1

Gregor Mendel

Biol 202: Lecture 2Genetics

Meiosis

• Cell division which results in halpoid “sex” cells(i.e., egg and sperm)

• One replication of the genetic material (DNA)during interphase, but two nuclear divisions(meiosis I and meiosis II).

• Results in haploid (N) cells (= gametes in animals)from an initial diploid (2N) cell

• Very similar to mitosis except that the cellsproduced are not genetically identical.

M phase

Chromosome Structure(cartoons)

sister chromatids

sister chromatids

centromere: region of thechromosome wherechromatids attach.

2

Synthesis

A- a-

B- b-

-A -a

-b-Brepresents gene A,from one parent. represents the

same gene fromthe other parent,codes for the~same protein,may have aslightly differentDNA sequence.Homologous Chromosomes

AllelesAlternative formsof the same gene.

Alleles occur at the same locations (loci) on homologous chromosomes.

A- a-

B- b-

C- C-

Heterozygous

Homozygous

Meiosis Prophase I

… Synapsis: the highly specific parallelalignment of homologous chomosomesduring the first division of meiosis,

A A a a

B B b b…tetrad: the two homologous

chromosomes become attachedalong their length in a structuretermed a tetrad.

3

Meiosis Prophase Icont.

Crossing Over: rearranges the genes from each parent.

A aA a

B B b b

a A

Chiasmata

Meiosis Metaphase I -- Telophase I

A aA a

B B b b

a A

AA

B B

a aa

b b

A

4

Meiosis Prophase II -- Telophase II

aA

BB

Aa

bb

A A

B B b b

aa

no DNA synthesis

2n combinations of chromosomes n = number of chromosomes

• n = 1, 2n = 2• n = 2, 2n = 4• n = 3, 2n = 8• n = 5, 2n = 32

• n = 23, 2n = 8,388,608 H. sapiens• n = 39, 2n = yikes! dog

Meiosis is critical for sexualreproduction in all diploid

organisms

...meiosis leads to the formation of gametes,

– gametes (one from each parent) conjugate toform a zygote,

...meiosis is the basis for extensive variationamong members of a population.

5

A comparison of mitosis and meiosis: summary

Gregor Mendel

Mendelian Genetics

Mendel Insight 1

• Used the pea,

6

Insight 2• alternate forms,

Insight 3• True breeding lines,

– “Permit me to state that, as an empirical worker, I must define constancyof type as the retention of character during the period of observation”. -Mendel

– Mendel observed his ‘true-breeding’ lines for up to 8 generations.

• Used the pure-breeding line to form hybrid lines,

– offspring of genetically dissimilar parents.

Insight 4• Expert plant breeder,

– carefully controlled the matings,

• prevented the intrusion of any pollen foreign to the desired mating,

– made reciprocal crosses:

• reversing the traits of the male and female parents,

– male wrinked x female smooth,

– female wrinkled x male smooth.

7

Insight 5

• Used large numbers of subjects,

– applied statistical analysis to his data!

• uncovered the patterns of transmission that we willtake for granted.

Insight 6

• Controlled for environmental factors,

– for example, when looking at the short and tallplants, he made sure that all subjects receivedequal light,

• from his studies of plant physiology, he knew thatlight mediates stem elongation.

Insight Summation• Used the pea,• Identified alternate forms,• Identified and used true breeding lines,• Expert plant breeder,• Used statistical analysis,• Controlled for environmental factors.

Set up a simple ‘black and white’ system, andthen figured out how it worked.

8

Monohybrid Cross

• Mating between individuals that differ in only onetrait,– yellow pea x green pea,

– violet flower x white flower

– tall x dwarf

– round seed x wrinkled seed

– full pod x constricted pod

– etc.

Monohybrid Cross

GenerationParental (P)

First Filial (F1)

Second Filial (F2)

yellow pea green pea (pollen) (eggs)

x

all yellow

grow plants, crosspollinate

grow, allow to self-fertilize

6022 yellow : 2001 green

3 : 1

9

Reappearance of Trait in F2Generation Disproves Blending

• Blending did not occur, in fact over 2000 peasretained the information necessary to make greenpeas,

• Mendel concluded that there must be two types ofyellow peas,

– those that breed true like the parent plant,

– those that can yield some green peas, like some of theF1 hybrids.

Dominant vs. Recessive Traits

x P

F1

The trait that appears in the F1 generation is the DOMINANT trait.

The trait that disappears in the F1 generation is termed RECESSIVE.

Mendel’s First Postulate

Unit Factors in Pairs• Genetic characteristics are controlled by unit

factors (Genes) that exist in pairs in individualorganisms,– each individual receives one unit factor from each parent,

– in a monohybrid cross, three combinations of unit factors arepossible,

10

Definitions to Know

• Homozygous: the unit factors thatdetermine a particular trait are the same,– YY = homozygous dominant,– yy = homozygous recessive,

• Heterozygous: the unit factors thatdetermine a particular trait are different,– Yy = heterozygous.

Mendel’s Second Postulate

Dominance/Recessiveness

• When two unlike unit factors are present ina single individual, one unit factor isdominant to the other, which is said to berecessive.

Unlike Unit Factors=

Alternate Forms of the Same Gene

=

Alleles

11

When Unit Factors Separate

Two Unit Factors = Diploid

One Unit Factor = Haploid

During Gamete formation, Unit Factors Separate

More Definitions to Know

• Phenotype: an observable trait,

• Genotype: the actual composition of allelespresent in an individual.

12

Monohybrid CrossP: GG x gg

F1: Gg (GgxGg)

Gametes: G g G g1/2 1/2 1/2 1/2

F2: GG1/2 x 1/2

1/4

Gg1/2 x 1/2

1/4

gG1/2 x 1/2

1/4

gg1/2 x 1/2

1/4

Random Segregation

F2: GG Gg gG gg

1/4 GG 1/4 Gg 1/4 gG 1/4 gg

1/4 GG 1/2 Gg 1/4 gg

Punnett Squares

Y y

Y

Y

YY

YY

Yy

Yy

gametesParent 1

gametesParent 2

PredictedOffspring InSquares

13

Mendel’s Third Postulate

Segregation

• During the processes of heredity, the paired unitfactors separate so that the offspring receives one unitfactor from each parent,

• The unit factors segregate to offspring randomly.

Postulates 1-3 AppliedF1 Generation

P1: Yellow Green Phenotype YY yy Genotype

Gametes: Y y

Yellow PhenotypeF1: Yy Genotype

Postulates 1-3 AppliedF2 Generation

YellowF1: Yy

F1 Self-Cross: Yy Yy

Gametes: Y or y Y or y

F2: Yy Yy yyYY

14

Dihybrid Crosses

• Monohybrid Cross,– one set of contrasting traits,

• Y (yellow) versus y (green).• S (smooth) versus s (wrinkled).

• Dihybrid Cross,

– SSYY x ssyy

Mendel’s Forth Postulate

Independent Assortment

• How do two traits segregate in the offspringof an individual that is heterozygous forboth traits?

15

Watson and Crick

On the last day of February1953, Francis Crick announced to the patrons of the Eagle pub in Cambridge

“We have discovered thesecret of life”

Figure 16.5 The double helix

Genetic Code• Three DNA letters are transcribed (process of

transcription) into three mRNA letters called aCodon

• A specific codon will code for a specific aminoacid = subunit building block of proteins that arecovalently linked together by peptide bonds(amino group to carboxyl group).

• Polypeptide - a molecule made up of aminoacids.

• Protein - a molecule (gene product) made of oneor more polypeptides, 3D structure, specificfunction.

16

Genetic Code• In DNA there are 4 bases: A,C,G,T• In a codon there are 3 bases• Thus, 64 possible codons (43)= 64• However, only 20 different amino acids, plus one

start codon and three stop codons• Genetic code = nucleotide triplets

17

Characteristics of the Code• Written in linear form in mRNA letters (A,C,G,U)• mRNA transcribed from 3’-5’ template strand• Each mRNA “word” contains three letters• Each group of three letters = codon• One codon specifies one amino acid• Code = Triplet• Code is degenerate (more than one codon can specify a

given amino acid)• Code is non-overlapping• Code is universal/nearly universal• Amino acid is a “sub-unit” building block of protein

Autosomal Recessive Traits• AA=normal, Aa=normal, aa=affected• 90% of all autosomal genetic defects• Trait not carried on sex chromosomes, but on other 22

pairs (autosomes)• Many arise from relatives mating - increased chance

for recessive alleles to pair• Two affected parents will always produce affected

offspring• Parents can be carriers (Aa) and have a 25% chance of

producing affected offspring• Trait often skips a generation• Males and females affected equally

Autosomal Dominant Traits• AA=affected, Aa=affected, aa=normal• ~10% of autosomal genetic defects (chin dimple)• Not carried on sex chromosomes• Two normal parents can only have normal offspring• Two affected parents who are heterozygotes (Aa) have

a 25% chance of producing normal offspring• An affected offspring must have at least one affected

parent• Traits will not skip a generation• Trait should appear in almost equal numbers• When an affected person mates with a normal person,

50% of the offspring are expected to be affected

18

X-Linked Recessive Traits• XB=normal, Xb=affected• Affects more males (50% chance of inheriting

recessive allele from mother = hemizygous)• Traits can skip a generation• Affected mother produces affected sons• Normal mother produces normal daughter,

unless mother is a carrier and father is affectedthen daughter could be affected

• Two affected parents will have affected children

X-Linked Dominant Traits• XB=affected, Xb=normal• Affects more females.• Traits does not skip a generation• Affected males must come from affected

mothers• Two normal parents will have normal children• All the daughters, but none of the sons, of an

affected father are affected• Approximately 50% of the children of an

affected heterozygous female are affected

Y-linked Traits• Only males affected• If one male in pedigree is affected then all

related males must be affected• If one male is normal then all related males are

normal