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Patterns of Inheritance

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Title: Patterns of Inheritance


1
Chapter 9
  • Patterns of Inheritance

2
Heritable Variations and Patterns of Inheritance
  • 1860 Gregor Mendel analyzed inheritable patterns
    and deduced the fundamental laws of genetics
    using garden peas
  • studied flower color and seed shape
  • Inheritance follows a few simple rules
  • Parents pass on to offspring to discrete
    heritable factors which we now know as genes
  • retain individual identities generation to
    generation no matter what

3
Peas
  • Easy to grow with distinguishable varieties
  • Could control matings
  • The egg is within the carpel and the sperm is the
    stamen
  • peas are self-fertilizing
  • sperm falls on carpel and move to the egg and
    fertilize it
  • Mendel controlled by placing bag over flower

4
Cross-Pollination
  • Mendel knew the parentage of his plants because
    he controlled the breeding
  • Choose traits that had 2 distinct forms
  • Worked until he had true breeding varieties
  • when cross you get off-spring identical to parents

5
Genetic Terms
  • Hybrids offspring of 2 true-breeding parents
  • Cross-fertilization of one parent by another
    parent is referred to as a cross
  • Parents are P generation
  • Hybrid offspring is F1 generation (F for filial)
  • When F1 self-fertilize or cross-fertilize the
    offspring are F2 generation

6
Traits Studied
7
Mono-Hybrid Crosses
  • Parents only differ in one trait

8
Cross Between F1 Generation
  • White flower crossed with purple flower F1 was
    all purple Was the ability to make a white
    flower lost
  • F1 x F1 showed that 1 in 4 had white flowers
  • White didnt disappear but purple flower was
    affecting F1 offspring
  • each carried 2 factors for flower color 1 white
    and 1 purple

9
4 Hypotheses Developed by Mendel
  • There are alternative forms of genes that
    determine heritable traits called alleles
  • Alleles are inherited 1 from each parent
  • if the same (2 purple) they are homozygous
  • if different (1 purple, 1 white) they are
    heterozygous
  • If alleles are different and 1 determines
    appearance that allele is dominant (use capital
    letter to ID) and the other is the recessive
    allele (use lowercase letter)
  • Alleles separate during meiosis so eggs or sperm
    have only one allele Law of Segregation
  • fertilization reunites the alleles

10
F1 Generation Cross
  • Egg has a Pp genotype as F1
  • Sperm has a Pp genotype as F1
  • Offspring from a P egg with a P sperm or p sperm
    both will be purple flowered (PP or Pp)
  • Offspring from p egg with a P sperm or p sperm
    one will be purple (Pp) and one white (pp)
  • Combinations of equal likelihood
  • Phenotype is 3 purple 1 white genotype is 1 PP
    2 Pp 1 pp

11
More Terms
  • Phenotype the physical appearance of an
    organism
  • purple or white
  • Genotype the genetic make-up of the organism
  • PP, Pp or pp
  • Law of Segregation applies to all sexually
    reproducing organisms

12
Homologous Chromosomes
  • Carry alleles for the same gene (alleles
    alternative forms of gene) one from each parent
  • Each band is a loci specific location of genes
    on chromosomes
  • genes are on the same spot on both homologues

13
Dihybrid Crosses
  • Also studied seed shape R round, r wrinkled
    and seed color Y yellow, y green
  • Could do dihybrid cross mating of parental
    varieties differing in 2 characteristics
  • Do they come together or are they independent of
    one another?

14
F1 Followed by F2 Cross
  • F1 cross resulted in all yellow round seeds
  • Crossing the F1 offspring should have resulted in
    a 31 phenotype if they came together but that is
    not what was seen

15
F2 Offspring
  • Showed 4 phenotypes (YR, Yr, yR, yr) with 9
    genotypes (9331)
  • Saw 2 new phenotypes wrinkled yellow and round
    green as well as parent types
  • Each characteristic is like a monohybid cross
  • 124 for green is same as 31 if done alone

16
Law of Independent Assortment
  • Each pair of alleles assorts independently of
    other pairs of alleles during gamete formation
  • Inheritance of one trait has no effect on
    inheritance of another trait
  • must be on separate chromosomes

17
Rules of Probability
  • Same as flipping a coin
  • Heads vs tails is essentially dominant vs
    recessive
  • What happens on one toss isnt affected by
    previous tosses
  • Probability of tossing 2 heads
  • 1 of 2 for first and 1 of 2 on the second
  • ½ x ½ ¼

18
Dominant/Recessive Traits
  • Mendels laws apply to many human characteristics
    that are determined by simple dominant recessive
    inheritance
  • Dominant does not mean normal or more common
  • wild-type traits most common in nature is not
    necessarily specified by dominant allele
  • recessive is more common in nature most dont
    have freckles

19
Pedigrees
  • Cant cross test humans so we go backwards and
    look at family history
  • Example of inherited disease such as deafness
  • People with copy of recessive allele but without
    symptoms are called carriers

20
Disease Controlled by Single Gene
21
Recessive Diseases
  • Relatively harmless to very severe
  • Most are born to parents that are carriers but
    normal phenotype
  • Use the Punnett square to determine likelihood to
    have child with recessive disorder
  • ¼ as ½ from each parent
  • 2/3 are carriers as the one that is DD is neither
    a carrier or affected
  • Most disorders are not evenly distributed among
    ethnic groups

22
Not Many Affected
  • Relatively unlikely that 2 people with recessive
    harmful alleles will meet and mate to make a baby
  • See more often when close relatives mate
    inbreeding
  • see in animal populations when trying to get
    certain characteristics but also get weak hips
    and eye problems

23
Dominant Disorders
  • Some are non-lethal extra fingers or toes or
    webbing between toes
  • Some are more serious such as achondroplasia a
    form of dwarfism that has normal body but
    shortened arms and legs
  • homozygous of this allele is usually lethal and
    so only heterozyogotes have chance of passing on
    allele
  • gt99.9 of population has recessive allele

24
Dominant Alleles
  • Dominant alleles that cause a lethal disorder are
    really much rarer than recessive lethal
  • usually results in spontaneous abortion or an
    individual that doesnt live to reproduce
  • this results in elimination of the allele from
    the population

25
Escape of Elimination
  • Some diseases such as Huntingtons disease is not
    evident until an advanced age
  • death occurs 10-20 years from onset
  • by the time symptoms arise they already have
    children and 50 of them will have the disease

26
Variations on Mendels Laws
  • Mendels laws require discrete factors many
    characteristics cannot be explained by Mendel
  • Incomplete dominance phenotype is between that
    of the parents
  • red x white snapdragons yield pink flowered
    offspring
  • 1 red 2 pink 1 white

27
Hypercholesterolemia
  • Caused by recessive allele
  • Normal is HH normal cholesterol
  • Heterozygous Hh 2x normal cholesterol
  • Recessive hh 5x normal cholesterol
  • heart attack by age 2
  • Allele H is a receptor for taking up cholesterol,
    heterozygous has half as many and so increase
    cholesterol level, hh have no receptors

28
ABO Blood Type
  • Most genes occur in more than 2 forms multiple
    alleles
  • Blood type has 3 alleles and co-dominance in
    making up blood type
  • 4 phenotypes A, B, AB and O
  • alleles are responsible for sugars on the surface
    of red blood cells

29
Determining Blood Type
  • Have antibodies to sugar not on your blood in
    your serum
  • 3 alleles are IA, IB and i so have 6 possible
    genotypes
  • Different alleles are co-dominant both are
    expressed in type AB
  • See in cattle and horses with roan coloring (red
    and white hairs to make up roan)

30
  • Dont confuse co-dominance (both alleles
    expressed) with incomplete dominance (express one
    intermediate trait)

31
Pleiotropy
  • Many cases have 1 gene that influences several
    characteristics
  • See in sickle cell disease
  • make abnormal hemoglobin because of 1 amino acid
    exchanged in the protein
  • many side effects in patient
  • only homozygous for allele have disease
  • heterozygous will make both normal and abnormal
    hemoglobin co-dominant

32
Sickle Cell
  • 1 in 500 African-American are homozygous, 1 in 10
    are heterozygous
  • Most common in tropical Africa, in high malaria
    areas
  • Malaria has part cycle in the RBCs and when have
    a sickle crisis, removed from circulation
  • heterozygotes live longer with malaria and
    therefore can pass the gene on to children

33
Polygenic Inheritance
  • Additive effect of 2 or more genes on a single
    characteristic (opposite of pleiotropy)
  • Skin color uses at least 3 genes A, B, C dark
    and a, b, c light
  • AABBCC would be very dark and aabbcc would be
    very light
  • AaBbCc would be in between above but the same
    color as AABbcc
  • 7 levels of pigmentation

34
Effects of Environment
  • In real world can see more than 7 shades of
    skin color
  • Based on environmental factors such as sun
    exposure
  • influenced by genes and environment

35
Chromosomal Basis of Inheritance
  • Mitosis and meiosis were figured out after Mendel
    died
  • Then made the connection between heritable
    factors and chromosomes which was the gene
  • Genes are at specific spots on chromosomes and
    behavior of chromosomes during meiosis and
    fertilization
  • Chromosomal Theory of Inheritance

36
Mendel and Chromosomes
37
Linked Genes
  • Genes located near each other on same chromosomes
  • tend to be inherited together and dont follow
    Mendel's Law of Independent Assortment
  • Used the fruit fly to study genetics and
    discovered this

38
Fly Studies
  • Made observations of gray body with long wings x
    black body with short wings thought we would
    see Mendelian distribution 25 of each
    offspring
  • Of 2300 flies only 17 where the non-wild type or
    non-parent
  • Believed that the genes were on 2 chromosomes
    rather than 4

39
Crossing Over Review
40
Genetic Recombination Crossing Over
  • Gray/short and black/long come from crossing over
    because if linked you wouldnt see these 2
    phenotypes new combinations of alleles
  • Explains the non-parental offspring

41
Recombination Frequencies
  • Can calculate by adding the offspring that comes
    from recombination and divide by total offspring
  • (206 185)/2300
  • Can also be done with parental like offspring as
    well but not called recombinant frequency

42
  • All the inheritance patterns we have talked about
    deal with autosomal chromosomes but some diseases
    are linked to the sex chromosomes

43
Review of Sex Determination
  • Each gamete has 1 sex chromosomes and 22
    autosomes
  • All eggs have 1 X and half the sperm have X and
    half have Y
  • XX girl XY boy
  • SRY gene on the Y chromosome triggers the
    development of testes without developing ovary
    other genes on Y play a role in sperm formation

44
Sex-Linked Genes
  • Also have genes that have nothing to do with
    maleness or femaleness
  • Genes on X or Y are sex-linked most not related
    to gender are on X-chromosomes
  • In flies red eyes are wild type and white are
    rare and on X chromosome (X-linked)

45
Fly Eyes
  • Mate homozygous red eye female to white eye male
  • All offspring are red eyed because eye color is
    determined by X chromosome

46
Additional Crosses
  • Most white eyed flies are male only see a white
    eyed female when you cross a heterozygous female
    mating with white eyed male
  • 22 phenotype and 1111 genotype as they are
    all have different alleles

47
Color Blindness
  • Red-green color blindness is more prominent in
    males than females
  • Allele is on X chromosomes so males do not have
    another one to counteract the bad allele on the X

48
Hemophilia
  • Bleeding disorder that causes bleeding with only
    minor injury
  • See in the royal families of Europe Queen
    Victoria was a carrier and so was her daughter to
    married and passed on to her daughter who
    married and gave to her son

49
Muscular Dystropy
  • A disease that causes weakening and loss of
    muscle tissue
  • The type called Duchenne is sex-linked to the X
    chromosome (Note not all types are sex-linked)
  • Almost all affected are male 1 in 3500 male
    births
  • missing protein found in normal muscle tissue
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