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Prior history

To appreciate Mendels work, one must keep in mind the prevailing theories of inheritance

Preformationism: the idea that gamete contains an intact organism, was first proposed in the late 1600s.

Blending inheritance: essences of both sperm and egg mixed to form offspring intermediate between the parents.

Mendel ignored development, focused solely on transmission of traits

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Darwin: Among individuals of any species, there are differences (variations)

Evolution cannot occur unless there are differences among individuals

Variation is important

Variation must be inherited

Mechanism of inheritance of variations is important for understanding evolution

Mechanism of inheritance not understood

Darwin and Heredity

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Pangenesis: (1860s)Whole organism reproduces itself

Gemmules determine characteristics (traits) of organism

Germ cells contain gemmules of all sort and these are transmitted to next generation.

Fertilization- gemmules unite and produce new cells of the types from which they are produced

Pangenesis

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Cytology:Organisms are composed of cells

The most distinct structure in a cell is the nucleus. ALL CELLS have a nucleus

Cells are formed by the division of pre-existing cells (Mitosis). Mitosis described nuclear and chromosome dynamics

Sperm and Ovum are cells. This links cells to heredity!

Fertilization in sea urchin showed that sperm and ovum fuse. This links parents to offspring

Ploidy?

Cytology

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Meiosis

Meiosis described Chromosomal changes-Reductional division of chromosomes keeps number of chromosomes constant.

Nuclei of embryo and parents are diploids, but nuclei of germ cells are haploid

Similar phenomenon not seen in any other cellular organelle.

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Cytology’s contribution prior to Mendel

Heredity is a consequence of genetic continuity of cells by division

Germ cells are the vehicle of transmission from one generation to the next

Nucleus is crucial. During division it resolves into long chromosomes that split lengthwise

Fertilization involves union of sperm and eggsFertilization involves union of nuclei

Chromosomes do not lose their individuality. They are inherited intact.

Germ cells contain half the number of chromosomes found in body cells.

Diploid embryo descends from maternal/paternal fusion of haploid chromosome groups

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The origin of genetics:

The study of genetics begins when Gregor Mendel, in 1865, addressed the question :

"How are characters passed on from one generation to the next?”

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Model organisms

The genomes of many organisms have been sequenced. www.genomenewsnetwork.org

A model organism is a species that has been widely studied, usually because it is easy to maintain and breed in a lab and/or has particular experimental advantages.

Remember: processes are conserved!!!!!

Model organisms are used to obtain information about other species – including humans – that are more difficult to study directly.

Genetic model organisms

Experimental model organisms

Genomic model organisms

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Model organisms

Human model organisms

Modern genetics:

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The Pea

Mendel chose the common garden pea to study patterns of inheritance. This was a excellent choice as a model system for the following reasons:

1

2

3

4

He identified over 20 traits and studied 7

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True breeding

He identified over 20 traits that bred true and studied 7

The first two years of Mendel's work were devoted to selecting lines that breed true (pure lines) for a particular character or trait.

Breeding True:

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Mendel’s first cross

P

F1

F2

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Mendel’s first cross

P

F1

F2

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Keys to success

Mendel crossed pure breeding yellow pea plants to pure breeding green pea plants. All of the progeny were yellow pea plants.

Next he selfed these yellow plants allowing the pollen to fall on its own stigma.

He obtained 6022 yellow pea plants and 2001 green pea plants.

He did these experiments with all seven traits!!!!

The ratios obtained were between 2.82:1 and 3.15:1

A cross involving only one character, seed color, is called a

monohybrid cross.

Keys to success:

1

2

3

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Conclusions

Although others were doing similar experiments at the time, Mendel work was unique

Results and Conclusions:

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2

3

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Terms:

Phenotype and genotype: The parental yellow pea plants are a pure line- they only produce yellow pea plants when selfed.

However the F1 yellow pea plants produce some green pea plants when selfed. Therefore it is necessary to make the distinction between the appearance of an organism and its genetic make-up.

Phenotype-

Genotype-

Would say the parental yellow pea plants and F1 yellow pea plants have the same phenotype but a different genotype.

P

F1

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Terms:

Dominant and recessive: All F1 seeds were yellow but when selfed the F2 produced some green seeds.

Mendel termed the trait that is expressed in the F1 as dominant

The trait that is hidden but re-expressed in the F2 as recessive.

The F1 plants must contain factors for green and yellow since both are found in the F2.

From the fact the reciprocal crosses produce the same result, Mendel concluded that male and females contribute equally

With these assumptions the simplest model is that the F1 contains two hereditary factors

One for green and another for yellow

Mendel used the Uppercase Y to represent the dominant yellow factor and the lower case y to represent the recessive green.

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The Principle of Segregation:

Notice that while the parents have two factors, they produce gametes containing only a single factor

Mendel reasoned that without a mechanism to halve the number of factors in each generation, that factors would multiple with each generation and become unmanageable.

Mendel reasoned that during gamete formation the paired factors separate and each gamete receives one of the two factors.

Parent

Gamete

F1

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Mendel's assumption of two factors and segregation makes a strong prediction concerning the genetic make-up of the of F2 yellow pea plants:

F1 Yy X Yy

Y y Y y

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Selfing

Mendel selfed each of the F2 plants

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Test cross

If instead of selfing the F2 plants, they are crossed to pure breeding green plants, what are the expected outcomes:

F2

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More terms:

Mendel's factors are now known as genes

Alternative forms of a gene that determine different traits are known as __________

Individuals with two identical alleles are said to be __________

Individuals with two different forms of alleles are said to be _____________

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The dihybrid cross and the principle of independent assortment:

In the second set of experiments Mendel investigated the pattern of inheritance for two sets of characters simultaneously. A cross involving two sets of characters is called a dihybrid cross.

Pea shape: smooth, wrinkled (Smooth is dominant to wrinkled)

Cotyledons color: yellow, green (Yellow is dominant to green)

P x

F1

selfed

F2

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The 9:3:3:1 ratio.

The 9:3:3:1 ratio is a lot more complex than the 3:1 ratios of the monohybrid crosses.

Mendel's insight was to realize the 9:3:3:1 ratio is nothing more than two 3:1 ratios combined at random. That is if one examinedthe traits individually they formed a 3:1 ratio.

To determine the mode of inheritance of the two genes in this dihybrid cross Mendel examined each of the traits separately:

If we examine seed shape (smooth, wrinkled) and ignore cotyledon color (yellow, green), in the F2, we expect to find:

3/4 smooth and 1/4 wrinkled:

# Smooth =  

#wrinkled =

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In addition, if we only examine cotyledon color, we expect 3/4 Yellow to 1/4 green.

#Yellow =

#green =

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Monohybrid---dihybrid

3/4 Yellow 3/4x3/4= 9/16

3/4 Smooth

1/4 Green 3/4x1/4= 3/16

3/4 Yellow 1/4x3/4= 3/16

1/4 Wrinkled

1/4 Green 1/4x1/4= 1/16

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Branched Diagram

If you combine the monohybrid ratios for two traits you get:

3/4 yellow 3/4x3/4 = 9/16

3/4 smooth

1/4 green 3/4 x 1/4 = 3/16

3/4 yellow 1/4x3/4 = 3/16

1/4 wrinkled

1/4 green 1/4x1/4 = 1/16

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Each trait behaves as a standard recessive found in amonohybrid cross.

They do not affect one another

Genes segregate independently!!!!

GeneA in a gamete does not affect the segregation of geneB in that gamete.

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Genes line up in two ways during gamete formation

SYsy

or SysY

Gamete SY sy Sy sY

In a heterozygous individual

SsYy x SsYy

SsYy SsYy

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Independent assortment

If independent assortment is occurring, four different kinds of gametes will be produced in equal frequencies.

The only rule is that S and s segregate to separate gametes and Y and y segregate to separate gametes; (that is one does not get an Ss gamete or a Yy gamete)

SsYy males and SsYy females can produce four types of gametes in equal frequencies:

SY, Sy, sY, sy

The male and female gametes randomly combine to restore diploidy

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Punnet diagram of a dihybrid cross

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What is the biological significance of the 9:3:3:1 ratio? This ratio is only produced if the different genes pairs assort independently of each other during gamete formation.

That is the presence of one gene in a gamete does not influence the probability of another gene being found in that gamete

Principle of segregation: for one gene, each individual has two copies. These two copies segregate from one another during gamete formation.

Independent assortment: Segregation of one gene pair is independent of the segregation of any other pair of gene.

Hence the 9:3:3:1 ratio and the biological significance of Mendel's second law-

Different gene pairs assort independently during gamete formation.

The presence of a Y in a gamete does not influence the probability of a S or s being in that gamete.

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Mendels laws

1. The principle of segregation: Each individual carries two

copies of a given gene and these segregate from one another

during gamete formation.

2. The principle of independent assortment: The segregation of

one pair of genes is independent of the segregation of any

other pair of genes during gamete formation

(as we will find their are important exceptions to this rule)!!!!!

 

By applying these rules Mendel concluded that SsYy individuals

produced the following gametes in a 1:1:1:1 ratio

SY Sy sY sy

As described above, he inferred these gamete ratios by selfing

SsYy individuals.

He could also have inferred these gamete ratios by crossing

SsYy individuals to ssyy individuals.

Crossing to the homozygous recessive individuals is known as a

test cross

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Test cross

A test cross is easier that a self cross for the F2

SsYy x ssyy

What are the expected genotypic and phenotypic ratios of the progeny produced from this cross?

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Gene number

Power of Mendel phenotype ratios:

The ratio tells you the number of genes involved in determining a phenotype

3;1 ratio (selfing) = 1 gene9:3:3:1 ratio (selfing) = 2 genes27:9:9:9:3:3:3:1 ratio (selfing) = 3 genes

Say seed color is controlled by two genes- GeneS and GeneT

Green seeds are ssttAll others are yellow ( if either S or T are present, the seed is yellow)

True breeding yellow x true breeding greenSSTT sstt

F1 SsTt(yellow)

SelfSsTt x SsTt

How many green will you get out of this cross:

These conclusions about gene number become very important when applied to human traits like size, behavior, temperament etc.

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Dog Genome

Dogs have been breed for specific traits for 10,000 yearsAbout 150 breeds have been generated through selective breeding

Diversity in Physical makeup:

Coat colorheightmassmuscle

Behavior:herdingtrackingretrievalGuarding

Intelligence:

The individual breeds can mate with one another and produce viable fertile offspringThey can also mate with Cayotes and wolves

Border collies do not like water, Newfoundlands love water.

In the cross swimming is dominant!

This complex trait is likely mediated by a small number of genes (~2)

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Hybrid vigor

The first cross between two purebred lines is often healthier than either parent

Breed1 Breed2a-B-C-d-E A-b-c-d-Ea-B-C-d-E A-b-c-d-E

F1 a-B-C-d-EA-b-c-d-E

If GeneA causes narcolepsy and geneC causes haemophiliaF1 will be normal.

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Corn

Commercial corn is a F1 hybrid because of hybrid vigor

Two inbred lines are mated to generate a F1 that is sold

AAbb x aaBB

F1 AaBbhybrid vigor