Patterns of Inheritance

1
Patterns of Inheritance

• Chapter 23

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Gregor Mendel

• 1822 1884
• Austrian monk
• Experimented with garden peas
• Provided a basis for understanding heredity

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Mendel contd

• Published a paper in 1866 stating that parents
  pass discrete heritable factors on to their
  offspring
• Factors retain individuality generation after
  generation
• Identified that each trait is inherited by a pair
  of factors, one from each parent
• One form of a factor may be dominant over an
  alternative form
• Reasoned that each egg and sperm must contain
  only 1 copy of a factor for each trait

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Mendel contd.

• Mendels law of segregation
• Each individual has two factors (genes) for each
  trait
• The factors segregate (separate) during the
  formation of gametes
• Each gamete contains only one factor from each
  pair of factors
• Fertilization gives each new individual 2 factors
  for each trait

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Modern Genetics Genes

• Sections of chromosomes which give instructions
  for one characteristic or protein.
• Located at the same point or locus, on each
  member of a homologous pair
• All together make up the organisms genome.
• Controls the physical characteristics of a species

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Modern Genetics - Alleles

• Alternative forms of the same gene on each
  chromosome
• One allele comes from each parent
• Dominant allele
• Masks other traits present
• Only 1 dominant allele needs to be present for a
  certain trait to be expressed
• Represented by a capital letter
• Recessive Allele
• 2 copies of the recessive allele need to be
  present for trait to be expressed
• Represented by a lower case letter

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Modern Genetics Alleles contd

• Gene locus
• Fig. 23.2

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Modern Genetics Alleles contd

• Genotype
• Genetic composition of a specific trait
• Homozygous dominant 2 dominant alleles
• Heterozygous 1 dominant allele and 1 recessive
  allele
• Homozygous recessive 2 recessive allele
• Phenotype
• Physical expression of a specific trait
• Homozygous dominant or heterozygous ? Dominant
  Trait
• Homozygous recessive ? recessive trait

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Single Gene Inheritance

• Simplest situation
• One gene carries all the information responsible
  for one trait
• Widows Peak
• Alternative forms of alleles for hairline shape
• Widows peak is dominant to straight hair line
• Wallele for widows peak
• w allele for straight hairline

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Widows peak

• Fig. 23.3

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Genotype related to phenotype

• Table 23.1

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Single Gene Inheritance contd.

• Monohybrid cross
• Looks at inheritance of one trait only
• A punnett squares used to find all possible
  combinations of alleles.

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Single Gene Inheritance contd.

• Example 1
• If a homozygous woman with a widows peak
  reproduces with a man with a straight hairline,
  what kind of hairline will their children have?

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Single Gene Inheritance contd.

• Example
• If two heterozygous parents reproduce what kind
  of hairline will their children have?

W w
W WW Ww
w Ww ww
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Single Gene Inheritance contd.

• Genetic Ratios
• Express ratios of possible outcomes
• Genotypic ratio
• Homozygous Dominant Heterozygous Homozygous
  Recessive
• Phenotypic ratio
• Dominant trait recessive trait
• Often expressed as probability
• The probability resets and is the same for each
  pregnancy!
• Having one child with a trait has no effect on
  future children.

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Single Gene Inheritance contd.

• Look back to example 1
• What are the genotypic and phenotypic ratios?
• WW x ww
• Genotypic Ratio
• 0 4 0
• Phenotypic Ratio
• 10
• 100 Widows peak

• Ww x Ww
• Genotypic Ratio
• 1 2 1
• Phenotypic Ratio
• 3 1
• Probability is ¾ Widows Peak
• 75
• Probability ¼ Straight
• 25

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Single Gene Inheritance contd

• Determining Genotype
• No way to distinguish between a homozygous
  dominant individual and a heterozygous individual
  just by looking
• They are phenotypically the same
• Test cross may help us determine
• Used in breeders of plants and animals
• Cross unknown with a recessive individual
• We know one parent genotype, this will help us
  determine the other genotype
• If there are any offspring produced with the
  recessive phenotype, then the dominant parent
  must be heterozygous

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Single Gene Inheritance contd

• Example In rats, large ears is dominant to
  small ears. A rat breeder has a female rat with
  large ears, which she breeds with a male rat with
  small ears. In the first litter, all rats are
  born with large ears.
• What is the genotype of the female rat?
• In a second litter from the same parents, 4 baby
  rats have large ears, one has small ears.
• What is the genotype of the female rat?

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Single Gene Inheritance contd

• Practice problems
• Both a man and a woman are heterozygous for
  freckles. Freckles are dominant over no freckles.
  What is the chance that their child will have
  freckles?
• Both you and your sibling have attached ear
  lobes, but your parents have unattached lobes.
  Unattached earlobes (E) are dominant over
  attached (e). What are the genotypes of your
  parents?
• A father has dimples, the mother of his children
  does not, and all 5 of their children have
  dimples. Dimples (D) are dominant over no dimples
  (d). Give the probable genotypes of all persons
  concerned.

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Homework (WHAT??? Its BIO!)

• Bikini Bottom Beach Genetics

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Independent Assortment

• Mendel reasoned from the results of his pea plant
  crosses that each pair of factors assorts
  independently into gametes
• Each trait is passed down individually. The
  allele you receive for any one gene is not
  related to any other alleles you receive.
• We can now explain this through independent
  alignment and crossing over in meiosis. (Fig
  23.6)

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Independent Assortment contd

• Fig. 23.6

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Independent Assortment contd

• Called the Law of Independent Assortment
• Each pairs of factors assorts independently
  (without regard to how the others separate)
• All possible combinations of factors can occur in
  the gametes

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Independent Assortment contd

• Practice problems
• For each of the following genotypes, give all
  possible gametes
• WW
• WWSs
• Tt
• Ttgg
• AaBb
• For each of the following, state whether the
  genotype or a gamete is represented
• D
• Ll
• Pw
• LlGg

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Dihybrid Cross

• Punnet squares, considering two-trait crosses at
  one time.
• Example
• The traits for hairline and finger length are
  both single gene traits. As before, widows peak
  is dominant over straight hairline. Having short
  fingers is considered dominant over long fingers.
  
• Two parents who are both heterozygous for both
  traits have children.
• Determine the genotypic and phenotypic ratios for
  their children.

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Dihybrid cross contd

• Possible Gametes?
• Genotypic Ratio?
• Phenotypic Ratio?

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Dihybrid Crosses contd

• Determining Ratios
• Product rule of probability
• The chance of 2 or more independent events
  occurring together is the product of their chance
  of occurring separately
• In our example
• Probability of widows peak ¾
• Probability of short fingers ¾
• What is the probability of widows peak AND short
  fingers?
• ¾ x ¾ 9/16

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Dihybrid Crosses contd

• Recall from our single trait crosses
• Probability of widows peak ¾ Probability of
  short fingers ¾
• Probability of straight hairline ¼ Probability
  of long fingers ¼
• Using the product rule
• Probability of widows peak and short fingers
  X
• Probability of widows peak and long fingers
  X 3/16
• Probability of straight hairline and short
  fingers ¼ X ¾ 3/16
• Probability of straight hairline and long fingers
  ¼ X ¼ 1/16

These values are standard for all heterozygous
crosses! You dont need to memorize them, but
should be able to figure them out in your head!
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Dihybrid Crosses contd

• Using the product rule
• Probability of widows peak and short fingers
• Probability of widows peak and long fingers
• Probability of straight hairline and short
  fingers
• Probability of straight hairline and long fingers
  

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Dihybrid Crosses contd

• Two-trait test cross
• Cross an individual with the dominant phenotype
  for each trait with an individual with the
  recessive phenotype of both traits
• W?S? x wwss

WS W? ?S ??
wwss WwSs Ws?s ?wSs ?w?s
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Dihybrid Cross contd

• Attached earlobes are recessive, What genotype do
  children have if one parent is homozygous for
  earlobes and homozygous dominant for hairline,
  and the other is homozygous dominant for
  unattached earlobes and homozygous recessive for
  hairline?
• If an individual from this cross reproduces with
  another of the same genotype, what are the
  chances that they will have a child with a
  straight hairline and attached earlobes?
• A child who does not have dimples or freckles is
  born to a man who has dimples and freckles (both
  dominant traits) and a woman who does not. What
  are the genotypes of all persons concerned?

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Polygenic Inheritance

• Not fully understood by geneticists.
• Generally
• One trait controlled by 2 or more genes at
  different loci
• The higher the number of dominant alleles you
  possess, the stronger the expression of the
  trait.
• Result is a continuous range of phenotypes
• Distribution resembles a bell curve
• The more gene pairs involved, the more continuous
  the pattern of variation
• Ex human height, skin pigmentation, eye colour

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Polygenic inheritance contd

• Fig. 23.9

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Polygenic Inheritance contd

• Skin color
• Controlled by many gene pairs and many alleles
• Lets assuming a simple model of two alleles at 2
  loci
• A and B
• If two heterozygous parents have children,
  children can range from very light to very dark

Genotype Phenotype
AABB Very Dark Skin
AABb or AaBB Dark Skin
AaBb, AAbb, or aaBB Medium brown skin
Aabb, or aaBb Light Skin
aabb Very light skin
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Polygenic Inheritance contd

• Eye colour is controlled by 3 genes we have
  identified
• We suspect there are more
• Not a clear dominant and recessive
• Brown allele

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Environmental Influences on Inheritance

• Environment can influence gene expression and
  therefore phenotype
• Ex sunlight exposure on skin coat color in
  Himalayan rabbits
• Human twin studies
• Polygenic traits are most influenced
• nature vs. nurture
• Identical twins separated at birth are studied
• If they share a trait in common even though
  raised in different environments, it is likely
  genetic

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Coat color in Himalayan rabbits

• Fig. 23.10

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Incomplete Dominance

• Incomplete dominance
• Heterozygous individual has a phenotype
  intermediate to the two homozygous individuals
• Ex Curly-haired Caucasian woman and a
  straight-haired Caucasian man produce wavy-haired
  children
• When 2 wavy-haired people have children, the
  phenotypic ratio is 1 curly 2 wavy 1 straight

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Incomplete dominance

• Fig. 23.11

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Codominance

• Multiple allele inheritance
• The gene exists in several allelic forms, but
  each person still has only 2 of the possible
  alleles
• Occurs when both alleles are equally expressed
• Ex type AB blood has both A antigens and B
  antigens on red blood cells

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Codominance contd.

• ABO blood types
• IA A antigens on RBCs
• IB B antigens on RBCs
• i has neither A nor B antigens on RBCs
• Both IA and IB are dominant over I, IA and IB are
  codominant

Phenotype Genotype
A IAIA or IAi
B IBIB or Ibi
AB IAIB
O ii
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Codominance contd.

• Paternity testing- ABO blood groups often used
• Can disprove paternity but not prove it
• Rh factor- another antigen on RBCs
• Rh positive people have the antigen
• Rh negative people lack it
• There are multiple alleles for Rh negative, but
  all are recessive to Rh positive

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Inheritance of blood type

• Fig. 23.12

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Practice Problems

• A polygenic trait is controlled by three pairs of
  alleles. What are the two extreme genotypes for
  this trait?
• What is the genotype of the lightest child that
  could result from a mating between two
  medium-brown individuals?
• A child with type O blood is born to a mother
  with type A blood. What is the genotype of the
  child? The mother? what are the possible
  genotypes of the father?
• From the following blood types determine which
  baby belongs to which parents
• Baby 1 type O Mrs. Doe type A Mrs. Jones
  type A
• Baby 2 type B Mr. Doe type A Mr. Jones
  type AB

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Sex-linked inheritance

• Sex chromosomes
• 22 pairs of autosomes, 1 pair of sex chromosomes
• X and y
• In females, the sex chromosomes are XX
• In males, the sex chromosomes are XY
• Note that in males the sex chromosomes are not
  homologous
• Traits controlled by genes in the sex chromosomes
  are called sex-linked traits
• X chromosome has many genes, the Y chromosome
  does not

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Sex-linked inheritance contd.

• X-linked traits
• Red-green colorblindness is X-linked
• The X chromosome has genes for normal color
  vision
• XB normal vision
• Xb colorblindness

Genotypes Phenotypes
XBXB female with normal vision
XBXb carrier female, normal vision
XbXb colorblind female
XBY male with normal vision
XbY colorblind male
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Cross involving an X-linked allele

• Fig. 23.13

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Practice Problems

• Both the mother and the father of a colorblind
  male appear to be normal. From whom did the son
  inherit the allele for colorblindness? What are
  the genotypes of the mother, father, and the son?
• A woman is colorblind. What are the chances that
  her son will be colorblind? If she is married to
  a man with normal vision, what are the chances
  that her daughters will be colorblind? Will be
  carriers?
• Both the husband and the wife have normal vision.
  The wife gives birth to a colorblind daughter. Is
  it more likely the father had normal vision or
  was colorblind? What does this lead you to deduce
  about the girls parentage?
• What is the genotype of a colorblind male with
  long fingers is slong fingers? If all his
  children have normal vision and short fingers,
  what is the likely genotype of the mother?

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Inheritance of linked genes

• The sequence of individual genes on a chromosome
  is fixed because each allele has a specific locus
• All genes on a single chromosome form a linkage
  group
• When linkage is complete, a dihybrid produces
  only 2 types of gametes
• Any time traits are inherited together, a linkage
  group is suspected
• If very few recombined phenotypes appear in
  offspring, linkage is also suspected

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Inheritance of linked genes

• Crossing over between 2 alleles of interest, can
  result in 4 types of gametes
• Occurrence of crossing over can indicate the
  sequence of genes on a chromosome
• More frequent between distant genes

Fig. 23.14
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Practice Problems

• When AaBb individuals reproduce, the phenotypic
  ratio is about 31. What ratio was expected? What
  may have caused the observed ratio?
• The genes for ABO blood type and for fingernails
  are on the same homologous pair of chromosomes.
  In an actual family, 45 of offspring have type B
  blood and no fingernails, and 45 have type O
  blood and fingernails 5 have type B blood and
  fingernails, and 5 have type O blood and no
  fingernails. What process accounts for the
  recombinant phenotypes?