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. Fathers pass sex-linked alleles to all of their daughters but to none of their sons. In contrast, mothers can pass sex-linked alleles to both sons and daughters (see figure below).” (Text quoted from page 283 of the textbook) Examples of sex-linked traits in humans are: color blindness, Duchenne muscular dystrophy, Hemophilia.
Neither Jon nor Pam is color blind, but their son Phillip is color blind. What is the probability that this couple could have another son who is also color blind? From which parent did Phillip inherit the gene for colorblindness? Since Jon has normal color vision we know that his genotype is XAY. We also know that Pam must be heterozygous since she has a son Phillip with color blindness. Thus Pam is XAXa. Since Phillip is color blind we know his genotype is Xa Y. Now let’s look at the possible offspring using a Punnett square. XA Xa XA XA XA XA Xa Y XA Y Xa Y
From the Punnett square on the left we expect two of the children to be male. Of these male children we expect one to be color blind. Thus ½ of the male children are expected to be color blind. ½ is 50%.
Since the male child receives the Y chromosome from his father, male individuals inherit the recessive allele for color blindness from their mother since the allele is on the X chromosome and the male individual receives the X chromosome from his mother.
Why are males more likely to be affected by X-linked traits? “If a sex-linked trait is due to a recessive allele, a female will express the phenotype only if she is a homozygote. Because males have only one locus, the terms homozygous and heterozygous lack meaning for describing their sex- linked genes (the term hemizygous is used in such cases). Any male receiving the recessive allele from his mother will express the trait. For this reason, far more males than females have sex-linked recessive disorders. However, even though the chance of a female inheriting a double dose of the mutant allele is much less than the probability of a male inheriting a single dose, there are females with sex-linked disorders. For instance, color blindness is a mild disorder inherited as a sex-linked trait. A color-blind daughter may be born to a color-blind father whose mate is a carrier. However, because the sex-linked allele for color blindness is relatively rare, the probability that such a man and woman will mate is low.” Text quoted from page 283 of the textbook)
What is a Barr Body? List an example of x-chromosome inactivation in mammals. A Barr body is only found in the nucleus of female somatic cells. The small black dot is an inactivated X chromosome. Which X chromosome of each allelic pair that inactivates is unpredictable.
“The tortoiseshell gene is on the X chromosome, and the tortoiseshell phenotype requires the presence of two different alleles, one for orange fur and one for black fur. Normally, only females can have both alleles, because only they have two X chromosomes. If a female is heterozygous for the tortoiseshell gene, she is tortoiseshell. Orange patches are formed by populations of cells in which the X chromosome with the orange allele is active; black patches have cells in which the X chromosome with the black allele is active. (“Calico” cats also have white areas, which are determined by yet another gene.) “(Text quoted from page 284 of the textbook)
How many autosomes should a normal human somatic cell have? How many autosomes should a normal human gamete have? A normal human somatic cell is a diploid cell. Human diploid cells typically have 23 pair of homologous chromosomes. A normal human gamete is a haploid cell so there should be only 22 chromosomes present in the normal human gamete.
How many sex-chromosomes should a normal human somatic cell have? Which sex-chromosomes are present in a normal male? Which sex- chromosomes are present in a normal female? Since the somatic cell is normal, there should be one pair of sex chromosomes and 22 pair of autosomes in this somatic cell. In a normal human sperm, there would only be one sex chromosome. Since the human male determines the sex of the individual, the sperm could either contain an X chromosome or it could contain the y chromosome. For the human egg, there would only be one X chromosome if the egg were normal.
Explain nondisjunction. “Ideally, the meiotic spindle distributes chromosomes to daughter cells without error. But there is an occasional mishap, called a nondisjunction, in which the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II. In these cases, one gamete receives two of the same type of chromosome and another gamete receives no copy (see figure below).The other chromosomes are usually distributed normally. If either of the aberrant gametes unites with a normal one at fertilization, the offspring will have an abnormal number of a particular chromosome, a condition known as aneuploidy.” (Text quoted from page 285 of the textbook) How is monosomy (monosomic condition) different from trisomy (trisomic condition)? How can these conditions occur? “If a chromosome is present in triplicate in the fertilized egg (so that the cell has a total of 2n + 1 chromosomes), the aneuploid cell is said to be trisomic for that chromosome. If a chromosome is missing (so that the cell has 2n − 1 chromosomes), the aneuploid cell is monosomic for that chromosome. Mitosis will subsequently transmit the anomaly to all embryonic cells. If the organism survives, it usually has a set of symptoms caused by the abnormal dose of the genes associated with the extra or missing chromosome. Nondisjunction can also occur during mitosis. If such an error takes place early in embryonic development, then the aneuploid condition is passed along by mitosis to a large number of cells and is likely to have a substantial effect on the organism.” (Text quoted from page 285 of the textbook)
Which chromosome is normally involved with Down’s syndrome? “Down syndrome is usually the result of an extra chromosome 21, so that each body cell has a total of 47 chromosomes. Because the cells are trisomic for chromosome 21, Down syndrome is often called trisomy 21. Down syndrome includes characteristic facial features, short stature, heart defects, susceptibility to respiratory infection, and mental retardation. Furthermore, individuals with Down syndrome are prone to developing leukemia and Alzheimer’s disease. Although people with Down syndrome, on average, have a life span shorter than normal, some live to middle age or beyond. Most are sexually underdeveloped and sterile. Which chromosomes are involved and how many chromosomes are typically present for the following conditions: Turner syndrome, Klinefelter syndrome, Trisomy X, Jacob’s syndrome? What is the cause for these conditions? “Nondisjunction of sex chromosomes produces a variety of aneuploid conditions. Most of these conditions appear to upset genetic balance less than aneuploid conditions involving autosomes. This may be because the Y chromosome carries relatively few genes and because extra copies of the X chromosome become inactivated as Barr bodies in somatic cells.” (text quoted from page 287 of the textbook)
An extra X chromosome in a male, producing XXY, occurs approximately once in every 2,000 live births. People with this disorder, called Klinefelter syndrome, have male sex organs, but the testes are abnormally small and the man is sterile. Even though the extra X is inactivated, some breast enlargement and other female body characteristics are common. The affected individual is usually of normal intelligence. Males with an extra Y chromosome (XYY) do not exhibit any well-defined syndrome, but they tend to be somewhat taller than average. Females with trisomy X (XXX) are called poly-x females. Poly-x females occur once in approximately 1,000 live births, are healthy and cannot be distinguished from XX females except by karyotype. Monosomy X, called Turner syndrome, occurs about once in every 5,000 births and is the only known viable monosomy in humans. Although these X0 individuals are phenotypically female, they are sterile because their sex organs do not mature. When provided with estrogen replacement therapy, girls with Turner syndrome do develop secondary sex characteristics. Most have normal intelligence.” (Text quoted from page 287 of the textbook)
Identify organisms which have the following chromosomal basis of sex- determination: A. X-Y system “In mammals, the sex of an offspring depends on whether the sperm contains an X chromosome or a Y. (Text quoted from page 282 of the textbook) B. X-O System “In grasshoppers, roaches and some other insects, there is only one type of sex chromosome, the X. Females are XX ; males have only one X chromosome (XO). Sex of the offspring is determined by whether the sperm has an x chromosome or no sex chromosome. .“(Text quoted from page 282 of the textbook) C. Z-W System “In birds, some fishes and some insects, the sex chromosome present in the ovum (not the sperm) determines the sex of offspring. The sex chromosomes are designated Z and W. Females are ZW and males are ZZ.“(Text quoted from page 282 of the textbook) D. Haplo-diploid system “There are no sex chromosomes in most species of bees and ants. Females develop from fertilized ova and thus are thus diploid. Males develop from unfertilized egg sand are haploid. They have no fathers” (Text quoted from page 282 of the textbook)