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Sex-linked Traits

Sex-linked Traits. A gene that is found only on the X chromosome and not the Y chromosome Examples: Red-green colorblindness Male Pattern Baldness Hemophilia Duchenne Muscular Dystrophy. Female Male. Sex-linked Traits. More common in men because they only have one X chromosome

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Sex-linked Traits

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  1. Sex-linked Traits • A gene that is found only on the X chromosome and not the Y chromosome • Examples: • Red-green colorblindness • Male Pattern Baldness • Hemophilia • Duchenne Muscular Dystrophy Female Male

  2. Sex-linked Traits • More common in men because they only have one X chromosome • Females have two X chromosomes and would need the gene on both chromosomes to have the trait • Females that are heterozygous for the trait are called carriers • Carriers appear normal but can pass the trait to their offspring

  3. Sex-linked traits

  4. Colorblindness • Recessive trait on X chromosome • Condition in which certain colors cannot be distinguished • Red/Green color blindness is most common (about 99%) • Causes problems in distinguishing reds and greens • No treatment • Life is normal

  5. Hemophilia • Recessive trait on the X chromosome • Bleeding disorder in which the blood does not clot normally • Persons with hemophilia may bleed for a longer time than others after an injury or accident • They also may bleed internally, especially in the joints • Live life cautiously, receive treatments to help blood clot normally

  6. Duchenne Muscular Dystrophy • Caused by a recessive gene on the X chromosome • Rapidly-worsening muscle weakness that starts in the legs and pelvis, and later affects the whole body • No cure • Treatment is aimed at control of symptoms to maximize the quality of life

  7. Sample Problem: Male Pattern Baldness • Unlike all the other crosses there is information that we know before we ever read the problem. We know that a XX female is going to mate with a XY male. So, you can set up your square ahead of time. X Y X X

  8. Next, you have to read the problem and figure out what the sex-linked condition is and write down the allele you want to use for the problem and set up a key! Baldness is a sex-linked trait. The allele for baldness is recessive and is carried on the X chromosome. The Griffin family lives in a town called Quahog. The father, Peter, has a gene for normal hair. The wife, Lois, is a carrier for baldness. Peter and Lois have 3 kids: Chris, Meg, and Stewie. The kids want to know what is the probability they will develop baldness?

  9. Setting up your key • What letters will Peter have since he is a male? • What letters will Lois have since she is a female? • Decide what letter you will use for the trait • B = normal, b= bald

  10. Reread the problem and find the parents genotypes using the key you just made • So, Peter is XY and Lois is XX • Now you have to add the allele for baldness to all of the X chromosomes as described in the problem • Peter XBY • Lois XBXb

  11. Now, set up the Punnett square and do the cross…Don’t forget the genotype and phenotype ratios!

  12. XB Y XB Geno: ¼ XBXB ¼ XBXb ¼ XBY ¼ XbY Pheno: ¼ female, normal ¼ female, carrier ¼ male, normal ¼ male, bald Xb

  13. Answer the question: • What is the probability the children will develop baldness? • Females = 0% chance • Males = 50% chance

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