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Mendelian Exceptions

Mendelian Exceptions. Not all genes show simple patterns of dominant and recessive alleles Because the majority of traits are controlled by more than one gene/alleles. Cases in which both alleles are expressed White cow x Brown cow = Speckled (white and brown) cow. Codominance.

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Mendelian Exceptions

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  1. Mendelian Exceptions • Not all genes show simple patterns of • dominant and recessive alleles • Because the majority of traits are controlled by more than one gene/alleles

  2. Cases in which both alleles are expressed White cow x Brown cow = Speckled (white and brown) cow Codominance

  3. Neither allele is dominant Red flower x White flower = Pink flower Incomplete dominance

  4. Practice Problems • In a certain cactus, prickly spines can be two pronged or one pronged. If a true breeding (homozygous) one-pronged cactus is crossed with a true breeding two-pronged cactus, the F1 generation has a mixture of spines, some are two-pronged, some are one-pronged. • Is this an example of codominance or incomplete dominance? codominance

  5. In snapdragons, the combined expression of both alleles for flower color produces a new phenotype that is pink. This illustrates incomplete dominance. The Punnett square above shows that both the white and red snapdragons are homozygous. Which of the following would be the correct product from a cross between two heterozygous pink snapdragons? [VA05 EOC] • A 1 red, 2 pink, 1 white • B 2 red, 2 white • C 2 red, 1 pink, 1 white • D 1 red, 1 pink, 2 white

  6. Sex-linked Genetics

  7. Sex Determination X Y Male Section 12.2 Summary – pages 315 - 322 • If you are female, your 23rd pair of chromosomes are homologous, XX. XY Male X Y X XX Female XY Male X X XX Female Female X • If you are male, your 23rd pair of chromosomes XY, look different. XY Male XX Female

  8. Section 12.2 Summary – pages 315 - 322 Sex-linked inheritance • Traits controlled by genes located on sex chromosomes are calledsex-linked traits. • The alleles for sex-linked traits are written as superscripts of the X or Y chromosomes.

  9. Sex Linked Traits • When genes are located on the X chromosome, females receive two alleles for these genes, but males only receive one. • In males, the genotype is automatically known. • Colorblind male - XbY (recessive) • Normal male - XBY (dominate). • A female can be: • XBXB - normal • XBXb - carrier • XbXb - colorblind

  10. The following shows a cross between a normal man and a woman who is a carrier.

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