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HUMAN GENETICS AND INHERITANCE PATTERNS

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Title: HUMAN GENETICS AND INHERITANCE PATTERNS Author: Mona Curington Last modified by: Corsicana ISD Created Date: 10/30/2000 5:45:28 PM Document presentation format – PowerPoint PPT presentation

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Title: HUMAN GENETICS AND INHERITANCE PATTERNS


1
HUMAN GENETICS AND INHERITANCE PATTERNS
  • Biology
  • Corsicana High School

2
Linkage
  • Genes are located all along chromosomes
  • A gene is a group of base triplets on the DNA
    molecule
  • Therefore, there are hundreds of genes on each
    chromosome

3
Linkage (continued)
  • During meiosis, one of each homologous pair of
    chromosomes goes into each of the gametes formed
  • All the genes on that chromosome are carried with
    it
  • These genes are linked---they are distributed to
    the same gamete

4
Linkage (example)
  • If genes for seed shape (R, r) and stem length
    (T, t) were carried on same chromosome
  • Genes on the same chromosome do not separate
  • Only 2 possible gametes RT and rt

5
Linkage (continued)
  • linkage---when two genes are carried on the same
    chromosome, they do not separate independently
    when forming gametes
  • Law of Independent Assortment was modified to
    account for this

6
Law of Independent Assortment
  • When the gene pairs on a given pair of
    chromosomes are separated, they are distributed
    to the gametes completely independently of the
    way other gene pairs on other chromosomes are
    distributed.

7
Human Genetics is Hard to Study Because
  • humans have a long generation span
  • humans produce relatively few offspring
  • the influence of the environment is strong on
    many traits
  • many possible gene combinations
  • you cannot do experimental crosses on humans

8
Population Genetics
  • based on finding gene frequencies---how often a
    particular gene occurs in a given population
  • Much of the study of human genetics is population
    genetics

9
Population Sampling
  • testing a cross-section of a population for a
    given trait
  • in certain populations, there are high gene
    frequencies for certain traits

10
Gene Pool
  • all the genes present in a given population
  • any individual member of the population is
    considered to have a random sample of genes from
    the gene pool
  • examples

11
Changes in Gene Pools
  • gene pools change as populations shift
  • individuals move into or out of the population
  • new combinations of genes occur

12
Pedigree
  • a chart which traces the inheritance of a trait
    in a family
  • another way to study human genetics

13
Sex Chromosomes
  • Humans have 22 pairs of autosomes---regular
    chromosomes
  • Humans have 1 pair of sex chromosomes---determine
    if individual is male or female
  • Male XY
  • Female XX

14
Sex Determination Cross
P1 XY x XX
Genotypes 2 XX, 2 XY
X
X
X
X
X
X
X
Phenotypes 2 females, 2 males
Y
X
Y
X
Y
15
Which Parent Determines the Sex of a Baby?
  • the gamete from the father does (the sperm)
  • The sperm can contribute either
  • an X chromosome (for a female)
  • or a Y chromosome (for a male)
  • The egg from the mother can contribute only an X
    chromosome

16
Sex Linkage
  • Some traits can be found mostly in one sex or the
    other
  • These traits are carried on the sex chromosomes

17
Hemophilia
  • free-bleeders
  • lack a clotting factor in the blood
  • can result in great loss of blood from a minor
    wound
  • most hemophiliacs are male (a sex-linked trait)

18
Genetics of Hemophilia
  • gene for hemophilia is a recessive gene carried
    on the X chromosome
  • gene for normal blood (with clotting factor) is
    dominant
  • there is no gene for this trait on the Y
  • Males get only one gene for the trait (on X)
    females get two (two X chromosomes)

19
Genetics of Hemophilia (continued)
  • XhY---hemophiliac male
  • he has only one gene for the trait
  • there is no gene on the Y
  • XHY---normal male

20
Genetics of Hemophilia (continued)
  • XHXh---carrier female
  • she has normal blood because of dominant gene
  • she carries the recessive gene for hemophilia,
    and can pass it on
  • XHXH---normal female
  • XhXh---hemophiliac female (not likely)

21
Sample CrossSex Linkage
normal male x carrier female
XHY
XHXh
XH
Xh
XH
XH
XH
XH
Xh
Y
Y
Y
XH
Xh
22
Genotypes 1 XHXH, 1 XHXh, 1 XHY, 1 XhY
Phenotypes 1 normal female, 1 carrier female 1
normal male, 1 hemophiliac male
23
Sex-Influenced Traits
  • the presence of male or female hormones
    influences the expression of the trait
  • example pattern baldness--dominant in males and
    recessive in females
  • due to the influence of the male hormone
    testosterone

24
Polygenic traits
  • determined by more than one set of genes
  • phenotypes results from the combination of all
    the sets of genes
  • examples hair color, eye color, skin
    pigmentation

25
Human Blood Types
  • determined by genes
  • important in blood transfusions
  • a transfusion with the wrong blood type can cause
    blood cells to clump inside blood vessels, which
    can be fatal

26
Blood Groups
  • there are many blood groups Rh, MN, etc.
  • ABO blood group is the most familiar

27
Genetics of ABO Blood Types
  • multiple alleles---more than two alleles for a
    trait
  • three alleles in this group
  • A and B are codominant
  • O is recessive

28
ABO Blood Types
  • There are four blood types in this group
  • Type A genotypes AA, AO
  • Type B genotypes BB, BO
  • Type AB genotype AB
  • Type O genotype OO

29
Antigens
  • genes result in formation of proteins in the red
    blood cells called antigens
  • People with type A blood have A antigen, those
    with type B have B antigen
  • Those with AB blood have both antigens
  • Those with type O have neither

30
Blood Type Cross
type AB x type O
AB
OO
Genotypes 2 AO, 2 BO
O
O
A
A
A
O
O
Phenotypes 2 type A, 2 type B
B
B
B
O
O
31
Twins
32
Fraternal Twins
  • dizygous
  • formed when two different sperms fertilize two
    different eggs
  • Genetically, they are no more alike than regular
    siblings

33
Identical Twins
  • monozygous
  • one sperm fertilizes one egg. Later, the mass of
    cells formed from the zygote splits into 2
    masses, each growing up into a separate
    individual
  • Genetically identical

34
Genetic Disorders
  • autosomal recessive---examples sickle-cell
    anemia, cystic fibrosis
  • autosomal dominant---example Huntingtons
    disease
  • sex-linked---carried on X chromosome. example
    hemophilia

35
Nondisjunction
  • inherited disorder resulting from failure of
    homologous chromosomes to separate properly in
    meiosis
  • gametes formed have too few chromosomes or extra
    chromosomes

36
Downs Syndrome
  • individual has an extra 21st chromosome in body
    cells
  • also called trisomy-21
  • causes mental retardation, weakness, etc.
  • more common in older mothers

37
Other Conditions from Nondisjunction
  • Turners syndrome---only one X chromosome
    (X_)---results in a short, sterile female
  • Klinefelters syndrome---XXY---sterile male,
    mentally retarded
  • Trisomy X---XXX---female, some mentally retarded,
    some show male traits

38
Genetic Screening
  • examining a persons genetic makeup
  • karyotype---a picture of an individuals
    chromosomes
  • genetic counseling---informs people of genetic
    disorders that could affect offspring

39
Amniocentesis
  • a test for detecting some genetic defects in
    unborn babies
  • sample of amnionic fluid is taken, loose cells
    are cultured and studied

40
Mutation
  • a change in a gene---results in a new trait that
    can be inherited
  • most mutations are harmful (some are lethal)
  • mutations are nearly always recessive

41
Types of Mutations
  • somatic mutation---occurs in body cells. Is not
    passed on to offspring
  • germ mutation---occurs in reproductive cells.
    May be passed on to offspring

42
Cancer
  • uncontrolled growth of cells
  • caused by disorders in gene expression, which
    result in cell division disorders
  • oncogene (onco cancer)---genes that, when
    expressed, can cause a normal cell to become
    cancerous

43
Tumor
  • an abnormal mass of cells resulting from
    uncontrolled cell division
  • benign---cells remain withing the mass no threat
    to life
  • malignant---cells break away and cause new tumors
    in other locations (metastasis)
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