Genetics: Mendel and Beyond

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Genetics Mendel and Beyond
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Genetics Mendel and Beyond

• By the end of this chapter you should be able to
• Describe Mendels Experiments and the Laws of
  Inheritance
• Predict inheritance patterns of monohybrid and
  dihybrid crosses using a punnett square
• Explain examples of gene interactions
• Distinguish between genes and chromosomes
• Explain sex determination and
• Predict sex-linked inheritance

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The Foundations of Genetics

• Applied genetics (plant and animal breeding) has
  been used for five thousand years ago or more.
• The foundation for the science of genetics is
  credited to Gregor Mendel who used varieties of
  peas to conduct experiments on inheritance.
• Mendels research was ignored until the turn of
  the twentieth century.
• Meiosis provides an explanation for Mendels
  theory.

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The Foundations of Genetics

• Plants have some desirable characteristics for
  genetic studies
• They can be grown in large quantities.
• They produce large numbers of offspring.
• They have relatively short generation times.
• Many have both male and female reproductive
  organs, making self-fertilization possible.
• It is easy to control which individuals mate.

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Figure 10.1 A Controlled Cross between Two
Plants
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Mendels Experiments and the Laws of Inheritance

• Mendel looked for characters that had
  well-defined alternative traits and that were
  true- breeding.
• Mendel developed true-breeding strains to be used
  as the parental generation, designated P.
• The offspring from the cross of the P parents are
  called the first filial generation, designated
  F1.
• When F1 individuals are crossed to each other or
  self-fertilized, their progeny are designated F2.

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Mendels Experiments and the Laws of Inheritance

• Mendels experiment 1
• A monohybrid cross involves one character (seed
  shape) and different traits (spherical or
  wrinkled).
• SS x ss ? Ss
• The F1 seeds were all spherical the wrinkled
  trait failed to appear at all.
• Because the spherical trait completely masks the
  wrinkled trait when true-breeding plants are
  crossed, the spherical trait is considered
  dominant and the wrinkled trait recessive. We
  represent the dominant trait with a capital
  letter and the recessive trait with a small case
  letter.

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Mendels Experiments and the Laws of Inheritance

• Mendels experiment 1 continued
• The F1 generation was allowed to self-pollinate
  to produce F2 seeds.
• Ss x Ss ? 3Ss, 1 ss
• 3 are heterozyous smooth and 1 is homozygous
  wrinked.
• In the F2 generation, the ratio of spherical
  seeds to wrinkled seeds was 31.

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Figure 10. 3 Mendels Experiment 1 (Part 1)
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Figure 10. 3 Mendels Experiment 1 (Part 2)
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Mendels Experiments and the Laws of Inheritance

• From these results, Mendel reached several
  conclusions
• The units responsible for inheritance are
  discrete particles that exist in pairs and
  separate during gamete formation this is called
  the particulate theory.
• Each pea has two units of inheritance for each
  character.
• During production of gametes, only one of the
  pair for a given character passes to the gamete.
• When fertilization occurs, the zygote gets one
  unit from each parent, restoring the pair.

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Mendels Experiments and the Laws of Inheritance

• Mendels units of inheritance are called genes (a
  portion of the chromosomal DNA that resides at a
  specific locus and codes for a particular
  function) different forms of a gene are called
  alleles.
• True-breeding individuals have two copies of the
  same allele (i.e., they are homozygous).
• Some smooth-seeded plants are Ss or heterozygous.
• The physical appearance of an organism is its
  phenotype (what it looks like) the actual
  composition of the organisms alleles for a gene
  is its genotype. Homozygous dominant
  heterozygous dominant homozygous recessive

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• An organisms trait does not always reveal its
  genetic composition. Why?
• Heterozygous genotypes yield phenotypes showing
  the dominant trait.
• The same phenotype can result from different
  genotypes.

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Mendels Experiments and the Laws of Inheritance

• Mendels first law is called the law of
  segregation The two alleles of a trait segregate
  (separate) during meiosis.
• Each gamete receives one member of a pair of
  alleles.
• Determination of possible allelic combinations
  can be accomplished by a Punnett square.

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Punnett Square

• Another way to demonstrate this is through the
  use of a punnett square
• S S S s
• s Ss Ss S SS Ss
• s Ss Ss s Ss ss
• Since one character with two contrasting traits
  are being crossed, this is called a monohybrid
  cross

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Figure 10.4 Mendels Explanation of Experiment 1
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Figure 10.5 Meiosis Accounts for the Segregation
of Alleles (Part 1)
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Figure 10.5 Meiosis Accounts for the Segregation
of Alleles (Part 2)
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Mendels Experiments and Laws of Inheritance

• How do we determine if a purple flowering plant
  is SS or Ss?
• We could cross the purple flowering plant of
  unknown genotype with a true breeding purple
  flowering plant or a white flowering plant
• SS x SS (unknown) or SS x Ss (unknown)
• all spherical all spherical
• ss x SS (unknown) or ss x Ss (unknown)
• all spherical 2 spherical 2 wrinkled

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Mendels Experiments and the Laws of Inheritance

• Crossing an unknown with a homozygous recessive
  is called a test cross.
• An individual with a dominant trait is crossed
  with a true-breeding recessive (homozygous
  recessive).
• The appearance of the recessive phenotype in half
  the offspring indicates that the parent is
  heterozygous.

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Figure 10.6 Homozygous or Heterozygous?
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Mendels Experiments and the Laws of Inheritance

• Mendels second law, the law of independent
  assortment, states that alleles of different
  genes assort into gametes independently.
• This can be shown by using a dihybrid crosses.
• For example, in pea plants purple flowers are
  dominant over white flowers and green pods are
  dominant over yellow pods.
• Cross a purebreeding purple flowering plant with
  green pods with a white flowering plant with
  yellow pods.
• Random fertilization of gametes results in all
  heterozygous offspring.

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Dihybrid Crosses

• PPGG x ppgg PpGg
• If we allow these plants to self-pollinate, PpGg
  x PpGg, what are the possible offspring?
• Each parent could produce four different gametes
  PG, Pg, pG, or pg

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Dihybrid Crosses

• PG Pg pG pg
• PG
• Pg
• pG
• pg

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Dihybrid Crosses

• PG Pg pG pg
• PG PPGG PPGg PpGG PpGg
• Pg PPGg PPgg PpGg Ppgg
• pG PpGG PpGg ppGG ppGg
• pg PpGg Ppgg ppGg ppgg
• Putting these into a punnett square results in a
  9331 ratio

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Figure 10.7 Independent Assortment
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Mendels Experiments and the Laws of Inheritance

• Humans cannot be studied using planned crosses.
• Therefore, human geneticists rely on pedigrees.
• Human pedigrees do not show clear proportions.
• Outcomes for small samples fail to follow the
  expected outcomes closely.

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Mendels Experiments and the Laws of Inheritance

• If neither parent has a given phenotype, but it
  shows up in their offspring, the trait is
  recessive and the parents are heterozygous.
• Half of the children from such a cross will be
  carriers (heterozygous for the trait).
• The chance of any one childs getting the trait
  is 1/4.

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Figure 10.11 Recessive Inheritance
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Mendels Experiments and the Laws of Inheritance

• A pedigree analysis of the dominant allele for
  Huntingtons disease shows that
• Every affected person has an affected parent.
• About half of the offspring of an affected person
  are also affected (assuming only one parent is
  affected).
• The phenotype occurs equally in both sexes.

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Figure 10.10 Pedigree Analysis and Dominant
Inheritance
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Polydactyl
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Figure 10.11
figure 10-11.jpg

• Figure 10.11

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Albinism
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Huntingtons Disease
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Marfans
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Marfans
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Alleles and Their Interactions

• Differences in alleles of genes consist of slight
  differences in the DNA sequence at the same
  locus, resulting in slightly different protein
  products.
• This is a mutation. Alleles can mutate randomly.

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Alleles and Their Interactions

• A population can have more than two alleles for a
  given gene.
• In rabbits, coat color is determined by one gene
  with four different alleles. Five different
  colors result from the combinations of these
  alleles.
• Even if more than two alleles exist in a
  population, any individual can have no more than
  two of them one from the mother and one from the
  father.
• Also send in ABO blood typing.

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Figure 10.12 Inheritance of Coat Color in Rabbits
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Alleles and Their Interactions

• A white snapdragon crossed with a red snapdragon,
  gives an intermediate phenotype pink
• Incomplete Dominance
• In the case of snapdragons, one allele codes for
  an enzyme that leads to the formation of red
  pigment. The other allele does not code for
  pigment production.

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Figure 10.13 Incomplete Dominance Follows
Mendels Laws
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• The F2 offspring, however, demonstrate Mendelian
  genetics. For self-fertilizing F1 pink flowers
  the F2 progeny have a phenotypic ratio of 1
  red2 pink1 white.
• Other examples include roan cattle and blue
  Andalusian chickens
• Tay Sachs Disease is also an example of
  incomplete dominance.

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Incomplete Dominance

• Persons with Tay Sachs lack a crucial enzyme to
  metabolize a type of lipid. The lipids
  accumulate in the brain interfering with normal
  function. Causes regression of nervous system, -
  blind, deaf, bedridden, inability to move limbs
• Cherry red spot in retina is one indication
• Heterozygotes have an intermediate level of the
  lipid metabolizing enzyme

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Tay Sachs
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Tay Sachs
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Alleles and Their Interactions

• In codominance, the two different alleles are
  both expressed in the heterozygotes.
• In the human ABO blood group system the alleles
  for blood type are IA, IB, and IO. We inherit
  two of these three alleles.
• Two IA, or IA and IO, results in type A.
• Two IB, or IB and IO, results in type B.
• Two IO results in type O.
• IA and IB results in type AB. The alleles are
  called codominant.

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Figure 10.14 ABO Blood Reactions Are Important
in Transfusions
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ABO Genetic Problems

• A couple have their blood typed before marriage.
  They both are AB. What types of blood might
  their children have? Explain
• A woman sues a man for child support. She has
  type A blood, her child type O, and the man type
  B. Could the man be the father? Why or why not?

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ABO Genetic Problems

• A wealthy, elderly couple die together in a car
  accident. Soon a young man shows up to claim
  their fortune, contending that he is their only
  son who ran away from home when he was a young
  man. Other relative dispute his claim. Hospital
  records show that the deceased couple were blood
  types AB and O. The claimant is type O. Do you
  think the claimant is an impostor? Explain.

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Pleiotropic alleles

• Most genes have multiple phenotypic effects.
• An example is the coloration pattern and crossed
  eyes of Siamese cats, which are both caused by
  the same allele.
• These unrelated characters are caused by the same
  protein produced by the same allele.
• Another example is sickle cell anemia.

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Sickle Cell Anemia
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Gene Interactions

• In epistasis a gene at one locus alters the
  phenotypic expression of a gene at a second
  locus.
• An example is coat color in mice
• The B allele produces a banded pattern, called
  agouti. The b results in unbanded hairs.
• The genotypes BB or Bb are agouti or banded. The
  genotype bb is black.
• Another locus determines if any coloration
  occurs. The genotypes AA and Aa have color and aa
  are albino.
• Cross two AaBb mice. What is the phenotype?

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Figure 10.15 Genes May Interact Epistatically
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Gene Interactions

• When two homozygous strains of plants or animals
  are crossed, the offspring are often
  phenotypically stronger, larger, and more
  vigorous than either parent.
• This phenomenon is called hybrid vigor.
  Hybridization is now a common agricultural
  practice used to increase production in plants.

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Figure 10.16 Hybrid Vigor in Corn
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The Environment Affects Gene Action

• Genotype and environment interact to determine
  the phenotype
• Environmental variables such as light,
  temperature, and nutrition can affect the
  translation of genotype into a phenotype
• Examples Siamese cats, hydrangea flowers
• Twin Studies

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Genes and Chromosomes

• Homologous chromosomes can exchange corresponding
  segments during prophase I of meiosis (crossing
  over).
• Genes that are close together tend to stay
  together.
• The farther apart on the same chromosome genes
  are, the more likely they are to separate during
  recombination.

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Figure 10.19 Crossing Over Results in Genetic
Recombination
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Sex Determination in Humans

• Sex chromosomes carry genes that determine
  whether male or female gametes are produced.
• In humans, the Y chromosome has a sex-determining
  region - SRY
• The SRY gene codes for a functional protein. If
  this protein is present, testes develop if not,
  ovaries develop.
• Some XY individuals lacking a small portion of
  the Y chromosome are phenotypically female.
• Some XX individuals with a small piece of the Y
  chromosome are male.

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Sex linked traits

• The Y chromosome carries few genes (20). The X
  chromosome carries many genes. This difference
  generates a special type of inheritance called
  sex-linked inheritance
• Two well-know traits carried on the X chromosome
  are colorblindness and hemophilia.
• Sex linked traits tend to be expressed with
  greater frequency in males.
• Can you explain why?
• This is due to the fact that many of these
  diseases are only on the X chromosome and not the
  Y

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Hemophilia
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Colorblindness
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Figure 10.24 Red-Green Color Blindness Is a
Sex-Linked Trait in Humans
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Sex Determination and Sex-Linked Inheritance

• Pedigree analysis of X-linked recessive
  phenotypes
• The phenotype appears much more often in males
  than in females.
• A male with the mutation can pass it only to his
  daughters.
• Daughters who receive one mutant X are
  heterozygous carriers.
• The mutant phenotype can skip a generation if the
  mutation is passed from a male to his daughter
  and then to her son.

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Sex Determination and Sex-Linked Inheritance

• Disorders can arise from abnormal sex chromosome
  constitutions.
• Turner syndrome is characterized by the XO
  condition and results in females who physically
  are slightly abnormal but mentally normal and
  usually sterile.
• The XXY condition, Klinefelter syndrome, results
  in males who are taller than average and always
  sterile.

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Non-Nuclear Inheritance

• Mitochondria, chloroplasts, and other plastids
  possess a small amount of DNA.
• Some of these genes are important for organelle
  assembly and function.
• Mitochondria and plastids are passed on by the
  mother only, as the egg contains abundant
  cytoplasm and organelles.
• A cell is highly polyploid for organelle genes.
• Organelle genes tend to mutate at a faster rate.