Chapter 11 Notes: Mendelian Genetics

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Chapter 11 Notes Mendelian Genetics

• Genetics is the scientific study of heredity that
  involves how genes are passed from parents to
  their offspring.

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History of Genetics

• Gregor Mendel was an Austrian monk and scientist
  who was in charge of the monastery garden.
  Mendel studied garden peas.

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Pea plants happened to be a good choice to study
because

• They are self-pollinating.
• He had different pea plants that were
  true-breeding.
• True-breeding - means that they are homozygous
  for that trait.
• EX. if the plants self-pollinate they produce
  offspring identical to each other and the
  parents.

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Pea plants happened to be a good choice to study
because

• He developed a technique of producing seeds from
  a process called cross-pollination, in which he
  dusted the pollen of one pea plant onto another
  plant.
• He was in control of which plants crossed with
  each other.

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Genes and Dominance

• A trait is a specific characteristic that varies
  from one individual to another.
• Mendel studied seven different pea plant traits
  including seed shape, seed color, seed coat
  color, pod shape, pod color, flower position, and
  plant height.

• Mendel studied two alleles, or different
  versions, of each trait (wrinkled or smooth pea
  shape, green or yellow seed color, etc.)

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When discussing generations traits, we label
them as following

• The true-breeding parental generation is called
  the P generation.
• The offspring of the two parental plants is
  called the F1 generation.
• A cross between F1 generation would be called F2
  generation.

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Original cross
Cross pollination
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Mendels Investigations

• Mendel wanted to cross (or breed) two plants with
  different versions of the same trait. He wanted
  to know if the characteristics of the plants were
  blended in the offspring.

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Mendels Investigations

• Mendel saw that when he crossed plants with
  different versions of the same trait (P
  generation), the F1 offspring were NOT blended
  versions of the parents.
• The F1 plants resembled only one of the parents.

Tall x short ? all tall
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Mendel concluded

• 1. Biological inheritance is determined by
  factors that are passed from one generation to
  the next.
• Factors were later defined as genes-
• Mendel discovered all of this without the
  knowledge of DNA!

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Mendel concluded

• In Mendels plants, there was one gene for each
  trait. For example, there was one gene for
  plant height.
• But, there were two versions of this gene one
  for a tall plant and one for a short plant.

• Alleles Different versions of the same gene
• Remember, genes are used to make proteins.
• Each allele contains the DNA that codes for a
  slightly different version of the same protein
• This gives us the different characteristics for
  each trait

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2. Principal of dominance

• Some alleles are dominant and some alleles are
  recessive.
• Recessive alleles are able to be masked
• Dominant alleles mask recessive alleles
• The trait that was represented in the F1
  generation was the dominant trait.

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2. Principal of dominance

• How many alleles do you have for each gene?
• Where do they come from?

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3. Segregation

• Experiment Mendel self-pollinated the F1
  plants, or crossed the F1 plants with each other,
  to produce the F2 generation. From his F1
  crosses, Mendel observed
• The versions of the traits coded for by recessive
  alleles reappeared in the F2 plants.
• The recessive trait was still there!

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3. Segregation

• About 25 (or ¼) of the F2 plants exhibited the
  recessive version of the trait. In this case the
  recessive phenotype is short. The dominant
  phenotype, tall, was found in 75 (or ¾) of the
  F2 plants.

P generation F1 generation F2 generation
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Segregation of alleles during meiosis

• When the F1 plants produce gametes (sex cells)
  and self-pollinate, the two alleles for the same
  gene separate from each other so that each gamete
  carries only one copy of each gene.
• Remember, gametes are haploid. In the example,
  we use T to represent the dominant, tall allele
  and t to represent the recessive, short allele.

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4. Law of Independent Assortment

• Law of Independent Assortment- genes for each
  trait can be inherited independently from each
  other. For example
• not all tall plants have green pea pods and
• not all people with brown hair have brown eyes.

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Key Terms in Mendelian Genetics

• Dominant- allele that can mask represented by
  capital letters (B, D, F, etc.)
• Recessive- alleles that can be masked
  represented by lower case letters (b, d, f, etc.)

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Key Terms in Mendelian Genetics

• Phenotype- observable traits (brown eyes, yellow
  seed pods)
• Genotype- actual alleles describes the genetic
  characteristics (BB, dd, Ff)

Phenotype brown eyes Genotype could be BB,
or Bb
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Key Terms in Mendelian Genetics

• Homozygous (True-Breeding)- having two identical
  alleles for the same trait (TT, tt) homo means
  same
• Heterozygous- having two different alleles from
  the same trait (Tt) hetero means different

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• Punnett Squares must be taught before continuing ?

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Beyond dominant and recessive alleles

• There are some exceptions to Mendels principles.
  Luckily, none of these exceptions are exhibited
  in pea plants.
• If so, Mendel would not have been able to figure
  out inheritance.

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• Some alleles are neither dominant nor recessive.
• Incomplete Dominance
• Codominance

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Incomplete dominance

• situation in which one allele is not completely
  dominant over another the phenotype is a
  blending of the two alleles
• Example In some plants, when a true-breeding
  plant with red flowers is crossed with a
  true-breeding plant with white flowers, pink
  flowers are produced. Neither red nor white is
  dominant over the other.

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• Consider thisPunnett square

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Codominance

• situation in which both alleles of a gene
  contribute to the phenotype of the organism both
  alleles are expressed but NOT blended
• Example In cows, the allele for red fur is
  codominant with the allele for white fur.
  Heterozygous cows carrying one red and one white
  allele have spotted fur, known as roan.

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• Consider thisPunnett square

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• Many traits are controlled by multiple alleles or
  multiple genes.
• Multiple alleles (more than 2 choices)
• Polygenic (multiple genes control a single trait)

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Multiple alleles

• the case where three or more alleles of the same
  gene exist. Remember, an organism will have only
  two of these alleles (one from mom and one from
  dad).
• Examples Coat color in rabbits, blood type in
  humans

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Multiple alleles
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Polygenic traits

• traits that are determined by alleles from more
  than one gene these traits usually have a range
  of phenotypes
• Examples skin color in humans, height in humans

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Mapping Genes

• Its easy to imagine that genes on different
  chromosomes assort independently, but what about
  genes that occur on the same chromosome? Dont
  they always appear together?
• Not always due to crossing over. Genes that
  occur together on a chromosome will be separated
  when homologous chromosomes exchange genes.
• The frequency of genes occurring together can
  help us generate a gene map.

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• The more often two genes occur together, the
  closer they are to each other on the chromosome.
  
• If the genes are never separated by crossing
  over, they always occur together. All offspring
  will look like one of the parents (in reference
  to the genes in question).

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• If half of the offspring are parental and half
  are recombinations of the parents (in reference
  to the genes in question), then they are said to
  be independent. This means they are either on
  separate chromosomes or they are almost always
  separated during meiosis.
• You will learn to calculate distances and create
  a map in AP Bio, or in college

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Human chromosomes

• There are two types of chromosomes.
• Autosomes Of the 46 chromosomes, 44 of them (22
  pairs of chromosomes) are called autosomes
  (non-sex chromosomes).
• Sex chromosomes The last two chromosomes are
  called the sex chromosomes because they determine
  the sex of the person. Females have two X
  chromosomes (XX) and males have one X and one Y
  chromosome (XY).

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• Gametes
• All gametes are haploid. In humans, that means
  each egg cell and each sperm cell has 1 copy of
  each chromosome for a total of 23 chromosomes.
• Egg cells All human egg cells carry 23
  chromosomes, one of which is a single X
  chromosome. This is written as 23, X.
• Sperm cells In males, there are two types of
  sperm cells- one carries an X chromosome (23, X)
  and one carries a Y chromosome (23, Y).

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• When a sperm and egg cell combine, half of the
  time the fertilized eggs (also called zygotes)
  are female (46, XX) and half of the time they are
  male (46, XY).

X
X
eggs
X
XX
XX
female
female
Y
XY
XY
male
male
sperm
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• Sex Linked traits traits that are determined by
  alleles that are found on the X or Y chromosome.
• The Y chromosome is shorter and does not carry
  all the same alleles as the X chromosome.

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• Females are XX and males are XY.
• Females can be homozygous or heterozygous for a
  trait carried on the X chromosome, but males
  (having only one X chromosome) are hemizygous.

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• If they inherit a defective gene from the parent,
  then they will exhibit the trait because they
  cannot inherit a second gene to mask it.
• Conversely, a healthy male cannot be hiding a
  bad recessive allele because they only have one X
  chromosome.

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• Example of a sex-linked Punnett square
• XBXb (heterozygous female with normal vision)
  crossed to XBY (hemizygous male with normal
  vision)

XBY
Y
XB
XB XB
XBY
XB
XB Xb
XbY
XB Xb
Xb
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Genetics and the Environment

• Characteristics are determined by both genes and
  the environment.
• External While genes will influence the height
  of a plant, the amount of water, sun, and other
  climate conditions will also affect the height.

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Genetics and the Environment

• Internal There are recent findings that
  proteins involved with DNA can turn genes on or
  off based on environmental factors.
• Certain chemical exposure can turn genes on or
  off (make the traits show up or not) for
  generations after exposure, but there are no
  changes to the DNA (no mutations).
• This new understanding of how genes are expressed
  is called epigenetics.