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Topic 10: Genetics HL

This version was saved 13 years, 1 month ago View current version     Page history
Saved by Darrell Sharp
on February 25, 2011 at 6:26:50 pm
 

 

    

 

Human Sex Chromosomes, Biology, McDougal Littell, 2008.

  

 

Dihybrid Punnett Square, http://cccmkc.edu.hk/index.en.php

 

10.1 Meiosis

 

 

 

10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis. 

10.1.2 Outline the formation of chiasmata in the process of crossing over. 

10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.

 

Meiosis Simulation

 

 

 

 

                        Meiosis II 

http://www.biologycorner.com/APbiology/inheritance/10-1_meiosis.html

 

 

Chiasmata

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Sadava, D. et al. Life: The Science of Biology, 8th ed

 

 

 

DNA is replicated before meiosis starts

 

Meiosis I - key points: crossing over and independent assortment

 

  Prophase I

  • Chromosomes condense - "supercoiling"
  • Homologous chomosomes make pairs
  • Chiasmata form - positions where non-sister chromatids cross over (singular: chiasma)
  • Nuclear membrane breaks down 
  • Spindle fibers attach to centromeres 

  Metaphase I

  • Homologous pairs move to the middle of the cell
  • Each pair lines up randomly - Mendel called this "independent assortment"
  • Every time meiosis happens the pairs line up differently

  Anaphase I

  • Spindle fibers separate homologous pairs

  Telophase I and Cytokinesis

  • Spindle fibers break down
  • Cytoplasm and plasma membrane are divided into two cells
  • In some species, new nuclear membranes form 

 

  Result: two haploid cells 

 

 

Meiosis II - key point: sister chomatids are separated

 

 Prohpase II

  • Spindle fibers attach to centromeres 

 Metaphase II

  • Chromosomes line up individually in the middle of the cell 

  Anaphase II

  • Spindle fibers separate suster chromatids

 Telophase II and Cytokinesis

  • Spindle fibers break down
  • New nuclear membranes form
  • Cytoplasm and plasma membrane are divided into two cells

 

  Result: four unique haploid cells

 

 

 

Important Ideas

  

Crossing over and independent assortment create unique gametes.

 

  • Because of crossing over, the four chromatids of homologous chomosome pairs are different
  • Because of independent assortment, the result of meiosis is different every time it happens 
  • = genetic variation in gametes

 

Random fertilization happens during sexual reproduction - one random sperm and one random egg meet and create one unique offspring.

 

Sexual reproduction's most important characteristic is its ability to create genetic variation in offspring and increase the evolutionary success of the species.

 

 

Thanks meiosis and random fertilization!

 

 

 

10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.

 

 

 

10.1.4 State Mendel’s law of independent assortment.

 

 

 

10.1.5 Explain the relationship between Mendel’s law of independent assortment and meiosis.

 

 

 

 

 

 

 

 

10.2 Dihybrid crosses and gene linkage

 

 

Dihybrid crosses follow the inheritance pattern of two traits.

   (Di- = two)

In the following cross, two traits of each parent are observed:

        1) seed color:  Y=yellow / y=green

        2) seed shape:  R=round / r=wrinkled

 

P1: Phenotypes = Yellow/Round  X  Green/Wrinkled

      Genotypes =              YYRR  X  yyrr 

 

F1: Phenotypes = 100% Yellow/Round

     Genotypes  =  100%       YyRr

 

                          Yellow/Round X Yellow/Round

                                        YyRr X YyRr

 

F2: Phenotypes = 9 Yellow/Round

                          3 Yellow/Wrinkled

                          3 Green/Round

                          1 Green/Wrinkled

      Genotypes =

 

 

9:3:3:1 ratio

 

 

 

http://cccmkc.edu.hk/index.en.php

 

10.2.1 Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.

 

 

 

10.2.2 Distinguish between autosomes and sex chromosomes.

 

 

 

10.2.3 Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.

 

 

10.2.4 Define linkage group.

 

 

 

10.2.5 Explain an example of a cross between two linked genes.

 

Alleles are usually shown side by side in dihybrid crosses, for example, TtBb. In representing crosses involving linkage, it is more common to show them as vertical pairs, for example

Insert alt text

Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

Alleles are usually shown side by side in dihybrid crosses, for example, TtBb. In representing crosses involving linkage, it is more common to show them as vertical pairs, for example

Insert alt text

 

 

 

10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

 

In a test cross of

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the recombinants will be

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and

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In a test cross of

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the recombinants will be

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and

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10.3 Polygenic inheritance

 

 

 

10.3.1 Define polygenic inheritance

 

 

 

10.3.2 Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour. 

 

 

 

 



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