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10.2 gene linkage
 

10.2 gene linkage

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IB Biology Genetics

IB Biology Genetics

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    10.2 gene linkage 10.2 gene linkage Presentation Transcript

    • 10.2 Gene Linkage (autosomal) Topic 10 Genetics
    • Gene Linkage  10.2.4 Define linkage group.  10.2.5 Explain an example of a cross between two linked genes.  10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.
    • Chromosomes  Human body cells consist of 46 chromosomes in their nucleus.  Another way of putting that is 23 pairs of chromosomes  23 from the mother  23 from the father  Two of the 46 chromosomes are called the sex chromosomes. They determine the sex of an individual.  Females XX  Males XY  The other 44 chromosomes are referred to as autosomes.  So from each parent you get:  22 autosomes and 1 sex chromosome
    • Gene Linkage  When Gregor Mendel studies his pea plants, he chose factors that occur on separate chromosomes.  Thus he was able to come up with his law of Independent Assortment of factors.  He claimed that all genes assort independently of each other.  Today we know that this is not always the case.  Alleles on the same chromosome are inherited together.  Genes (alleles) which lie on the same chromosome are said to be linked. They form a linkage group.
    • Recombination in Unlinked Genes  Unlinked genes are found on different chromosomes.  During meiosis (metaphase I), when homologous pairs line on the equator of the cell, the pairs line up independently of each other (independent assortment)  Independent Assortment allows for recombination of unlinked genes because either of a pair of alleles can combine randomly with either of another pair.  Linked genes do not obey this law because they do not assort independently due to being linked on the same chromosome.  Instead they pass together into the same gamete.
    • Gene Linkage Representation  Alleles are normally shown side-by-side in dihybrid crosses.  eg: AaBb  In representing crosses involving linkages groups, it is more common to show them as vertical pairs.  A B a b It allows us to see that the above genotype could also be:  A b a B
    • An example:  Large head and red body are dominant characteristics found on chromosome 5. The recessive characteristics are small head and black body. A beetle homozygous for large head and red body is crossed with one that is homozygous for small head and black body. Deduce the genotypes and phenotypes of the offspring of a test cross on the F1 beetles. Let L = large, l = small;  R = red, r = black.
    • Recombination in Linked Genes  Pairs of linked genes are on the same chromosome.  Recombination of linked genes can only occur when crossing over occurs.  Crossing over occurs during prophase I of meiosis.
    • Crossing Over Ref: Biology Key Ideas
    • A test cross uses a homozygous recessive parent for both genes. Ref: Biology for the IB Diploma, Allott Notice that recombinants are formed as a result of crossing over.
    • IBO guide:  10.2.4 Define linkage group.  10.2.5 Explain an example of a cross between two linked genes.  10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.