BIOL 1020 – CHAPTER 15 LECTURE NOTES
Chapter 15: The chromosomal basis of inheritance
I.
Chromosome theory of inheritance Mendelian factors (genes) have specific locus on the chromosomes
 chromosomes undergo segregation and independent assortment
 Morgan used Drosophila as a model system and demonstrated gene chromosome relationships
 Morgan demonstrated that linked genes tend to be inherited together because they are near each other on the same
chromosomes
A. genetic linkage – independent assortment does not always occur
1. independent segregation of chromosomes during meiosis I leads to independent assortment
2. independent assortment leads to recombination
 recombination – any process that leads to combinations of genotypes not seen in the parents
 recombinant gametes – gametes that display a recombinant genotype
 recombinant offspring – offspring whose phenotype reveals that they inherited genes from a recombinant gamete
3. genes that are on the same chromosome may not sort independently; such genes are said to be linked
4. an example will be used in class to show the effect of linkage on the results of a genetic cross
5. crossing over breaks linkages between genes
 recall crossing over during prophase I between homologous chromosomes; it is the only way to get genetic
recombination between genes that are on the same chromosome
 the further apart two genes are, the more likely they are to have crossing over occur between them (thus leading to
genetic recombination)
6. genetic maps of chromosomes
 percentage of crossing over or recombination is calculated from 100 times the number of recombinant offspring
divided by the total number of offspring
 map unit – by convention, one map unit = 1% recombination (the term cM or centiMorgan is sometimes used for
map units, in honor of a pioneer in gene mapping)
 map distances between genes on the same chromosome are measured in map units
 linkage group = all genes on a particular chromosome; tend to be inherited together
 placement of a gene into a position in a linkage group is genetic mapping
 map distances get less meaningful as they get large
 as genes get further apart, the odds of multiple crossing over events between them increase
 when distances approach 50 map units, the genes appear essentially unlinked
 many chromosomes have an overall map length of well over 50 map units
genetic maps are useful in locating the actual physical location of genes
II.
Sex determination and sex chromosomes
A. sex determination varies between species
1. hermaphroditic organisms – have both sexes in the same individual
2. many animals have sex determined in response to environmental signals
3. most animals have sex determined by genetic inheritance; sex chromosomes are involved
B. sex chromosomes
1. homogametic sex – has a pair of similar sex chromosomes; all gametes that individual produces get that kind of sex
chromosome
2. heterogametic sex – has two different sex chromosomes, and makes gametes with two different types of sex
chromosome
3. all the other, non-sex chromosomes are called autosomes
4. usually, the sex chromosome found in the homogametic sex is considerably larger, and the shorter sex chromosome found
only in the heterogametic sex has few genes
5. in humans, females are XX and males are XY (not all do it this way – birds are essentially reversed in this)
6. X and Y chromosomes have regions of homology (sequence similarity) that allow for pairing during meiosis I
7. usually, but not always, the sex determining gene is on the Y chromosome
 XXY humans are male (Klinefelter syndrome)
 X_ humans are female (Turner syndrome)

XXX humans are female (likely to give birth to a XXY child)

XYY humans are male (Jacob’s syndrome)
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BIOL 1020 – CHAPTER 15 LECTURE NOTES
C. sex-linked traits
1. genes on sex chromosomes show inheritance patterns that do not fit traditional Mendelian ratios that describe what
happens to genes on autosomes
2. a number of inherited diseases in humans are located in sex chromosomes. Examples: Hemophilia, color blindness
3. in humans (and other species with XY sex determination), a gene found only on the X chromosome is said to be X-linked
(which is a type of sex-linked)
 males only get one X chromosome, from the mother, and are hemizygous at every locus found only on the X
chromosome
 thus, recessive X-linked alleles are expressed more often in males than in females
 X-linked alleles are written with superscripts
 make sure to try some of the sex linked gene problems on your own
D. dosage compensation
1. it is not always good to have twice as much of a chromosome, or half as much
2. dosage compensation – a mechanism for equalizing the overall expression of an sex-linked genes in both males and
females
 some organisms (like fruit flies) ramp up X-linked gene expression in the heterogametic sex
 some (like humans and other mammals) use inactivation of most of one of the X chromosomes
3. Barr body – condensed, mostly inactivated X chromosome visible during interphase in most mammalian cells
4. variegation or mosaicism – mixes in phenotypic appearance in an organism due to expression of X-linked genes and
variable, random inactivation patterns for X chromosomes (example: calico cat)
III. Alterations in chromosomes number or structure cause some genetic disorders in humans
1. Abnormal chromosome numbers
 aneuploidy- nondisjucntion of homologous chromosomes during meiosis may lead to formation of an embryo with
one extra chromosomes called trisomic (2n+ 1) or less chromosomes called monosomics (2n-1)
 some organisms may have more than two complete copies of chromosomes sets – Poloploidy. Polyploidy is
uncommon and lethal in most animals, but more prevalent in plants
 Example: Trisomy for 21: Down’s syndrome
2. Alterations in chromosomes structures:
 deletion or loss of a part of chromosomes- usually lethal
Loss of a piece of chr 5 leads to a child with cri-du chat (Cries of a cat) syndrome
 translocation of a part chromosomes to another sites
a piece of chromosome 9 to chromosome 22 is associated with CML leukemia
 Fragile sites - thin chromosomes at a particular point
fragile X has a CGG repeats more than 1000 times leading to break in X xhromosomes
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