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 TestisDiploidzygote(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-pollination

CROSS-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