Mendel and Meiosis

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Mendel and Meiosis

• Unit 4
• Chapter 10

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Gregor Mendel

• Austrian monk
• Studied patterns of heredity (passing on of
  characteristics from parent to offspring)
• Used the common garden pea in experiments

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Why did Mendel use peas?

• Sexually reproducing able to isolate both male
  and female gametes
• Easy to identify traits (characteristics that are
  inherited)
• Short life cycle able to be grown quickly

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Hybrid

• Any offspring of parents with different traits
  (ex tall plant x short plant)
• Monohybrid cross cross-pollination (breeding)
  between two parents with only one variation
  difference (ex tall plant x short plant)
• Dihybrid cross cross-pollination (breeding)
  between two parents with two variation
  differences (ex tall, green plant x short,
  yellow plant)

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Pea cross-pollination experiments
PARENT GENERATION (P1) Tall true breed x short
true breed
FILIAL GENERATION (F1) All tall hybrids
FILIAL GENERATION (F2) 75 tall hybrids, 25
short hybrids
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Phenotypes from P1 to F2
Dihybrid Cross
round yellow x wrinkled green
P1
Round yellow
Wrinkled green
All round yellow
F1
F2
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3
3
1
Round yellow
Round green
Wrinkled yellow
Wrinkled green
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What did Mendel observe?

• When a true-breeding tall plant is crossed with a
  true-breeding short plant in the P generation,
  the F1 height trait is always predictable. 100
  are tall plants.

P generation F1
F2
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What happens when the F1 tall plants are crossed
together?

• Mendel observed that the F2 generation, the
  offspring of F1 plants, are always in a fixed
  ratio of 31 tallshort.
• Why?

P generation F1
F2
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Pea traits that Mendel identified

• Through multiple crosses, Mendel determined that
  all these traits displayed a mathematical
  predictability for inheritance.

Seed Shape
Flower Position
Seed Coat Color
Seed Color
Pod Color
Plant Height
Pod Shape
Round
Yellow
Gray
Smooth
Green
Axial
Tall
Wrinkled
Green
White
Constricted
Yellow
Terminal
Short
Round
Yellow
Gray
Smooth
Green
Axial
Tall
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Mendels conclusions

• There must be two variations for every trait,
  where each variation is called an allele.
• Each offspring inherits only one allele from each
  parent.
• The alleles are either dominant or recessive.
• To show the recessive trait, two recessive
  alleles must be inherited.

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Dominant and recessive traits

• The traits that seem to mask other traits when
  present are called dominant traits.
• The traits that seem to be hidden in the presence
  of dominant traits are called recessive traits.

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Dominant and recessive traits
Seed shape
Seed color
Flower color
Pod color
Flower position
Pod shape
Plant height
Dominant trait
axial (side)
round
yellow
purple
green
tall
inflated
Recessive trait
terminal (tips)
green
short
white
yellow
wrinkled
constricted
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Homozygous vs. Heterozygous

• Homozygous inherits two similar alleles from the
  parents for a particular gene
• Ex tall allele and tall allele, written as TT
• Ex short allele and short allele written as tt
• Heterozygous inherits two different alleles from
  the parents for a particular gene
• Ex tall allele and short allele, written as Tt

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Law of Segregation

• Mendel concluded only one allele is passed from
  parent to offspring for each trait.
• F1 plants must be heterozygous because the P
  generation only passed on one tall allele and one
  short allele.
• The F1 plant will then pass on to its offspring
  either a tall or a short allele, never both.

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Using a Punnett square

• AA x aa ? 100 Aa
• Each of the four squares represents 25 chance of
  inheritance for one offspring.

A A
a A a A a
a A a A a
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Phenotype vs. Genotype

• Phenotype physical appearance of the trait
• Ex purple flowers
• Genotype homozygous or heterozygous inheritance
• Ex PP, Pp, pp

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Law of independent assortment

• Because organisms are made up of more than one
  trait, Mendel concluded that the inheritance of
  one trait does not influence the inheritance of a
  second trait.
• Example Height of the pea plant does not
  influence the color of the peas
• Height is independently assorted from color.

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Using dihybrid crosses to show independent
assortment

• A smooth, yellow pea (RrYy) can pass on these
  combinations of genes to its offspring RY, Ry,
  rY, or ry.

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Section 10.1 Summary pages 253-262
Punnett Square of Dihybrid Cross
Dihybrid crosses
Gametes from RrYy parent
Ry
RY
rY
ry

• A Punnett square for a dihybrid cross will need
  to be four boxes on each side for a total of 16
  boxes.

RRYY
RRYy
RrYY
RrYy
RY
RRYy
RRYy
RrYy
Rryy
Ry
Gametes from RrYy parent
RrYY
RrYy
rrYY
rrYy
rY
RrYy
Rryy
rrYy
rryy
ry
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Punnett Square of Dihybrid Cross
Dihybrid crosses
Gametes from RrYy parent
Ry
RY
rY
ry
RRYY
RRYy
RrYY
RrYy
RY
F1 cross RrYy RrYy
RRYy
RRYy
RrYy
Rryy
round yellow
Ry
Gametes from RrYy parent
round green
RrYY
RrYy
rrYY
rrYy
rY
wrinkled yellow
RrYy
Rryy
rrYy
rryy
ry
wrinkled green
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Modernizing Mendelian genetics

• DNA is the basis for inheritance.
• DNA are coiled into chromosomes.
• Parts of the DNA that code for a trait are called
  genes.
• Some genes have only two alleles and other have
  more.

Gene for hairline Allele A
Genotype Aa
Gene for hairline Allele a
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How do these pictures compare?
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Variations on Mendel

• Incomplete dominance the heterozygous genotype
  shows a blend of the two parents and not the
  dominant allele

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Variations on Mendel

• Codominance the heterozygous genotype shows both
  inherited alleles
• Example of roan horse coat AA (dark red) x aa
  (white) ? Aa (dark red and white)

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Variations on Mendel

• Multiple alleles when there are more than two
  alleles that code for a trait
• Example ABO blood type
• A type AA or Ao
• B type BB or Bo
• O type oo
• AB type AB

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Blood typing
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Variations on Mendel

• Polygenic trait when more than one gene codes
  for a particular trait
• Example fur color, human height, human skin
  color, eye color

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Variations on Mendel

• Linked genes Mendel concluded that traits are
  assorted independently, but some traits are
  linked.
• This means that two genes are almost always
  inherited together (ex red hair, green eyes).

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Cells and chromosomes

• A cell with two of each kind of chromosome is
  called a diploid cell and has diploid, or 2n,
  number of chromosomes.
• Organisms produce gametes that contain one of
  each kind of chromosome

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Homologous chromosomes

• The two chromosomes of each pair in a diploid
  cell are called homologous chromosomes.

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Homologous chromosomes

• On homologous chromosomes, the same types of
  genes are arranged in the same order.
• Because there are different possible alleles for
  the same gene, the two chromosomes in a
  homologous pair are not always identical to each
  other.

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Making haploid cells

• Meiosis is the process of producing haploid
  gametes with a ½ the amount of DNA as the parent
  cell.
• A cell with one of each kind of chromosome is
  called a haploid cell and has a haploid, or n,
  number of chromosomes.
• Meiosis enables sexual reproduction to occur.

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Sexual reproduction
Haploid gametes (n23)
Sperm Cell
Meiosis
Meiosis
Egg Cell
Fertilization
Diploid zygote (2n46)
Mitosis and Development
Multicellular diploid adults (2n46)
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Interphase

• During interphase, the cell replicates its
  chromosomes.
• After replication, each chromosome consists of
  two identical sister chromatids, held together by
  a centromere.

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Prophase I

• The chromosomes coil up and a spindle forms.
• Homologous chromosomes line up with each other
  gene by gene along their length, to form a
  four-part structure called a tetrad.

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Prophase I crossing over

• Chromatids are wrapped so tightly the chromosomes
  can actually break and exchange genetic material
  in a process known as crossing over.
• Crossing over results in new combinations of
  alleles on a chromosome.

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Metaphase I

• The centromere of each chromosome attaches to a
  spindle fiber.
• The spindle fibers pull the tetrads into the
  middle, or equator, of the spindle.

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Anaphase I

• Homologous chromosomes separate and move to
  opposite ends of the cell.
• This critical step ensures that each new cell
  will receive only one chromosome from each
  homologous pair.

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Telophase I

• The spindle is broken down, the chromosomes
  uncoil, and the cytoplasm divides to yield two
  new cells.
• Each cell has half the DNA as the original cell
  because it has only one chromosome from each
  homologous pair.

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Prophase II

• A spindle forms in each of the two new cells and
  the spindle fibers attach to the chromosomes.

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Metaphase II.

• The chromosomes, still made up of sister
  chromatids, are pulled to the center of the cell
  and line up randomly at the equator.

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Anaphase II

• The centromere of each chromosome splits.
• The sister chromatids to separate and move to
  opposite poles.

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Telophase II

• Finally nuclei reform, the spindles breakdown,
  and the cytoplasm divides.
• Four haploid cells have been formed from one
  diploid cell

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Why meiosis is important

1. Forms gametes for sexual reproduction
2. Crossing over during meiosis which rearranges
   allele combinations so that the offspring
   generations are genetically different than the
   parents.

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Nondisjunction leading to trisomy

• This can lead to gamete formations having too
  many or too few chromosomes.
• Ex A gamete with 2 copies of 21 chromosome
  fertilizes a gamete with 1 copy of 21. The
  result is an embryo with trisomy 21. This causes
  Down Syndrome in humans.

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Trisomy leading to monosomy

• A gamete with one copy of the X chromosome
  fertilizes a gamete missing a copy of the X
  chromosome.
• The result is monosomy X, which in humans causes
  Turner Syndrome.
• Affects 1 in every 2,500 girls.
• Most girls with Turner Syndrome are infertile.

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Nondisjunction leading to polyploidy

• When a gamete with an extra set of chromosomes is
  fertilized by a normal haploid gamete, the
  offspring has three sets of chromosomes and is
  triploid.
• The fusion of two gametes, each with an extra set
  of chromosomes, produces offspring with four sets
  of chromosomes and is a tetraploid.
• This occurs often in flowering plants, leading to
  larger fruit production.