The Chromosomal Basis of Inheritance and
Mutations Study Guide

Home Test your knowledge

Mendelian inheritance has its physical basis in the behavior of chromosomes

  • In the early 1900s, several researchers proposed that genes are located on chromosomes and that the behavior of chromosomes during meiosis accounts for Mendel’s laws of segregation and independent assortment.

  • Morgan's Experimental Evidence: Morgan's discovery that transmission of the X chromosome in Drosophila correlates with inheritance of the eye-color trait was the first solid evidence indicating that a specific gene is associated with a specific chromosome.


    Drosophila Mutant

    Linked genes tend to be inherited together because they are located near each other on the same chromosome

  • How linkage affects inheritance: Each chromosome has hundreds of thousands of genes. Genes on the same chromosome whose alleles are so close together that they do not assort independently are said to be linked. The alleles of unlinked genes are either on separate chromosomes or so far apart on the same chromosome that they assort independently.

  • Genetic Recombination and Linkage: recombinant offspring exhibit new combinations of traits inherited from two parents. Because of the independent assortment of chromosomes and random fertilization, unlinked genes exhibit a 50% frequency of recombination. Even with crossing over between non sister chromatids during the first meiotic division, linked genes exhibit recombination frequencies less than 50%.

    inheritance

    Sex-linked genes exhibit unique patterns of inheritance

    Inheritance of Sex-linked Genes: The sex chromosomes carry certain genes for traits that are unrelated to maleness or femaleness. For instance, recessive alleles causing color blindness, hemophilia, and Duchenne muscular dystrophy are carried on the X chromosome. Fathers transmit such sex-linked alleles to all daughters but no sons. Any male who inherits a single sex-linked recessive allele from his mother will express the trait.

    Alterations of chromosome number or structure cause some genetic disorders

    Human Disorders Due to Chromosomal Alterations: Changes in the number of chromosomes per cell or in the structure of individual chromosomes can affect phenotype. Such alterations cause Down syndrome (usually due to trisomy of chromosome 21), certain cancers associated with chromosomal translocations, and various other human disorders.

    Some inheritance patterns are exceptions to the standard chromosome theory

    Inheritance of Organelle Genes: The inheritance of traits controlled by the genes present in mitochondria and chloroplasts depends solely on the maternal parent because the zygote's cytoplasm comes from the egg. Some diseases affecting the nervous and muscular systems are caused by defects in mitochondrial genes that prevent cells from making enough ATP (energy).

    Sex-linked genes in humans follow the same pattern for the eye-color locus in Drosophila. Fathers pass sex-linked alleles to all of their daughters but to none of their sons. In contrast, mothers can pass sex-linked alleles to both sons and daughters.

    Karyotype of Down's Syndrome
    • A karyotype shows the full set of chromosomes of an individual arranged in sequence. Here the karyotype shown is from the cell of a male with Down's syndrome. Chromosomes one to 20 and 22 are shown to be present as homologous pairs. There are three copies of chromosome 21 and such individuals develop to show Down's syndrome. The sex chromosomes are shown as X and Y and such individuals develop as male.
    • Gene and chromosome mutations occur naturally and generally have a low frequency. However, certain chemicals and rays can increase the rate of frequency of mutation. These are called mutagenic agents and include:
      • chemicals such as mustard gas, caffeine and colchicine
      • irradiation such as U-V radiation, X-rays and gamma rays


    Vocabulary
    Mendel's Law of Segregation (there are four parts): Alternative versions of genes account for variations in inherited characters. For each characteristic, an organism inherits two alleles, one from each parent. If the two alleles differ, then one, the dominant allele, is fully expressed in the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance. The two alleles for each characteristic segregate during gamete production.
    Independant Assortment:

    the process of random segregation and assortment of chromosomes during gametogenesis to form genetically unique gametes.

    Trisomy: the presence of three, instead of the normal two chromosomes of a particular numbered type in an organism.
  • Source: Campbell, Neil A., and Reece, Janes B. Biology. Sanfrancisco: Pearson Education, 2005.