Beyond Mendel. Extending Mendelian Genetics. Incomplete Dominance. Think about this. Beyond Mendel. Chapter 12

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1 Beyond Mendel Extending Mendelian Genetics Chapter 12 Mendel s work did, however, provide a basis for discovering the passing of traits in other ways including: Incomplete Dominance Codominance Polygenic Traits Sex-Linked Traits Think about this You are walking around outside and you notice a bush with two distinctly colored flowers: red and white. However, you notice a pink flower here and there. Is this possible considering what you have just learned? If not, what do you think is happening? Incomplete Dominance Incomplete Dominance is when one allele is not completely dominant over the other. Thus, the heterozygous phenotype is a blend of the effects of the two alleles. The pink flower is a common example of incomplete dominance. Beyond Mendel Mendel s discoveries obviously did not work in this case and in many others. Mendel s work focused on traits that were either/ or traits-- they would be round or wrinkled, red or white. He didn t focus on anything that didn t follow such a pattern. Is this a wrinkled or round seed? RR WW RW RW RW RW White is not the recessive trait therefore, we simply use different letters to represent the two colors 1

2 Punnett Squares and Incomplete Dominance In snapdragons, when crossing a red flower(rr) with a white flower(ww), a pink flower(rw) can be produced. The alleles that control this trait in snapdragons are incompletely dominant. Other Codominance Examples Cross a pink snapdragon with a red snapdragon to find the probable genotypes and phenotypes. Use R for the red allele and W for the white allele. Punnett Squares and Codominance In chickens, crossing a black feathered chicken with a white feathered chicken can result in black and white speckled chicken. Cross a black feathered chicken with a white feathered chicken to find the probable genotypes and phenotypes. Label the allele for black feathers as (B) and the allele for white feathers as (W). Codominance is when both alleles contribute to the resulting phenotype. For example, some chickens have both black and white feathers. Codominance 2

3 Multiple Alleles Multiple alleles indicate that there may be more than two possible alleles for a certain trait in a population. Coat color in rabbits is an example Blood Type in humans is another example Blood Typing Activity Go to the following website to play a game on blood typing dicine/landsteiner/index.html Multiple Alleles and Blood Typing What is blood-typing? Determining what blood type you are. Depends on the presence or absence of specific proteins (antigens) on your red blood cells. Governed by multiple alleles. More than two alleles exist for blood types. (A, B & O) Human blood types: A, B, AB, or O A and B are both dominant alleles; this is also an example of codominance. O = recessive allele. Polygenic Traits Polygenic traits are physical characteristics that are determined by more than one gene. Examples: eye color, skin color (generally any trait that has several variations) Chart of Blood Types Phenotype is a combination of genotype and environment Examples: sea turtles male or female height amount of nutrition hair color effects of sunlight Universal Receiver Universal Donor 3

4 Sex-Linked Traits Sex-linked traits are traits that are passed through genes located on the X and Y chromosomes. (usually the X chromosome!) Some classic examples are color blindness and hemophilia. An example of a test used to determine color blindness is pictured here. Sex-linked Traits: Problems Working out a sex-linked traits problem is similar to completing a monohybrid Punnett square. However, instead of using only capital and lowercase letters, we must also include X and Y to represent female and male. Example of a sex-link cross: X r Y male r X R femalex X R X r X R Y X r X r X r Y The allele for a trait is always shown as a superscript letter on the X chromosome The Y chromosome never has a superscript letter with it! Sex Determination Recall: How many pairs of chromosomes do humans have? 22 pairs of our chromosomes are autosomes. These chromosomes contain genetic information for somatic (body) cells. The last pair of chromosomes, our 23 rd pair, are our sex chromosomes. Sex chromosomes are designated with the letters, X and Y. Female sex chromosomes = XX Male sex chromosomes = XY Thomas Hunt Morgan In 1910, discovered traits linked to sex chromosomes. Studied fruit flies found a male fruit fly with white eyes rather than the normal red. Crossed the white-eyed male fly with a homozygous redeyed female fly. All of the F1 offspring had red eyes, indicating that white eyes was a recessive trait. Click to watch video Sex Determination, cont. Knowing the sex chromosomes for males and females allows us to determine the chances of having a male or female child. X Y X X XX XX XY XY There will always be: 50% chance of a female child 50% chance of a male child Thomas Hunt Morgan, cont. As Morgan continued to study fruit flies, he found that when he crossed flies from the F1 generation, white eyes only appeared in males; thus he hypothesized that: Red eyes are dominant to white eyes. The gene for eye color was located on the X chromosome and not the Y chromosome. Because heterozygous females had two X chromosomes, the dominant allele for red eyes masked the recessive allele for white eyes. Males, however, only had one X chromosome; thus if males were white-eyed, they had inherited the X chromosome carrying the white-eyed gene. 4

5 Morgan s Experiments -- First Cross: Red-eyed female (X R X R ) x white-eyed male (X r Y) Pedigrees A pedigree chart is used to trace the phenotypes and genotypes throughout a family. Boxes = males Circles = females Shaded shape = person shows the trait White shape = person doesn t show trait Half-shaded = person is a carrier -- Second Cross: used offspring from first cross X R X r x X R Y Sex-Linked Traits Morgan s discoveries led to the concept of sex-linked traits. Sex-linked traits are traits controlled by genes located on sex chromosomes. Karyotypes A karyotype is a picture of all the chromosomes in a cell. Females are carriers of sex-linked traits if they have the heterozygous genotype. (X R X r = normal carrier) Female parents who are carriers can pass sex-linked traits to children, but males are usually the ones who express the trait. Males cannot be carriers because they only have one X chromosome. (X R Y = normal X r Y = colorblind) X Y 5

6 Karyotypes can show changes in chromosomes. deletion or loss of chromosome extra chromosomes or duplication of part of a chromosome deletion duplication 6