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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 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)

  14. Mapping Genes • It’s easy to imagine that genes on different chromosomes assort independently, but what about genes that occur on the same chromosome? Don’t they always appear together? If the genes for eye & hair color were on the same chromosome, would we always get the same combinations?

  15. Mapping Genes • Not always due to crossing over! Genes that occur together on a chromosome will be separated when homologous chromosomes exchange genes. • The frequency of genes occurring together can help us create a gene map!

  16. Mapping Genes • The more often 2 genes occur together, the closer they are to each other on the chromosome. • If the genes are never separated by crossing over, they always occur together. All offspring will look like 1 of the parents (in reference to the genes in question). The closer 2 genes are to each other on the chromosome, there’s less of a chance that they’ll be separated during crossing over.

  17. A--B---------------C • For example, here’s a chromosome with genes A, B, & C. • Gene A: red hair • Gene B: freckles • Gene C: height Where is the enzyme most likely to cut this chromosome during crossing over? Genes A & B are very close together on the chromosome, so there’s less of a chance that the enzyme will cut between them!

  18. Why do most people with red hair also have freckles? The genes are linked! They hardly ever separate during crossing over, so you always see the 2 genes together!

  19. Mapping Genes • If half of the offspring are parental & half are recombinants of the parents (in reference to the genes in question), then the genes are said to be independent. • This means they are either on separate chromosomes or they are always separated during meiosis.

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