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Topic 10: Genetics HL

Page history last edited by Darrell Sharp 10 years, 12 months ago




Human Sex Chromosomes, Biology, McDougal Littell, 2008.



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


10.1 Meiosis



   Modeling Meiosis Activity 


Cartoon Guide to Genetics 1, 2, 3

Gonick, Larry, and Mark Wheelis. The Cartoon Guide to Genetics. New York, NY: Collins Reference, 2007. Print.



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 




















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!




10.1.4 State Mendel’s law of independent assortment.


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




Mendel observed the inheritance of many traits in his pea plants.

The inheritance of one trait was not connected to the inheritance of another trait.


For example, seed color could be yellow or green, and seed shape could be round or wrinkled.


The inheritance of seed color did not affect the inheritance of seed shape. 


The offspring could have yellow and round seeds, yellow and wrinkled seeds, green and round seeds, or green and wrinkled seeds.



From these observations, Mendel wrote his Law of Independent Assortment which predicted the random assortment of homologous chromosomes during metaphase I of meiosis !!!!




    Why can't I be more like Mendel?






10.2 Dihybrid crosses and gene linkage




   Homework Questions 


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






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



Practice Problems

Write the genotypic and phenotypic ratios for the following crosses:



A = long wings

a = short wings

B = wide beak

b = thin beak



AABB X aabb

aaBB X AAbb

AaBb X AaBb

AaBB X Aabb


Determine the genotype of a long winged, wide-beaked bird.

It could be AABB or AaBb. Use a testcross.



10.2.2 Distinguish between autosomes and sex chromosomes.



Autosomes are chromosomes that do not determine male or female characteristics.

Sex chromosomes have genes for determining male or female characteristics.



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.



Linkage groups are genes that are on the same chromosome.

They do not separate by independent assortment - genes on different chromosomes do separate randomly during independent assortment.

They can separate by crossing over.


Studying how often linkage groups separate can show where the genes are located on the chromosome.



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



Recombinant: An organism, cell, or chromosome that is the result of crossing over ("re-combined")


Linkage groups are written horizontally to show which alleles are on the same chromosome:

Insert alt text

T and B are on one chromosome; t and b are on the other homologous chromosome.


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


In a test cross of

Insert alt text

the recombinants will be

Insert alt text


Insert alt text



Usually linked genes are inherited together.

If crossing over occurs between linked genes, the result is a recombinant.


In a testcross, TtBb X ttbb usually produces offspring like TtBb and ttbb.

Same cross written differently:


TB       tb                                                   TB            tb

__   X  __  usually produces offspring like  __    and   __

tb        tb                                                   tb            tb


But if crossing over occurs, then the same cross can produce recombinants:


TB       tb                                                            Tb           tB

__   X  __  produces recombinant offspring like   __   and   __

tb        tb                                                            tb           tb


Without crossing over, a TtBb parent produces TB and tb gametes.

With crossing over, a TtBb parent produces Tb and tB gametes.



Recombinants can be identified because they

  • occur less frequently
  • show unexpected phenotypes
  • do not follow the expected ratios, like 3:1 or 9:3:3:1







10.3 Polygenic inheritance








10.3.1 Define polygenic inheritance



Polygenic inheritance: more than one gene controls the trait. ("poly" = many, "genic" = genes)



  • Human skin color
  • Human hair color
  • Human eye color



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



Human skin color is controlled by at least 4 genes.


This Punnett square shows the possible variation for two parents that are heterozygous for 3 skin color genes.



This variation creates a continuous range of phenotypes.






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