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Extensions on Mendelian Genetics

Extensions on Mendelian Genetics. Genotype vs. Phenotype. Since an individual has two copies of each gene, a heterozygous individual carries the recessive gene without showing it. This is referred to as a carrier . Some human disorders are caused by recessive alleles (ex: cystic fibrosis).

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Extensions on Mendelian Genetics

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  1. Extensions on Mendelian Genetics

  2. Genotype vs. Phenotype • Since an individual has two copies of each gene, a heterozygous individual carries the recessive gene without showing it. • This is referred to as a carrier. • Some human disorders are caused by recessive alleles (ex: cystic fibrosis). • Other disorders, such as Huntington’s disease, are caused by dominant alleles. • Which one do you think is more common?

  3. Cystic Fibrosis

  4. Sex-Linked Genes • Genes located on sex chromosomes (X or Y) are called sex-linked genes. • Y chromosome is smaller and has fewer genes • Mostly just for sex determination • Females are XX; males are XY • Males express all genes from both chromosomes because there is not another copy to “mask” recessive traits

  5. Color Blindness • Recessive trait on the X chromosome • Females need two copies to express the trait • Males only need one copy

  6. Practice • The gene for colorblindness is carried on the X chromosome and is recessive. A colorblind man marries a woman of normal vision. They have a daughter who is colorblind. • What are the genotypes of each parent? • What is the daughter’s genotype? • Show a Punnett square predicting the possible genotypes and phenotypes that could be produced if the couple has another child.

  7. Practice • Hemophilia is carried on the X chromosome and is recessive to normal blood type. Cross a carrier female to an affected man. • What percentage of the offspring will be carrier females? • What percentage will be normal males? • What percentage will be hemophiliac females? • How many genotypes are possible among the offspring?

  8. Complex Inheritance Patterns • Sometimes, phenotype results as an interaction between both alleles • Incomplete dominance • Codominance

  9. Incomplete Dominance • Both traits are mixed in a heterozygous offspring • Example: four o’clock plant • Homozygous red x homozygous white results in pink offspring

  10. Incomplete Dominance Practice • In radishes, the gene that controls color exhibits incomplete dominance. Pure-breeding red radishes (RR) crossed with pure-breeding white (WW) radishes make purple radishes (RW). What are the genotypic and phenotypic ratios when you cross a purple radish with a white radish?

  11. Codominance • Both alleles are expressed completely • Homozygous red crossed with homozygous white would result in red and white spotted offspring • Example: roan cows

  12. Codominance Practice In shorthorn cattle hair color is decided by a pair of codominant traits. R is the allele for red hair color and W is the allele for white hair color. A cow with the heterozygous genotype is roan in color, meaning its coat contains both white and red hairs. What would be the expected phenotypes of the offspring if a white cow is bred with a roan cow?

  13. Polygenic Traits • Traits produced by more than one gene • Skin color, eye color, blood type • Blood type is polygenic andcodominant!

  14. Blood Type • A man with type AB blood marries a woman with type B blood. Her mother has type O blood. List the expected phenotype & genotype frequencies of their children.

  15. Blood type Practice • What are the expected genotypes and phenotypes of the offspring if the father is type A heterozygous and the mother is type B heterozygous. • What are the expected genotypes and phenotypes of the offspring if the father is type O and the mother is type AB? • Ralph has type B blood and his wife Rachel has type A blood. They are very shocked to hear that their baby has type O blood, and think that a switch might have been made at the hospital. Can this baby be theirs? Explain why or why not (use a Punnett square to help).

  16. Pedigrees • Pedigrees help trace phenotypes and genotypes in a family

  17. Pedigrees • Pedigrees can be analyzed to help you answer several different questions: • Is this trait dominant or recessive? • Recessive traits will be less common • Is this trait autosomal or sex linked? • Autosomal traits should affect equal numbers of males and females • Sex linked traits will affect more males

  18. Hemophilia in the Royal Family

  19. Practice • What are the genotypes of individuals 1 and 2? • How many of their offspring are colorblind? • Are any carriers? • What is the genotype of individual 3? How do you know? 1 2 3

  20. Karyotyping • A karyotype is a picture of all the chromosomes in a cell • Chromosome mapping can help diagnose genetic disorders such as Down syndrome (having an extra copy of the 21st chromosome) or Klinefelter’s (XXY)

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