LECTURE 3
Finish mutations, introduction to Mendel

Today’s topics:
1.  Inversions
2.  Chromosomal mutations in humans
3.  The frequency of mutations
4.  Gregor Mendel
5.  Segregation of a gene in the F1 generation
6.  Phenotypes in the F2 generation

Readings:176-178, 187-189, 34-43

Inversions
An inversion occurs when the order for a group of genes is reversed.


Inversion loop




If there is no crossing over, the inversion loop will not create any structural problems during chromosome separation.

If there is a crossover, and the inversion is paracentric, the two chromatids involved will have either 0 or 2 centromeres.
Paracentric inversions:



Pericentric inversions:


Chromosomal mutations in humans

Many examples, including:
Down Syndrome

Trisomy X


Origin of the Y chromosome

One of the best examples of multiple chromosomal mutations is the origin of the Y chromosome.



The Y-chromosome originated 300-350 million years ago, shortly after mammals diverged from birds.

inversions:

accumulation of mutations:

Origin of human chromosome 2




Spider silk genes











Frequency of mutations

Mutation rate



High variation between genes.




Frequency of chromosomal mutations in humans




Gregor Mendel





Garden peas Pisum sativum
Advantages:

- Selective breeding


- Categorical traits

- Short generation time

- Large population size

see Figures 2.1, 2.3

Segregation of a single gene

"true breeding" lines




homozygous individuals


heterozygous individuals



The monohybrid cross


AA  x  aa



F1 generation

What will the heterozygous (hybrid) offspring look like?




The "molecular phenotype" for some types of molecular markers is codominant.



Dominance
Complete dominance     vs.     codominance     vs.     incomplete dominance



Segregation in the F1 generation






Reciprocal crosses?



Phenotypes in the F2 generation

Problem:  How do we verify the genotype of the offspring?





Answer:  breed the F1 progeny with each other and record the phenotype.

see Figure 2.5, Table 2.1





For next time:  pp. 44-51, 107-108