GeneticsHow do organisms reproduce?

How does hereditary information pass from one generation to the next?

Animal Reproduction• Each organism has TWO SETS of genetic

information (2 sets of chromosomes)– One set from one parent– One set from other parent – This condition is called DIPLOID

• One set comes from the nucleus of the EGG and the other from the nucleus of the SPERM (= gametes)

Chromosomes• Each of us, therefore, has two of each

chromosome (one #3 from mom and one #3 from dad etc.)

• Pairs are called homologous chromosomes• Humans have 46 chromosomes, but actually

23 PAIRS– 22 SOMATIC chromosome pairs (#1-22)– 1 pair of SEX chromosomes (males XY and

females XX)

“The Paradox”

• But (unless you are an identical twin), each individual is different! Unique!

• How, then, does each parent donate in their gamete only 1 SET of chromosomes (from a diploid or 2N organism)?

• How can such extensive VARIATIONcome about?

MEIOSIS is the answer!

ANIMAL(2N)

MEIOSIS

GAMETE (N)

SYNGAMY

GAMETE (N)(usually from

another individual)

Human Life Cycle

MEIOSIS• A two step process (Meiosis I and

Meiosis II) with two end results:– gametes or spores with one set of

chromosomes (N) instead of the original two sets (2N)

– all cells (gametes or spores) are genetically different. Lots of variation.

MEIOSIS (cont.)

• Takes place in sex organs-- ovaries and testes in higher animals

• In the ovules and anthers in higher plants• In sporangia in ferns and related groups• In basidia and asci in many fungi• In single cells in yeasts, protozoa and some

algae• etc…...

MEIOSIS I

• Chromosomes double during preparation for meiosis (S-phase) and begin to condense

• During PROPHASE I homologous chromosomes “pair up” - a process called SYNAPSIS

• Then CROSSING OVER takes place between homologous chromosomes

Condensation of DNA

Crossing Over

MEIOSIS I (cont.)

• At METAPHSE I the doubled homologous pairs randomly line up on either side of the “equator” of the cell (mom’s #3 one one side, dad’s #3 on the other side)

• THIS ACTIVITY CREATES MASSIVE VARIATION IN THE RESULTING CELLS

MEIOSIS I (cont.)

• At ANAPHASE I one each homologous pair (still with two chromatids) moves to opposite poles

• TELOPHASE I nuclei reform as in mitosis• Cytokinesis may or may not occur

depending on the organism

MEIOSIS I

MEIOSIS II• Between the end of Meiosis I and the

beginning of Meiosis II (interphase) NO S-PHASE occurs - no doubling of the chromosome material (DNA)

• Prophase II and Metaphase II are essentially identical to mitosis

• At Anaphase II chromatids separate. New cells are formed in Telophase II with normal cytokinesis

MEIOSIS II

MEIOSIS II

Results of Meiosis• Four cells are derived from the original cell

(2N) • Each of these cells is HAPLOID (N)• These cells may function as gametes or

spores (depending on the group)• All the cells are genetically different - no

two are the same!

Animation of meiosis:http://www.johnkyrk.com/meiosis.html

Human Meiosis• In male testes primary spermatocytes are

the cells which undergo meiosis• Each spermatocyte produces four sperm

cells• In the female ovary primary oocytes are the

cells which undergo meiosis• However, cytokinesis is unequal at Meiosis

I and Meiosis II and only one egg cell is produced

Human Meiosis (cont.)

Male Spermatocyte (2N)

Sperm Cells (N)

Female Oocyte (2N)

Egg Cell (N)

First Polar Body

Second Polar Body

MEIOSIS I

MEIOSIS II

Principles of Genetics

Gregor Mendel Barbara McClintock

Mendelian Genetics

How is hereditary information passed from one generation to the next?

“Mr. Darwin and I were asking different questions. He was concerned with the process of natural selection, that is, why certain traits were passed from generation to generation. I was working with something much more fundamental, namely the mechanism by which these traits are passed. Not why, but how! ” NOVA video: “The Garden of Inheritance”

Some Terminology

• Each chromosome carries information for thousands of GENES (sequences of DNA)

• In diploid organisms, you have two versions of each gene =ALLELES

• One allele on each homologous chromosome at some precise location called the LOCUS of the gene

Terminology (cont.)

• If the alleles are the same = homozygous or true breeding

• If the alleles are different = heterozygous

yYcentromere

YY

Mendel: Law of Segregation

• Mendel was first to postulate that genes (which he called particles) would segregate during the formation of gametes– Y and y would end up in different gametes– distribution would be random, based on simple

statistical principles• He also introduced the concept of

dominant vs. recessive alleles

A Monohybrid Cross• Mendel crossed a true-breeding pea plant

with yellow seeds with a true-breeding pea plant with green seeds (P generation)

• All the offspring (F1 generation = first filial generation) had yellow seeds

• He then crossed two of the F1 generation plants and got F2 plants with about 75% yellow seeds and 25% green seeds. Diagram the cross.

Monohybrid Cross: Solution

Y

Y

y yYY

PARENT

yy

PARENT

Yy

YyYy

Yy

Result: All F1 plants are yellow (dominant)

Monohybrid Cross: Solution (cont.)

Y

y

Y y

yY

F1 Plant

yY

F1 Plant

YY

yyYy

Yy

Results: 3 yellow to 1 green = PHENOTYPIC RATIO

1 (YY) : 2 (Yy) : 1 (yy) = GENOTYPIC RATIO

The Test Cross• If you have a yellow-seeded plant, how can

you tell if it is true-breeding or not? • It could be Yy (yellow) or YY (yellow)• Perform a “test cross” with your unknown

genotype by crossing it with the homozygous recessive type (yy)– If you get all yellow offspring your unknown

was YY– If you get 1/2 yellow and 1/2 green, you

unknown was Yy

Law of Independent Assortment• When Mendel considered TWO traits at a time, he

discovered that the inheritance of one did not seem to effect the inheritance of the other

• Getting yellow seeds do not predict whether or not they would be wrinkled!

• Mendel postulated that these traits were inherited INDEPENDENTLY

• We now say that, “As long as the traits are on separate chromosomes, inheritance of one does not influence the inheritance of the other.”

The Dihybrid Cross• We have two true-breeding guinea pigs• One has black fur and with short hair, the other

brown fur with long hair• We cross these two PARENTS and get all black

fur, short hair offspring (F1). What is probably dominant?

• Now we cross two of these F1 animals and get: 17 black, short hair ones, 7 black, long hairs, 6 brown, short hairs, and 2 brown long-haired individuals. Explain.

Dihybrid Cross: Solution

GAMETESBB

PARENT

black, short hair

S S

All BS

All bs

bb

PARENT

brown, long hair

s s

BbSs

Dihybrid Cross: Solution (cont.)

Meiosis produces 4 possible types of gametes (in equal proportions/probabilities) for each F1 individual

BS Bs bS bs

BbSs BbSsF1 Offspring

Dihybrid Solution (cont.)

BS

Bs

bSbs

BS Bs bS bs

BBSS

bbss

bbSS

BBss

RESULTS of DIHYBRD CROSS

9

Black, Short Hair

3

Black, Long Hair

3

Brown, Short Hair

1

Brown, Long Hair

F1Both

Black, Short Hair

F2 Generation: FOUR PHENOTYPES in ratio of 9:3:3:1

What if the genes are linked?

BB

SS

bb

ss

PARENTS

bB

sSF1 GENERATION

Linkage (cont.)

bB

sS

bB

sS

F1 Generation

BS bs

Possible Gametes?

RESULTS with LINKAGE

bs

BS

bsBS

BBSS

BbSs

BbSs

bbss

3 black, short hair

1 brown, long hair

Linkage (cont.)

• But what if you got the following results:– 58 black, short hair– 22 brown, long hair– 3 black, long hair– 4 brown, short hair

• How would you explain these results?

CROSSING OVER

Bb

sS

Crossover F1

bB

sS

Regular F1

Now sometimes two new gametes are in the mix (Bs and bS) allowing for the possibility of the other two phenotypes:

brown, short hair and black, long hair.

Variations on Mendelian Inheritance

• X-Linked (sex-linked) genes• Incomplete Dominance• Co-Dominance• Pleiotropy• Polygenic Inheritance• Polyploidy

X-Linked Genes

• Two human examples are hemophilia and colorblindness (both recessive traits)

• Male individuals (XY) have only one allele since the Y chromosome does not carry the trait

• Females (XX) have two alleles

X-Linked Traits (cont.)

• Sample Problem:–A colorblind man has children with

a normal-visioned woman whose father was colorblind. What percent of their male offspring will be expected to be colorblind? How about their female children?

X-Linked Genes (cont.)

• Solution:

x

XcY

XcY XcX?

Answer: 1/2 of male and 1/2 of female children are predicted to be colorblind.

Incomplete Dominance

• In the interaction of two alleles, neither is dominant

• Result is a “blending” of the two characteristics

• In snapdragons, cross a homozygous red flowered (RR) plant with a homozygous white (rr) flowered plant and all offspring are pink (Rr)

Co-Dominance• In a series of alleles, more than one allele is

“dominant” to a “recessive” allele• When two dominants alleles occur together,

neither is dominant• Example: A,B,O blood groups in humans

– “A” and “B” alleles are both dominant to the recessive “O” allele

Co-Dominance (cont.)

• A man with type A blood has children with a woman with type B blood. The man’s mother had type O blood and the woman’s mother also had type O blood. What possible blood types could their children have?

Co-Dominance (cont.)

A B

AB

BO

AO

OO

A

O

B O

OO OO

O O

All 4 blood types are equally possible!

Human Genetics

Check out this web site for everything you could possibly want to know about human genetic traits!

http://www3.ncbi.nlm.nih.gov/Omim/searchomim.html

Pleiotropy

• A genetic condition where one gene effects several traits

• Example:– In wheat plants, the crop “yield” can be

predicted by the length of the awn (a bristle) on the lemma (bracts) of the wheat florets. The single gene effects both traits.

Polygenic Inheritance

• Genetic system where many genes effect a single trait

• Most examples involve CONTINUOUS VARIATION (a bell-shaped curve) rather than the DISCRETE VARIATION seen in most Mendelian problems

• Examples: human skin color and height, color of wheat grains etc...

Continuous Variation

PolygenicInheritance

Polyploidy• Genetic condition with additional complete

SETS of chromosomes– 2N = diploid– 3N = triploid– 4N = tetraploid

• More common in plants than in animals• Many common crops and ornamentals are

polyploids, e.g. modern wheats are 8N• Example: seedless watermelons!

Seedless Watermelons• Seeds are expensive, up to $150.00

per 1,000 seeds. Here is why …..

2N=22 4N=44

Pollen from one group(diploids) is used to pollinate the other (tetraploids).

1N=11 + 2N=22 forms seeds with 3N=33 chromosomes.

Plant these seeds, and the plants are sterile

(NO SEEDS).

Mutations

Can be caused by may mechanisms:– spontaneous errors in meiosis or mitosis

• non-disjunctions• polyploidy

– ionizing radiation– chemicals

Example: Non-Disjunction

Mutations (cont.)

• Chromosomal Mutations– Inversions

• A section of a chromosome is cut and reinserted “backwards”

Mutations (cont.)

– Duplications• A section of a chromosome is “copied” and

occurs more than once on the chromosome– Deletions

• A section of a chromosome is deleted (lost)

Mutations (cont.)

– Translocations• A section of one

chromosome is “swapped” for a piece of another non-homologouschromosome

Mutations (cont.)

• DNA Mutations– Deletions– Duplications– Inversions