Chromosomal basis of inheritance
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Chromosomal Basis of Inheritance. Chapter 15. Mendel & Chromosomes. Today we know that Mendel’s “hereditary factors” are located on chromosomes So we can link Mendelian genetics to modern genetics through the genes that lie on the chromosome Chromosome Theory of Inheritance

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Mendel chromosomes
Mendel & Chromosomes

  • Today we know that Mendel’s “hereditary factors” are located on chromosomes

    • So we can link Mendelian genetics to modern genetics through the genes that lie on the chromosome

  • Chromosome Theory of Inheritance

    • Mendelian genes have specific loci (positions) on chromosomes

    • Chromosomes undergo Segregation & Independent Assortment

  • Converting to morgan speak
    Converting to Morgan speak

    • Phenotype

      • Dominant = wild type

      • Recessive = mutant

  • Wild-type – normal or typical

    • W+

  • Mutant type – not normal

    • W

  • For example, vg+ = wild type for body size (full body)

    vg = mutant (vestigial body, smaller size)

  • T h morgan studied flies
    T.H. Morgan studied flies

    • He studied Drosophila Melanogaster – Fruit fly

      • They _____ like flies

      • Take 2 weeks to breed

      • Hundreds of offspring per brood

      • Only 4 pairs of chromosomes

        • 3 autosomes and 1pair of sex chromosomes

    Morgan s results
    Morgan’s Results

    • He crossed wild type (red eyes) with a mutant (white eyes), but did not get Mendelian results, or did he?

    • Gender or sex differences

      • Called Sex-linked gene or trait

    • ONLY F2 MALES = white eyes !!

    Chromosomal basis of inheritance

    • Cross the other 3 combinations

    • What are the other combinations?

    • Are there any patterns?

    What happens to produce
    What happens to produce?

    • All males are mutant, but all females are wild-type?

    • All females are wild type, but only 50% of males are?

    • 50% are wild-type, 50% are mutant?

    Think about it
    Think about it…

    • IF the mother is homozygous dominant, then sons are

    • IF the mother is heterozygous, then sons are

    • IF the mother is homozygous recessive, then sons are

    Think about it page 2
    Think about it… (Page 2)

    • IF the father is wild-type, then daughters are

    • IF the father is mutant, then daughters are

    • So mothers determine ______ &

      wild type fathers produce ______ daughters

    Known sex linked disorders
    Known Sex-Linked Disorders

    • Duchenne muscular dystrophy

      • Progressively weakening of muscles and loss of coordination

  • Hemophilia

    • Blood that is unable to clot normally

    • Due to absence of proteins required for proper clotting

  • Sex linked disorders
    Sex-linked Disorders?

    • Which gender do you think is afflicted at a higher rate?

    • Why?

    Chromosomal basis of inheritance

    • If XX is female & XY is male, but the Y chromosome contains virtually no genetic material, do females have more genetic information than males?

    Females x inactivation
    Females = X Inactivation

    • Although females receive 2 copies of alleles, one chromosome becomes inactivated during embryonic development

      • Due to XX

  • Chromosome inactivation is Random

  • Inactivation is due to methylation

  • So BOTH females and males are operating on only 1 sex chromosome

  • Barr Body – inactivated chromosome condenses

    • Lies on the inside of the nuclear envelope

  • More fly stuff
    More fly stuff

    2 Characters: body color & wing size

    Body Color

    b+ = Grey (wild type)

    b = black (mutant)

    Wing Size

    vg+ = normal wings

    vg = vestigial wings (Reduced wing size)


    • Linked Genes – located on same chromosome

      • Tend to be inherited together

  • Genetic Recombination – Offspring with new combination of genes inherited from parents

  • Parental Phenotype - at least one of the parental phenotypes

  • Recombinants – NOT either of the parental phenotypes

  • What should have happened
    What should have happened?

    • What should have been the ratio if the characters were inherited via a Mendelian pattern?

    • How do the recombinants form?


    • 2,300 offspring

    • Far higher proportion of parental phenotypes than expected from independent assortment

      • Genes are inherited together

  • There were also recombinants or non-parental phenotypes as well

  • Conclusion = Partial linkage & Genetic recombination (recombinants or recombinant types)

  • Recombination frequency
    Recombination frequency

    • Calculate by (Total Recombinants / Offspring * 100)

    • If the genes are located on different chromosomes, then the recombination frequency should be 50%

    • In the flies, the recombinant frequency was less than 50%; it was about 17%

      • Evidence of that the 2 genes lied on the same chromosome

  • So some linkage but incomplete

  • More recombinants = less linkage

  • Linkage maps
    Linkage Maps

    • Crossing Over explains why some linked genes get separated during meiosis

    • Crossing Over occurs in Meiosis I

    • Farther apart 2 genes = Higher P(Crossing Over)

    • Linkage Map – genetic map based on the percentage of cross-over events

    • Map unit – 1% recombination frequency

      • Used ONLY for relative distances on the chromosome


    • We know that Mendel’s seed color and flower color were on the SAME chromosome, but they did not behave as linked genes. Explain.

    Chromosomal abnormalities
    Chromosomal Abnormalities

    • Nondisjunction – mishap where pairs of homologs do not move apart properly during meiosis

      • Could happen in Meiosis I or when Sister chromatids fail to separate correctly in Meiosis II

  • One gamete receives 2 of the same type of chromosome, while another receives no copy

  • Abnormal numbers of chromosomes
    Abnormal NUMBERS of chromosomes

    • Aneuploidy – abnormal number of chromosomes

      • Nondisjunction could result in a cell with 2n+1

      • Here this cell would be considered aneuploid, and considered trisomic for that individual chromosome

  • -somy = different number of an INDIVIDUAL chromosomes

  • Trisomy – 3 copies of a chromosome

    • 2n + 1

  • Monosomy – only 1 copy of a chromosome

    • 2n – 1

  • Abnormal number of chromosome sets
    Abnormal Number of Chromosome SETS

    • Alteration of an ENTIRE CHROMOSOMAL SET

      • Called polyploidy

      • Triploid = 3n

      • Tetraploid = 4n

    • Polyploidy plants are fairly common

      • animals are less common

    • Polyploids are more normal than aneuploids

      • Hence, 1 chromosome extra or fewer is more disruptive, than an entire set of chromosomes extra or fewer

    Down syndrome
    Down Syndrome

    • 1 of 700

    • Trisomy 21 (each cell has 47 chromosomes, not 46)

    • Risk increases with maternal age

    Klinefelter s syndrome
    Klinefelter’s Syndrome

    • Male have extra X chromosome

    • Possess male sex organs, but are sterile

    • IF Female, 3 chromosomes (XXX) = healthy & normal

    Turner syndrome
    Turner Syndrome

    • Female with only 1 X

    • Only viable monosomy in humans