MENDEL

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CHAPTER 10

• MENDEL MEIOSIS

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MENDELS LAWS OF HEREDITY

• I. WHY MENDEL SUCCEEDED
• Gregor Mendol father of genetics
• 1st studies of heredity the passing of
  characteristics to offspring
• Genetics study of heredity
• The characteristics passed on called traits

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1. MENDEL CHOSE HIS SUBJECT CAREFULLY

• Used garden peas to study
• Have male female gametes (sex cells)
• Male female same flower
• Know what pollination fertilization mean
• He could control the fertilization process
• Not many traits to keep track of

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2. MENDEL WAS A CAREFUL RESEARCHER

• USED CAREFULLY CONTROLLED EXPERIMENTS
• STUDIED ONE TRAIT AT A TIME
• KEPT DETAILED DATA

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II. MENDELS MONOHYBRID CROSSES

• MENDEL STUDIED 7 TRAITS CAREFULLY
• Mendel crossed plants w/ diff. traits to see what
  traits the offspring would have
• These offspring are called hybrids offspring of
  parents w/ different traits
• A monohybrid cross is one that looks at only one
  trait (lets look at plant height tall or short)

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A. THE 1ST GENERATION

• Mendel crossed two plants 1 tall 1 short
  (they came from tall short populations)
• These plants are called the parental generation
  (P generation)
• The offspring were all called the 1st filial
  generation (F1 generation)
• All the offspring were tall (the short plants
  were totally excluded)

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B. THE 2ND GENERATION

• Next, Mendel crossed two plants from the F1
  generation
• The offspring from this cross are called the 2nd
  filial generation (F2 GENERATION)
• Mendel found that ¾ of the offspring were tall
  ¼ were short (the short plants reappeared!!!!!!)

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TO GO ANY FURTHER, WE MUST UNDERSTAND ALLELES,
DOMINANCE, SEGREGATION

• Genes a section of DNA that codes for one
  protein
• These genes are what control produce traits
• The genes Mendel studied came in two forms
  (tall/short - round/wrinkled -
  yellow/green.etc.)
• Alternate forms of a gene are called alleles
• Alleles are represented by a one or two letter
  symbol (e.g. T for tall, t for short)

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ALLELES CONTD

• THESE 2 ALLELS ARE NOW KNOWN TO BE FOUND ON
  COPIES OF CHROMOSOMES ONE FROM EACH PARENT

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THE RULE OF DOMINANCE

• A dominant trait is the trait that will always be
  expressed if at least one dominant allele is
  present
• The dominant allele is always represented by a
  capital letter
• A recessive trait will only be expressed if both
  alleles are recessive
• Recessive traits are represented by a lower case
  letter

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DOMINANCE CONTD

• LETS USE TALL SHORT PEA PLANTS FOR AN EXAMPLE
• WHICH OF THESE WILL SHOW THE DOMINANT RECESSIVE
  TRAIT?

TT Tt
tt DOMINANT TRAIT
RECESSIVE TRAIT
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THE LAW OF SEGREGATION

• MENDEL ASKED HIMSELF..HOW DID THE RECESSIVE
  SHORT PLANTS REAPPEAR IN THE F2 GENERATION?
• HE CONCLUDED THAT EACH TALL PLANT FROM THE F1
  GENERATION CARRIED TWO ALLELES, 1 DOMINANT TALL
  ALLELE ONE RECESSIVE SHORT ALLELE
• SO ALL WERE Tt

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SEGREGATION CONTD

• HE ALSO CONCLUDED THAT ONLY ONE ALLELE FROM EACH
  PARENT WENT TO EACH OFFSPRING
• HIS CORRECT HYPOTHESIS WAS THAT SOMEHOW DURING
  FERTILIZATION, THE ALLELES SEPARATED (SEGREGATED)
  COMBINED WITH ANOTHER ALLELE FROM THE OTHER
  PARENT
• The law of segregation states that during gamete
  formation, the alleles separate to different
  gametes

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FATHER
MOTHER
F1 GENERATION
T t
T t
t t
T T
T t
F2 GENERATION
- the law of dominance explained the heredity of
the offspring of the f1 generation - the law of
segregation explained the heredity of the f2
generation
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PHENOTYPES GENOTYPESPG. 264

• PHENOTYPE THE WAY AN ORGANISM LOOKS AND BEHAVES
  ITS PHYSICAL CHARACTERISTICS (i.e. TALL,
  GREEN, BROWN HAIR, BLUE EYES, ETC.)
• GENOTYPE THE GENE COMBONATION (ALLELIC
  COMBINATION) OF AN ORGANISM (i.e. TT, Tt, tt,
  ETC.)
• HOMOZYGOUS 2 ALLELES ARE THE SAME
• HETEROZYGOUS 2 ALLELES DIFFERENT

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ANSWER ON YOUR SHEET

• TRAITS BLUE SKIN YELLOW SKIN
• BB IS THIS HOMOZYGOUS OR HETEROZYGOUS?
• IS BLUE SKIN OR YELLOW SKIN DOMINANT?

HOMOZYGOUS
BLUE
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MENDELS DIHYBRID CROSSES

• MONOHYBRID MENDEL LOOKED AT ONE TRAIT
• IN HIS DIHYBRID CROSSES HE LOOKED AT 2 TRAITS
• WANTED TO SEE IF TRAITS ARE INHERITED TOGETHER OR
  INDEPENDENTLY

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DIHYBRID CROSS

• TOOK TWO TRUE BREEDING PLANTS FOR 2 DIFFERENT
  TRAITS (ROUND/WRINKLED SEEDS ------- YELLOW/GREEN
  SEEDS)
• 1ST GENERATION
• WHAT WOULD HAPPEN IF HE CROSSED JUST TRUE
  BREEDING ROUND W/ TRUE BREEDING WRINKLED (ROUND
  IS DOMINANT)

ALL THE OFFSPRING ARE ROUND
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DIHYBRID CROSS 1ST GENERATION CONTD

• SO WHAT DO YOU THINK HAPPENED WHEN HE CROSSED
  TRUE BREEDING ROUND/YELLOW SEEDS WITH TRUE
  BREEDING WRINKLED/GREEN SEEDS

ALL THE F1 WERE ROUND AND YELLOW
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DIHYBRID CROSS 2ND GENERATION

• TOOK THE F1 PLANTS AND BRED THEM TOGETHER
  (PHENOTYPE WAS ROUND/YELLOW X ROUND/YELLOW)
• 2ND GENERATION
• FOUND ROUND/YELLOW - 9
• FOUND ROUND/GREEN - 3
• FOUND WRINKLED/YELLOW - 3
• FOUND WRINKLED/GREEN - 1
• ( 9 3 3 1 RATIO)

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EXPLANATION OF 2ND GENERATION

• MENDEL CAME UP W/ 2ND LAW THE LAW OF
  INDEPENDENT ASSORTMENT
• GENES FOR DIFFERENT TRAITS ARE INHERITED
  INDEPENDENTLY FROM EACH OTHER
• THIS IS WHY MENDEL FOUND ALL THE DIFFERNENT
  COMBONATIONS OF TRAITS

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PUNNETT SQUARES

• A QUICK WAY TO FIND THE GENOTYPES IN UPCOMING
  GENERATIONS
• 1ST DRAW A BIG SQUARE AND DIVIDE IT IN 4S

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PUNNETT SQUARE

• CROSS T T X Tt

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CONTD

• T T X T t

T
T
T
T
T
T
T
t
T
t
T
t
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DIHYBRID CROSSES

• A LITTLE DIFFERENT
• H h G g X H h G g
• MUST FIND OUT ALL THE POSSIBLE ALLELIC
  COMBONATIONS
• USE THE FOIL METHOD LIKE IN MATH

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H h G g X H h G g
FOIL FIRST, OUTSIDE, INSIDE, LAST
1. HG
BOTH PARENTS ARE THE SAME
2. Hg
3. hG
4. hg
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NOW LETS DO A DIHYBRID CROSS

• H h G g X H h G g

HG
Hg
hG
hg
HG
HHGG
HHGg
HhGG
HhGg
Hg
HHGg
HHgg
HhGg
Hhgg
hG
HhGG
HhGg
hhGG
hhGg
hg
HhGg
Hhgg
hhGg
hhgg
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WHAT ARE THE PHENOTYPIC RATIOS?

• H h G g X H h G g

DD Dr rD rr
9 3 3 1
HG
Hg
hG
hg
HG
HHGG
HHGg
HhGG
HhGg
Hg
HHGg
HHgg
HhGg
Hhgg
hG
HhGG
HhGg
hhGG
hhGg
hg
HhGg
Hhgg
hhGg
hhgg
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PROBABILITY

• WILL REAL LIFE FOLLOW THE RESULTS FROM A PUNNETT
  SQUARE?
• NO!!!!!! A PUNNETT SQUARE ONLY SHOWS WHAT WILL
  PROBABLY OCCUR
• ITS A LOT LIKE FLIPPING A COIN YOU CAN
  ESTIMATE YOUR CHANCES OF GETTING HEADS, BUT
  REALITY DOESNT ALWAYS FOLLOW PROBABILITY

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MEIOSIS

• GENES, CHROMOSOMES, AND NUMBERS
• CHROMOSOMES HAVE 100S OR 1000S OF GENES
• GENES FOUND ON CHROMOSOMES

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DIPLOID HAPLOID CELLS

• ALL BODY CELLS (SOMATIC CELLS) HAVE CHROMOSOMES
  IN PAIRS
• BODY CELLS ARE CALLED DIPLOID CELLS
  (2n)
• HUMANS HAVE THE 2n OF CHROMOSOMES

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DIPLOID AND HAPLOID CELLS CONTD

• HAPLOID CELLS
• ONLY HAVE 1 OF EACH TYPE OF CHROMOSOME (DIPLOID
  CELLS HAVE 2 OF EACH TYPE)
• SYMBOL IS (n)
• SEX CELLS HAVE THE n OF CHROMOSOMES

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HOMOLOGOUS CHROMOSOMES

• HOMOLOGOUS CHROMOSOMES ARE THE PAIRED CHROMOSOMES
  THAT CONTAIN THE SAME TYPE OF GENTIC INFORMATION,
  SAME BANDING PATTERNS, SAME CENTROMERE LOCATION,
  ETC.
• THEY MAY HAVE DIFFERENT ALLELES, SO NOT PERFECTLY
  IDENTICAL
• WHY DO THEY HAVE DIFFERENT ALLELES?

CAME FROM DIFFERENT PARENTS
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WHY MEIOSIS?

• MITOSIS RESULTS IN GENETICALLY IDENTICAL
  OFFSPRING INCLUDING THE CHROMOSOMES
• WHAT WOULD HAPPEN IF THE EGG AND SPERM HAD THE
  SAME OF CHROMOSOMES AS THE BODY CELLS?

EGG 46 CHROMOSOMES SPERM 46 CHROM. ZYGOTE
46 46 92 CHROMOSOMES NOT HUMAN
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MEIOSIS

• A TYPE OF CELL DIVISION WHICH PRODUCES GAMETES
  CONTAING HALF THE NUMBER OF CHROMOSOMES AS THE
  BODY CELLS
• 2 STAGES MEIOSIS I MEIOSIS II
• START W/ 1 DIPLOID CELL, END UP W/ 4 HAPLOID
  CELLS (GAMETES)
• 4 DAUGHTER CELLS ARE GENETICALLY DIFFERENT FROM
  EACH OTHER AND MOTHER CELL

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INTRO TO MEIOSIS CONTD

• SPERM MALE GAMETE (n)
• EGG FEMALE GAMETE (n)
• FERTILIZATION PRODUCES A ZYGOTE (2n)
• THIS TYPE OF REPRODUCTION IS CALLED SEXUAL
  REPRODUCTION

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STAGES OF MEIOSIS

• MEIOSIS I
• PROPHASE I, METAPHASE I, ANAPHASE I, TELOPHASE I
  (PMAT)
• MEIOSIS II
• PROPHASE II, METAPHASE II, ANAPHASE II, TELOPHASE
  II (PMAT)

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IMPORTANT THINGS TO KNOW

• CROSSING OVER OCCURS DURING PROPHASE I
• CREATES GENETIC VARIABILITY (RECOMBINATION OF
  GENES)
• IN MEIOSIS I, HOMOLOGOUS CHROMOSOMES SEPARATE
  (ANAPHASE I)
• IN MEIOSIS II, SISTER CHROMATIDS SEPARATE
• TETRAD WHAT THE HOMOLOGOUS CHROMOSOMES ARE
  CALLED WHEN THEY PAIR UP DURING PROPHASE I