Chapter 14: Genetics Mendelian

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Chapter 14 Genetics - Mendelian

• AP Biology Ms. Rader

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Father of Genetics Gregor Mendel

• Austrian Monk who used pea plants and selective
  breeding to track the inheritance pattern of
  certain characters from parent to offspring.
• Established two laws of genetics that are still
  used today
• The Law of Segregation
• The Law of Independent Assortment

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Important Genetics Terms

• Character Any heritable feature of an organism
  (Ex. Flower color)
• Trait the possible variants of a character
• (Ex. For the character flower color in pea
  plants the possible traits are purple flowers and
  white flowers)
• True-breeding plants When self pollinated, these
  plants always produce the same variety of plant
• Hybridization The crossing of two different,
  true-breeding plants
• P generation The initial cross of a genetic
  breeding. The parental generation.

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Important Genetics Terms

• F1 generation The offspring produced by the P
  generation. (Each subsequent generation in a
  breeding line is numbered after F1. Ex. F2, F3,
  etc)
• Dominant trait The variant that is expressed
  (seen) in an organism that carries both alleles
  for a character.
• Recessive trait The variant that is not
  expressed (seen) in an organism that carries both
  alleles for a character.
• Genotype The alleles that an organism inherits
  from its parents. (Ex. Pp, PP, pp)
• Phenotype The way a character appears (or is
  expressed) as a result of its genotype. (Ex.
  Genotype PP phenotype purple flowers, genotype
  pp phenotype white flowers)

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Important Genetics Terms

• Homozygous (homozygote) Organism that has
  inherited the same variant of allele from its
  parent. (Ex. PP or pp)
• Heterozygous (heterozygote) Organism that has
  inherited different alleles for the same
  character. (Ex. Pp)

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Monohybrid Cross

• When looking at one character we can do a
  monohybrid cross to examine inheritance.
• The punnett square can be useful in determining
  the potential offspring of a cross.

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Monohybrid Cross

• The Law of Segregation The two alleles from a
  parent will be separated into two different
  gametes.

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Dihybrid Cross

• We can look at the inheritance of two characters
  in a dihybrid cross.
• Y yellow seeds R round seeds
• y green seeds r wrinkled seeds
• P generation YYRR x yyrr YyRr
• F1 generation YyRr x YyRr (dihybrid cross)

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Dihybrid Cross

• The Law of Independent Assortment The alleles
  for different characters will segregate
  independent of one another into gametes.

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Dihybrid Cross

• In a dihybrid cross there are four possibilities
  for gametes YR, Yr, yR, and yr

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Probability and Genetics

• There is a equal chance (50/50) of a gamete
  receiving either one of a parents homologous
  chromosomes. Therefore there is a equal
  probability that the gamete will have either
  allele carried on that chromosome.

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Probability and Genetics

• The law of multiplication states that by
  multiplying the probabilities of two or more
  independent events you can determine the
  probability of the events occurring at the same
  time.

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Probability and Genetics

• The law of addition states that by adding the
  probabilities of the different ways an event can
  occur you can determine the overall probability
  of the event occurring.
• There are two ways to have the genotype Pp.
• P p or P p
• Mom Dad Dad Mom
• ¼ ¼ overall probability of getting Pp
  genotype 1/2

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Probability and Genetics

• The laws of multiplication and addition can be
  combined to figure out the probabilities for more
  complex genetics crosses without having to create
  a punnett square.
• Consider a cross with three characters being
  followed instead of just one.

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Probability and Genetics

• Ex.1 Trihybrid cross
• PpYyRr x Ppyyrr
• What is the probability of getting an offspring
  with the genotype ppyyRr?
• pp (1/2 x 1/2) 1/4
• yy (1/2 x 1) 1/2
• Rr (1/2 x 1) 1/2

¼ x ½ x ½ 1/16 The overall probability of
getting this genotype
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Probability and Genetics

• Ex.2 What is the probability of getting an
  offspring with the phenotype purple flowers,
  yellow seeds, and wrinkled seeds?
• Step1 What are the possible genotypes from this
  cross that will satisfy the desired phenotype?
• Answer PPYyrr or PpYyrr

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Probability and Genetics

• Ex.2 What is the probability of getting an
  offspring with the phenotype purple flowers,
  yellow seeds, and wrinkled seeds?
• Step2 Figure out the probabilities of getting
  each genotype.
• PPYyrr ( ½ x ½ ) x ( ½ x 1) x ( ½ x 1)
  1/16
• PpYyrr ( ½ x ½ ) ( ½ x ½ ) x ( ½ x 1) x ( ½ x
  1) 2/16
• overall probability of desired phenotype 3/16

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When dominance isnt that simple

• Incomplete Dominance Condition where neither
  allele is completely dominant over the other. The
  resulting phenotype of the heterozygote is an
  intermediate or blended phenotype of the two
  alleles.
• Ex. Snapdragon flower color.

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Incomplete Dominance
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Incomplete Dominance

•   Palomino (Ccr) A palomino is a red horse with
  one cream gene. Coats vary in shades of gold with
  manes and tails white.

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When dominance isnt that simple

• Codominance Condition where both alleles are
  dominant and are equally expressed. Therefore,
  the heterozygous genotype carries the results of
  both phenotypes. This is not a blending or
  intermediate phenotype as in incomplete
  dominance.
• Ex. Human M and N blood groups

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When dominance isnt that simple

• Ex. Human M and N blood groups
• There are three phenotypes with this character M
  proteins on RBC surface, N proteins on RBC
  surface, or M and N proteins on RBC surface.

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Codominance
Appaloosa Horses
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Other types of inheritance

• Multiple alleles When the phenotype of a
  character has more than two allele types.
• Ex. Human ABO blood groups.

Agglutination (bad!)
Universal recipient
Universal donor
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Multiple Alleles

• Ex. Human ABO blood groups
• alleles possible IA, IB, and i
• genotypes possible IAIA, IBIB, IAIB, IAi, IBi,
  and ii.
• blood types possible A, B, AB and O
• AB blood is also an example of codominance

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Multiple alleles

• Blood group inheritance

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Other types of inheritance

• Pleiotropy Where one gene can affect more than
  one characteristic in an organisms phenotype.

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Other types of inheritance

• Epistasis Where one gene determines whether
  another gene will be expressed at all. Also known
  as a stopping gene.
• Ex. Mouse coat color the mouse must have at
  least one dominant allele (C) for coat color to
  have the color expressed. The color of the fur is
  determined by another gene where B black fur
  (dominant) and b brown fur (recessive)

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Other types of inheritance

• Polygenic inheritance Where the phenotype of one
  character is determined by an additive effect of
  two or more genes.
• Ex. Human skin color is thought to be controlled
  by three separate genes and it is the interaction
  of those genes that determines what a persons
  skin color is.

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Studying human genetics

• Since it is not practical (or ethical) to do
  controlled breeding experiments in humans we can
  use pedigrees to track characters through human
  families.

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Studying human genetics

• Recessively inherited disorders Human disorders
  that follow the laws of simple dominance and the
  homozygous recessive genotype results in a person
  having the disease or disorder. A person that is
  heterozygous for this character is considered a
  carrier of the disease as they can pass it to
  offspring if they breed with a person that has
  the disorder is also heterozygous for the
  character.
• Ex. Cystic fibrosis (more prevalent in whites of
  European descent), Tay-Sachs disease (Jewish
  people that descended from central Europe), and
  sickle cell anemia (Africans and those of African
  descent)

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Studying human genetics

• Dominantly inherited disorders Human disorders
  that follow the laws of simple dominance and the
  homozygous dominant or heterozygous genotype
  results in a person having the disease or
  disorder.
• Ex. Huntingtons Disease (lethal disorder that
  continues to persist in the population due to the
  fact that the person shows no evidence of the
  phenotype until they are 35-45 years of age and
  may have already passed the allele on to their
  offspring) and achondroplastic dwarfism

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Genetic testing

• Carrier testing Persons that are high risk for
  certain disorders due to their heritage, familial
  affliction, etc, can be tested to see if they
  carry an allele for a disorder before they decide
  to produce offspring.
• Ex. Tests exist for Tay-Sachs, one form of cystic
  fibrosis, sickle cell disease, and Huntingtons
  disease.

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Genetic testing

• Fetal testing for genetic disorders and
  conditons
• Amniocentesis A small amount of amniotic fluid
  is removed from the uterus and karyotyped to look
  for known genetic abnormalities. This process
  takes several weeks due to the small number of
  fetal cells recovered which need to be cultured
  to increase the amount of DNA present.
• Chorine Villus Sampling (CVS) Fetal tissue fro m
  the placenta is removed by suction. This tissue
  has enough cells present for immediate
  karyotyping and is much faster than
  amniocentesis.
• Ultrasound and fetoscopy Less invasive ways to
  visualize fetus in utero and look for large,
  obvious disorders.