SEX




      What is Sex?
      What is Mitosis?
      What is Mendelian Genetics?




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" Life is sexually transmited, but Sex is only one means of reproduction!"

Reproduction I

    • Asexual reproduction: Cell division and tissue growth
      = SAME GENETIC MAKE-UP

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Reproduction II

Sexual reproduction
    • Recombination of genetic information
    • Male and female gametes (haploid)   combine to form a cell with a full   complement of genetic material (diploid)
    • This cell is call a zygote

      = NEW, UNIQUE GENETIC MAKE-UP

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Both Asexual and Sexual reproduction involve the duplication of genetic material (chromosomes) DNA

Mitosis: duplication of genetic material - one cell to two (identical) cells (diploid)

Meiosis: production of gametes - ½ the normal number of chromosomes (haploid)



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Stages of mitosis

    • Condensation of chromosomes that consist of paired chromatids (prophase)
    • Migration & alignment of chromosomes at cell´s equator (metaphase)
    • Splitting of chromosomes into chromatids & migration (anaphase)
    • Chromatids migrated to opposite ends of the cell (telophase)
    • (Cytokenisis, Replication) Review Animation

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Reproduction II Sexual reproduction

Question: What do you get if you cross a smooth pea & a wrinkled pea?





Hypotheses?




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Mendel's Observations of Peas

  Cross a pure smooth pea & a wrinkled pea
      - get all smooth pea offspring.
  Cross any two of these offspring
      - get 25% wrinkled.


SMOOTH

WRINKLED

Parents




SMOOTH

SMOOTH

SMOOTH

SMOOTH
First Generation



SMOOTH

SMOOTH

SMOOTH

WRINKLED
Second Generation
Appearance (smooth, wrinkled)="Phenotype"

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Hypothesis: inheritance controlled by discrete paired units (genes)

    alleles: one of the alternate forms of a gene

S S

s s

Parents




S s

S s

S s

S s
First Generation F1



S S

S s

S s

s s
Second Generation F2

Genetic makeup(S, s)="Genotype"

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Inheritance (Mendelian)

Phenotype is controled by Genotype
The offspring get one gene from each parent.
  - S (smooth) is dominant.
  - s (wrinkled) is recessive.
Wrinkled phenotypes are only ss genotypes
  - "homozygous recessive only"
Smooth phenotypes may be SS or Ss genotypes
  -"homozygous" OR "heterozygous"

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Why were the offspring of the smooth pea - wrinkled pea cross (1st generation) all smooth?
  -   Homozygous smooth peas only "S" gametes
  -   Homozygous wrinkled peas only "s" gametes
All Offspring (F1) have the Ss genotype and therefore all are smooth

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What about crossing two of the heterozygous offspring (F1)?
  - " They produce both "S" and "s" gametes
  - " The Punnett square includes All combinations
 

S

gamete

s

gamete

S

gamete

SS

smooth

Ss

smooth

s

gamete

Ss

smooth

ss

wrinkled
¼ of the (F2) have the "wrinkled" phenotype (ss)

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What if Mendel had crossed a smooth pea with a wrinkled pea and had gotten 50% - 50% mix?
 

?

gamete

?

gamete

s

gamete

??

??

s

gamete

??

??

Is the smooth pea homozygous (SS) or heterozygous (Ss)?

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Finally, we can use the Punnett square to consider two characters simultaneously
  -   S (smooth) is dominant to s (wrinkled)
  -   Y (yellow) is dominant to y (green)
  -   Let's cross 2 heterozygous pea plants
FIRST: what kinds of gametes can they produce?

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SY

gamete

sY

gamete

Sy

gamete

sy

gamete

SY

gamete

SSYY

SsYY

SSYy

SsYy

sY

gamete

SsYY

ssYY

SsYy

ssYy

Sy

gamete

SSYy

SsYy

SSyy

Ssyy

sy

gamete

SsYy

ssYy

Ssyy

ssyy

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  -   What are the four phenotypes?
  -   What is the ratio of the phenotypes?

Smooth Yellow 9
Smooth Green 3
Wrinkled Yellow 3
Wrinkled Green 1


This (Mendelian Genetics) only works if the color and wrinkledness genes are on different chromosomes; i.e., the are not "linked."

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What good is sex?
  1. VARIABILITY INCREASES CHANCES OF SURVIVAL IN A HETEROGENEOUS AND CHANGING ENVIRONMENT
  2. MEIOSIS (THE METHOD BY WHICH GAMETES ARE PRODUCED) AND FERTILIZATION (RECOMBINATION) WEED OUT HARMFUL MUTATIONS

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