Chapter 15
Chromosomal Basis of Inheritance
Discovery
• 1900 – cytology and genetics converge:
correlation between chromosomes and
Mendelian genetics.
• Mendelian genes have certain loci on
chromosomes that undergo segregation and
independent assortment
Thomas Hunt Morgan
• 1st to associate a specific gene with a
_____________.
• Worked with Drosophila melanogaster
(__________)
– Hundreds of offspring, can be bred every two
weeks
– Only four chromosome pairs; 3 ___________ and
one _____ pair (F-XX M-XY)
Terms
• Wild type: most common in a population (+)
• Mutant type: alternative to wild type
Example
•
•
•
•
Red eyes (+) X White eyes
Red is dominant to white
Only males had white eyes
Morgan concluded that the gene must be on the
X chromosome and not the Y
• Females: XRXW  _______
• Males: XWY  _________
• Sex linked genes: genes located on a sex
chromosome.
Linked Genes
• Genes located on the same chromosome that tend to
be inherited together.
• Ex. Drosophila body color and wing size
• b+ gray b black vg+ normal vg vestigial
• b+ b vg+ vg X b b vg vg
• Expected: 4 phenotypes in equal ratios
• Found: More gray normal and black vestigial than gray
vestigial and black normal
• Conclusion: Body color and wing shape are inherited
together on the same chromosome (found more
parental phenotypes than nonparental)
Genetic Recombination
• Parental types: match a parent
• Recombinant types: don’t match a parent
(when ½ of the offspring are recombinants,
there is a 50% frequency of recombination).
• 50% recombination is observed for any 2
genes located on different chromosomes (due
to independent assortment of alleles at
Metaphase I of meiosis).
• Recombination frequency = #/total * 100%
Crossing Over
• If genes on the same chromosome, we would
expect them to always be linked. However, we
saw that this is not true (we saw some gray
vestigial and black normal). This incomplete
linkage can be accounted for by crossing over
in meiosis I. (SHOW MEIOSIS)
Using recombination for genetic
mapping
• Alfred H. Sturtevan (student of Morgan’s) created a genetic
map.
• Recombinant frequencies reflects distances on
chromosomes: greater distances = higher chance of
crossing over
• Ex. Body color (b), wing size (vg) and cinnabar (cn)
• Recombination frequency: cn and b is 9%, cn and vg is
9.5% and b and vg is 17%
• Order on chromosomes must be:
• Map unit = 1% recombination or 1 centimorgan
• Maximum value = 50% recombination, so far apart they
could be on different chromosomes.
Types of Maps
• Linkage map: sequence of genes, not actual
locations
• Cytological map: locate genes with respect to
chromosomal features (staining patterns)
• DNA nucleotide map
Sex chromosomes
• Mammals: Males are _____ females are ____
• Males: in meiosis there is little crossing over
between homologous chromosomes because
they are different (X and Y). Development:
embryo starts differentiating at about 2 months:
Y chromosome has ____: sex determining region
of the Y discovered in 1990. In the absence of
____, gonads develop into ____________. (many
other genes responsible)
Sex linked genes
• Humans: usually X linked
• Fathers pass X linked genes to daughters, not
sons
• Mothers pass X linked genes to both sons and
daughters
• If recessive: female must be homozygous to
express trait. Males are ____________ (only one
X); any male receiving a trait from mom will
express it. Consequences: more __________
than _______________ are affected, although
both can be affected.
Human Disorders
• Duchenne Muscular Dystrophy: (1/3500
_________ in the US) Progressive weakening of
muscles, loss of coordination, rarely live past 20’s
(absence of muscle protein dystrophin, gene on X
chromosome)
• Hemophilia: on X, absence of protein required
for _____________________, excessive
_____________ (used to ________________
from small _______). Royal Family: starting with
Queen Victoria, spread to Prussia, Spain and
Russia (Anastasia).
X Inactivation
• Females inactivate one X, which condenses
into a Barr body inside the nucleus and most
genes are not expressed.
• Mary Lyon: discovered the Barr body
assignment is random, females are a mosaic of
the two types of cells (active X from mom and
active X from dad)
• Ex. Calico Cat: heterozygous for black and
orange