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12.1 Sex Linkage

12.1 Sex Linkage. Thomas Hunt Morgan: Sex Determination : Studied (used) 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)

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12.1 Sex Linkage

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  1. 12.1 Sex Linkage • Thomas Hunt Morgan: • Sex Determination: Studied (used) 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

  2. 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)

  3. 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

  4. 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

  5. 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:

  6. 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

  7. Morgan’s Work on Linkage • G = Gray L = Long • g = Black 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

  8. 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 overthan genes that are far apart.

  9. 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

  10. Types of Mutations • Germ Cell Mutation – mutation that occurs in the gametes (sex cells). • 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.

  11. Chromosome Mutations • Deletion – loss of piece of chromosome due to breakage • Inversion – piece of chromosome breaks off, flips, and attaches to that chromosome backwards • Translocation – piece of chromosome breaks off and attaches to a non-homologue • Nondisjunction – homologous chromosomes fail to separate during gamete formation examples  

  12. 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)

  13. 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

  14. 3 Types of Point Mutations • 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.

  15. 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

  16. Insertion – A single nucleotide is added to DNA • 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.

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