Mendel

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Chapter 10

• Mendel
• and
• Meiosis

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California and AUHSD standards for this chapter
5. Mutation and sexual reproduction lead
to genetic variation in a population.
Students should know
a. meiosis is an early step in sexual
reproduction in which the pairs of chromosomes
separate and are segregated randomly during cell
division to produce gametes containing only one
chromosome of each type. b. that only certain
cells in a multicellular organism undergo meiosis.
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c. how random chromosome segregation explains the
probability that a particular allele will be in a
gamete. d. new combinations of alleles may be
generated in a zygote through fusion of male and
female gametes (fertilization). e. why
approximately half of an individual's DNA
sequence comes from each parent.
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Mendel and his Laws

• Austrian monk
• Left from teaching high school science to study
  heredity
• Chose to use garden peas as his subject
• They produce sexually
• Male and female produce sex cells -- gametes
• Male and female parts are in the same flower in
  pea plants
• Chose 7 traits (with distinct opposite
  expressions) to study
• Tall vs. short purple flowers vs. white
  green seeds vs. yellow, etc. (See p. 262)

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Pea flower parts
Mendel hand-pollinated (transferred pollen to
carpel) each flower fertilization occurs
shortly after pollination uniting of male and
female gametes
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Experiment Cross white flower by purple flower
plants
This is called a monohybrid cross involving a
single trait flower color
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Writing out Mendels flower color experiment
P PP x pp (pure (pure purple)
white)
Mendel called genes factors and showed that
each parent contributed one factor each to
produce the offspring.
F1 Pp (purple hybrids)
2nd Law Law of Dominance Dominant allele
(gene) will be expressed (show) when paired with
a recessive allele
F2 Pp x Pp
PP 2 Pp pp (pure (hybrid) (pure purple) purp
le) white)
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Other genetics terms

• Phenotype the appearance of a trait
• Tall, yellow, round, purple, etc.
• Genotype the allele combination which causes
  the phenotype alleles (genes) are designated by
  letters (A, b, K, m, Z, etc.)
• PP, Tt, tt, etc.
• Types of genotypes
• Homozygous (pure) PP, tt, KK, MM, qq, etc.
• Heterozygous (hybrid) Pp, Tt, Kk, Mm, Qq, etc.

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What ratio of phenotypes will appear in the F2
generation if you cross these two traits below?
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move one allele (gene/factor) from each
parent into F1 generation line
3rd Law Law of Segregation. Parent allele
pairs separate into different gametes.
F1 all round seeds
Rr
F2 Rr x Rr
Use a Punnett Square to solve the predicted F2
generation ratios
Rr
RR
rr
Rr
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Predict next generation phenotype ratios if
P Yellow x Green (Yy) (yy)
Yellow Green
F1
Answer 11
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Test Cross (a variation of a monohybrid cross)
is performed when you dont know the genotype of
a dominant-looking phenotype
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Incomplete dominanceIn heterozygous condition,
dominant allele is not completely expressed.
Example Sweet pea flower colors
P RR x RR (red) (white)
F1 RR (pink)
RR x RR
F2 RR 2 RR RR
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Another example of incomplete dominance which
occurs in short horn cattle (Texas)
P RR x RR Red White (looks auburn)
F1 all RR (Roan color)
Phenotype Ratio ? Genotype Ratio ?
F2
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Dihybrid cross

• Used to predict outcome when crossing 2 traits
  simultaneously
• Ex. Plant height and seed shape

P TTRR x ttrr tall,round short, wrinkled
4th Law Law of Independent Assortment. Alleles
for each trait are inherited independently of
each other one allele for each trait enter
into a gamete. can only be observed in dihybrid
(and larger) crosses
F1 all TtRr tall, round
Gametes of F1 must each have one t and one
r TR, Tr, ? , ?
tR
tr
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Dihybrid cross results
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Chromosomes and Meiosis (Sex Cell Formation)
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Meiosis

• Chromosomes contain genes lined up and connected
  together (linked)
• Chromosomes occur in sets designated by n
• Garden pea plant cells contain a total of 14
  chromosomes
• There are 7 chromosomes in each set
• Cells which contain 2 sets of chromosomes are
  said to be diploid and the symbol 2n is used.
• Organisms produce gametes which contain only one
  set of chromosomes and are said to be haploid or
  n.
• Only gametes are haploid, all other (somatic)
  cells are diploid.

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• If the diploid number for a frog is 26, how many
  chromosomes will be in its gametes?
• If dogs produce gametes with 39 chromosomes, how
  many chromosomes will occur in a skin cell?
• Every species has its own unique number and sizes
  of chromosomes.

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Homologous chromosome pairs
similar chromosomes in pairs from separate
parents
Paternal set
Maternal set
Each pair of chromosomes contains the same
position (locus) for a traits allele as its
homologous mate contains. (See Fig. 10.10 on p.
271)
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Why meiosis is necessary

• Mitosis produces genetically identical offspring
  cells as the parent 2n parent -----gt 2
  (2n daughters)
• 2n cells cannot be used in sexual reproduction
• So, theres a need for a special kind of cell
  division which will reduce the chromosome number
  by 1/2 MEIOSIS (reduction division)
• Sperm (n) Egg (n) --------------gt Zygote (2n)

fertilization
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Basics of meiosis

• A sequence of 2 cell divisions

(Homologous chromosome pair)
Meiosis I
Meiosis II
daughter cells
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Meiosis Animation
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Two Phases of meiosis

• Germ cells (the starting cells in sex organs) go
  through a cell cycle which enters meiosis instead
  of mitosis.
• G1 ---gt S ---gt G2 ---gt Meiosis I and II
• Meiosis I
• Prophase I, Metaphase I, Anaphase I, Telophase I
• No G1, S, or G2 -- cell goes directly into II
• Meiosis II
• Prophase II, Metaphase II, Anaphase II, Telophase
  II
• Cytokinesis produces 4 haploid gamete

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Prophase I
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Metaphase I
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• Things to notice in meiosis- -
• crossing over in Prophase I
• independent assortment of homologous chromosomes
  during Metaphase I
• that Meiosis I ends with sister chromatids still
  joined.
• Sister chromatids line up singly in Metaphase II
  (just like in mitosis)
• Gametes contain one each of the beginning sister
  chromatids

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Meiosis Animation Replay
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Methods of Genetic Recombination

• Genetic recombination making new combinations
  of genes in offspring
• 1. Crossing over during Prophase I
• 2. Independent mixing of paternal/maternal
  homologues during Metaphase I (remember
  homologous pairs can come to lie one of 2 ways
  across the equator)

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Genetic mistakes good or bad?

• Nondisjunction in plants
• Sister chromatids fail to separate during
  Anaphase II the centromere doesnt divide
• Researchers can force this to occur by treating
  plant cells with the drug colchicine
• usually results on whole additional sets of
  chromosomes remaining in a gamete.
• leads to polyploidy of some kind triploid,
  tetraploid, hexaploid, octoploid (p. 278 shows
  4n)
• Can only happen in plants (bananas 3n,
  daylilies 4n, pansies 6n, apples 3n, wheat
  4n)

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Nondisjunction in humans

• Is a meiotic accident.
• Usually involves only one chromosome
• Final gamete will contain either 2 chromosomes of
  a kind or no chromosome for that kind.

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• Total count of persons chromosomes will either
  be
• 45 (2n-1) or 47 (2n1)chromosomes.
• 45 chromosomes means that somewhere among
  the chromosomes there is a single (not paired)
  chromosome MONOSOMY
• Turner Female Monosomy X (infertile female not
  lethal)
• 47 chromosomes means that somewhere among the
  chromosomes there is a triplet of chromosomes
  TRISOMY
• Down syndrome is a trisomy of chromosome 21.
• Klinefelter male Trisomy (XXY)(not lethal)
• Patau Syndrome Trisomy 13 (many physical
  defects lethal by 3 days avg.)
• Edwards Syndrome Trisomy 18 (severe mental
  retardation only 10 live beyond 1 year)

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Quick check

• How do the following word combinations relate?
• Pollination fertilization
• Allele dominant recessive
• Genotype phenotype
• Homozygous heterozygous
• Monohybrid dihybrid
• Diploid haploid
• Homologous chromosomes allele
• Sperm egg zygote
• Meiosis gamete
• Crossing over genetic recombination

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Easy questions
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Question 1 In order to assure himself of only the
crosses he wanted, Mendel transferred male pollen
grains to the female parts of pea plants in a
process called A. fertilization B.
pollination C. allele transfer D. A, B, and C
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Question

• Which of the following desribes an organism that
  has the genotype Bb?
• Homozygous
• Heterozygous
• Inbreed
• All of these

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Question

• The gamete that contains genes contributed only
  by the mother is ______.
• The sperm
• An egg
• A zygote
• dominant

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• Question 2
• Mendel's crosses dealing with one trait are
  called
• monohybrid crosses
• dihybrid crosses
• fertilization
• A and B

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Question 3 Mendel defined dominant traits as
those that A. "stand down" in the first
generation B. disappear in the first
generation C. completely disappear forever D.
appear even in hybrid situations
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• Question 4
• If an organism is a homozygote, it may
• show the dominant trait
• show the recessive trait
• both A and B
• neither A nor B

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Question

• The ______ produced by each parent are shown
  along the sides of a Punnett Square.
• Zygotes
• Offspring
• Gamtes
• Hybrids

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• Question 5
• The process of meiosis produces
• gametes
• specialized haploid cells
• sperm and eggs
• A, B, and C

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Medium Difficulty questions
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• Question 1
• Homologous pairs of chromosomes line up at the
  equator of a cell during ____ .
• prophase I
• metaphase I
• metaphase II
• telophase II

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Question

• The only cells that undergo the process of
  meiosis are ______?
• Somatic cells
• Viral cells
• Sex cells
• Bacterial cells

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Question 2 Four haploid cells are produced at
the end of ____ . A. prophase I B. metaphase I
C. metaphase II D. telophase II
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Question 3 A process that explains Mendel's
results is ____ . A. fertilization B. zygote
formation C. meiosis D. mitosis
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Question 4 Body cells are A. diploid B. n C.
A and B D. neither A nor B
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Question

• Cells containing two alleles for each trait are
  described as ______.
• Hapliod
• Gametes
• Diploid
• Homozygous

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Question

• A useful device for predicting the possible
  offspring of crosses between different genotypes
  is the ______.
• Law of dominance
• Law of independent assortment
• Punnett square
• Testcross

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Question 5 Gametes are A. haploid B. n C.
sperm and eggs D. A, B, and C
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Difficult questions
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• Question 1
• If Mendel crossed peas that were heterozygous for
  two dominant traits, purple flowers and green
  pods, how many pea plants would be produced that
  have purple flowers and yellow pods?
• 0
• 1/16
• 3/16
• D. 9/16

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Question

• The statement In meiosis, the way in which a
  chromosome pair separates does not affect the way
  other pairs separate, is another way of
  expressing Mendels law of ______.
• Dominance
• First filial generations
• Independent assortment
• Punnett squares

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Question 2 An organism has the following
genotype KkLLMMNn. How many different kinds of
gametes can be formed? A. 16 B. 8 C. 4 D. 2
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• Question 3
• If two organisms are hybrid for three traits, how
  many boxes would you need to put in the Punnett
  square to show all the possible offspring?
• 64
• 32
• 16
• 9

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Question

• The numbers in Figure 10-1 represent the
  chromosome number found in each of the dog cells
  shown. The processes that are occurring at A and
  B are ____.
• Mitosis and fertilization
• Meiosis and fertilization
• Mitosis and pollination
• Meiosis and pollination

A
B
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Fig. 10-1
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