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Patterns of Inheritance


1860 Gregor Mendel analyzed inheritable patterns and deduced the fundamental ... See in cattle and horses with roan coloring (red and white hairs to make up roan) ... – PowerPoint PPT presentation

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Title: Patterns of Inheritance

Chapter 9
  • Patterns of Inheritance

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

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

  • 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

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

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

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

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
  • 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

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

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

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

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?

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

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

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

Rules of Probability
  • Same as flipping a coin
  • Heads vs tails is essentially dominant vs
  • 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

Dominant/Recessive Traits
  • Mendels laws apply to many human characteristics
    that are determined by simple dominant recessive
  • 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

  • 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

Disease Controlled by Single Gene
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

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
  • see in animal populations when trying to get
    certain characteristics but also get weak hips
    and eye problems

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
  • gt99.9 of population has recessive allele

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

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

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
  • 1 red 2 pink 1 white

  • 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

ABO Blood Type
  • Most genes occur in more than 2 forms multiple
  • 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

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
  • 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)

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

  • Many cases have 1 gene that influences several
  • 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

Sickle Cell
  • 1 in 500 African-American are homozygous, 1 in 10
    are heterozygous
  • Most common in tropical Africa, in high malaria
  • 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

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

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

Chromosomal Basis of Inheritance
  • Mitosis and meiosis were figured out after Mendel
  • 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
  • Chromosomal Theory of Inheritance

Mendel and Chromosomes
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

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
  • Of 2300 flies only 17 where the non-wild type or
  • Believed that the genes were on 2 chromosomes
    rather than 4

Crossing Over Review
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

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

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

Review of Sex Determination
  • Each gamete has 1 sex chromosomes and 22
  • 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

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)

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

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

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

  • 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

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
  • missing protein found in normal muscle tissue