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Beyond Dominant & Recessive Alleles Mendel had no knowledge of DNA, chromosomes, or meiosis. In fact, DNA wasnt even discovered until the 1950s, almost 100 years later! Since then, our knowledge of genetics has grown exponentially.

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Beyond Dominant & Recessive Alleles Since Mendels work, scientists have learned that not all genes follow a simple inheritance pattern. We have found many exceptions to Mendels rules. Most traits do NOT follow simple Mendelian patterns.

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Non-Mendelian Genetics

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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.

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Multiple Alleles

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Polygenic Traits Traits that are determined by MORE than 1 gene These traits usually have a range of phenotypes. Most human traits are complex & determined by several genes.

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Polygenic Traits Skin color, hair color, eye color, & height are all traits that are determined by several genes.

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Polygenic Traits So far, as many as 15 genes have been associated with contributing to human eye color!

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Incomplete Dominance Cases in which there is no dominant allele a heterozygous phenotype is somewhere in between the 2 homozygous phenotypes

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Codominance Both alleles contribute to the phenotype & can be observed in the offspring. X=

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Codominance X

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Incomplete Dominance or Codominance? X Incomplete DominanceCodominance

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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). Males have an X & a & chromosome (XY). Does this karyotype belong to a male or female?

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Sex Linked Traits Traits that are determined by alleles that are found on the X or Y chromosome The Y chromosome is smaller than the X chromosome. They are 2 different chromosomes & do not carry the same genes!

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Sex Linked Traits Females are XX & males are XY. Females can be homozygous or heterozygous for traits found on the X chromosome, because they get 2 copies of them. HOWEVER, males can only be hemizygous for traits found on the X chromosome, because they only get 1 copy.

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Sex Linked Traits If males inherit a defective gene on a sex chromosome (X or Y) from a parent, they will express the trait, because they cannot inherit a second gene to mask it. Remember that the X & Y chromosomes carry different genes. SO, a male cant hide a bad allele, even if its recessive, because they only have 1 copy of that gene. Bad allele

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Baldness is a sex linked trait (recessive) on the X chromosome. B = not bald, b = bald A woman can be a carrier of the bald allele, but she may not express it if shes heterozygous. Men inherit their single X chromosome from their mother. So if the mother is a carrier, she could pass that on to her son (50% chance). Bb b If inherited, a man will express the bald allele, even though it is recessive (he is hemizygous). This is why mostly men are affected by sex linked traits (but not always)!

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With sex linked traits, we use XX or XY in the genotype to show that the gene is on the sex chromosomes. Mom is a carrier for hemophilia, which is located on the X chromosome & recessive. So, moms genotype is X R X r. Dads genotype is X R Y. She is heterozygous, so she is not affected by the disease. She may or may not pass this allele on to her offspring. If she passes this allele on to her son, the son will have hemophilia. XRYXRYXRXrXRXr

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Lets say mom is a carrier for color blindness (recessive). This is a sex linked trait found on the X chromosome. She has normal vision, because shes heterozygous & has 1 good allele on the other X chromosome. There is a 50% chance that she will pass on the color blind allele to her son. If her son inherits the color blind allele, he will be color blind, even though the allele is recessive.

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Sex Linked Punnett Square Color Blindness (B=normal, b=color blind) X B X b (heterozygous female with normal vision) X B Y (hemizygous male with normal vision)

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Blood Typing

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In humans, blood type is determined by the Rh blood group & the ABO blood group. The Rh blood group determines if your blood is positive or negative. Rh + is the dominant allele; Rh - is recessive. If your blood is positive, this just means that there are certain markers on your RBCs. If its negative, the markers are absent.

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The ABO Blood Group When determining the ABO blood group, there are 3 alleles: I A, I B, and i. I A & I B are codominant. They cause the expression of antigens on the surface of RBCs. The i allele is recessive to I A & I B. Produces O blood with no antigens. Antigens

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Antigens are proteins on the surface of your red blood cells. If you have type A blood, you have A antigens. If you are type AB, you have A & B antigens. If you are type O, you dont have any antigens.

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Antigens Markers on the surface of cells that could trigger your immune system to attack Your body knows the antigens on your own cells, so your immune system wont attack your own cells. If a foreign antigen enters your body, your immune system reacts & your body will attack!

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Blood Cell Antigens If the body has never been exposed to an blood cell antigen (A or B) & that antigen enters the body, it will cause an immune reaction. This means that your body will begin attacking those blood cells with the foreign antigen, which is deadly. Why is this important when it comes to blood transfusions?

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Blood Transfusions In emergency rooms, if theres not enough time to find out the blood type of the patient, the patient will receive type O blood. WHY? Type O blood cells have no A or B antigens. The body will not attack the blood cells if there are no antigens. People with blood type O are considered universal donors.

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Blood Transfusions People with AB blood can receive any blood type. Their bodies all ready contain A & B antigens. Theyre the universal recipient.

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Mother Father What is the mothers blood type? Blood Type B What is the chance that their offspring will be Type AB? 50%

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What influences characteristics more, genes or the environment? BOTH

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External & internal factors can affect genes. External: while genes influence the height of a plant, the amount of water, sun & other climate conditions will also affect the height

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External & internal factors can affect genes. Internal: there are recent findings that proteins involved with DNA can turn genes on/off based on environmental factors Certain chemical exposure can turn genes on/off for generations after exposure, but there are no mutations. This new understanding of how genes are expressed is called epigenetics.

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Epigenetics Example Females are born with all of the egg cells theyll ever have. In fact, females start producing egg cells while theyre still in the embryo. Female embryos can have up to 7 million germ cells (early eggs). So, your journey began when your grandmother was still pregnant with your mother! Fetal ovary w/ early egg cells.

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BLAME IT ON GRANDMA