Chapter+Twelve


 * __ CHAPTER 12 __**** : Human Genetics **

1. Review some of the key terms and concepts related to DNA and Genetics:  A. **__ Genome __**: the entire set of genetic information contained in a cell in the form of a diffuse mass of very long, very thin strands of DNA & protein molecules. DNA material within the nucleus is in this form the majority of the time. B. **__ Chromosome __**: a single DNA molecule (and associated proteins). **1.** Most human cells (somatic cells) have a total of **__ 46 __** (one set from each parent) so it can also be said that most human cells have “23 pairs”. Because these cells contain two sets of genetic instructions, they are also called **__ Diploid __** cells. **2.** These “23 pairs”, are similar but NOT identical to each other and are commonly called **__ Homologous __** chromosomes. __**3.** They are divided into two groups: __ a. **__ Autosomes __** are those chromosomes that are the same in both sexes. They generally provide the information for making eyes, ears, hair, internal organs, etc. b. **__ Sex Chromosomes __** are those that differ between males and females and determine a person’s gender. Males and females have different genotypes: 1. Female: XX (homologous pair, two “long” chromosomes) 2. Male: XY (not homologous pair, one “long” and one “short”)

**4.** Gamete cells (sperm and egg) that are produced by germ cells in the testes and ovaries have only ONE set of chromosomes so that after fertilization the resulting cell will have the full complement of 46 chromosomes. Gamete cells therefore are also called **__ haploid __** cells (half the number). a. In the process of Meiosis as the gamete cells are being produced, the homologous chromosomes unite, forming unique structures called **__ tetrads __**. Once formed there is a “exchange” of genetic information in a process called **__ crossing over __**. This is the first cause of genetic variation observed in sexual reproduction. b. During Meiosis, the alignment of the homologous pairs is random, therefore as the chromosomes separate, they do so in a random manner. This leads to a phenomenon called independent assortment that is the second cause of genetic variation.  c. During Meiosis, the homologous chromosomes separate from each other independently. This leads to a phenomenon called **__ segregation __** that is the third cause of genetic variation.

d. Each sperm or egg is unique, different from each other and different from the original parent cell. At fertilization there is a random chance as to which egg and which sperm unite. This results in the final cause of genetic variation.

<span style="font-family: 'Times New Roman','serif'; font-size: 12pt;">5. A visual “mapping” of chromosomes as viewed under a microscope during late prophase and early metaphase is called a **__ karyotype __**. A blood sample is centrifuged to “separate” the RBCs and WBCs from the plasma (fluid portion of blood). The blood cells are placed in a hypotonic solution that causes the RBCs to swell and burst. The WBCs swell but don’t burst and their chromosomes are able to spread out. The cells are stained and the chromosomes take up the stain easily, which allows them to be identified by their size and shape. The image viewed under the microscope is sent to a computer, where the images of the chromosomes are organized by size and shape.

<span style="font-family: 'Times New Roman','serif'; font-size: 12pt;">C. **__ Gene __****__:__** a specific section of a DNA molecule that codes for a specific protein or physical trait.

<span style="font-family: 'Times New Roman','serif'; font-size: 12pt;">1. **__ Allele __****__:__** different molecular forms found on the “23 pairs”.

<span style="font-family: 'Times New Roman','serif'; font-size: 12pt;">a. The one that masks the physical trait expressed by the one it is paired with is called **__ dominant __**.

<span style="font-family: 'Times New Roman','serif'; font-size: 12pt;">b. The one that remains “hidden” and has no noticeable effect on the organism’s physical traits when paired with other alleles is called **__ recessive __**. <span style="font-family: 'Times New Roman','serif'; font-size: 12pt;">Instructors Note: Due to time constraints, you will not be tested on material covered in sections 12.3-12.7; however, there are some important concepts covered in section 12.8 <span style="font-family: 'Times New Roman','serif'; font-size: 12pt;">2. There are several changes (mutations) that can occur in Chromosomal Structure. It is possible that changes occur in the DNA sequencing or chromosomal structure. In some cases it does not cause a severe change in the phenotype, while in some cases it results in severe or lethal consequences. We can live without hair, but we can not live without a heart. The following are the major categories of structural changes observed in chromosomes during the process of cell division: <span style="font-family: 'Times New Roman','serif'; font-size: 12pt;"> A. **__ Duplication __**: when crossing over occurs, one fragment attaches without the complimentary strand leaving. Now there are two copies of the “instructions”. <span style="font-family: 'Times New Roman','serif'; font-size: 12pt;"> B. **__ Inversion __**: when the crossing over occurs, one fragment attaches in the reverse order. Information is still there, just backwards. <span style="font-family: 'Times New Roman','serif'; font-size: 12pt;"> C. **__ Translocation __**: when crossing over occurs, it occurs between different (non-homologous) chromosomes; between 2 different tetrads, not within one tetrad. Information is still there, just on a different chromosome.

D. **__ Deletion __**: when crossing over occurs, one fragment fails to attach to the other chromosome, and is lost. This means some of the genetic information doesn’t remain. It may involve minor or major information; large deletions tend to be the most serious.

<span style="font-family: 'Times New Roman','serif'; font-size: 12pt;">E. Occassionally, abnormal events occur before or during cell division, resulting in gamete cells receiving the wrong number of chromosomes. This generally happens during Anaphase I or Anaphase II when the chromosome pairs or sister chromatids are separating. If they do not split correctly the chromosomal defect is called a ​**__ Nondisjunction __**. Usually this results in a spontaneous abortion/ termination of pregnancy however there are some exceptions. ​
 * __<span style="color: red; font-family: 'Times New Roman','serif'; font-size: 12pt; line-height: 115%;">Down __**<span style="font-family: 'Times New Roman','serif'; font-size: 12pt; line-height: 115%;"> SYNDROME (TRISOMY 21) occurs when chromosome #21 doesn’t split equally. When the 4 sister chromatids were together in Meiosis I, one of them didn’t split like it was suppose to, resulting in one of the daughter cells having 3 copies of chromosome #21 and one having only 1 copy. During Meiosis II, one of the gamete cells will end up with 2 copies. It is also possible that the error occurred in MEIOSIS II, where the sister chromatids failed to separate, so both ended up in one gamete cell. In either case, at the time of fertilization the offspring will end up with 3 copies.

//Sample Test Questions for chapters 9-12://


 * 1. When a eukaryotic cell is in the mitotic phase of cell division, its genetic material is generally found**
 * 1) in the nucleolus
 * 2) in the form of RNA
 * 3) in the form of chromatin
 * 4) in the form of sister chromatids
 * 5) none of the above

2. Which of the following phases of mitosis is essentially the opposite of prophase in terms of the nuclear changes observed? a. interphase b. metaphase c. telophase d. anaphase

3. Sister chromatids are a. found only when a cell is actively dividing b. tightly linked together at the centrioles c. formed when chromatids separate during cell division d. made of RNA molecules e. only observed during the process of meiosis

4. If a liver cell of a field mouse contains 20 chromosomes, a field mouse’s sperm cell would contain __chromosomes. a. 10 b. 20 c. 40 d. 46

5. Which of the following occurs during the “G1 Phase” of the cell cycle? a. the cytoplasm of the cell begins to divide b. sister chromatids begin to move to opposite sides of the cell c. organelles and the cell membrane increase in number and size d. DNA in the chromatin begins to replicate (make copy of itself) e. none of the above

6. The haploid chromosome number of a human cell consists of how many chromosomes? a. 1 b. 2 c. 23 d. 46 e. 92

7. TRUE or FALSE : Mitosis is also known as sexual reproduction.

8. The phase of mitosis during which the nuclear envelope (membrane) reforms, the nucleoli reappear, and distinct chromosomes are no longer clearly visible is called a. interphase b. metaphase c. anaphase d. prophase e. telophase

9. Which of the following occurs during metaphase of mitosis? a. the centromeres of each chromosome divide (separate from each other) b. sister chromatids begin to move toward opposite poles of the cell c. sister chromatids line up near the middle d.DNA in the chromatin replicates of the cell e. A and B are correct

10. Which of the following occurs during interphase of the cell cycle? a. formation of a cleavage furrow b. duplication of the chromosomes c. cell growth d. both B and C e. all of the above

11. In terms of its chromosomal number, a fertilized egg (zygote) is__ while the sperm and ovum (gamete or germ cell) are __a. haploid…….diploid b. diploid……..haploid c. diploid…….diploid d. haploid…….haploid

12. Two chromosomes in a nucleus that carry the information for the same traits in the same positions on the chromosome but specify different versions of some traits are a. homologous chromosomes b. sister chromatids c. heterologous chromosomes d. dominant chromosomes

13. Which of the following is a difference between mitosis and meiosis? a. in mitosis two daughter cells are produced, whereas in meiosis four daughter cells are produced b. cells produced by mitosis are diploid; cells produced by meiosis are haploid c. in mitosis cytokinesis occurs once, whereas in meiosis cytokinesis occurs twice. d. All of the above

14. During which phase of meiosis does crossing over occur? a. interphase I b. prophase I c. metaphase I d. prophase II e. metaphase II

15. Sister chromatids a. experience crossing-over during prophase II of meiosis b. separate from each other during anaphase I of meiosis c. become visible under a microscope during prophase of the cell cycle d. are also called homologous chromosomes e. are similar copies of each other, but not identical during mitosis

16. The expressed or physical traits of an organism are referred to as a. genotypes b. karyotypes c. phenotypes

17. The science of heredity is referred to as a. karyotyping b. inheritance c. genetics d. mad cow disease e. a breeding program

18. An organism with the same gene for a single physical trait may be referred to as__ _ for that trait. a. homozygous recessive b. heterozygous recessive c. homozygous dominant d. heterozygous dominant e. more than one of the above

a. at the same locus on homologous mitochondria b. at corresponding loci on a pair of homologous chromosomes c. at a different locus on homologous chromosomes d. none of the above are true
 * 19. Alleles for a gene are found

20. Whether an allele is dominant or recessive depends on a. how common the allele is, relative to other alleles b. whether it is inherited from the mother or the father c. which chromosome it is located d.** whether it or another allele determines the phenotype when both are present ** e. whether or not it is linked to other genes 21. Which of the following will contribute to genetic diversity in a population? a. fertilization of an egg by a sperm cell during sexual intercourse b. independent assortment of chromosomes during meiosis c. crossing over of genetic information between homologous chromosomes during meiosis d. segregation of the chromosomes during meiosis e. all of the above

22. When a gene for a given trait comes in alternative forms (versions) that specify different forms of the trait (for example black hair vs. blond hair), the versions of the gene are called a. sister chromatids b. gametes c. super genes d. alleles e. karyotypes

23. A carrier of a genetic disorder who does not show symptoms of the disorder is most likely to transmit the disorder to offspring. a. heterozygous for the trait and able b. heterozygous for the trait and unable c. homozygous for the trait and unable d. homozygous for the trait and able

24. TRUE or FALSE: In order to run a “test cross” to determine the unknown genotype of an organism, it is necessary to cross the organism with one that has a homozygous recessive genotype.

25. A black cow (DD) is mated with a red cow(dd). All of their offspring are black. If two of these offspring are mated, what fraction of their offspring will be red(dd)?
 * 1) all of them
 * 2) none of them

26. Most genetic disorders of humans are caused by (dominant/ recessive ) alleles. 27. Which of the following terms refers to the situation where a single phenotypic characteristic is determined by the additive effects of two or more gene pairs a. incomplete dominance b. codominance c. pleiotropy d. blending inheritance e. epistasis