Chapter 12-1 Chromosomes
and Inheritance patterns
Objectives
• Explain the role of sex chromosomes in
sex determination
• Describe how sex linkage affects the
inheritance of traits
• Explain the effect of crossing over on the
inheritance of genes in linkage groups
• Summarize the procedure involved in
constructing a chromosome map
• Distinguish between chromosome
mutations and gene mutations
• How does the
inheritance of sex
chromosomes result in
approximately equal
numbers of males and
females among the
offspring of fruit flies.
• Shown in a punnet
square
• Offer an explanation for
why morgan did not find
white-eyed female
drosophilia in the F2
generation when he
crossed white eyed
males with red eyed
females.
• The eye color gene is
located on the X
chromosome and any
female offspring would
be heterozygotes
• How does
crossing over
show that genes
are found on
chromosomes
• Because of the
expression of
those genes in
the offspring
• How can crossing over between
two alleles be used to map their
locations on chromosomes
• To make the map scientists mate
organisms together and watch
how often the genes get split up
• Each percent of the time they get
split up is one map unit.
• By comparing many genes
scientists can figure out the order
they are in
• What are point mutations
• Changes in one nucleotide in DNA
Sex Determination
• T.H. Morgan bred the fruit fly Drosophilia
Melanogaster to study genetics
– Fruit Flys are good to study
– They have lots of offspring
– They live a short time
– They are easy to take care of
– They only have four pairs of chromosomes
• In fruit flies XX are females, and XY are
males
– Just like humans
• Like other chromosomes the X and Y pairs
segregate during Meiosis
Sex Linkage
• Since the X is bigger than the Y, Morgan
hypothesized that there would be more
genes on the X
• Genes on the X chromosome are called Xlinked Genes
• Genes are the Y chromosome are called
Y- linked genes
• This is sex linkage
Inheritance of sex linked genes
• For example in fruit flies the
gene for eye color is on the
X-chromosome
• Red is dominant and white is
recessive
• In Sex-linked genes the
recessive trait shows up
more often in males than in
females
• Why?
Y-linked
• SRY gene
• SRY (Sex-determining Region
Y) is a sex-determining gene on
the Y chromosome in humans
and other primates.
• One of the most controversial
uses of this discovery was as a
means for gender verification at
the Olympic Games,
• Athletes with a SRY gene were
not permitted to participate as
females, although all athletes in
whom this was "detected" at the
1996 Summer Olympics were
ruled false positives and were not
disqualified. In the late 1990s, a
number of relevant professional
societies in United States called
for elimination of gender
verification,
Linkage groups
• Each chromosome carries many genes
• All the genes on a chromosome form
linkage groups
• Since they are linked on the same
chromosome these genes tend to get
inherited together
Fruit Flies again
• Morgan crossed Gray-long winged flies
with black-short winged flies
• Using two traits you’d think it would look
like the big dihybrid punnet squares
• The results actually looked like a little four
square punnet square because color and
wing length are linked on the same
chromosome
Crossing over happened
• Sometimes Morgan got
Black long winged and
Grey Short winged flies
• If color and wing length are
on the same chromosome
how did they get mixed?
• During meiosis some parts
of chromosomes switch off
Chromosome Mapping
• If two genes are far
apart on a chromosome
and crossing over
happens they are more
likely to get split up
• If two genes are close
together on a
chromosome they are
less likely to get split up
• A chromosome map is a diagram that shows the order of
genes on a chromosome.
• To make the map scientists mate organisms together
and watch how often genes of interest separate.
• 1 Map unit = 1 % recombination frequency. In other
words, divide the number of recombinant offspring by
total offspring, then convert to percent.
• By comparing many genes scientists can figure out the
relative positions of the genes on the chromosome.
Mutation
Mutations can involve an entire chromosome
or a single nucleotide (the ATGC letters)
Germ cell mutations occur in an organism’s
gametes. These are the mutations that
can be passed on.
Somatic mutations happen in your body cells
and are not passed on.
Chromosome mutations
• A deletion is a loss of a piece of a chromosome due to
breakage
•Inversion: a segment breaks off and reattaches in the
opposite direction
•Translocation: a segment breaks off and reattaches onto
another chromosome
•Nondisjunction: two chromosomes fail to separate during
meiosis
Gene mutations
• Point mutations: when a single nucleotide
mutations
• Substitution mutations happen when one
nucleotide is replaced with another
• This leads to a different codon dif.
Amino acid  dif shaped protein.
– Sickle Cell is an example
Frame-shift mutation
• The insertion or deletion of a nucleotide
switches the order in which the codon’s
are read and can be much more serious
than a substitution.
• How does the inheritance of sex
chromosomes result in approximately
equal numbers of males and females
among the offspring of fruit flies.
• Shown in a punnet square
• Offer an explanation for why morgan did
not find white-eyed female drosophilia in
the F2 generation when he crossed white
eyed males with red eyed females.
• The eye color gene is located on the X
chromosome and any female offspring
would be heterozygotes
• How does crossing over show that genes
are found on chromosomes
• Because of the expression of those genes
in the offspring
• How can crossing over between two alleles be
used to map their locations on chromosomes
• To make the map scientists mate organisms
together and watch how often the genes get split
up
• Each percent of the time they get split up is one
map unit.
• By comparing many genes scientists can figure
out the order they are in
• What are point mutations
• Changes in one nucleotide in DNA