THE CHROMOSOMAL

BASIS OF INHERITANCE

YOU MUST KNOW…• HOW THE CHROMOSOME THEORY OF

INHERITANCE CONNECTS THE PHYSICAL MOVEMENT OF CHROMOSOMES IN MEIOSIS TO MENDEL’S LAWS OF INHERITANCE

• THE UNIQUE PATTERN OF INHERITANCE IN SEX-LINKED GENES

• HOW ALTERATION OF CHROMOSOME NUMBER OR STRUCTURALLY ALTERED CHROMOMES (DELETION, DUPLICATIONS, ETC.) CAN CAUSE GENETIC DISORDERS

CONCEPT 15.1• MENDELIAN INHERITANCE HAS

ITS PHYSICAL BASIS IN THE BEHAVIOR OF CHROMOSOMES

CHROMOSOME THEORY OF INHERITANCE

• STATES THAT GENES HAVE SPECIFIC LOCATIONS (LOCI) ON CHROMOSOMES AND THAT IT IS CHROMOSOMES THAT SEGREGATE AND

ASSORT

INDEPENDENTLY• SEX-LINKED GENE –

LOCATED ON A

SEX CHROMOSOME

CONCEPT 15.2• SEX-LINKED GENES EXHIBIT

UNIQUE PATTERNS

OF INHERITANCE

SEX-LINKED GENES• LOCATED ON THE X CHROMOSOME• SEX IS DETERMINED BY THE MALE SPERM

CELL• FATHERS PASS SEX-LINKED GENES ON TO

THEIR DAUGHTER BUT NOT TO THEIR SONS• FEMALES WILL EXPRESS A SEX-LINKED

TRAIT LIKE ANY OTHER TRAIT BUT MALES WITH ONLY ONE X CHROMOSOME, WILL EXPRESS THE ALLELE ON THE X CHROMOSOME THEY INHERITED FROM THEIR MOTHER

• THE VAST MAJORITY OF GENES ON THE X CHROMOSOME ARE NOT RELATED TO SEX

SEX-LINKED DISORDERS• DUCHENNE MUSCULAR DYSTROPHY

– PROGRESSIVE WEAKING OF THE MUSCLES AND LOSS OF COORDINATION, RARELY LIVE PAST THEIR EARLY 20’S

• HEMOPHILIA – BLOOD WITH AN INABILITY TO CLOT NORMALLY, CAUSED BY THE ABSENCE OF PROTEINS REQUIRED FOR BLOOD CLOTTING

• X-INACTIVATION – REGULATES GENE DOSAGE IN FEMALES

• ONE OF THE X CHROMOSOMES IN EACH CELL OF THE BODY BECOMES INACTIVATED DURING EMBRYONIC DEVELOPMENT BY METHYLATION

• MALES AND FEMALES HAVE THE SAME EFFECTIVE DOSE OF GENES WITH LOCI ON THE X CHROMOSOME

• THE INACTIVE CHROMOSOME CONDENSES INTO A BARR BODY WHICH LIES ALONG THE INSIDE OF THE NUCLEAR ENVELOPE

• FEMALES ARE NOT AFFECTED AS HETEROZYGOTE CARRIERS OF PROBLEMATIC ALLELES BECAUSE HALF OF THEIR SEX CHROMOSOMES ARE NORMAL AND PRODUCE THE NECESSARY PROTEINS

CONCEPT 15.3• LINKED GENES TEND TO BE

INHERITED TOGETHER BECAUSE THEY ARE LOCATED NEAR EACH OTHER ON THE SAME CHROMOSOME

• LINKED GENES ARE LOCATED ON THE SAME CHROMOSOME AND THEREFORE TEND TO BE INHERITED TOGETHER DURING CELL DIVISION

• GENETIC RECOMBINATION IS THE PRODUCTION OF OFFSPRING WITH A NEW COMBINATION OF GENES INHERITED FROM THE PARENTS

• SAME PHENOTYPE AS ONE OF THE PARENTS (PARENT TYPES)

• DIFFERENT PHENOTYPE THAN PARENTS (RECOMBINANTS)

• CROSSING OVER – CAN EXPLAIN WHY SOME LINKED GENES GET SEPARATED DURING MEIOSIS

• DURING MEIOSIS UNLINKED GENES FOLLOW INDEPENDENT ASSORTMENT BECAUSE THEY ARE LOCATED ON DIFFERENT CHROMOSOMES

• LINKED GENES ARE LOCATED ON THE SAME CHROMOSOME AND WOULD NOT BE PREDICTED TO FOLLOW INDEPENDENT ASSORTMENT

• LINKAGE MAP – GENETIC MAP THAT IS BASED ON THE % OF CROSS-OVER EVENTS

• MAP UNIT – IS EQUAL TO A 1% RECOMBINATION FREQUENCY, USED TO EXPRESS RELATIVE DISTANCES ALONG THE CHROMOSOME

CONCEPT 15.4• ALTERATIONS OF CHROMOSOME

NUMBER OR STRUCTURE CAUSE SOME GENETIC DISORDERS

NONDISJUNCTION• OCCURS WHEN THE MEMBERS OF A

PAIR OF HOMOLOGOUS CHROMOSOMES DOES NOT SEPARATE PROPERLY

DURING MEIOSIS I OR SISTER CHROMATIDS DON’T SEPARATE PROPERLY DURING MEIOSIS II

• ONE GAMETE RECEIVES TWO COPIES OF THE CHROMOSOME WHILE THE OTHER GAMETE RECEIVE NONE

• IF THE FAULTY GAMETES ENGAGE IN FERTILIZATION, THE OFFSPRING WILL HAVE AN INCORRECT CHROMOSOME NUMBER KNOWN AS ANEUPLOIDY

• TRISOMY – FERTILIZED EGG RECEIVES 3 COPIES OF THE CHROMOSOME (trisomy 21, 18, 13)

• MONOSOMY – FERTILIZED EGG RECEIVES ONLY ONE COPY OF A CHROMOSOME

• POLYPLOIDY – CONDITION OF HAVING MORE THAN TWO COMPLETE SETS OF CHROMOSOMES, FORMING A 3n OR 4n INDIVIDUAL

MUTATIONS• DELETION – WHEN A CHROMOSOMAL

FRAGMENT IS LOST (MISSING GENES)• DUPLICATION – WHEN A

CHROMOSOMAL FRAGMENT THAT BROKE OFF BECOMES ATTACHED TO ITS SISTER CHROMATID (ZYGOTE GETS A DOUBLE DOSE OF THE GENES)

MUTATIONS• INVERSION – WHEN A CHROMOSOME

FRAGMENT BREAKS OFF AND REATTACHED TO ITS ORIGINAL POSITION BUT BACKWARD

• TRANSLOCATION – WHEN A DELETED CHROMOSOME FRAGMENT JOINS A NONHOMOLOGOUS CHROMOSOME

• DOWN SYNDROME – TRISOMY 21 – CHARACTERISTIC FACIAL FEATURES, SHORT STATURE, HEART DEFECTS, MENTAL RETARDATION

• KLINEFELTER SYNDROME – XXY – HAVE MALE SEX ORGANS BUT ARE STERILE

• TURNER SYNDROME – X – FEMALES ARE STERILE BECAUSE REPRODUCTIVE ORGANS DON’T MATURE