Gregor Mendel (1822-1884)

Mendel developed a strategy of research before beginning his experiments. His goals were:

• to determine the number of different forms of hybrids produced • to arrange the forms according to generations (F1, F2 etc.)• to attempt to evaluate the statistical relations (i.e. the proportions) of the various forms

Seven characteristics of Peas studied by Mendel

• Round or wrinkled ripe seeds• yellow or green seed interiors• green or yellow unripe pods• purple or white flowers• inflated or pinched pods• axial or terminal flowers• long or short stems

Mendel also found that parental, F1, and F2 peas

behaved differently when crossed: P1 (yellow)

X -> F1 (all yellow)

P2 (green)

 F1 (yellow)

X -> ½ yellowP2 (green) ½ green

 F2 (yellow)

X -> 2/3 yellowP2 (green) 1/3 green

When he examined each F2 (yellow) family

separately, he found that:• 2/3 of the F2 yellows gave ½ yellow and ½

green offspring

•1/3 of the F2 yellows gave all yellow offsping

x

x

=

=

Mendel’s explanation P1 (yellow)

XP2 (green)

YY

yy

Yy Yy

Yy Yy

YY

yyy

y

Y Y

All offspring (F1 generation) = Yy Yy

When he crossed the F1 generation: (F1 x F1)

Yy Yyx

Y

y

Y y

Yy

Yy

=

¼

½

¼

Yy

YY

yy

F2 generation

YY Yy

Yy yy

When he examined each F2 (yellow) family separately,

he found that:• 2/3 of the F2 yellows gave ½ yellow and ½ green

offspring

•1/3 of the F2 yellows gave all yellow offsping

x

x

=

=

F2 yellow individuals are either YY or Yy(two different punnet squares with the yellow genotypes)We can use a conditional probability.Pr(Yellow F2 is Yy) = Pr(Yy in F2)/Pr(F2 is yellow) = (1/2)/(3/4) = 2/3

Pr(Yellow F2 is YY) = Pr(YY in F2)/Pr(F2 is yellow) = (1/4)/(3/4) = 1/3

Mendel’s Law of Independent Segregation: the genetic basis of any trait is determined by one particle from each parent (one from the mother and one from the father). These particles that Mendel refers to are now known as genes on chromosomes.

Mendel’s Law of Independent Assortment: particles (chromosomes) are distributed randomly into gametes during meiosis. (i.e. each of these particles is equally likely to be transmitted when gametes are formed)

If a trait is dominant, an individual has to carry only one allele for that trait for it to be expressed in the phenotype.

For a recessive trait to be expressed, both alleles must be recessive.

Incomplete dominance: Expression of a phenotype that is intermediate between those of the parents

Codominance: full phenotypic expression of both alleles in the heterozygous condition

Walter Sutton and Theodore Boveri (1903) separately proposed that chromosomes were the cellular components that physically contained the genes.

locus: the position on a chromosome where a given gene (or other structure) occurs.

A gene codes for a protein or a portion of a protein

alleles: alternate forms of a gene

The Cell cycle:Interphase: the portion of the cell cycle during which metabolic processes and other cellular activities occur.  Mitosis is the second major part of the cell cycle.

Mitosis: A form of cell division that produces two identical daughter cells, each with the same complement of chromosomes as the parent cell.

At the same time as mitosis, the process of cytokinesis occurs.

Cytokinesis is the division of the cytoplasm.

Mitosis takes place in four stages:

Prophase: the chromosomes condense (they are more threadlike during interphase so that protein transcription can occur) and become visible as chromatids. Spindle fibers begin to form.

Metaphase: the nuclear membrane dissolves. The chromosomes are pulled by the spindle fibers (which attach at the centromere) and align at the equator of the cell.

Mitosis (continued)

Anaphase: the centromeres divide, and each chromatid is pulled toward opposite poles

Mitosis (continued)

Telophase: the chromosomes arrive at opposite poles and begin to uncoil so that they are no longer visible. The nuclear membrane begins to form and the cytoplasm divides during cytokinesis.

Mitosis (continued)

Meiosis: the process of cell division during which one cycle of chromosome replication is followed by two successive cell divisions to produce four haploid cells.

Meiosis occurs in two division events:

Meiosis I: like mitosis, the chromosomes have replicated during interphase.

Prophase I : the chromosomes coil and condense and then the members of a chromosome pair physically associate in a process known as synapsis (i.e. the sister chromatids are joined by a single centromere). Each chromosome pair associates with its homologous chromosome pair forming a tetrad. At this point crossing-over occurs

Chromatid(a chromosome)

Chromosome pair(sister chromatids)

tetrad

Figure 02.13

Crossing-over: a process of recombination where nonsister (but homologous) chromatids physically overlap (or cross-over) and then exchange material.

Metaphase I: the nuclear membrane disappears, the tetrads align at the center of the cell

Meiosis (continued)

Anaphase I: the tetrad is pulled apart with one pair of sister chromatids (still attached by one common centromere) being pulled to one pole and the other pair being pulled to the other. This process is called disjunction.

Meiosis (continued)

Telophase I: the nuclear membrane may reform or the cell may go directly into meiosis II. (after cytokinesis)

Meiosis (continued)

Prophase II: Chromosome duplication does not occur again but the chromosomes do condense again and spindle begins to formMetaphase II: The chromosomes line up (sister chromatids are still attached together at a single centromere) in the middle of the cell.Anaphase II: The centromere of each sister chromatid pair divides, the sister chromatids separate and start to move toward opposite poles (pulled by spindle fibers)Telophase II: The chromosomes uncoil, the nuclear membrane forms and the cytoplasm begins to divide (cytokinesis). RESULT = four HAPLOID cells

Meiosis II

At the start of Meiosis II

At the end of Meiosis

Chromosomal Theory of Inheritance: The theory that genes are carried on chromosomes and that the behavior of chromosomes during meiosis is the physical explanation for Mendel’s observations on the segregation and independent assortment of genes.

The structure of DNA of identified in 1953 by Francis Crick and James Watson.

Transcription: the gene is copied into messenger RNA

What is RNA?• RNA = ribonucleic acid• RNA is single stranded• RNA has a different type of sugar• instead of the base T (thymine), RNA uses U (uracil) which is complementary to A.

mRNA = messenger RNA

transcription translationDNA RNA Protein

The central dogma of biology

Point mutation:

Hb

DNA CAC GTG GAC TGA GGA CTC CTC TTC

RNA GUG CAC CUG ACU CCU GAG

Amino Acids

val his leu thr pro glu

Hb S

DNA CAC GTG GAC TGA GGA CAC CTC TTC

RNA GUG CAC CUG ACU CCU GUG

Amino Acids

val his leu thr pro val

Human Genome Project (HGP)

• aids in basic understanding of how the body functions (this has medical applications)• helps the understanding of human evolution because we can compare the information with that from other species to see how we differ and we can look at the diversity (or extent of variation) within humans.

The Four Forces of Evolution:

1. Mutation2. Gene Flow (or migration)3. Genetic Drift4. Natural Selection

From a modern genetic perspective, evolution is a a change in the frequency of alleles from generation to the next.

population: a community of individuals where mates are usually found

Microevolution: small changes in allele frequencies

Macroevolution: change over many generations

The Four Forces of Evolution:

1. Mutation: a change in the DNA sequence2. Gene Flow (or migration): the exchange

of genes between populations3. Genetic Drift: changes in allele frequency

that occur at random4. Natural Selection: the process that

produces adaptation (provides directional allele frequency change relative to specific environmental factors)

Genotype: the combination of alleles that characterizes an individual at some set of genetic loci. For example: someone may have the genotype AO at the ABO locus.

Phenotype: the observable characteristics of organisms. For example, a person with the genotype AO, would have the phenotype of type A blood.

Random mating: mating takes place at random with respect to the gene or trait in question

Hardy-Weinberg Model Assumptions

• random mating• nonoverlapping generations• large population size (i.e. genetic drift isn’t an issue)• no migration• no mutation • no natural selection