Erika Veidis
AP Biology – Pd. 1
November 29, 2009
CHAPTER 15 NOTES: The Chromosomal Basis of Inheritance
I.
II.
III.
IV.
Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual
life cycles
a. Cytology – study of mitosis
b. Around 1900, cytology and genetics converged
i. Chromosomes and genes both present in pairs in diploid cells
ii. Homologous chromosomes separate and alleles segregate during meiosis
iii. Fertilization restores the paired condition for both chromosomes and genes
c. Chromosome theory of inheritance – Sutton, Boveri – Mendelian genes have a specific
loci on chromosomes, and it is the chromosomes that undergo segregation and
independent assortment
Morgan traced a gene to a specific chromosome: science as a process
a. Morgan – first to associate a specific gene with a specific chromosome; experimental
evidence that chromosomes are the location of Mendel’s inheritable factors
b. Experimental organism: Drosophila melanogaster (fruit fly)
i. Prolific breeders – a single mating will produce hundreds of offspring, and a new
generation can be bred every two weeks
ii. Only has four pairs of chromosomes, which are easily distinguishable with a light
microscope (3 pairs of autosomes and 1 pair of sex chromosomes)
c. Wild type – the phenotype most common in natural populations (red eyes in fruit flies)
d. Mutant phenotypes – traits that are alternatives to the wild type (white eyes)
e. Sex-linked genes – genes located on a sex chromosome (eye color in fruit flies)
i. Morgan’s experiments with fruit flies evidenced that a specific gene is carried on
the X chromosome – added credibility to the chromosomal theory of
inheritance
Linked genes tend to be inherited together because they are located on the same
chromosome
a. Each chromosome has hundreds or thousands of genes
b. Linked genes – genes located on the same chromosome – tend to be inherited together
because the chromosome is passed along as a unit
i. When dealing with linked genes in breeding experiments, the results deviate
from those expected according to the Mendelian principle of independent
assortment
Independent assortment of chromosomes and crossing over produce genetic recombinants
V.
VI.
VII.
VIII.
a. Genetic recombination – the production of offspring with new combinations of traits
inherited from two parents
b. Parental types – the offspring that inherit a phenotype that matches one of the parental
phenotypes
c. Recombinants - the offspring that have different combinations of traits than either
parent
d. Recombinant offspring result from events of meiosis (crossing over, independent
assortment of chromosomes) and random fertilization
e. A recombination frequency of less than 50% indicates that the genes are linked but that
crossing over has occurred
i. Crossing over – homologous chromosomes in synapsis during prophase of
meiosis I break at corresponding points and switch fragments, creating new
combinations of alleles that are then passed on to the gametes
Geneticists can use recombination data to map a chromosome’s genetic loci
a. The farther apart genes are on a chromosome, the more likely they are to be separated
during crossing over
b. Genetic map – an ordered list of the genetic loci along a particular chromosome
c. Some genes on a chromosome are so far apart from each other that crossovers between
them occur very often. The frequency of recombination measured between such genes
can have a maximum value of 50%, a result indistinguishable from that for genes on
different chromosomes.
d. Linkage map – a genetic map based on recombination frequencies – portrays the
sequence of genes along a chromosome, but it does not give the precise locations of
genes
e. Cytological maps – locate genes with respect to chromosomal features, such as stained
bands – pinpoints the physical locus of a gene by associating a mutant phenotype with a
chromosomal defect seen in the microscope
f. The sequence of genes between a linkage map and a cytological map is the same, but
the spacing between them is not.
The chromosomal basis of sex varies with the organism
a. Sex is an inherited phenotypic character usually determined by the presence or absence
of special chromosomes – in humans and fruit flies, this is determined by the XY system
(XX = female, XY = male)
b. The offspring’s sex is determined at conception by whether the sperm carries X or Y
Sex-linked genes have unique patterns of inheritance
a. The sex chromosomes carry certain genes for traits that are unrelated to maleness or
femaleness
i. Ex: Hemophilia – sex-linked recessive disorder whose gene is on the X
chromosome
Alterations of chromosome number or structure cause some genetic disorders
a. Errors during meiosis can change the number of chromosomes per cell or the structure
of individual chromosomes
IX.
X.
b. Nondisjunction – the members of a pair of homologous chromosomes do not move
apart properly during meiosis I, or in which sister chromatids fail to separate during
meiosis II – one gamete receives two of the same type of chromosome and the other
gamete receives no copy
c. Aneuploidy – an abnormal chromosome number
i. Can arise when a normal gamete unites with one containing two copies or no
copies of a particular chromosome as a result of nondisjunction during meiosis
d. Polyploidy – more than two complete sets of chromosomes
i. Can result from complete nondisjunction during meiosis
ii. The fertilization of an abnormal diploid egg produced by nondisjunction of all its
chromosomes (triploidy – 3n)
iii. The failure of a diploid zygote to divide after replicating its chromosomes
(tetraploidy – 4n)
e. Deletion – occurs when a chromosomal fragment lacking a centromere is lost during cell
division – the chromosome from which the fragment originated will then be missing
certain genes
f. Duplication – a deleted fragment attaches to the homologous chromosome
g. Inversion – the deleted fragment reattaches to the original chromosome but in the
reverse orientation
h. Translocation – a rearrangement in which the fragment joins a nonhomologous
chromosome
i. Down syndrome (Trisomy 21) – an aneuploid condition in which there is an extra
chromosome 21, so that each body cell has a total of 47 chromosomes
The phenotypic effects of some genes depend on whether they were inherited from the
mother or the father (imprinting)
a. Imprinting – the same alleles may have different effects on offspring depending on
whether they arrive in the zygote via the ovum or the sperm
b. Individuals imprint certain parts of chromosomes in their gamete-producing cells with
either a male or a female “stamp” in the form of methylation – affects the way some
genes are expressed in offspring
c. Explains the inheritance patterns of some hereditary disorders, including fragile X
syndrome
Extranuclear genes exhibit a non-Mendelian pattern of inheritance
a. Mitochondria and chloroplasts contain some of their own genes.
b. Because the zygote’s cytoplasm comes from the ovum, certain features of the
offspring’s phenotype depend solely on these maternal cytoplasmic genes.
c. Some diseases affecting the nervous and muscular systems are caused by defects in
mitochondrial DNA that prevent cells from making enough ATP.