CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE

1. CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE History • It was not until 1900 that biology finally caught up with Gregor Mendel. • Independently, Karl Correns, Erich von Tschermak, and Hugo de Vries all found that Mendel had explained the same results 35 years before. • Around 1900, cytologists and geneticists and the behavior of Mendel’s factors. • Mendel’s hereditary factors are the __________ located on chromosomes.

2. Around 1902, Walter Sutton, Theodor Boveri, and others noted these parallels and a ____________ theory of inheritance began to take form. Fig. 15.2

3. Red eyes White eyes Normal is called___________ Non-normal is called__________ 2. Morgan traced a gene to a specific chromosome • Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome in the early 20th century. • Morgan’s model-Drosophila melanogaster, a fruit fly • - have three pairs of _______________and a pair of ______ chromosomes (XX in females, XY in males). Fig. 15.3

4. Know these symbols = _______ = ________ • Morgan’s experiments- Red eye X White eye F1 All Red eye 3:1 Red:white F2 Conclude- classic _______________ genetics But- all __________were white, all females red Morgan concluded that a fly’s eye color was linked to the ___________________. Fig. 15.4

5. Genes located on the same chromosome,__________________, tend to be inherited together because the chromosome is passed along as a unit. Chromosome- 1.5 x 108 base pairs containing about ________ genes 3. Linked genes tend to be inherited together because they are located on the same chromosome 0.4% of a chromosome, containing 10 genes • Results of crosses with linked genes deviate from those expected according to ____________________________.

6. Linked genes Gray/wings X Black/wingless P • Morgan observed this linkage in ___________ and ____________. • Gray with wings is wild type F1 X Black/wingless Gray/wings _______ Equal numbers of each Thus, body color and wing size genes must be_________ _______ 5:5:1:1 Fig. 15.5

7. A _____frequency of recombination is observed for any two genes located on different (nonhomologous) chromosomes. 4. Independent assortment of chromosomes and crossing over produce genetic recombinants • In contrast, _________genes, genes located on the same_______________, tend to move together through meiosis and ______________. • Morgan’s testcross for body color and wing shape did not _____________to either independent assortment or complete linkage. • Independent assortment- expect 1:1:1:1 ratio • Completely linked- expect 1:1:0:0 • But observe _______of the flies were recombinants (5:5:1:1) , suggesting _______________linkage.

8. The actual mechanism, crossing over during_____________, results in the production of more types of gametes than one would predict by _____________ rules alone. Fig. 15.6 These unexpected numbers (5:5:1:1) is the basis for constructing a ________________________.

9. ________________an ordered list of the genetic loci along a chromosome. 5. Geneticists can use recombination data to map a chromosome’s genetic loci Sturtevant hypothesis- the ________________of recombinant offspring reflects the distances between genes on a chromosome. These genes are less likely to travel together These genes are more likely to travel together Sturtevant used _____________________________from fruit fly crosses to map the relative position of genes along chromosomes, a ___________________________.

10. Recombination frequencies (Rf) determine the relative __________ of genes Example: 3 fruit fly genes: body color (b), wing size (vg), and eye color (cn). Rf between cn and b is______ Rf between cn and vg is_______. Rf between b and vg is _________ The only possible arrangement of these three genes places the ___________ gene between the other two. Fig. 15.7 • Sturtevant expressed the distance between genes, the recombination frequency, as______________. • One map unit (sometimes called a_________________) is equivalent to a _____ recombination frequency.

11. Some genes on a chromosome are so far apart that a _____________between them is virtually certain. Rf reaches is its maximum value of ______ genes act as if found on separate chromosomes and are inherited independently. For example, seed color and flower color are far enough apart that linkage is not observed. A’ A A a Meiosis B b B b

12. Genes located far apart on a chromosome are mapped by adding the ___________________________ between the distant genes and ____________________genes. How do we map a gene if linkage is 50% ?? Answer- find another gene that does _______________ Fig. 15.8 0 MU Short antennae Long antennae Black body Grey body 48 MU 22 ____or 22% ______ or 22 ___________ Normal wings Short wings 67 MU 37 mu Red eyes Brown eyes 104 MU

13. Map units indicate _____________distance and order, not precise locations of genes. The frequency of crossing over is not actually uniform over the length of a chromosome. A _____________________indicates the positions of genes with respect to chromosomal features. A stained chromosome 15.3 15.2 15.1 14 13.3 13.2 13.1 12 11 11.1 11.2 12 13.1 13.2 13.3 14 15 21 22 23.1 23.2 23.3 31.1 31.2 31.3 32 33.1 33.2 33.3 34 35.1 35.2 35.3 Gene A is located on this band The distance between A and B can be_________ Gene B is located here Both ________maps and physical maps are used to locate disease genes

14. 6. The chromosomal basis of sex Mammals use the X-Y system to determine sex Other creatures may use other “systems” Female XX XY or XX x Male XY Male Female • Each _____ has one X chromosome. • 1/2 the _________have an X chromosome • 1/2 sperm have a Y chromosome • Thus, each conception has about a __________ chance of producing a particular sex.

15. Answer- the ____ gene (sex determining region of the Y chromosome) determines sex (male). located on the ____________________ regulates many other “male” genes. If lack the SRY gene, __________are produced. What determines sex in mammals?? Female is the____________________

16. The X chromosomes is loaded with________. One defective gene spells trouble for male Why? Because male has only one copy of the gene 7. Sex-linked genes have unique patterns of inheritance • In contrast, for a recessive sex-linked trait, a female will likely have one ______________ gene and one normal one • Heterozygous females will be_______________. • Example- Duchenne muscular dystrophy affects one in 3,500 males born in the United States. • Affected individuals rarely live past their early 20s. • Absence of an X-linked gene called _____________ • The disease is characterized by a progressive weakening of the muscles

17. Another example: Hemophilia is a __________________ trait defined by the absence of one or more clotting factors. prolonged bleeding because a firm clot forms slowly. Bleeding in muscles and joints can be painful and lead to serious damage. Individuals can be treated with intravenous injections of the missing protein.

18. In female mammals only _____ X chromosome is active. Therefore, males and females have the same effective _______ (one copy ) of genes on the X chromosome. The inactivated X chromosome is a _______________. This inactivates most of its genes. reactivated in ovarian cells that produce ova. After Barr body formation, all descendent cells have the same inactive X. • If a female is _______________________for a sex-linked trait, approximately half her cells will express one allele and the other half will express the other allele.

19. A _______________________mutation- A heterozygous woman will have patches of normal skin and skin patches lacking sweat glands. Examples of X-linked diseases • _________________cats -orange and black pattern • ½ cells express an orange allele while others have a nonorange allele. Fig. 15.11

20. How is the X chromosome inactivated? • Answer- One X chromosome expresses the XIST gene (X-inactive specific transcript) and XIST RNA _______ the chromosome. XIST RNA • Xist is necessary and ________________ for X inactivation (using 450kb YAC) Figure not in text • Evidence: If insert Xist transgene on autosome results in an ______________autosome

21. ________________occurs when problems with the meiotic spindle cause errors in daughter cells. This may occur if tetrad chromosomes do not separate properly during meiosis I. Alternatively, sister chromatids may fail to separate during meiosis II. 8. Alterations of chromosome number or structure cause some genetic disorders Fig. 15.12

22. Nondisjunction results in too many or to few chromosomes termed ________________. some gametes receive two of the same type of chromosome and another gamete receives no copy. ___________ cells - three copies of a particular chromosome type and have 2n + 1 total chromosomes. _____________cells - one copy of a particular chromosome type and have 2n - 1 chromosomes. Normally results in embryonic death, but some survive Organisms with more than two ____________sets of chromosomes, have undergone ________________e.g. 3n or 4n (rare in mammals) • Polyploids are more nearly normal in phenotype than____________

23. A deletion - a chromosome fragment lacking a ________________ is lost during cell division. A _____________- a fragment becomes attached as an extra segment to a sister chromatid. Other chromosome problems Fig. 15.14c & d • An inversion - a chromosomal fragment ____________ to the original chromosome but in the reverse orientation. • In_________________, a chromosomal fragment joins a _________________ chromosome. • Some translocations are ___________others are not.

24. Homozygous embryos with a large ___________ normally die A deletion in the X chromosome is ____________in males A _______________________________ can alter phenotype because a gene’s expression is influenced by its location. Results of chromosome errors Example- Leukemia is due to a growth gene being placed next to an active region resulting in cancer • Approx. ________ of human embryos are aneuploid and die early in development • Some are viable- • Chromosome 15 trisomy- die at birth • Chromsome 21 trisomy- Down’ssyndrome

25. Down syndrome- trisomy 21. One in 700 children born in the United States. result from nondisjunction during gamete production • correlates with the age of the mother. Fig. 15.15 Karyotype Phenotype

26. _______________________________an XXY male, occurs once in every 2000 live births. Male sex organs, but are__________. Feminine characteristics; normal intelligence. Males with an extra Y chromosome (XYY) tend to somewhat _________ than average. Trisomy X (XXX), which occurs once in every 2000 live births, produces _______________ females. Monosomy X or Turner’s syndrome (X0), which occurs once in every 5000 births, produces phenotypic, but immature females. Extra sex chromosomes are allowed • XYY and XXYY- males often found in ____________.

27. A few genes (not most!!) are expressed only if derived from the mother (or from the father) 9. The phenotypic effects of some mammalian genes depend on whether they were inherited from the mother or the father________________ • _____________________________ • A gene on ________ homologous chromosome is silenced • The ________________ status of a given gene depends on whether the gene resides in a female or a male. 35.3

28. The difference between the disorders is due to___________________________ Examples of imprinted genes 15.3 15.2 15.1 14 • 1. Prader-Willi syndrome and Angelman syndrome are due a deletion of a specific segment of ______________________. • Prader-Willi syndrome - mental retardation, obesity, short stature, and unusually small hands and feet. • Inherit the abnormal chromosome from their father. • Angelman syndrome -spontaneous laughter, jerky movements, and other motor and mental symptoms. • This is inherited from the mother. 13.3 13.2 13.1 12 11 11.1 11.2 12 13.1 13.2 13.3 14 15 21 22 23.1 23.2 23.3 31.1 31.2 31.3 32 33.1 33.2 33.3 34 35.1 35.2 2. ________________ syndrome, which leads to various degrees of mental retardation, also appears to be subject to genomic imprinting

29. ________________genes are found on small circles of DNA in mitochondria and ____________. These organelles________________ themselves. Their cytoplasmic genes do not display Mendelian inheritance. They are not distributed to offspring during meiosis. 10. Extranuclear genes exhibit a non-Mendelian pattern of inheritance • A zygote inherits all its mitochondria only from the_______. • Mitochondrial mutations may contribute to diabetes, heart disease, and other diseases of aging.