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 73 73

    Sex Chromosomes and Their Genes X chromosome

    has over 1,500 genes

    most genes on the X chromosome do not havecorresponding alleles on the Y chromosome

    Y chromosome has only 231 protein-encoding genes

    some genes are unique only to the Y chromosome

    Sex-linked Genes Y-linked genes are transmitted only from father to


    X-linked genes are transmitted from father todaughter or from mother to daughter or son

    Hemophilia A is a sex-linked disorder

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 74

    Sex Determination

    The basic rule: if the Y chromosome is present, the person is male If the Y chromosome is absent, the person is

    female In meiosis, the X and Y chromosomes

    separate and go into different sperm cells: the sperm carry the X and the other half carry the Y.

    All eggs have one of the mothers Xchromosomes, so when they are fertilized, of the zygotes are XX (female), and are XY (male).

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 75

    Sex Determination

    The Y chromosome has the main sex-determining gene on it, called SRY.

    About 4 weeks after fertilization, an embryothat contains the SRY gene developstestes, the primary male sex organ. Thetestes secrete the hormone testosterone.Testosterone signals the other cells of theembryo to develop in the male pattern.

    If the embryo does not have theSRYgene,it develops ovaries instead, which secreteestrogen and causes development in thefemale pattern.

    A few oddities

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 76

    It is possible to be XY and female through twoways i.e.

    1. the SRY gene can be inactivated by a mutation.If SRY doesnt work, testes dont develop andthe embryo develops as a normal female.

    2. In a condition called androgen insensitivity,

    the person is XY with a functional SRY gene,but her cells lack the testosterone receptorprotein, so the cells dont ever get the messagethat the testosterone is sending. Testesdevelop in the abdominal cavity (=rongga

    perut), and no ovaries, fallopian tubes, oruterus develop. At puberty, the internal testessecrete testosterone, which gets converted intoestrogen and the body develops as a normal(but sterile) adult female.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 77

    4. Hermaphrodites

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 78

    Hermaphrodites In some cases, androgen insensitivity is only partial: the

    cells respond a little bit to testosterone produced by the

    testes.The embryo develops with ambiguous genitalia,

    neither completely male not completely female. Sucha person is sometimes called ahermaphrodite.

    Another condition, congenital adrenal dysplasia, causesthe adrenal glands to produce an abnormally large

    amount of testosterone in a female embryoThis can also cause development of ambiguousgenitalia, ahermaphrodite.

    Another rare condition: a chimera occurs when twoseparate embryos fuse together.

    This can result in a person with some XX cells andsome XY cells. Such a person can have both testesand ovaries, atrue hermaphrodite. This conditionis extremely rare: more people say they have it thanactually do

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 79

    5. Down syndrome

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 80

    Chromosome Variations Changes in number and structureare possible: first look at numbervariations.

    Aneuploidy: having an extra ormissing chromosome is fairlycommon in sperm and eggs. Non-disjunction in meiosis causeschromosomes to not separate

    equally into the gametes. The rate of non-disjunction in

    males is constant: 1-2% of spermhave an extra of missingchromosome. But in females, therate increases marked with age.

    This is illustrated by the frequencyof Down syndrome births atdifferent ages of mother. Downsyndrome is the most frequentresult of non-disjunction.

    Chromosome Number Variations

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 81

    Except for the X and Y, humans dont survive

    with only 1 copy of any chromosome. Also, 3copies is lethal in most cases.

    Aneuploidy is a major cause of spontaneousabortion in early pregnancy.

    Down Syndrome is the most common humananeuploidy. It is also called trisomy-21,

    meaning 3 copies of chromosome number 21. People with Downs have a characteristic

    appearance: flattened face, turned up nose,epicanthal folds at the outer corners of the eyes. Inmost cases the diagnosis is made immediately atbirth. Heart defects, protruding tongue, and mentalretardation are also found in most people withDowns. Occurs about 1 in 1000 births.

    There are also translocational and mosaic forms ofDown syndrome

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 82

    Chromosome Number Variations

    Translocational involves a chromosome structurechange (2 chromosomes get hooked together) and isinherited. With translocational Downs, if one child ina family has it, others are likely to also get it. Occurs

    in about 5% of Downs cases. Mosaic Downs means having some cells with

    trisomy 21 and other cells normal. The personsphysical appearance and mental condition dependson exactly which cells are which. About 3% of all

    Down syndrome cases.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 83

    Chromosome Number Variations

    S S Ch A l idi

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 84

    Some Sex Chromosome Aneuploidies

    Non-disjunction can also resultin a person with 2 Xs and a Y:47,XXY. This is calledKlinefelter Syndrome.

    The Y chromosome makes aperson with Klinefelters male:possessing testes.

    Symptoms: female body hairpattern, breast development,sterile, can be somedevelopmental delay orretardation, especially forverbal skills.

    Often not diagnosed, or diagnosed only accidentally. Most symptoms are helped by testosterone treatment.

    T S d

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 85

    Turner Syndrome Also called XO, because people with

    Turners have only 1 X chromosome: 45, X.

    No Y means Turners people are female.However, no ovaries develop, so they dontundergo the body changes of puberty andthey are sterile.

    Hormone treatment curesall but the sterility.

    Other symptoms: shortstature, webbed skin andlow hairline at the neck,

    some oddities of spatialperception. Not retarded.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 86

    Other Number Variations

    Triplo-X, having 3 X chromosomes. No Ychromosome means female. Many with thissyndrome are undiagnosed because they haveno symptoms. Some have slight social anddevelopmental problems, especially language-related. Occasional fertility problems, but manyhave normal fertility. Not well studied.

    XYY: having 2 Y chromosomes plus an X. Malebecause they have a Y. Many are never

    diagnosed due to a lack of symptoms. Tend tobe taller, more physically active, slightly retarded,prone to acne.

    Chromosome Structure

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 87

    Chromosome StructureVariations

    Chromosomes can be broken by X-rays and by certain chemicals. Thebroken ends spontaneously rejoin,but if there are multiple breaks, the

    ends join at random. This leads toalterations in chromosome structure. Problems with structural changes: breaking the

    chromosome often means breaking a gene. Sincemost genes are necessary for life, many chromosomebreaks are lethal or cause serious defects.

    Also, chromosomes with structural variations often havetrouble going through meiosis, giving embryos withmissing or extra large regions of the chromosomes.This condition is aneuploidy, just like the chromosomenumber variations, and it is often lethal.

    Chromosome Structure Variations

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 88

    The major categories:duplication (an extra copy of aregion of chromosome), deletion (missing a region ofchromosome), inversion (part of the chromosome isinserted backwards, and translocation (two differentchromosomes switch pieces).

    There are lots of ways

    chromosomes can changestructure, so the syndromesare not as well defined aswith number variations.

    Cri-du-chat syndrome

    comes from a deletion of oneend of chromosome 5, so theperson only has 1 copy of allthe genes on this end of thechromosome.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 89

    Structure Variation Examples

    The name means cats cry, because their crysounds vaguely like a cats meow. People withthis condition are severely retarded, as well ashaving a variety of physical problems.

    Translocational Down syndrome is caused bymost of chromosome 21 becoming joined withchromosome 14. Some children of a personwith this translocation will inherit thetranslocation as well as 2 normal chromosome21s. This results in trisomy-21: having 3copies of the chromosome, which gives Downsyndrome.

    S li k d G

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 90

    Sex-linked Genes

    Genes on the X chromosome are called sex-linked,because they expressed more often in males than infemales

    There are very few genes on the Y chromosome. Since males only have one X chromosome, all genes on

    it, whether dominant or recessive, are expressed. In contrast, a mutant gene on an X chromosome in a

    female is usually covered up by the normal allele on theother X. Most mutations are recessive. So, most peoplewith sex-linked genetic conditions are male.

    Another fact about sex-linked genes. Males produce their sperm with their X chromosome, and half with their Ychromosome. The X-bearing sperm lead to daughtersand the Y-bearing sperm lead to sons. So, sons get theironly X from their mothers, and the fathers X goes only todaughters.

    The Y chromosome is passed from father to son.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 91

    6. Hemophilia

    H hili

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 92


    Hemophilia is a disease in which the blood does not clot

    when exposed to air. People with hemophilia can easilybleed to death from very minor wounds. Hemophilia isanother sex-linked trait.

    Hemophilia is treated by injecting the proper clottingproteins, isolated from the blood of normal people. Inthe early 1980s, the blood supply was contaminated byHIV, the AIDS virus, and many hemophiliacs contractedAIDS at that time.

    Queen Victoria of England, who lived through most of

    the 1800s, apparently had a mutation on one of her Xchromosomes that caused many of her descendants tohave hemophilia. Most importantly, Alexis, son of theCzar of Russia had it, which contributed to the RussianRevolution and the rise of communism.


  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 93

    S I fl d T it

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 94

    Sex-Influenced Traits Some traits appear to be specific to

    one sex, but are not sex-linked: theirgenes are not on the X chromosome.

    Such a trait is called sex-influenced.More specifically, a trait that isdominant in one sex but recessive inthe other is a sex-influenced trait.

    The best human example is malepattern baldness.

    Baldness is dominant in males:heterozygotes and homozygotesboth become bald

    In females, baldness is recessive: only homozygotes (which arerelatively rare) become bald. Also, females tend to lose hair moreevenly than men, giving a sparse hair pattern rather than completelybaldness.

    BUT: this may be an oversimplification.

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 95 95

    Gender Effects on Phenotype

    Sex-limited trait affects a structure or function of the body that

    is present in only males or only females examples are beards(=jenggot) or growth of

    breasts (dada)Sex-influenced inheritance

    an allele is dominant in one sex andrecessive in the other

    baldness is an example heterozygous males are bald but

    heterozygous females are not

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 96

    7. Gene Therapy

    Gene Therapy

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 97 97

    Gene Therapy

    group of experimental techniques that alter, replace,silence, or augment a genes function to improve, delay

    or prevent symptoms heritable gene therapy

    introduces the genetic change into a sperm, egg, orzygote changes passed to future generationscommon in plants; not done in humans

    nonheritable gene therapy targets only affected cells changes not passed to future generations

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 98 98

    Gene TherapyTargets

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 99



  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 100


    The altering of the character of anorganism by inserting genes fromanother organism

  • 8/3/2019 Genetic Inheritance Krt


    KRT-2008 101

  • 8/3/2019 Genetic Inheritance Krt