CHROMOSOMES & HEREDITY CHAPTER 15 CHROMOSOMES & HEREDITY

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

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

CHROMOSOMAL BASIS OF MENDEL’S LAWS

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

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

SEX-LINKED INHERITANCE

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

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

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

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

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)

RECOMBINATION DUE TO CROSSING OVER

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%

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

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

A PARTIAL GENETIC MAP OF DROSOPHILA

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

DIFFERENT CHROMOSOMAL SYSTEMS OF GENDER

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

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)

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

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

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

SEX-LINKED RECESSIVE TRAITS

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

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

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

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

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

MEIOTIC NONDISJUNCTION

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

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

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

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

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

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

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