Introduction to Genetic Variation
Or, lame photo montage thinly disguised as illustration of genetic variation
MeiosisKey
Haploid (n)Diploid (2n)
Haploid gametes (n = 23)Egg (n)
Sperm (n)MEIOSIS FERTILIZATION
Ovary Testis
Diploidzygote(2n = 46)
Mitosis anddevelopment
Multicellular diploidadults (2n = 46)
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• 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!
MaternalPaternaln=2 chromosomes
M1/M2 P1/P2 M1/P2 P1/M2
Separation of HomologsExample: individual who is heterozygous at two genes
Allele thatcontributesto greeneyes
Allele thatcontributesto blue eyes
Eye color gene
Allele thatcontributesto red hued
hairAllele thatcontributesto dark hair
Hair color gene
During meiosis I, tetrads can line up two differentways before the homologs separate.
OR
Green eyesRed hues
Blue eyesDark hair
Green eyesDark hair
Blue eyesRed hues
Crossing Over- Genetic Recombination• Recombinant chromosomes
– combine genes from each parent.
• Prophase I– Chromosomes pair up gene by gene– Chiasma
• Homologous portions of two nonsister chromatids 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
Centromere
Sister chromatids
Chromosomes
One homolog
Synaptonemalcomplex
Second homolog
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.
Non-sisterchromatids
Protein complex
Fig. 13-12-5Prophase Iof meiosis
Pair ofhomologs
Nonsisterchromatidsheld togetherduring synapsis
Chiasma
Centromere
Anaphase I
Anaphase II
Daughtercells
Recombinant chromosomes
TEM
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!
Gregor Mendel (1822-1884)• Lots of training– Augustine monk– Beekeeper– Physics teacher– Meteorologist
• Monastery garden– Pea plants
“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• 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 (Pisum sativum).
Trait PhenotypesSeed shape
Seed color
Pod shape
Pod color
Flower color
Flower and pod position
Stem length
Tall Dwarf
Terminal (at tip)Axial (on stem)
Purple White
YellowGreen
Inflated Constricted
GreenYellow
Round Wrinkled
Why Peas are Awesome Genetic Models for 1860s
• 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
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
TECHNIQUE
RESULTS
Parentalgeneration(P) Stamens
Carpel
1
2
3
4
Firstfilialgener-ationoffspring(F1)
5
How was this Technically Done?
• 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
Cross-Pollination (“Forced outcrossing”)• Control over matings– Allows observations and predictions– Great approach for genetics at large
Self-pollinationFemale organ(receives pollen)
EggsMale organs(produce pollengrains, whichproduce malegametes)
Cross-pollinationCROSS-POLLINATION
3. Transfer pollen to the female organs of the individual whose male organs have been removed.
2. Collect pollen from adifferent individual.
1. Remove male organsfrom one individual.
SELF-POLLINATION
Particulate Inheritance: Dominant and Recessive Traits
• 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
RR or Rr
always rr