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Name: _____________________ Date: _________________________ SOLUTIONS Chapter 15 – Chromosomes and Heredity Section A: Linked and Sex-linked Genes 1. Who were the scientists in 1902 that discovered that the behavior of chromosomes during meiosis was related to the behavior of “factors” or alleles in Mendel’s experiments? Walter Sutton and Theodor Boveri 2. Linked Genes are those genes that are located on the same chromosome. A a. Would you expect the results of breeding experiments to follow those expected from Mendel’s laws? Explain why or why not. Mendel ran simple breeding experiments where alleles for different characteristics were located on separate chromosomes (e.g seed colour and seed texture are on different chromosomes). Therefore, the alleles assorted independently of one another. If alleles are located on the same chromosome (and are close together), then they will be inherited together, as opposed to independently. Therefore, the results of breeding experiments involving linked genes will not coincide with the results of Mendel’s experiments. b C b. Based upon what you know about genetics so far, what event or process during meiosis might occasionally “break” the physical connection between genes on the same chromosome? Crossing over, which occurs during Prophase I in Meiosis occasionally breaks the physical connection between genes on the same chromosome. (Crossing over segments from the paternal non-sister chromatid trade places with segments from the maternal non-sister chromatid.) c. In reality, the genes for seed colour and seed shape in peas are said to be linked (i.e. on the same chromosome). Why do the alleles for seed colour and seed shape always segregate and assort independently? The alleles for seed colour always segregate and assort independently of the allele for seed shape because the distance between these genes on the chromosome is so great, that crossing over almost always occurs. In general, the probability of crossing over is proportional to the distance between genes on the same chromosome. This means that the greater the distance between genes, the higher the probability of crossing over and the lesser the distance between genes, the lower the probability of crossing over. 3. Define: sex-linked inheritance. Why are very few Y-linked traits known? Sex-linked inheritance is the transmission of genes that are located on one of the sex chromosomes. Few Y-linked traits are known because the Y chromosome is a lot smaller than the X, thus containing less genes. 4. Explain two reasons why fruit flies are excellent experimental organisms for Morgan? 1. Fruit flies are prolific breeders – a single mating can produce hundreds of offspring and a new generation can be bred every two weeks 2. Fruit fly genome – only has four pairs of chromosomes that are easily distinguishable under a microscope 5. In Drosophila flies, describe the genotype and phenotypes of the mutant and wild-type flies for eye colour only. Trait(s) G/T Phenotype Genotype Phenotype Genotype Eye Colour Body Colour and Wing Type Wild-Type (Dominant) Red Eyes + w w+ or w+ w Normal colour and wings bl+ bl+ vg+ vg+ Mutant (Recessive) White Eyes ww Black, vestigial wings bl bl vg vg 6. Explain the full experiment and results that Morgan used which proved that some genes are sex-linked. Use Punnett Squares to demonstrate the crosses. Use appropriate genotype notation. F2 Generation F1 Generation Xw Y Xw+ Xw+ Xw Xw+ Y Xw+ Xw+ Xw Xw+ Y Xw+ Y Xw+ Xw+ Xw+ Xw+ Y Xw Xw+ Xw Xw Y All female have red eyes Eye colour gene is located on X, no locus for eye colour on Y In Morgan’s first cross (when he mated red eyed female with a white eyed male), all offspring has red eyes (both males and females). This led him to conclude that red eyes was dominant over white eyes. Then, he took male and female offspring from this cross (F1) and mated them. The result was the typical 3:1 ratio of dominant (red) to recessive (white) phenotypes. He noticed that no female had white eyes, and only half of the males had white eyes. If eye colour was not sex-linked, then you would expect there to be 50% female white eyes and 50 % male. Therefore, Morgan concluded that the locus for eye colour was located on the X chromosome only. It is possible for females to have white eyes, but their father must have white eyes and their mother must either be a carrier or have white eyes as well. The white eyed trait is passed down through transmission of X chromosomes, not Y. 7. If females have two X chromosomes and males only have one, why is it that both males and females produce the same amounts of proteins coded by genes on X chromosomes? Males and females produce the same amount of proteins coded for by genes on X chromosomes because one of the X chromosomes in females is inactivated and becomes a BARR BODY (which one that becomes inactivated is random). The X chromosome will become reactivated when a cell turns into an egg cell (meiosis). Section B – Patterns of Inheritance and Nondisjunction 1. Based upon the readings, hypothesize the genotypes of individuals with genetic diseases for each type of inheritance pattern. Assume allele A is dominant over allele a. a. Autosomal Dominant i. disorder: AA, Aa ii. normal: aa c. X-Linked Dominant i. disorder: MALE: XAY FEMALE: XAXA, XAXa ii. normal: MALE: XaY FEMALE: XaXa b. Autosomal Recessive i. disorder: aa ii. normal: Aa, AA iii. carrier: Aa d. X-Linked Recessive i. disorder: MALE: XaY FEMALE: XaXa ii. normal: MALE: XAY FEMALE: XAXA, XAXa iii. carrier: FEMALE: XAXa 3. Some genetic diseases are caused by nondisjunction. a. Define nondisjunction. When can non-disjunction occur during meiosis? Nondisjunction occurs when members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II. Specifically, this can occur during anaphase I or anaphase II. b. Hypothesize the meaning of: trisomy and monosomy. Normal individuals, following interphase, have 2 of each chromosome Trisomy – the condition of having an extra chromosome (3) Monosomy – the condition of having one less chromosome (1) Meiosis I Nondisjunction Meiosis II Nondisjunction Gametes n+ n+ n1 n+1 n –1 n–1 Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I n n (b Nondisjunction of sister chromatids in meiosis II c. The following are karyotypes of different individuals. For each answer the following questions: i. What problem is shown? ii. What is the resulting disorder? iii. Would this be labeled as monosomy, trisomy or neither? iv. What is the gender of the individual? Condition Syndrome Trisomy 21 Down Trisomy 18 Edward Trisomy 13 Patau XXY Klinefelter XYY Jacobs XXX XO (1 X chromosome only) Triple X Turner Characteristics Autosome Intellectual disabilities, abnormal patterns of palm creases, almond-shaped eyes, flattened face, short stature Intellectual and physical disabilities, facial abnormalities, extreme muscle tone, early death Intellectual and physical disabilities, wide variety of defects in organs, large triangular nose, early death Sex Chromosome Sexual immaturity (inability to produce sperm), breast swelling Typically no unusual symptoms; some individuals may be taller than average Tall and thin, menstrual irregularities Short stature, webbed neck, sexually underdeveloped A. i. This individual has two X chromosomes and 1 Y chromosome ii. This individual has Klinefelter Syndrome iii. This is an example of a trisomy of the sex chromosomes iv. This individual is male due to the presence of the Y chromosome B. i. This individual has two Y chromosomes and 1 X chromosome ii. This individual has Jacob’s Syndrome iii. This is an example of a trisomy of the sex chromosomes iv. This individual is male due to the presence of the two Y chromosomes C. i. This individual has an extra chromosome 18. ii. This individual has Edward’s Syndrome iii. This is an example of a trisomy of chromosome 18 (Trisomy 18) iv. This individual is male