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MENDEL AND MEIOSIS
Chapter 10
 Zebra stripes are like
fingerprints.
 Every zebra’s stripe pattern
is unique.
 Stripes are genetically
determined
Mendel’s Laws of Heredity
 Gregor Mendel
 Austrian monk who carried out studies in heredity
 Chose pea plants to study
 Heredity
 Passing on of characteristics from parents to offspring
 Traits
 Inherited characteristics
 Hair color, eye color
Why Pea Plants?
 Reproduce sexually
 Usually self pollinate
 Gametes – male and female sex cells
 Fertilization – when male gamete units with female
gamete
 Zygote – fertilized cell, develops into a seed
 Able to control pollination!
 Pollination – transfer of pollen grains from male to
female
Mendel used good
scientific method
 Only studied one trait at a time
 Like plant height, flower color, type of seed coat
 Used math to calculate data
Mendel’s Monohybrid
Crosses
 Mendel crossed a tall plant and a short plant
 He called the offspring a hybrid
 Offspring from parents that have different forms of a
trait
 “mono” meaning one trait
 “hybrid” see above definition
Results of monohybrid cross
1st Generation
nd
2
Generation
 Mendel allowed tall plants
to self pollinate
 ¾ were tall
 ¼ were short
 Like the short trait had
reappeared from nowhere
 “P” parent
 “F” filial or son/daughter
Terms
 “P1” – your parents
 “F1” – you
 “F2”- any children you have
Rule of unit factors
 Two factors control traits
 Traits are controlled by genes
 Genes are locations on chromosomes
 Different forms of a gene are called “alleles”
 Like two alleles for height
 Possibilities
 Two alleles for tall height
 Two alleles for short height
 One allele for tall height and one allele for short height
Rule of Dominance
 Dominant – the trait that you see
 Recessive – the trait that is not expressed if
dominant allele is present
 Use same letter for allele
 Capital letter for dominate
 Lower case letter for recessive
 Dominant (capital letter) is written first
Law of Segregation
 Every individual has two alleles of each gene and
when gametes are produced, each gamete receives
one of these alleles
Phenotypes and Genotypes
 Two organisms can look alike (tall pea plants) but
have different underlying allele combinations.
 Phenotype – the way an organism looks and behaves
– PHYSICAL
 Tall plant (phenotype) even though it could be TT or Tt
 Genotype – the allele combination an organism
contains – GENETIC
 TT or Tt or tt – the actual genetic makeup of organism
Homozygous or Heterozygous
 Homozygous – when alleles for a trait are the same
 TT or tt – homozygous for tall or short
 Heterozygous – when alleles for trait are different
 Tt – one allele for tall height, one allele for short
height
Practice
 Give phenotype and genotype of the following (R is
dominant for round seed, r is recessive for wrinkled
seed)






TT
Tt
Tt
RR
Rr
rr
 Heterozygous or homozygous?
Dihybrid Cross
 Differing in 2 traits
 Mendel paired plants that were RRYY (round and
yellow) and rryy (wrinkled and green)
 He found the offspring were in a definite ratio of
phenotypes
 9 round yellow
 3 round green
 3 wrinkled yellow
 1 wrinkled green
Law of Independent
Assortment
 Genes for different traits are inherited
independently of each other
 Having the allele for a green seed color does not effect
the type of seed coat or height of plant
Punnett Squares
 A method to predict offspring outcomes based on
knowledge of parent genotypes.
 PROBABILITY
 Punnett Squares show likelihood, not reality
 Closer to actual numbers when the number of
offspring is large
FOIL METHOD
 FIRST, OUTSIDE, INSIDE, LAST
 AaBb




F – AB
O – Ab
I – aB
L – ab
EXAMPLE
 RRYy X Rryy
Practice
 Complete a Punnett Square for the following crosses
 List possible genotypes and phenotypes
 Traits for flower color and plant height




P – purple flower
p – white flower
T – tall
t – short
 PpTt X PPtt
MEIOSIS
CHAPTER 10 SECTION 2
Genes, Chromosomes and
Numbers
 Pea plants had 14 chromosomes or 7 pairs
 Most chromosomes appear in pairs
 One from male parent, one from female parent
 Diploid
 Cell that contains a diploid or 2n, number of chromosomes
 Gametes contain one of each kind of chromosome
 Haploid
 Cell that contains one of each kind of chromosome, haploid
or n
Chromosome Numbers
Organism
Body cells (2n)
Gamete (n)
Fruit fly
8
4
Corn
20
10
Human
46
23
Dog
78
39
Homologous Chromosomes
 Have genes for the same trait
 But may have different alleles
 Mendel Pea Plants
 Get one from each parent
Why Meiosis?
 Difference between Mitosis and Meiosis?
 End product – Mitosis
 2 identical daughter cells, with same genetic makeup as
parent and the same number of chromosomes
 End product – Meiosis
 Gametes with half the number of chromosomes as parent
 Why does this work?
 When gametes unite – you are back to the right number of
chromosomes
Meiosis
 Occurs in the specialized body cells of each parent
 Male gametes – sperm – haploid (n)
 Female gametes – egg – haploid (n)
 Joining of sperm and egg – diploid (2n) number of
chromosomes
 Zygote then undergoes mitosis to develop into
multicellular organism
 Sexual Reproduction
 Recombination of genetic material
Phases of Meiosis
 Meiosis I
 Interphase
 Chromosomes replicated, held together by centromere
 Prophase I
 DNA thickens
 Homologous chromosomes line up
 Four part structure is called a tetrad
 Crossing Over


When chromosomes break and exchange genetic material
Approx 2-3 crossovers for each pair of homologous
chromosomes, in humans
Meiosis I
 Metaphase I
 Spindle fiber pulls tetrad to middle of cell
 Homologous chromosomes are side by side
 Anaphase I
 Homologous chromosomes separate and move to opposite
ends of the cell
 Centromeres are still holding sister chromatids together
 Telophase I




Spindles break down
Chromosomes uncoil
Cytoplasm divides to yield two new cells
Cell is still at 2n, because of the replication of DNA
Meiosis II
 We need to get to n, or haploid, to a gamete
 Prophase II
 Spindle forms and attach chromotids
 Metaphase II
 Chromatids line up randomly at equator
 Anaphase II
 Centromere splits, sister chromatids separate
 Move to opposite poles
 Telophase II
 Spindle break down
 Cytoplasm divides
End Products of Meiosis
 4 haploid (n) cells are formed
 These will become gametes
Meiosis Provides for
Genetic Variation
 Genetic Variation - reassortment of chromosomes and
genetic information they carry by c.o. or independent
segregation
 Crossing Over
 Provides variation by giving new combinations of alleles
 Pea Plant Sperm possibilities
 Seven pairs of chromosomes can each line up 2 different
ways, same for eggs
 2n = 2 7 = 128
 Humans
 2n = 223 or 8 million possible egg or sperm
Nondisjunction
 Events usually go smoothy during Meiosis, but,
sometimes, homologous chromosomes fail to
separate
 This is called nondisjunction
 For example, when nondisjunction occurs
 One gamete has an extra chromosome
 One gamete is missing a chromosome
 When the gamete with the extra chromosome is
fertilized by a normal gamete Trisomy occurs
 Now 47 not 46 chromosomes, location chromosome
21, Down Syndrome occurs
 Turner Syndrome
 Monosomy – missing a X chromosome, occurs in
females
Polyploidy
 More than the usual number of chromosomes
 Usually results in death for zygote in ANIMALS
 But, in plants, used to plant breeders advantage
 Plants are healthier, larger
 Hexaploid wheat, triploid apples, polyploid
chrysanthemums
Gene Linkage and Maps
 If genes are close together, the usually are inherited
together
 They are said to be “linked”
 All the genes on a chromosome are usually linked
 Chromosomes themselves follow I.A.
 Affected by Crossing Over