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Ch 14-15 Review Questions

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Ch 14-15 Review Questions Genetics II Answered Review Questions 1. Explain the incomplete dominance inheritance pattern. “Alleles can show different degrees of dominance and recessiveness in relation to each other. We refer to this range as the spectrum of dominance. One extreme on this spectrum is seen in the F1 offspring of Mendel’s classic pea crosses. These F1 plants always looked like one of the two parental varieties because of the complete dominance of one allele over another. In this situation, the phenotypes of the heterozygote and the dominant homozygote are indistinguishable.” “The alleles for some characters fall in the middle of the spectrum of dominance. In this case, the F1 hybrids have a phenotype somewhere in between the phenotypes of the two parental varieties. This phenomenon, called the incomplete dominance of either allele, is seen when red snapdragons are crossed with white snapdragons: All the F1 hybrids have pink flowers (see figure below). This third phenotype results from flowers of the heterozygotes having less red pigment than the red homozygotes (unlike the situation in Mendel’s pea plants, where the Pp heterozygotes make enough pigment for the flowers to be a purple color indistinguishable from those of PP plants).” (Text quoted from page 260 of the textbook) With incomplete dominance, there is no dominant and recessive trait. As a result, there are three different phenotypes. The ratio for two hybrids parents would be 50% like the parents, 25 % with one homozygous trait expressed and 25% with the other homozygous trait expressed. Looking at snapdragon flower color, if both parents had a pink flower, we would expect 50% of their offspring to have the pink color flower (heterozygous pink), 25% would be expected to have a red color (homozygous red) and we would expect 25% to have the white flower color (homozygous white). The genotype ratios for a cross between two incompletely dominant parents would be the same as the phenotype ratio. Going back to the snapdragon flower color, we would expect 25% of the offspring to be homozygous red, 50% to be heterozygous pink and 25% to be homozygous white. This is a 1:2:1 ratio the same as the phenotype ratio for incomplete dominance. 2. Explain the codominance inheritance pattern. With codominance, there are two not one dominant alleles and each one dominates over the recessive allele but not each other. With incomplete dominance there is neither a dominant nor recessive allele but two different homozygous individuals. When a heterozygous condition exists both alleles are expressed. This is similar to codominance in a way, but unlike codominance, neither allele is dominant or recessive. 3. How is ABO blood type an example of the codominance inheritance pattern? “The ABO blood group in humans, for instance, is determined by multiple alleles of a single gene. There are four possible phenotypes for this character: A person’s blood group may be either A, B, AB, or O. These letters refer to two carbohydrates—A and B—that may be found on the surface of red blood cells. A person’s blood cells may have carbohydrate A (type A blood), carbohydrate B (type B), both (type AB), or neither (type O), as shown schematically below.”(Text quoted from page 262 of the textbook) “Not only are the ABO blood groups determined by multiple alleles, the allele for A dominates over O and the allele for B dominates over O, but neither A nor B dominate over each other. They are considered codominant.” 4. What are the genotypes for the following? A. Type A blood Individuals with type A blood may be homozygous for type A blood (two alleles for A) or heterozygous for type A blood (one allele for A and one allele for O) B. Type B blood Individuals with type B blood may be homozygous for type B blood (two alleles for B) or heterozygous for type A blood (one allele for B and one allele for O) C. Type AB blood Individuals with type AB blood have one A allele and one B allele. D. Type O blood Individuals with type O blood have two O alleles. 5. If a woman has type AB blood, could she be the biological mother of a child with type O blood? Explain your answer. Since the woman’s blood type is AB she would have alleles for A and B. She could produce eggs containing either an allele for A or an allele for B. Since both the A and B alleles dominate over the allele for O she could either be the biological mother of an individual with either type A or Type B blood. It would not be possible for her to be the biological parent of a child with type O blood. 6. Explain the polygenic inheritance pattern. Many genetic traits are controlled by many genes. Polygenic traits act like incomplete dominance. There is no dominant or recessive. The effect of each gene and each allele is additive. For example, kernel color in wheat ranges from white through shades of pink to dark red. The color is dictated by three genes (ABC). Imagine that there is a light allele (abc) and a dark allele for each gene (ABC). The more dark pigment alleles present in the genotype the more red the kernel. Likewise, the more light pigment alleles in the genotype makes the kernels whiter. The trait is continuous. The more genes that are involved in a trait the less of a difference there is between phenotypes. Genotype Phenotype AABBCC Dark red AaBBCC or AABbCC or AABBCc Red aaBBCC or AaBbCC or AAbbCC or AABbCc or AABBcc etc… Dark pink aabBCC or AabbCC or AAbbCc or AABbcc etc.. Pink aabbCC or AAbbcc or aaBBcc etc… Light pink aabbCc or Aabbcc or aaBbcc etc… Pinkish-white aabbcc White Polygenic inheritance is a common inheritance pattern for many traits like: skin color, eye color, height, weight, temperament, personality, intelligence, heart disease, mental illness, diabetes etc… 7. Explain multifactorial inheritance. Many traits, especially polygenic traits, have an environmental component. For example, one member of a set of identical twins may have schizophrenia while the other is perfectly normal. If schizophrenia was completely genetic, then both twins would have the disease. Twin studies are often used to determine how much of given trait is genetic and how much is environment. Researchers compare the inheritance in identical twins with fraternal twins. If the chances that identical twins share a trait are greater than fraternal twins, then the trait has a genetic component. 8. Who associated genes with chromosomes and what specific experimental organism did this person use. What characteristics made this organism convenient for genetic studies? “For his work, Morgan selected a species of fruit fly, Drosophila melanogaster, a common, generally innocuous insect that feeds on the fungi growing on fruit. Fruit flies are prolific breeders; a single mating will produce hundreds of offspring, and a new generation can be bred every two weeks. These characteristics make the fruit fly a convenient organism for genetic studies. Morgan’s laboratory soon became known as ‘the fly room.’ Another advantage of the fruit fly is that it has only four pairs of chromosomes, which are easily distinguishable with a light microscope. There are three pairs of autosomes and one pair of sex chromosomes. Female fruit flies have a homologous pair of X chromosomes, and males have one X chromosome and one Y chromosome.” (Text quoted from page 276 of the textbook) 9. How many pairs of chromosomes does the fruit fly have? Which sex chromosomes are found in a female Drosophila? Which sex chromosomes are found in a male Drosophila? The fruit fly has only 4 pairs of chromosomes. A female has two X chromosomes while a male fruit fly has an X and a Y chromosome. 10. What does the term “wild type” phenotype refer to with the Drosophila? What is the variant to the “wild type” called? “The normal phenotype for a character (the phenotype most common in natural populations), such as red eyes in Drosophila, is called the wild type. Traits that are alternatives to the wild type, such as white eyes in Drosophila, are called mutant phenotypes because they are due to alleles assumed to have originated as changes, or mutations, in the wild-type allele.” (Text quoted from page 276 of the textbook) 11. On which chromosome did Morgan conclude that eye color was located in the Drosophila? “Morgan’s finding of the correlation between a particular trait and an individual’s sex provided support for the chromosome theory of inheritance: namely, that a specific gene is carried on a specific chromosome (in this case, the eye-color gene on the X chromosome). In addition, Morgan’s work indicated that genes located on a sex chromosome exhibit unique inheritance patterns, which we will discuss later in this chapter. Recognizing the importance of Morgan’s early work, many bright students were attracted to his fly room.” (Text quoted from page 276 of the textbook) What is a sex-linked trait? What are some examples of X-linked traits in humans? “In addition to their role in determining sex, the sex chromosomes, especially X chromosomes, have genes for many characters unrelated to sex. A gene located on either sex chromosome is called a sex-linked gene, although in humans the term has historically referred specifically to a gene on the X chromosome. (Note the distinction between the terms sex-linked gene, referring to a gene on a sex chromosome, and linked genes, referring to genes on the same chromosome that tend to be inherited together.) Sex-linked genes in humans follow the same pattern of inheritance that Morgan observed for the eye-color locus in Drosophila.
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