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 P1: P2: 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 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

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 Germ Cell Mutation – mutation that occurs in the gametes. 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 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   

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 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.

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.

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