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Introduction to Genetic Variation. Or, lame photo montage thinly disguised as illustration of genetic variation. Key. Haploid gametes ( n = 23 ). Meiosis. Haploid ( n ). Egg ( n ). Diploid (2 n ). Sperm ( n ). MEIOSIS. FERTILIZATION. Testis. Ovary. Diploid zygote (2 n = 46).

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introduction to genetic variation

Introduction to Genetic Variation

Or, lame photo montage thinly disguised as illustration of genetic variation

meiosis

Key

Haploid gametes (n = 23)

Meiosis

Haploid (n)

Egg (n)

Diploid (2n)

Sperm (n)

MEIOSIS

FERTILIZATION

Testis

Ovary

Diploid

zygote

(2n = 46)

Mitosis and

development

Multicellular diploid

adults (2n = 46)

contributors to genetic variation
Contributors to Genetic Variation
  • Independent assortment
    • Which chromosome does a gamete get?
  • Crossover events (“recombination”)
    • Chimeric alleles (remember chiasma formation?)
  • Random fertilization
    • Any sperm can fertilize any egg
independent assortment
Independent assortment
  • Whose chromosome did I get in Meiosis I?
    • 50-50 shot at maternal or paternal per gamete
  • Independence of pairs
    • Each homologous pair is sorted independently from the others
  • For humans (n = 23) there are about 8 million possible combinations of chromosomes!

Maternal

Paternal

n=2 chromosomes

M1/M2

P1/P2

M1/P2

P1/M2

separation of homologs
Separation of Homologs

Example: individual who is heterozygous at two genes

Allele that

contributes

to red hued hair

Allele that

contributes

to dark hair

Allele that

contributes

to green

eyes

Allele that

contributes

to blue eyes

Hair color gene

Eye color gene

During meiosis I, tetrads can line up two different

ways before the homologs separate.

OR

Green eyes

Red hues

Blue eyes

Red hues

Green eyes

Dark hair

Blue eyes

Dark hair

crossing over genetic recombination
Crossing Over- Genetic Recombination
  • Recombinant chromosomes
    • combine genes from each parent.
  • Prophase I
    • Chromosomes pair up gene by gene
    • Chiasma
  • Homologous portions of two nonsisterchromatids traded
  • In Humans
    • two to three times per chromosome pair.
  • New combinations of alleles
    • combinations that did not exist in each parent.
  • Independent assortment builds on this variability
key events in prophase of meiosis i
Key Events in Prophase of Meiosis I
  • Prophase I
    • 2 pairs of sister chromatids are held tightly together
  • Crossing over can occur at many locations
  • Swapping of segments between maternal and paternal chromosomes.

Centromere

Non-sister

chromatids

Sister chromatids

Chromosomes

Protein complex

One homolog

Synaptonemal

complex

Second homolog

slide8

Fig. 13-12-5

Prophase I

of meiosis

Nonsister

chromatids

held together

during synapsis

Pair of

homologs

Chiasma

Centromere

TEM

Anaphase I

Anaphase II

Daughter

cells

Recombinant chromosomes

random fertilization
Random Fertilization
  • Any sperm can fuse with any egg.
  • Humans (n=23)
    • Each ovum is one of 8 million possible chromosome combinations
    • Successful sperm is one of 8 million different possibilities
    • Zygote (diploid offspring) is 1 of 70 trillion possible combinations of chrms
      • Amazing how similar siblings/offspring

can look!

  • Recombination adds even more variation to this.
  • Independent assortment builds on recombination
  • Mutations- ultimately create a population’s genetic diversity

…or not!

slide10

Gregor Mendel (1822-1884)

  • Lots of training
    • Augustine monk
    • Beekeeper
    • Physics teacher
    • Meteorologist
  • Monastery garden
    • Pea plants
experiments on plant hybridization
“Experiments on Plant Hybridization”
  • Published in 1866
    • Before 20th century, cited 3 times
    • NOT cited in “The Origin of Species” (1859)
  • Rediscovered
    • Hugo de Vries
    • Better publicity
mendel and the gene idea
Mendel and the Gene Idea
  • What he knew:
    • Heritable variations exist
    • Traits are transmitted from parents to offspring
  • Two main theories existed
    • Blending (mixing of traits)
    • Particulate inheritance (direct passage of one trait over another)
  • Where he started:
    • documented particulate inheritance with garden peas (Pisumsativum).
why peas are awesome genetic models for 1860s
Why Peas are Awesome Genetic Models for 1860s

Phenotypes

Trait

Seed shape

Round

Wrinkled

  • Lots of visible traits (“phenotypes”)
    • flower color, seed shape, pod shape, etc.
  • Controlled mating
    • Hermaphroditic
      • sperm-producing organs (stamens) and egg-producing organs (carpels)
    • Cross-pollination (fertilization between different plants) can be done intentionally

Seed color

Yellow

Green

Pod shape

Constricted

Inflated

Pod color

Green

Yellow

Flower color

Purple

White

Flower and   pod position

Axial (on stem)

Terminal (at tip)

Stem length

Tall

Dwarf

mendel focused on particulate inheritance
Mendel Focused on Particulate Inheritance
  • True-breeding varieties
    • Offspring of the same variety when they self-pollinate
  • Hybridization
    • mate two contrasting, true-breeding varieties
  • True-breeding parents P generation
  • Hybrid offspring of the P generation are called the F1 generation
  • F1 individuals self-pollinate, the F2 generation is produced
how was this technically done

TECHNIQUE

How was this Technically Done?

1

  • Peas normally self-fertilize
    • This is a problem…
  • Cut the stamen
    • Removes male gametes
    • Prevents selfing
  • Manually add pollen
    • Carpels fertilized by non-self plants
  • Forced outcrossing

2

Parental

generation

(P)

Stamens

Carpel

3

4

RESULTS

First

filial

gener-

ation

offspring

(F1)

5

cross pollination forced outcrossing
Cross-Pollination (“Forced outcrossing”)

Self-pollination

Female organ

(receives pollen)

  • Control over matings
    • Allows observations and predictions
    • Great approach for genetics at large

SELF-

POLLINATION

Male organs

(produce pollen

grains, which

produce male

gametes)

Eggs

CROSS-

POLLINATION

Cross-pollination

1. Remove male organs

from one individual.

3. Transfer pollen to the

female organs of the individual

whose male organs have

been removed.

2. Collect pollen from a

different individual.

particulate inheritance dominant and recessive traits
Particulate Inheritance: Dominant and Recessive Traits

RR or Rr

  • Mendel’s outcrossed plants
    • Seed shapes were either round or wrinkled
    • No “chimeric” version
    • NOT 50-50; round seeds were more common
  • Dominant trait
    • Round seeds
  • Recessive trait
    • Wrinkled seeds
  • Writing convention for alleles: R vs. r
    • Capital letter = dominant allele; lowercase = recessive allele
  • Individuals with two copies of the same allele (RR or rr) are homozygous, and those with two different alleles (Rr) are heterozygous.

If Dominant gene is present, offspring WILL have the trait without exception

alwaysrr