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Journal Chromosomal Theory of Inheritance: simply states that chromosomes are carriers of genetic information (Walter Sutton) Human Chromosomes (Karyotype) Humans have 22 pairs of autosomes, 1 pair of sex chromosomes Fruit flies have 3 pairs of autosomes, 1 pair of sex chromosomes What is the sex of this person? Female eggs = XX Male sperm = XY Possible offspring are shown in square: 50% male 50% female What happens when an allele is located on the X chromosome? Males only get ONE ! These are a few of the known alleles located on the X chromosome Disorders that are recessive are masked in females by the other X chromosome. Sex Linked Genes •Genes located on the X chromosome are inherited with that X. •When doing crosses you must include the sex chromosomes in your cross. •Use superscript letters for the allele. Example: In fruit flies, eye color is a sex linked trait. Red is dominant to white. FRUIT FLY CHROMOSOMES Females XRXR XRXr XrXr Males XR Y Xr Y Drosophila melanogaster Figure 12.6 Thomas Morgan used crosses with fruit flies. Why do you think he used them? Figure 12.1 What is an autosome? Show the cross between a white eyed male and a red eyed female: XrY x XRXR Show the cross between a red eyed male and a white eyed female: Figure 12.2 Human Sex Linked Disorders 1. Colorblindness 2. Muscular Dystrophy 3. Hemophilia 4. Fragile X Syndrome COLORBLINDNESS More Tests Muscular Dystrophy • characterized by the wasting away of muscles • life expectancy: ~20 yrs • females can be carriers for the disease, where they pass the disease to their sons only Show the cross of a female who is a carrier and a normal male: Fragile X Syndrome • caused by triplet repeats in a gene on the X chromosome • causes mental retardation • named because the X chromosome had an odd appearance - the tip of the chromosome seemed to be attached only by a small thread • the number of repeats of the gene determines the severity of the disease Figure 12Aa Hemophilia • also known as "bleeder's disease", blood does not clot properly • disease was present in the royal family, starting with Queen Victoria Show the cross between a female with hemophilia and a normal man: Pedigree Key • Squares represent males • Circles are used for females • A horizontal line from male to female represents mates that have produced offspring • The offspring are listed below their parents from oldest to youngest • A fully shaded individual posses the trait being studied – If the condition being studied is a monogenic recessive condition (rr), then those shaded gray have the genotype rr – If the condition being studied is dominant condition (Rr or RR), then those that are unshaded have the genotype rr • A line through a symbol indicated that the person is deceased How to analyze a pedigree Figure 12.5 Identify all of the CARRIERS