Transcript Chromosomes and Inheritance

Please pick up the notes, yeti
sheet, and x-linked traits
sheet.
Chromosomes and
Inheritance
Chromosome Theory of Inheritance
• A basic principle that states genes are
located on chromosomes and that the
behavior of chromosomes during meiosis
accounts for inheritance patterns.
– Supported by:
• Mendel’s three laws.
– Post-Mendelian Developments
• Gene linkage.
• Crossing-over
Sex Determination
• In the early 1900’s Thomas Hunt Morgan
discovered (while studying fruit flies (also known
as Drosophila) that one pair of chromosomes
was different in males from that of females.
• Gametes produced by females have only x
chromosomes.
• Gametes produced by males have an x and a y
chromosome.
• 50% Of the offspring will be female and 50% will
be male.
Sex Linkage
• Due to the different shape in the x and y
chromosomes. Morgan hypothesized that
more genes could be carried on the x
chromosome.
• The presence of a gene on a sex
chromosome is sex linkage.
• X-linked genes
• Y-linked genes
Sex Linkage
• For example:
• Morgan noticed that while
most fruit flies have red eyes some males
have white eyes.
• He proposed that white eyes was an xlinked trait.
• Do a Punnett square between a white
eyed male and a homozygous red-eyed
female.(Where R = red eyes)
Linkage Groups
• There are many more genes than there
are chromosomes.
• The genes on one chromosome form a
linkage group.
• Therefore two genes on the same
chromosome are said to be linked genes.
• Linked genes tend to be inherited
together.
Linkage Groups
• Morgan proved this by making dihybrid crosses
of Drosophila with genotypes of GgLl (where G
for gray body is dominant to g for black body and
L for long wings is dominant to l for short wings).
• If these genes assorted independently the
phenotypic ratio should be 9:3:3:1, instead they
produced a phenotypic ratio of 3:1, as if it were a
heterozygous monohybrid cross.
Linkage Groups
• While linked genes tend to stay together,
this doesn’t always occur.
• Genes may become separated during
crossing over.
• The farther apart the genes are on a
chromosome, the more likely it is that they
will assort independently.
Chromosome Mapping
• Scientists can conduct experiments to
determine the frequency that certain traits
occur together.
• This information is used to create a
chromosome map.
Genetic Traits and Disorders
• Multiple Allele Traits are controlled by 3
or more alleles.
– Example: Human blood types are controlled
by 2 of 3 different alleles IA IB and i.
• Polygenic Traits – Traits that are
controlled by multiple genes.
– Example: Eye color, skin color, height.
– Some can be modified by environmental
factors as well.
Mutations
• Mutation – A change in the DNA of an
organism.
• Mutations may occur in the entire
chromosome or a single nucleotide.
• Germ cell mutations occur in the
gametes.
• Somatic mutations occur in body cells.
• Which mutations can be passed to
offspring?
• Germ Cell
Mutations
• Lethal Mutations – Mutations that cause
death (many times before birth).
• Some mutations are beneficial
phenotypically to the organism. These
mutations give organisms an evolutionary
advantage.
Chromosome Mutations
• Can be a change in the structure of a
chromosome or the deletion of an entire
chromosome.
• A deletion is the loss of a piece of a
chromosome due to breakage.
• An inversion is when a segment breaks of
and reattaches to the same chromosome
inverted.
Chromosome Mutations
• Translocation – A piece of the
chromosome breaks off and reattaches to
another nonhomologous chromosome.
• Nondisjunction – The failure of a
chromosome to separate from its homolog
during meiosis.
Gene Mutations
• Involve large segments of DNA or a single
nucleotide within a codon.
• Point mutation – The substitution, addition, or
removal of a single nucleotide.
• Substitution – One nucleotide in a codon is
replaced by another nucleotide, resulting in a
new codon.
– If the substitution codes for the same amino acid then
there will be no affect.
– If it codes for something different problems will arise.
Gene Mutations
• An example of a genetic disorder arising
from a substitution is sickle cell anemia.
– An adenine is substituted for a thymine in a
single codon.
– This results in a defective form of hemoglobin.
– Prominent in African-Americans (1 in 500)
– Approximately 1 in 10 are heterozygous for
the sickle cell allele.
Gene Mutations
• A single nucleotide may also be deleted or
added.
• This results in a frame shift mutation.
The missing or additional nucleotide
throws all of the succeeding codons off.