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Download Sex Linked Traits - Thomas Hunt Morgan Fruit Fly Experiment
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Transcript
Sex-Linked Traits Some terms to understand: 1. Parental types – phenotype of offspring match parent phenotype. 2. Recombinant (types) – offspring have new combination of phenotypes different than parents. Ex. Heterozygous Yellow Round peas crossed with green wrinkled peas can yield green round peas (recombinant). Sex Linked genes = located on the sex chromosomes (23rd pair in humans; associated with the X chromosome Sex-linked genes are recessive. Examples: 1. Color blindness 2. Hemophilia – lacks protein(s) that create platelets to clot blood 3. Muscular dystrophy – lacks protein, dystrophin, for proper muscle structure 4. (Patterned) Baldness - Females must inherit two recessive alleles to express a sex linked trait (XX) o One from her mom; one from her dad XBXB = homozygous dominant = sees normal color XBXb = heterozygous = sees normal color XbXb = homozygous recessive = color blind - Male only inherits one recessive allele to express a sex linked trait (Xy) o One from his mom XBy = hemizygous = sees normal color Xby = hemizygous = color blind Example: Two parents were shocked to find out they had a color blind child when neither parent is color blind. What are the genotypes of the parents and what is the sex of the child. So,…sex-linked traits are more commonly seen in males; allele passed from mother to son. Thomas Hunt Morgan’s Experiment - First evidence of associating a specific gene on a specific chromosome. - Worked with fruit flies = Drosophila melanogaster o Only 4 pair of chromosomes – 3 pair of autosomes; 1 pair of sex chromosomes XX = female; Xy = male (like humans) Sex–linked example in Drosophila : *Note: notation is different in Drosophila - (many) multiple alleles) The most common phenotype in population = wild types - letter with superscripted plus = wild type [ex: v+ ] Alternatives to wild types = mutant phenotypes (assumed to have mutated from wild type) - lower case (same) letter = mutant allele (usually recessive) [ex: v ] y = y chromosome Example: Eye color ~ (wild type) red, (mutants) vermillion, white, prune So what did Morgan do see to determine that eye color is associated with sex chromosomes… - Cross true breeders to get F1 - Then, monohybrid cross ~ F1 X F1…What should this phenotypic ratio be?.... - You are correct! 3:1 … But…This did NOT happen v = vermilion eye color (mutant) v+ = wild type eye color (red) v v+ v v+ vv vv+ vv+ v+v+ This happened instead… cross: vv+ (female) wild type & v+y (male) wild type Still got a 3:1 ratio, but… males: 50% vermilion, 50% wild females: 100% wild (but 50% are heterozygous) If eye color wasn’t sex-linked then one would expect to get some vermillion (red) eyed females. X-chromosome Inactivation (XCI) Human females inherit two copies of every gene on the X chromosome, whereas males inherit only one. But for the hundreds of other genes on the X, are males at a disadvantage in the amount of gene product their cells produce? - The answer is no, because females have only a single active X chromosome in each cell. The Barr body is one of the X chromosomes. - Compact appearance reflects its inactivity. - So, the cells of females have only one functioning copy of each X-linked gene — the same as males. This affects expression of traits. Since X-inactivation is basically random, this means that the expression of traits in different cells of a woman's body is actually different, even if the cells are genotypically identical.