Download Morgan and Sex Linkage / Mutations

12.1 Sex Linkage
• Thomas Hunt Morgan:
– Sex Determination:
Studied fruit flies – 4 pairs of homologous chromosomes but
one pair was different between Male and Female.
- Female had 4 identical pairs
- Male had 3 identical pairs and 1 pair that was different (XY)
Morgan’s Hypothesis – A pair of chromosomes determines
sex  XX (female); XY (male) – Called sex chromosomes
• Morgan’s rationale:
– In meiosis each gamete gets only 1 sex chromosome –
either X or Y in males only X in females.
Egg (1 sex chromo) + Sperm (1 s.c. ) = Zygote (2 s.c.)
Because of this sex determination is 50/50 male : female
in sexual reproduction.
Male determines the sex of the offspring (can give X or Y)
Sex Linkage
• Morgan thought more genes can be held on X
than Y
– X-linked Genes – genes on X chromosome
– Y-linked Gene – genes on Y chromosome
– Sex-linked Genes – genes carried on sex chromosomes
Morgan’s Experiment
• Morgan found that most fruit flies had red eyes
but some MALES had white eyes
Crossed a red eyed female x white eyed male
Morgan’s Results
• F1: All red eyed fruit flies
– He let the F1 offspring mate
• F2: 3:1 red eyed to white eyed  but all white eyed ff
were males.
• Morgan proved that the gene for eye color is carried on
the X chromosome
P1:
P2:
Linkage Groups
• Each chromosome carries many genes
• Genes on 1 chromosome form linkage groups
• 2 or more genes on the same chromosome
are said to be linked  tend to be inherited
together
Morgan’s Work on Linkage
• G = Gray
• g = Black
L = Long
l = Short
– P1 : GGLL x ggll
– F1G: All GgLl
– F1P: All Gray Long
– P2: GgLl x GgLl
• He knew if genes were on different chromosomes
phenotype would be 9:3:3:1
• Found F2 results were 3:1 (3 gray long) (1 black short)
– Hypothesis: body color and wing length were linked
• Also produced gray short (Ggll) and black long
(ggLl) – found that this occurred because of
crossing over of the homologous chromosomes
• Crossing over does not create delete genes – it
does change location on chromosomes  leads to
new gene combinations (genetic recombination)
• Genes closer together are more likely to cross over
than genes that are far apart.
Linkage Maps
• Use recombination frequencies to determine
where genes are on chromosomes.
– Use frequencies (%) to lay out where each gene is
located on the chromosome.
• Higher % - further the 2 genes are and less likely to
cross over together.
• Outliers – 2 genes that are furthest apart (highest %)
– Each % = 1 map unit
Types of Mutations
1. Germ Cell Mutation – mutation that occurs
in the gametes.
2. Somatic Cell Mutation – mutation that
occurs in the somatic cells.
All mutations fall under the 2 above:
- Lethal Mutations – causes death
- Silent Mutations – not on a gene – does not
harm the organism
- Nonsense Mutation - is a point mutation in a
sequence of DNA that results in a premature stop
codon.
Chromosome Mutations
1. Deletion – loss of piece of chromosome due
to breakage
2. Inversion – piece of chromosome breaks off,
flips, and attaches to that chromosome
backwards
3. Translocation – piece of chromosome breaks
off and attaches to a non-homologue
4. Nondisjunction – homologous chromosomes
fail to separate during gamete formation
examples
  
• 1 gamete gets 2 copies of a chromosome and
the other gamete gets no copy.
– At fertilization: the zygote gets 3 H. C. = Down’s
Syndrome (Trisomy 21)
– At fertilization: the zygote gets H. C. = Turner’s
Syndrome (Monosomy)
Gene Mutations
• Could be:
- Large segments of DNA
- Single nucleotide in a codon
• Point Mutations – Addition, Subtraction (removal),
or substitution of nucleotide(s) in a codon
3 Types of Point Mutations
1. Substitution Point Mutation: (Missense Mutation)
1 nucleotide is replaced by a different nucleotide, results
in a new codon. It COULD affect one amino acid.
- If substituted nucleotide does not change AA, no
affect on organism
- If substituted nucleotide does change AA, resulting
protein will be altered, affecting the organism.
2. Insertion – A single nucleotide is added to DNA
3. Deletion – A single nucleotide is removed from
DNA
Both are more serious than substitution
By gaining or losing a nucleotide causes all
codons after this point to be altered
(incorrectly grouped) and affects the AA chain
This (#2,#3) is called a Frame Shift Mutations –
causes all AA from this point to be different
than intended by DNA template.
• Example: Sickle Cell Anemia
– Caused by Substitution Point Mutation
– Adenine is substituted for uricil in 1 codon 
causes defective hemoglobin
• This is a recessive allele disorder so you must have 2
copies of the defective allele to have Sickle Cell (aa)
• Affects circulation of blood
• Heterozygous for Sickle Cell (Aa) = Carrier, do not have
Sickle Cell but can pass defective allele to offspring.
The carrier is phenotypically normal