1. CHROMOSOMES & HEREDITY
CHAPTER 15
CHROMOSOMES & HEREDITY

2. GENETIC HISTORY 1860’S: MENDEL’S LAWS OF SEGREGATION AND ASSORTMENT
1875: CYTOLOGISTS WORKED OUT PROCESS OF MITOSIS
1890: MEIOSIS FIGURED OUT
1900: THREE BOTANISTS REDISCOVERED MENDEL’S LAWS
1902: MENDEL’S LAWS AND MEIOSIS WERE STUDIED AND IDEAS CONVERGED

3. CHROMOSOME THEORY OF INHERITANCE
ACCORDING TO THIS THEORY (DERIVED FROM THE WORK STATED)
1) MENDELIAN FACTORS OR GENES ARE LOCATED ON CHROMOSOMES
2) IT IS THE CHROMOSOMES THAT SEGREGATE AND INDEPENDENTLY ASSORT

4. CHROMOSOMAL BASIS OF MENDEL’S LAWS

5. EARLY 1900’S THOMAS MORGAN
MORGAN USED FRUIT FLIES TO STUDY CHROMOSOMES: THEY ONLY HAVE 4 PAIRS WHICH ARE EASILY SEEN WITH MICROSCOPE
MORGAN TRACED A GENE TO A SPECIFIC CHROMOSOME
THIS PROVIDED CONVINCING EVIDENCE THAT MENDEL’S INHERITABLE FACTORS ARE LOCATED ON CHROMOSOMES

6. DISCOVERY OF A SEX LINKAGE
MORGAN DEDUCED THAT EYE COLOR IS LINKED TO SEX AND THAT THE GENE FOR EYE COLOR IS LOCATED ONLY ON THE X CHROMOSOME.
WILD TYPE = NORMAL PHENOTYPE
MUTANT = ALTERNATIVE TO NORMAL
SEX-LINKED GENES = GENES LOCATED ON SEX CHROMOSOMES. THE TERM IS MOSTLY APPLIED ONLY TO GENES ON THE X CHROMOSOME

7. SEX-LINKED INHERITANCE

8. LINKED GENES
GENES THAT ARE LOCATED ON THE SAME CHROMOSOME AND THAT TEND TO BE INHERITED TOGETHER
LINKED GENES DO NOT ASSORT INDEPENDENTLY
A DIHYBRID CROSS FOLLOWING TWO LINKED GENES WILL NOT PRODUCE AN F2 PHENOTYPIC RATIO OF 9:3:3:1

9. LINKED GENES MORGAN PROPOSED THAT THESE RATIOS WERE DUE TO
LINKAGE. THE GENES FOR BODY COLOR AND WING SIZE
ARE ON THE SAME CHROMOSOME AND ARE USUALLY
INHERITED TOGETHER

10. LINKED GENES MORGAN PROPOSED THAT THESE RATIOS WERE DUE TO
LINKAGE. THE GENES FOR BODY COLOR AND WING SIZE
ARE ON THE SAME CHROMOSOME AND ARE USUALLY
INHERITED TOGETHER

11. GENETIC RECOMBINATION
THE PRODUCTION OF OFFSPRING WITH NEW COMBINATIONS OF TRAITS DIFFERENT FROM THOSE COMBOS FOUND IN THE PARENTS; RESULTS FROM THE EVENTS OF MEIOSIS AND RANDOM FERTILIZATION
PARENTAL TYPES= OFFSPRING THAT HAVE THE SAME PHENOTYPE AS ONE PARENT
RECOMBINANTS = OFFSPRING WHOSE PHENOTYPES DIFFER FROM EITHER PARENT

12. CROSSING OVER: RECOMBINATION OF LINKED GENES
IF GENES ARE TOTALLY LINKED, SOME POSSIBLE PHENOTYPIC COMBOS SHOULD NOT APPEAR; BUT SOMETIMES THE UNEXPECTED PHENOTYPES DO APPEAR
MORGAN DID A STUDY AND FOUND THAT 17% OF OFFSPRING WERE RECOMBINANTS, WHERE HE THOUGHT THE GENES WERE LINKED
CROSSING OVER ACCOUNTS FOR THIS RECOMBINATION (NOT KNOWN THEN)

13. RECOMBINATION DUE TO CROSSING OVER

14. RECOMBINATION DATA
SCIENTISTS USED RECOMBINATION FREQUENCIES BETWEEN GENES TO MAP THE SEQUENCE OF LINKED GENES ON A PARTICULAR CHROMOSOME
MORGAN’S DROSOPHILA STUDIES SHOWED THAT SOME GENES ARE LINKED MORE TIGHTLY THAN OTHERS
EX: THE RECOMBINATION FREQUENCY BETWEEN THE b and vg LOCI IS ABOUT 17%

15. GENETIC LOCI
ONE OF MORGAN’S STUDENTS THEORIZED THAT THE PROBABILITY OF CROSSING OVER BETWEEN TWO GENES IS DIRECTLY PROPORTIONAL TO THE DISTANCE BETWEEN THEM
RECOMBINATION FREQUENCIES BETWEEN GENES WERE USED TO ASSIGN THEM A LINEAR POSITION ON A CHROMOSOME MAP
IF LINKED GENES ARE SO FAR APART THAT THE RECOMBO FREQ. IS 50%, THEN THEY ARE NO DIFFERENT FROM UNLINKED GENES THAT ASSORT INDEPENDENTLY

16. RECOMBINATION FREQUENCIES
ONE MAP UNIT = 1% RECOMBINATION FREQUENCY, SO 17%
WILL EQUAL APPROX 17 MAP UNITS

17. A PARTIAL GENETIC MAP OF DROSOPHILA

18. SEX CHROMOSOMES
IN MOST SPECIES, SEX IS DETERMINED BY THE PRESENCE OR ABSENCE OF SPECIAL CHROMOSOMES
HETEROGAMETIC SEX = THE SEX THAT PRODUCES TWO KINDS OF GAMETES AND DETERMINES THE SEX OF THE OFFSPRING
HOMOGAMETIC SEX = THE SEX THAT PRODUCES ONE KIND OF GAMETE

19. DIFFERENT CHROMOSOMAL SYSTEMS OF GENDER

20. SEX DETERMINATION IN HUMANS
MALES ARE HETEROGAMETIC (XY)
FEMALES ARE HOMOGAMETIC (XX)
WHETER AN EMBRYO DEVELOPS INTO A MALE OF FEMALE DEPENDS UPON THE PRESENCE OF THE Y CHROMOSOME
SRY GENE: SEX DETERMINING REGION ON Y CHROMOSOME THAT TRIGGERS EVENTS THAT LEAD TO TESTE DEVELOPMENT; IN ABSENCE OF SRY, THE GONADS DEVELOP INTO OVARIES

21. SEX-LINKED DISORDERS
SEX-LINKED TRAITS USUALLY REFERS TO X-LINKED TRAITS
THE X CHROMOSOME IS MUCH LARGER THAN THE Y, GIVING MORE X-LINKED TRAITS
MOST X-LINKED GENES HAVE NO HOMOLOGOUS LOCI ON THE Y CHROMOSOME
MOST GENES ON THE Y NOT ONLY HAVE NO X COUNTERPARTS, BUT THEY ENCODE FOR STRICTLY MALE TRAITS (EX: TESTIS)

22. SEX-LINKED DISORDERS
FATHERS PASS X-LINKED ALLELES TO ALL THEIR DAUGHTERS ONLY
MALES RECEIVE THEIR X CHROMOSOME ONLY FROM THEIR MOTHERS
FATHERS CANNOT PASS SEX-LINKED TRAITS TO THEIR SONS

23. SEX-LINKED DISORDERS
MOTHERS CAN PASS SEX-LINKED ALLELES TO BOTH SONS AND DAUGHTERS
FEMALES RECEIVE TWO X CHROMOSOMES, ONE FROM EACH PARENT
MOTHERS PASS ON ONE X CHROMOSOME TO EVERY DAUGHTER AND SON

24. SEX-LINKED RECESSIVE
A FEMALE WILL EXPRESS TRAIT ONLY IF SHE IS HOMOZYGOUS
MORE MALES HAVE SEX-LINKED DISORDERS, AS THEY ONLY HAVE ONE X CHROMOSOME; FEMALES CAN BE A HETERZYGOUS CARRIER, BUT NOT SHOW THE TRAIT HERSELF
A CARRIER THAT MATES WITH NORMAL MALE WILL PASS THE TRAIT ON TO HALF HER SONS AND DAUGHTERS

25. SEX-LINKED RECESSIVE TRAITS

26. X-INACTIVATION IN FEMALE MAMMALS
IN FEMALE MAMMALS, MOST DIPLOID CELLS HAVE ONLY ONE FULL FUNCTIONAL X CHROMOSOME
EACH EMBRYONIC CELL INACTIVATES ONE OF THE TWO X CHROMOSOMES
THE INACTIVE X CHROMOSOME CONTRACTS INTO A DENSE OBJECT CALLED A BARR BODY

27. BARR BODY MOST BARR BODY GENES ARE NOT EXPRESSED
THEY ARE REACTIVATED IN GONADAL CELLS THAT UNDERGO MEIOSIS TO FORM GAMETES

28. FEMALE MAMMALIAN CELLS
FEMALE MAMMALS ARE A MOSAIC OF TWO TYPE OF CELLS: THOSE WITH AN ACTIVE MATERNAL X AND THOSE WITH AN ACTIVE PATERNAL X
WHICH OF THE TWO X’S WILL BE INACTIVATED IS DETERMINED RANDOMLY IN EMBRYONIC CELLS
AFTER AN X IS INACTIVATED, ALL MITOTIC DECENDNTS WILL HAVE THE SAME INACTIVE X
IF A FEMALE IS HETEROZYGOUS FOR A SEX-LINKED TRAIT, ABOUT HALF OF HER CELLS WILL EXPRESS ONE ALLELE AND THE OTHER CELLS WILL EXPRESS THE ALTERNATE

29. CHROMOSOMAL ERRORS AND EXCEPTIONS
MEIOTIC ERRORS AND MUTAGENS CAN CAUSE MAJOR CHROMOSOMAL CHANGES SUCH AS ALTERED CHROMOSOME NUMBERS OR ALTERED CHROMOSOMAL STRUCTURE

30. NONDISJUNCTION
MEIOTIC OR MITOTIC ERROR DURING WHICH CERTAIN HOMOLOGOUS CHROMOSOMES OR SISTER CHROMATIDS FAIL TO SEPARATE
THERE ARE 2 MAIN TYPES OF NONDISJUNCTION: ANEUPLOIDY AND POLYPLOIDY

31. MEIOTIC NONDISJUNCTION

32. ANEUPLOIDY HAVING AN ABNORMAL NUMBER OF CERTAIN CHROMOSOMES
WHEN AN ANEUPLOID ZYGOTE DIVIDES BY MITOSIS, IT TRANSMITS THE PROBLEM TO ALL EMBRYONIC CELLS
TRISOMIC = AN ANEUPLOID CELL THAT HAS A CHROMOSOME IN TRIPLICATE (DOWN’S SYNDROME)
MONOSOMIC- A CELL MISSING A CHROMOSOME

33. POLYPLOIDY
A CHROMOSOME NUMBER THAT IS MORE THAN TWO COMPLETE CHROMOSOME SETS
IS COMMON IN PLANTS AND IMPORTANT IN PLANT EVOLUTION
TRIPLOIDY = THREE HAPLOID CHROMOSOME SETS (3N);MAY BE PRODUCED BY FERTILIZATION OF ABNORMAL DIPLOID EGG PRODUCED BY NONDISJUNCTION OF ALL CHROMOSOMES
TETRAPLOIDY = FOUR HAPLOID CHROMOSOME SETS (4N);MAY RESULT IF A DIPLOID ZYGOTE UNDERGOES MITOSIS WITHOUT CYTOKINESIS. SUBSEQUENT NORMAL MITOSIS WOULD PRODUCE A 4N EMBRYO

34. ALTERATIONS OF CHROMOSOMES
CHROMOSOME BREAKAGE CAN ALTER THEIR STRUCTURE IN FOUR WAYS:
1) DELETION = CHROMOSOMES LOSE A FRAGMENT LACKING A CENTROMERE
2) DUPLICATION = FRAGMENTS MAY JOIN TO A HOMOLOGOUS CHROMOSOME
3) TRANSLOCATION = FRAGMENTS MAY JOIN TO A NONHOMOLOGOUS CHROMO.
4) INVERSION = FRAGMENTS MAY REATTACH TO THE ORIGINAL CHROMO. IN REVERSE ORDER

35. ALTERATIONS OF CHROMOSOMES
POSITION EFFECT - INFLUENCE ON A GENE’S EXPRESSION
BECAUSE OF ITS LOCATION AMONG NEIGHBOR GENES

36. HUMAN DISORDERS DUE TO CHROMOSOMAL ALTERATIONS
DOWN’S SYNDROME - TRISOMY 21
PATAU SYNDROME - TRISOMY 13
EDWARD’S SYNDROME - TRISOMY 18
KLEINFELTER SYNDROME - USUALLY XXY, BUT ALSO XXYY, XXXY, XXXXY
TRIPLE X SYNDROME - XXX
TURNER SYNDROME = XO
CRI DU CHAT - DELETION ON #5
CHRONIC MYELOGENOUS LEUKEMIA (CML)-TRANSLOCATION ON #22 WITH SMALL FRAGMENT ON #9

37. GENOMIC IMPRINTING
CAUSES CERTAIN GENES TO BE DIFFERENTLY EXPRESSED IN THE OFFSPRING DEPENDING UPON WHETHER THE ALLELES WERE INHERITED FROM THE OVUM OR THE SPERM
PRADER-WILLI SYNDROME AND ANGELMAN SYDROME-SAME DELETION ON #15; SYMPTOMS DIFFER DEPENDING ON WHICH PARENT GAVE THE GENE
FRAGILE-X SYNDROME - AN ABNORMAL X CHROMOSOME, THE TIP HANGS ON THE REST OF THE CHROMOSOME BY A THIN DNA THREAD; MOST COMMON GENETIC CAUSE OF MENTAL RETARDATION; MORE LIKELY TO APPEAR IF X IS INHERITED FROM MOTHER

38. GENOMIC IMPRINTING
NOTICE, THE
IMPRINTS ARE
NOT PASSED
FROM
GENERATION
TO GENERATION