1. CHAPTER 9
MENDEL & MEIOSIS

2. 10.1 MENDEL’S 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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Mendelian Genetics
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Gregor Johann Mendel
Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants
He found that the plants' offspring retained traits of the parents
Called the “Father of Genetics"
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5. Particulate Inheritance
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Particulate Inheritance
Mendel stated that physical traits are inherited as “particles”
Mendel did not know that the “particles” were actually Chromosomes & DNA
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Genetic Terminology
Trait - any characteristic that can be passed from parent to offspring
Heredity - passing of traits from parent to offspring
Genetics - study of heredity
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Designer “Genes”
Alleles - two forms of a gene (dominant & recessive)
Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R)
Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r)
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8. 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

9. Reproduction in Flowering Plants
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Reproduction in Flowering Plants
Pollen contains sperm
Produced by the stamen
Ovary contains eggs
Found inside the flower
Pollen carries sperm to the eggs for fertilization
Self-fertilization can occur in the same flower
Cross-fertilization can occur between flowers
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How Mendel Began
Mendel produced pure strains by allowing the plants to self-pollinate for several generations
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12. 2. MENDEL WAS A CAREFUL RESEARCHER
USED CAREFULLY CONTROLLED EXPERIMENTS
STUDIED ONE TRAIT AT A TIME
KEPT DETAILED DATA

13. Mendel’s Experimental Methods
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Mendel’s Experimental Methods
Mendel hand-pollinated flowers using a paintbrush
He could snip the stamens to prevent self-pollination
Covered each flower with a cloth bag
He traced traits through the several generations
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14. II. MENDEL’S 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 (let’s look at plant height – tall or short)

15. 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)

16. 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!!!!!!)

18. 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)

20. ALLELES CONT’D
THESE 2 ALLELS ARE NOW KNOWN TO BE FOUND ON COPIES OF CHROMOSOMES – ONE FROM EACH PARENT

21. 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

22. DOMINANCE CONT’D LET’S USE TALL & SHORT PEA PLANTS FOR AN EXAMPLE
WHICH OF THESE WILL SHOW THE DOMINANT & RECESSIVE TRAIT?
TT Tt tt
DOMINANT TRAIT RECESSIVE TRAIT

23. 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

24. SEGREGATION CONT’D
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

25. Applying the Law of Segregation
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Applying the Law of Segregation
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26. - the law of segregation explained the heredity of the f2 generation
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

29. PHENOTYPES & GENOTYPES PG. 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

30. ANSWER ON YOUR SHEET TRAITS = BLUE SKIN & YELLOW SKIN
BB – IS THIS HOMOZYGOUS OR HETEROZYGOUS?
IS BLUE SKIN OR YELLOW SKIN DOMINANT?
HOMOZYGOUS
BLUE

31. MENDEL’S 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

32. ALL THE OFFSPRING ARE ROUND
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

33. DIHYBRID CROSS – 1ST GENERATION CONT’D
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

34. DIHYBRID CROSS – 2ND GENERATION
TOOK THE F1 PLANTS AND BRED THEM TOGETHER (PHENOTYPE WAS ROUND/YELLOW X ROUND/YELLOW)
2ND GENERATION
FOUND ROUND/YELLOW
FOUND ROUND/GREEN
FOUND WRINKLED/YELLOW - 3
FOUND WRINKLED/GREEN
( 9 : 3 : 3 : 1 RATIO)

35. 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

36. Summary of Mendel’s laws
Mendelian Genetics
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Summary of Mendel’s laws
LAW
PARENT CROSS
OFFSPRING
DOMINANCE
TT x tt tall x short
100% Tt tall
SEGREGATION
Tt x Tt tall x tall
75% tall 25% short
INDEPENDENT ASSORTMENT
RrGg x RrGg round & green x round & green
9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods
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Incomplete Dominance
F1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties.
Example: snapdragons (flower)
red (RR) x white (rr)
RR = red flower
rr = white flower r R
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Codominance Problem
Example: homozygous male Type B (IBIB)
x heterozygous female Type A (IAi) IB IA i
IAIB
IBi
1/2 = IAIB
1/2 = IBi
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Codominance
Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?
boy - type O (ii) X girl - type AB (IAIB)
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Codominance
Answer: IB IA i
IAIB ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
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Sex-linked Traits
Traits (genes) located on the sex chromosomes
Sex chromosomes are X and Y
XX genotype for females
XY genotype for males
Many sex-linked traits carried on X chromosome
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Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
XX chromosome - female
Xy chromosome - male
fruit fly
eye color
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Mendelian Genetics
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Codominance
Answer: IB IA i
IAIB ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
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44. PUNNETT SQUARES
A QUICK WAY TO FIND THE GENOTYPES IN UPCOMING GENERATIONS
1ST DRAW A BIG SQUARE AND DIVIDE IT IN 4’S

45. PUNNETT SQUARE
CROSS T T X Tt

46. CONT’D
T T X T t T T T T T T T t T t T t

47. 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

48. H h G g X H h G g 1. HG BOTH PARENTS ARE THE SAME 2. Hg 3. hG 4. hg
FOIL – FIRST, OUTSIDE, INSIDE, LAST
H h G g X H h G g
1. HG
BOTH PARENTS ARE THE SAME
2. Hg
3. hG
4. hg

49. NOW LET’S 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

50. WHAT ARE THE PHENOTYPIC RATIO’S?
H h G g X H h G g
DD:
Dr:
rD:
rr: 9 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

51. PROBABILITY WILL REAL LIFE FOLLOW THE RESULTS FROM A PUNNETT SQUARE?
NO!!!!!! – A PUNNETT SQUARE ONLY SHOWS WHAT WILL PROBABLY OCCUR
IT’S A LOT LIKE FLIPPING A COIN – YOU CAN ESTIMATE YOUR CHANCES OF GETTING HEADS, BUT REALITY DOESN’T ALWAYS FOLLOW PROBABILITY

52. 10.2 MEIOSIS GENES, CHROMOSOMES, AND NUMBERS
CHROMOSOMES HAVE 100’S OR 1000’S OF GENES
GENES FOUND ON CHROMOSOMES

53. 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

54. DIPLOID AND HAPLOID CELLS CONT’D
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

55. 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

56. ZYGOTE = 46 + 46 = 92 CHROMOSOMES =
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 = = 92 CHROMOSOMES =
NOT HUMAN

57. 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

58. INTRO TO MEIOSIS CONT’D
SPERM – MALE GAMETE (n)
EGG – FEMALE GAMETE (n)
FERTILIZATION PRODUCES A ZYGOTE (2n)
THIS TYPE OF REPRODUCTION IS CALLED SEXUAL REPRODUCTION

59. STAGES OF MEIOSIS MEIOSIS I MEIOSIS II
PROPHASE I, METAPHASE I, ANAPHASE I, TELOPHASE I (PMAT)
MEIOSIS II
PROPHASE II, METAPHASE II, ANAPHASE II, TELOPHASE II (PMAT)

60. 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