IB Biology Review Chapter 14: Mendelian Genetics 4. A parent

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IB Biology Review

• Chapter 14 Mendelian Genetics

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Who is Gregor Mendel?

• Known as the father of modern genetics
• Conducted experiments with pea plants to discover
  how traits were passed on from generation to
  generation
• Identified the concept of dominant and recessive
  traits and several laws of heredity

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Mendelian Genetics Vocab

• Trait
• A heritable feature such as flower color
• Allele
• Alternate versions of a gene that produce
  different phenotypes
• Dominant Allele
• An allele that is fully expressed in the
  phenotype of a heterozygote
• Recessive Allele
• An allele whose phenotype is not observed in a
  heterozygote
• Homozygous
• Having two identical alleles for a given gene
• Heterozygous
• Having two different alleles for a given gene
• Carrier
• An individual who is heterozygous with one normal
  allele and one potentially harmful recessive
  allele. The individual is phenotypically normal
  but can pass on the harmful allele

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Mendelian Genetics Vocab cont.

• Genotype
• The genetic makeup or a set of alleles of an
  organism (ie. Aa, AA, aa)
• Phenotype
• The physical traits which are determined by
  genotype (many phenotypes are microscopic in the
  phenotype of a heterozygote)
• True Breeding
• Plants that-when self-pollinated- always produce
  the same phenotypic traits (homozygous dominant
  or recessive)
• Hyrbridization
• The mating or crossing of two true-breeding
  varieties (true breeding parents- P generation)
  Produces all heterozygous F1 generation offspring
• Test Cross
• Breeding an organism of unknown genotype
  (heterozygous or homozygous dominant) with a
  homozygous recessive individual to determine the
  unknown genotype. The Ratio of phenotypes in
  offspring determines unknown genotype
• Monohybrid Cross
• The cross between two heterozygotes (F1
  generation) for a single trait (ie. Bb x Bb)
• Dihybrid Cross
• The cross between two heterozygotes (F1
  generation) for two traits (ie. YyRr x YyRr)

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Mendelian Genetics Vocab cont.

• Complete Dominance
• The situation when the phenotypes of the
  heterozygote and the dominant homozygote are
  indistinguishable
• Incomplete Dominance
• The situation in which the phenotype of
  heterozygotes is in between the phenotypes of
  homozygous individuals for either allele
• Codominance
• The situation where the phenotypes of both
  alleles are exhibited in the heterozygote
• Multiple Alleles
• When there are three or more alleles for a single
  gene (as in ABO blood groups)
• Polygenic Inheritance
• An additive effect of two or more genes on a
  single phenotypic trait
• Pedigree Analysis
• Predicting the genotypes of individuals in a
  pedigree chart based on the phenotypes of
  offspring

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Example of Complete Dominance

• If you cross Tall (TT) and Short (tt), what do
  you expect?
• All talls?
• Some tall, some short?
• All shorts?
• Medium height?
• Since T is completely dominant, offspring are
  all tall

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Example of Incomplete Dominance

• If you cross CRCR and CWCW, what do you expect?
• All red (CRCR)
• All white? (CWCW)
• Some red and some white?
• All pink?
• Some red, some white, some pink?
• All pink, because the phenotype is a mixture of
  both traits
• What do you expect for the F2 offspring?

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Example of Codominance

• If you cross STST and SRSR,what do you expect?
• All triangle spots?
• All round spots?
• Some triangle, some round?
• No spots?
• Both spots?
• Both spots, because both traits are expressed
  without mixing

STST SRSR
STSR
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What is the law of segregation?

• Mendels first law- stating that Each allele in a
  pair (diploid set) separates into a different
  gamete (haploid) during gamete formation
• Offspring receive only one allele for a trait

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What is the law of independent assortment?

• Mendels second law that Each pair of alleles
  separates independently during gamete formation
• (Means which allele of Trait A a gamete receives
  is not at all related to which allele of Trait B
  the gamete receives)
• This law only applies when genes for two traits
  are located on different chromosomes.

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The influence of the law of segregation and the
law of independence on gamete formation.

• Both laws mix up the genes so that each gamete
  receives
• genetic diversity

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Punnet Square

• Used to examine how one trait will be passed down
  to offspring
• Use a Punnet Square with 4 cells

Parent/Organism One
Parent/Organism Two
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Monohybrid Cross

• Cross between two heterozygotes for a single
  trait
• What would the genotypes of the parents be in a
  monohybrid cross?
• Aa x Aa
• What would the genotypes of the offspring be?
• AA, Aa, and aa
• What would the ratio ofphenotypes in the
  offspring be?
• 121
• 1 Homozygous Dominant 2 Heterozygous 1
  Homozygous Recessive

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Test Cross

• Used to identify the unknown genotype of one
  individual
• Cross an unknown genotype with a
• Homozygous recessive phenotype
• Why homozygous recessive?
• Because you know the genotype
• Two recessive alleles

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Example of Test Cross

• In Bodine mice, blue fur is dominant and white
  fur is a recessive trait. A captured mouse has
  blue fur. How would you determine its genotype?
• Mate the blue mouse with a white mouse
• Possible genotypes and offspring
• If the offspring were all blue (Bb), what is the
  genotype of the original blue mouse?
• Homozygous dominant (BB)

Captured Blue Mouse
Captured Blue Mouse
Hom. Rec. White Mouse
Hom. Rec. White Mouse
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Two-Trait Punnet Squares

• Used to examine how two non-linked traits are
  passed down in relation to each other
• Use a Punnet Square with 16 cells
• Dihyrbid crosses use 16-cells cross between two
  heterozygotes

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How Dihybrid Crosses Work

• Remember dihybrid is a cross between two F1
  individuals (AaBb)
• Combine the traits like a regular 4-celled
  Punnet Square
• Work from left to right

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How would you set up a dihybrid cross between
BbEe and BbEe?

• Arrange the traits in pairs,
• one B / b per cell
• one E / e per cell
• How would the allele arrangement look for
  organism 1?
• Organism 2?
• Now solve the Punnet Square

Org. 1
Org. 2
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How many different types of offspring do we have?

• Dominant B, Dominant E
• BBEE, BbEE, BBEe, BbEe
• Dominant B, Recessive e
• Bbee, Bbee
• Recessive b, Dominant E
• bbEE, bbEe
• Recessive b, Recessive e
• bbee

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Different Genotypes in Dihybrid Cross

• How many different genotypes?
• Try to find them.
• Nine
• Homozygous dominant B and E
  (BBEE) (1)
• Homozygous dominant B, heterozygous E
  (BBEe) (2)
• Heterozygous B, homozygous dominant E (BbEE) (2)
• Heterozygous B, heterozygous E
  (BbEe) (4)
• Homozygous recessive B, homozygous dominant E
  (bbEE) (1)
• Homozygous recessive B, heterozygous E
  (bbEe) (2)
• Homozygous dominant B, homozygous recessive E
  (BBee) (1)
• Heterozygous B, homozygous recessive E
  (Bbee) (2)
• Homozygous recessive B, homozygous recessive
  E (bbee) (1)

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Different Phenotypes in Dihybrid Cross

• How many different phenotypes?
• Four
• 1. Dominant B and Dominant E
• 2. Dominant B and Recessive e
• 3. Recessive b and Dominant E
• 4. Recessive b and recessive e
• How many of each?
• 1. nine
• 2. three
• 3. three
• 4. one

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Recombinants

• Recombinants are the offspring that have a
  different phenotype than their parents
• If parents are XxHh and xxHh,
• Recombinants are
• Xxhh
• xxhh

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Sex Linkage / Sex-Linked Traits

• When the gene for a trait is on the X or Y
  chromosome (sex chromosomes)
• Sex traits express themselves more in one sex
  than the other- often more in males than females.
• In humans the term usually refers to X-linked
  characters genes located only on X chromosomes
• Fathers can pass X-linked alleles to their
  daughters, but not sons
• Mothers can pass sex-linked alleles to both sons
  and daughters

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Dominant Sex-Linked Traits

• Dominant gene on X chromosome
• Affected males pass to all daughters and none of
  their sons
• Genotype XAY
• If the mother has an X-linked dominant trait and
  is homozygous (XAXA) all children will be
  affected
• If mother is heterozygous (XAXa), there is a 50
  chance of each child being affected

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Recessive Sex-Linked Traits

• Gene located on the X chromosome
• A female will express the phenotype only if she
  is homozygous recessive
• XrXr
• If a male receives the recessive allele from his
  mother he will express the phenotype
• XrY
• Females can only inherit if the father is
  affected and mother is a carrier (hetero) or
  affected (homo)
• An affected female will pass the trait to all her
  sons
• Daughters will be carriers if father is not
  affected
• Males cannot be carriers (only have one X so
  either affected or not)
• More males than females affected (males inherit
  affected X from mother)
• Can skip generations
• Examples Colour blindness
• Haemophilia

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Dominant Sex-Linked Trait Pedigree

• Look for
• More males being affected
• Affected males passing onto all daughters
  (dominant) and none of his sons
• Every affected person must have an affected
  parent

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Recessive Sex-Linked Trait Pedigree

• Look for
• More males being affected
• Affected female will pass onto all her sons
• Affected male will pass to daughters who will be
  a carrier (unless mother also affected)
• Unaffected father and carrier mother can produce
  affected son

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Pedigree Analysis for Sex-Linked Trait

• Dominant or recessive?
• Recessive,III-4 does not have trait even though
  father gave her affected X chromosome

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Pedigree Analysis for Multiple Alleles

• Who was the father of 2nd-1? Why?
• 1st-3, because 1st-1 has no B allele to give
• If 2nd-3 marries a man with AB blood, what could
  their offspring have?
• If she is AA, offspring can be AA, AB
• If she is AO, offspring can be AA, AB, AO, BO

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Gene Linkage

• Genes are on the same chromosome
• Are usually inherited together
• for example, if blue eyes and freckles are
  linked, if your offspring have blue eyes, they
  will almost always have freckles
• Do these genes usually undergo independent
  assortment?
• No!
• Under what circumstance would the genes be
  inherited separately?
• If there is crossing over that mixes up the genes

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1. Define the terms gene and allele and explain
how they differ. (4 marks)
IB Exam Question

• gene is a heritable factor / unit of inheritance
• gene is composed of DNA
• gene controls a specific characteristic / codes
  for a polypeptide/protein
• allele is a form of a gene
• alleles of a gene occupy the same gene locus/same
  position on chromosome
• alleles differ (from each other) by one / a small
  number of base(s) / base pair(s)

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2. What is a difference between autosomes and sex
chromosomes? (1 mark)
IB Exam Question

• A. Autosomes are not found in gametes but sex
  chromosomes are.
• B. Sex chromosomes are found in animal cells and
  autosomes are found in plant cells.
• C. Autosomes are diploid and sex chromosomes are
  haploid.
• D. Sex chromosomes determine gender and
  autosomes do not.
• Correct answer D

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3. Explain the relationship between Mendels law
of segregation and meiosis. (3
marks)
IB Exam Question

• law of segregation states that one half of the
  alleles enter one gamete and the other half enter
  the other gamete
• meiosis reduces the chromosome number by half /
  diploid to haploid
• homologues carrying alleles separate (in anaphase
  I)
• end result is four cells, half with one
  allele/homologue and the other half with the
  other allele

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4. A parent organism of unknown genotype is mated
in a test cross. Half of the offspring have the
same phenotype as the parent. What can be
concluded from this result?
(1
mark)
IB Exam Question

• A. The parent is heterozygous for the trait.
• B. The trait being inherited is polygenic.
• C. The parent is homozygous dominant for the
  trait.
• D. The parent is homozygous recessive for the
  trait.
• Correct answer A

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5. In peas the allele for round seed (R) is
dominant over the allele for wrinkled seed (r).
The allele for yellow seed (Y) is dominant over
the allele for green seed (y).If two pea plants
with the genotypes YyRr and Yyrr are crossed
together, what ratio of phenotypes is expected in
the offspring?

(1 mark)
IB Exam Question

• A. 9 round yellow 3 round green 3 wrinkled
  yellow 1 wrinkled green
• B. 3 round yellow 3 round green 1 wrinkled
  yellow 1 wrinkled green
• C. 3 round yellow 1 round green 3 wrinkled
  yellow 1 wrinkled green
• D. 1 round yellow 1 round green 1 wrinkled
  yellow 1 wrinkled green
• Correct answer C

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6. In garden peas, the pairs of alleles coding
for seed shape and seed colour are unlinked. The
allele for smooth seeds (S) is dominant over the
allele for wrinkled seeds (s). The allele for
yellow seeds (Y) is dominant over the allele for
green seeds (y).If a plant of genotype Ssyy is
crossed with a plant of genotype ssYy, which
offspring are recombinants?
(1 mark)
IB Exam Question

• A. SsYy and Ssyy
• B. SsYy and ssYy
• C. SsYy and ssyy
• D. Ssyy and ssYy
• Correct answer C

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7. A polygenic character is controlled by two
genes each with two alleles. How many different
possible genotypes are there for this character?
(1 mark)
IB Exam Question

• A. 2
• B. 4
• C. 9
• D. 16
• Correct answer C
• I.e. BBEE, BbEE, BBEe, BbEe, BBee, Bbee, bbEE,
  bbEe, bbee

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8. A woman who is a carrier of hemophilia marries
a man who is not affected. What are the possible
genotypes of their children? (1 mark)
IB Exam Question

• A. XHXh, XHXH, XHY, XhY
• B. XHXh, XHXH, XHYh, XHYH
• C. XHXh, XhXh, XHYh, XhYh
• D. XHXh, XhXh, XHY, XhY
• Correct answer A
• woman can give normal or hemophilic X allele
• man can only give normal X allele
• trait is sex-linked, meaning nothing will be on
  the Y

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9. Two genes A and B are linked together as shown
below. If the genes are far enough apart such
that crossing over between the alleles occurs
occasionally, which statement is true of the
gametes?
IB Exam Question

• A. All of the gametes will be Ab and aB.
• B. There will be 25 Ab, 25 aB, 25 ab and
  25 AB.
• C. There will be approximately equal numbers of
  Ab and ab gametes.
• D. The number of Ab gametes will be greater than
  the number of ab gametes.
• Correct answer D
• Because Ab is the undisturbed gamete, ab is when
  crossing over occurs, which only happens
  occasionally

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10. Using an example you have studied, explain a
cross between two linked genes, including the way
in which recombinants are produced.
(9 marks)
IB Exam Question

• linked genes occur on the same chromosome /
  chromatid
• genes (tend to be) inherited together / not
  separated/do not segregate independently
• non-Mendelian ratio / not 9331 / 1111
• real example of two linked genes
• Award 1 for each of the following examples of a
  cross between two linked genes.
• key for alleles involved in the example of a
  cross
• homozygous parental genotypes and phenotypes
  shown
• F1 genotype and phenotype shown / double
  heterozygote genotype and phenotype
• possible F2 genotypes and phenotypes shown
• recombinants identified
• recombinants due to crossing over
• in prophase I of meiosis
• diagram / explanation of mutual exchange of parts
  of chromatids during crossing over