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Beyond Dominant & Recessive Alleles

Beyond Dominant & Recessive Alleles. Since Mendel’s work, scientists have learned that not all genes follow a simple inheritance pattern. There are some exceptions to Mendel’s principles!. Non-Mendelian Genetics. Multiple Alleles. When MORE than 2 alleles for a gene exist

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Beyond Dominant & Recessive Alleles

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  1. Beyond Dominant & Recessive Alleles • Since Mendel’s work, scientists have learned that not all genes follow a simple inheritance pattern. • There are some exceptions to Mendel’s principles!

  2. Non-Mendelian Genetics

  3. Multiple Alleles • When MORE than 2 alleles for a gene exist • Each individual organism can only carry 2 alleles (1 from mom & 1 from dad), BUT more than 2 alleles may exist for a gene! The gene for human blood type consists of 4 alleles (versions): A, B, AB, & O!

  4. Multiple Alleles

  5. Polygenic Traits • Traits that are determined by alleles from MORE than 1 gene • These traits usually have a range of phenotypes. Skin color & height in humans are both controlled by multiple genes!

  6. Incomplete Dominance • Cases in which there is no dominant allele • A heterozygous phenotype is somewhere in between the 2 homozygous phenotypes!

  7. Codominance • Both alleles contribute to the phenotype & can be observed in the offspring.

  8. Sex Determination • There are 2 types of chromosomes: • Autosomes: Of the 46 chromosomes, 44 of them (22 pairs) are non-sex chromosomes. • Sex Chromosomes: The last 2 chromosomes determine the sex of the person. Females have 2 X chromosomes (XX), while males have 1 X and 1 Y chromosome (XY).

  9. Sex Linked Traits • Traits that are determined by alleles that are found on the X or Y chromosome • The Y chromosome is shorter & does not carry the same genes as the X chromosome!

  10. Sex Linked Traits • Females are XX & males are XY. • Females can be homozygous or heterozygous for a trait carried on the X chromosome, but males (only having 1 X chromosome) are hemizygous.

  11. Sex Linked Traits • If males inherit a defective gene (on the sex chrom.) from a parent, they will express the trait, because they cannot inherit a second gene to mask it! A healthy male can’t “hide” a bad allele (even if it’s recessive), because they only have 1 X chromosome or 1 Y chromosome.

  12. Baldness is a sex linked trait (B=not bald, b=bald)! The baldness gene is located on the X chromosome. • A woman can be a carrier of the baldness allele, but she may not express it (if the other X chromosome carries the dominant allele). • Men inherit their single X chromosome from their mother, so if the mother is a carrier, she could pass that on to her son. • If inherited, a man will express the baldness allele, even though it’s recessive! This is why MOSTLY men are affected by sex linked traits (but not always).

  13. Sex Linked Punnett Square – Color Blindness (B=normal, b=color blind) • XBXb(heterozygous female with normal vision) • XBY (hemizygous male with normal vision)

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