e. the power of independent assortment 1. if you can assume that the genes assort independently,...

E. The Power of Independent Assortment

1. If you can assume that the genes assort independently, then you can calculate ‘single gene’ outcomes and multiply results together…

2. You can easily address more difficult multigene problems:

(female) AaBbCcdd x AABbccDD (male)

- how many types of gametes can each parent produce?

- What is the probability of an offspring expressing ABCD?

- How many genotypes are possible in the offspring? 2 x 3 x 2 x 1= 12

- how many phenotypes are possible in the offspring? 1 x 2 x 2 x 1 = 4

At A: At B: At C: At D:A A

A AA AA

a Aa Aa

B b

B BB Bb

b Bb bb

c

C Cc

c cc

D

d Dd

E. The Power of Independent Assortment

1. If you can assume that the genes assort independently, then you can calculate ‘single gene’ outcomes and multiply results together…

2. You can easily address more difficult multigene problems.

As you can see, IA produces lots of variation, because of the multiplicative effect of combining genes from different loci together in gametes, and then combining them together during fertilization… we’ll look at this again; especially with respect to Darwin’s 3rd dilemma.

F. WHY do these patterns occur?Meiosis: Gamete Formation

a. Overview:

In sexually reproducing species, gametes carry exactly ½ the genetic information as the parent; so that the fusion of gametes reconstitutes the correct genetic complement.

2n = 41n = 2

Fertilization (fusion)


a. Overview:


The divisional process that produces these special cells is called meiosis.

Thus, meiosis ONLY occurs in reproductive tissue (ovary, testis), and only produces gametes.

All other cells in multicellular organisms are produced by mitosis.

2n = 41n = 2


MEIOSIS

Diploid Haploid


a. Overview:


The divisional process that produces these special cells is called meiosis.

Thus, meiosis ONLY occurs in reproductive tissue (ovary, testis), and only produces gametes.

All other cells in multicellular organisms are produced by mitosis.

MEIOSIS has two divisional cycles, “reduction” and “division”

2n = 41n = 2


MEIOSIS

Diploid Haploid


a. Overview: b. Meiosis I: “The Reduction Cycle”

- Prophase I:condensation in pairspossible crossing over




- Metaphase I:Homologs align In PAIRS





- Anaphase I:Replicated chromosomes move

to opposite poles






to opposite poles

- Telophase – Prophase II transition






to opposite poles

- Telophase – Prophase II transition

PLOIDY REDUCED 2n parent cell 1n daughter cells


a. Overview: b. Meiosis I: “The Reduction Cycle” c. Meiosis II: “The Division Cycle”

Like mitosis, but a haploid cell… chromsomes line up in single file in Metaphase II, and sister chromatids (of replicated chromosomes) separate and move to opposite poles.


a. Overview: b. Meiosis I: “The Reduction Cycle” c. Meiosis II: “The Division Cycle” d. Variations in the Process:

In spermatogenesis: Karyokinesis is equal and cytokinesis is equal, resulting in 4 equal-sized sperm.

F. WHY do these pattern occur?Meiosis: Gamete Formation

a. Overview: b. Meiosis I: “The Reduction Cycle” c. Meiosis II: “The Division Cycle” d. Variations in the Process:

In oogenesis, karyokinesis is equal (dividing the genetic information exactly in half), but cytokinesis is unequal, with one daughter cell getting the majority of the cytoplasm and organelles. The smaller may/may not be able to divide. These are ‘polar bodies’

III. Uniting Genetics and Cell Biology

A. The Chromosomal Theory – Sutton and Boveri

Theodor Boveri

Walter Sutton

http://en.wikipedia.org/wiki/File:Theodor_Boveri.jpg



Saw homologous chromosomes separating (segregating). If they carried genes, this would explain Mendel’s first law.

Theodor Boveri

Walter Sutton

A a




And if the way one pair of homologs separated had no effect on how others separated, then the mvmnt of chromosomes would explain Mendel’s second law, also!

Theodor Boveri

Walter Sutton

A aA a

b BB b

AB ab Ab aB




This was a major achievement in science. The patterns of heredity had been associated with physical entities in biological cells. The movement of chromosomes correlated with Mendel’s patterns. Scientist now went about testing if this relationship was causal – and they found it was.

Theodor Boveri

Walter Sutton




B. Solving Darwin’s Dilemma – The Source of Variation




Independent Assortment produces an amazing amount of genetic variation.

Consider an organism, 2n = 4, with two pairs of homologs. They can make 4 different gametes (long Blue, Short Red) (Long Blue, Short Blue), (Long Red, Short Red), (Long Red, Short blue). Gametes carry thousands of genes, so homologous chromosomes will not be identical over their entire length, even though they may be homozygous at particular loci.

Well, the number of gametes can be calculated as 2n

or





Consider an organism with 2n = 6 (AaBbCc) ….There are 2n = 8 different gamete types.

ABC abcAbc abCaBC AbcAbC aBc






And humans, with 2n = 46? ABC abcAbc abCaBC AbcAbC aBc






And humans, with 2n = 46?

223 = ~ 8 million different types of gametes.

And each can fertilize ONE of the ~ 8 million types of gametes of the mate… for a total 246 = ~70 trillion different chromosomal combinations possible in the offspring.

ABC abcAbc abCaBC AbcAbC aBc




Independent Assortment produces an amazing amount of genetic variation.And each can fertilize ONE of the ~ 8 million types of gametes of the mate… for a total 246 = 70 trillion different chromosomal combinations possible in the offspring.

YOU are 1 of the 70 trillion combinations your own parents could have made. IA creates a huge amount of genetic variation, and that doesn’t include crossing over!!!!




C. Modification to Evolutionary Theory

Darwin’s Model

Sources of Variation Causes of Change

???????????????? VARIATION NATURAL SELECTION

(use and disuse??)


A. Early Studies

B. Divisional Processes

C. The Chromosomal Theory – Sutton and Boveri

D. Solving Darwin’s Dilemma – The Source of Variation

E. Modification to Evolutionary Theory

Darwin’s Model

Sources of Variation Causes of Change

Independent Assortment VARIATION NATURAL SELECTION

IV. Modifications to Mendelian Patterns


- Overview:

Mendels conclusions regarding dominance, equal segregation, independent assortment, and independent effects hold for many genes and traits. But they are not true of ALL traits.


- Overview:


Here, we will consider how the effect of a gene is influenced at three levels:


- Overview:



- Intralocular (effects of other alleles at this locus)


- Overview:




- Interlocular (effects of other genes at other loci)


- Overview:





- Environmental (the effect of the environment on determining the effect of a gene on the phenotype)


- Overview:





- Environmental (the effect of the environment on determining the effect of a gene on the phenotype)

And finally, we will examine the VALUE of an allele – are there “good genes” and “bad genes”?


A. Intralocular Interactions A a


A. Intralocular Interactions

1. Complete Dominance: - The presence of one allele is enough

to cause the complete expression of a given phenotype.



1. Complete Dominance:2. Incomplete Dominance: - The heterozygote expresses a

phenotype between or intermediate to the phenotypes of the homozygotes.



1. Complete Dominance:2. Incomplete Dominance:3. Codominance: - Both alleles are expressed completely;

the heterozygote does not have an intermediate phenotype, it has BOTH phenotypes.

Genotypic and phenotypic ratios of F1 x F1 crosses are both 1:2:1

ABO Blood Type:

A = ‘A’ surface antigens





Genotypic and phenotypic ratios of F1 x F1 crosses are both 1:2:1

ABO Blood Type:


B = ‘B’ surface antigens





ABO Blood Type:



O = no surface antigen from this locus





ABO Blood Type:



O = no surface antigen from this locus

Phenotype Genotypes

A AA, AO

B BB, BO

O O

AB ABcodominance

AB Phenotype



1. Complete Dominance:2. Incomplete Dominance:3. Codominance:4. Multiple Alleles: - While not really specifying an

‘interaction’, it does raise a complication of looking at a single trait.



1. Complete Dominance:2. Incomplete Dominance:3. Codominance:4. Multiple Alleles: - While not really specifying an

‘interaction’, it does raise a complication of looking at a single trait.

- You might presume that a ‘single-gene’ trait could only have a maximum of three phenotypes (AA, Aa, aa). But with many alleles possible for a gene (as for A,B,O), there are many diploid combinations and effects that are possible (as in the 4 phenotypes for the A,B,O system).



1. Complete Dominance:2. Incomplete Dominance:3. Codominance:4. Overdominance – the

heterozygote expresses a phenotype MORE extreme than either homozygote – see class notes.



- Summary and Implications: populations can harbor

extraordinary genetic variation at each locus, and these alleles can interact in myriad ways to produce complex and variable phenotypes.





-Consider this cross: AaBbCcDd x AABbCcDD

Assume:The genes assort independentlyA and a are codominantB is incompletely dominant to bC is incompletely dominant to cD is completely dominant to d

How many phenotypes are possible in the offspring?





-Consider this cross: AaBbCcDd x AABbCcDD

Assume:The genes assort independentlyA and a are codominantB is incompletely dominant to bC is incompletely dominant to cD is completely dominant to d

How many phenotypes are possible in the offspring?

A B C D

2x 3 x 3 x 1 = 18

If they had all exhibited complete dominance, there would have been only:

1x 2 x 2 x 1 = 4

So the variety of allelic interactions that are possible increases phenotypic variation multiplicatively. In a population with many alleles at each locus, there is an nearly limitless amount of phenotypic variability.



B. Interlocular Interactions:

The genotype at one locus can influence how the genes at other loci are expressed.





1. Epistasis: one gene curtails the expression at another locus; the phenotype in the A,B,O blood group system can be affected by the genotype at the fucosyl transferase locus.





1. Epistasis: one gene curtails the expression at another locus; the phenotype in the A,B,O blood group system can be affected by the genotype at the fucosyl transferase locus. This locus makes the ‘H substance’ to which the sugar groups are added to make the A and B surface antigens.





1. Epistasis: one gene curtails the expression at another locus; the phenotype in the A,B,O blood group system can be affected by the genotype at the fucosyl transferase locus. This locus makes the ‘H substance’ to which the sugar groups are added to make the A and B surface antigens.A non-function ‘h’ gene makes a non-functional foundation and sugar groups can’t be added – resulting in O blood regardless of the genotype at the A,B,O locus

Genotype at H

Genotype at A,B,O

Phenotype

H- A- A

H- B- B

H- OO O

H- AB AB

hh A- O

hh B- O

hh OO O

hh AB O





1. Epistasis: one gene curtails the expression at another locus; the phenotype in the A,B,O blood group system can be affected by the genotype at the fucosyl transferase locus. This locus makes the ‘H substance’ to which the sugar groups are added to make the A and B surface antigens.A non-function ‘h’ gene makes a non-functional foundation and sugar groups can’t be added – resulting in O blood regardless of the genotype at the A,B,O locus

So, what are the phenotypic ratios from this cross:

HhAO x HhBO?





1. Epistasis:

-example #2: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’.

Process:enzyme 1 enzyme 2

Precursor1 precursor2 product (pigment)





1. Epistasis:

-example #2: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’.For example, two strains of white flowers may be white for different reasons; each lacking a different necessary enzyme to make color.

Process:enzyme 1 enzyme 2

Precursor1 precursor2 product (pigment)

Strain 1:enzyme 1 enzyme 2

Precursor1 precursor2 no product (white)

Strain 2:enzyme 1 enzyme 2

Precursor1 precursor2 no product (white)





1. Epistasis:

-example #2: in a enzymatic process, all enzymes may be needed to produce a given phenotype. Absence of either may produce the same alternative ‘null’.For example, two strains of white flowers may be white for different reasons; each lacking a different necessary enzyme to make color.So there must be a dominant gene at both loci to produce color.

Genotype Phenotype

aaB- white

aabb white

A-bb white

A-B- pigment

So, what’s the phenotypic ratio from a cross:

AaBb x AaBb ?





1. Epistasis:

-example #3: Novel Phenotypes.Comb shape in chickens is governed by 2 interacting genes that independently produce “Rose” or “Pea” combs, but together produce something completely different (walnut).

Genotype Phenotype

rrpp single

R-pp rose

rrP- pea

A-B- Walnut





1. Epistasis:2. Polygenic Traits:

There may be several genes that essentially produce the same protein product; and the phenotype is the ADDITIVE sum of these multiple genes.

Creates continuously variable traits.

e. the power of independent assortment 1. if you can assume that the genes assort independently,...

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