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CHAPTER 9: FUNDAMENTALS OF GENETICS
I.
MENDEL’S LEGACY: GENETICS IS THE FIELD OF BIOLOGY DEVOTED TO UDERSTANDING HOW
CHARACTERISTICS ARE TRANSMITTED FROM PARENTS TO OFFSPRING
A.
GREGOR JOHANN MENDEL
1.
AN AUSTRIAN MONK (1842) WHO STUDIED SCIENCE AND MATH
2.
TAUGHT HIGH SCHOOL, KEPT A GARDEN, AND HAD AN INTEREST IN NATURE
AND EVOLUTION
3.
USED HIS KNOWLEDGE TO STUDY HEREDITY (TRANSMISSION OF TRAITS)
B.
MENDEL’S GARDEN PEAS
1.
OBSERVED 7 PEA PLANT CHARACTERISTICS (TRAITS)
a.
OCCURRED IN TWO CONTRASTING TRAITS
i)
LONG OR SHORT STEMS, GREEN OR YELLOW SEEDS, WHITE OR
PURPLE FLOWERS, ETC.
2.
USED HIS KNOWLEDGE OF STATISTICS TO ANALYLZE HIS ACTUAL OBSERVATIONS
C.
MENDEL’S METHODS
1.
PLANTS REPRODUCE BY POLLINATION (POLLEN COLLECTED FROM MALE
ANTHERS AND TRANSFERRED TO THE FEMALE STIGMA)
2.
SELF-POLLINATION OCCURS WHEN POLLEN IS TRANSFERRED TO THE STIGMA OF
THE SAME PLANT (NATURAL SELECTION)
a.
THIS IS HOW PEA PLANTS NORMALLY REPRODUCE
3.
CROSS-POLLINATION OCCURS WHEN POLLEN IS TRANSFERRED TO A DIFFERENT
PLANT
4.
MENDEL CAREFULLY CONTROLLED HIS PLANTS SO THEY COULD NOT SELFPOLLINATE (ARTIFICAL SELECTION)
D.
MENDEL’S EXPERIMENTS
1.
BEGAN BY PRODUCING A STRAIN OF PLANTS PURE FOR THE TRAITS HE WANTED
TO STUDY
a.
PURE PLANTS PRODUCE ONLY ONE KIND OF OFFSPRING
b.
MENDEL DEVELOPED 14 PURE STRAINS FOR STUDY
c.
EACH STRAIN WAS CALLED THE P1 GENERATION (or just P)
2.
MENDEL CROSS-POLLINATED THE STRAINS TO PRODUCE THE F1 GENERATION
3.
ALLOWED THE F1 TO SELF-POLLINATE TO PRODUCE THE F2
4.
PERFORMED THIS HUNDREDS OF TIMES FOR VERIFICATION OF HIS RESULTS
E.
MENDEL’S RESULTS AND CONCLUSIONS
1.
MENDEL CROSSED A PLANT PURE FOR GREEN PODS WITH A PLANT PURE FOR
YELLOW PODS
a.
THE RESULTING F1 OFFSPRING WERE ALL GREEN, EVEN THOUGH ONE
PARENT WAS YELLOW-PODDED
2.
MENDEL ALLOWED THE F1 TO SELF-POLLINATE
a.
THE F2 OFFSPRING WERE ¾ GREEN AND ¼ YELLOW
3.
RESULTING HYPOTHESIS FROM HIS OBSERVATIONS
a.
EACH TRAIT WAS CONTROLLED BY SOMETHING WITHIN THE PLANTS
b.
HE CALLED THE MEANS BY WHICH THEY WERE INHERITED FACTORS
c.
HE HYPOTHESIZED EACH TRAIT WAS INHERITED BY A SEPARATE FACTOR
AND THAT THERE MUST BE TWO FACTORS FOR EVERY TRAIT (I.E., GREEN
AND YELLOW)
F.
RECESSIVE AND DOMINANT TRAITS (COMPLETE DOMINANCE)
1.
IN THE P CROSSES, ONE OF THE TRAITS FAILED TO APPEAR IN THE F1
2.
3.
G.
H.
I.
IN EVERY CASE IT REAPPEARED IN THE F2 IN A 3:1 RATIO (GREEN TO YELLOW)
MENDEL CONCLUDED THAT THAT THE ONE THAT DISAPPEARED WAS RECESSIVE
TO THE ONE THAT APPEARED MOST OFTEN (DOMINANT)
a.
THE DOMINANT TRAIT ALWAYS MASKED THE RECESSIVE ONE IN ALL HIS
TRIALS
THE LAW OF SEGREGATION
1.
MENDEL CONCLUDED THAT THE PAIRS OF FACTORS SEPARATE DURING THE
FORMATION OF REPRODUCTIVE CELLS
2.
A PAIR OF FACTORS IS SEGREGATED, OR SEPARATED, DURING THE FORMATION
OF GAMETES
THE LAW OF INDEPENDENT ASSORTMENT
1.
MENDEL ALSO CROSSED PLANTS THAT DIFFERED IN TWO CHARACTERISTICS
(I.E., FLOWER COLOR, SEED COLOR)
2.
IN HIS CROSSES THE DOMINANT TRAITS DID NOT ALWAYS APPEAR TOGETHER
3.
FACTORS FOR DIFFERENT CHARACTERISTICS ARE DISTRIBUTED TO GAMETES
INDEPENDENTLY
CHROMOSOMES AND GENES
1.
MOLECULAR GENETICS IS THE STUDY OF STUCTURE AND FUNCTION OF
CHROMOSOMES AND GENES
a.
GENES ARE SEGMENTS OF DNA THAT CONTROLS A TRAIT
b.
EACH FORM OF A GENE IS CALLED AN ALLELE
i)
THESE ALLELES ARE WHAT MENDEL CALLED FACTORS
2.
LETTERS ARE USED TO REPRESENT ALLELES
a.
CAPITAL LETTERS REPRESENT DOMINANT ALLELES
b.
LOWER CASE LETTERS REPRESENT RECESSIVE ALLELES
WHAT YOU SHOULD KNOW AFTER STUDYING THIS SECTION:
1.
LIST THE STEPS INVOLVED IN MENDEL’S EXPERIMENTS ON GARDEN PEAS.
2.
DEFINE THE TERMS DOMINANT AND RECESSIVE
3.
DIFFERENTIATE GENES FROM ALLELES.
4.
STATE IN MODERN TERMINOLOGY THE TWO LAWS OF HEREDITY THAT RESULTED FROM
MENDEL’S WORK.
5.
HOW MIGHT MENDEL’S CONCLUSIONS HAVE DIFFERED IF HE HAD STUDIED TWO TRAITS
DETERMINED BY ALLELES CARRIED ON THE SAME CHROMOSOME?
6.
(DEMONSTRATE UNDERSTANDING) WHAT HAPPENS DURING MEIOSIS THAT WOULD
ALLOW GENES LOCATED ON THE SAME CHROMOSOME TO SEPARATE INDEPENDENTLY
OF EACH OTHER?
II.
GENETIC CROSSES
A.
GENOTYPE AND PHENOTYPE
1.
THE GENETIC MAKEUP OF AN ORGANISM IS ITS GENOTYPE
a.
CONSISTS OF ALL THE ALLELES AN ORGANISM INHERITS
b.
EACH TRAIT WILL HAVE A PAIR OF ALLELES
i)
IF THE ALLELES ARE THE SAME, THEY ARE HOMOZYGOUS (PP, pp)
ii
IF THEY ARE DIFFERENT, THEY ARE HETEROZYGOUS (Pp)
2.
PHENOTYPE IS THE APPEARANCE OF AN ORGANISM
i)
PP AND Pp ARE PURPLE, IF P REPRESENTS THE DOMINANT
PURPLE FLOWER COLOR
ii)
B.
pp IS WHITE, IF p REPRESENTS THE RECESSIVE ALLELE FOR WHITE
FLOWERS
PROBABILITY
1.
PROBABILITY IS THE LIKELIHOOD A SPECIFIC EVENT WILL OCCUR
a.
IS EXPRESSED AS A DECIMAML, A PERCENTAGE, OR A FRACTION
2.
PROBABILITY=NUMBER OF TIMES AN EVENT IS EXPECTED TO HAPPEN
NUMBER OF OPPORTUNITIES FOR AN EVENT TO HAPPEN
3.
EXAMPLE: ROLLING A DIE
# of ways to roll a 6
P(6)
=
total # of sides
P(even) =
C.
=
1
6
# ways to roll an even number 3 1
= =
total # of sides
6 2
PREDICTING RESULTS FOR MONOHYBRID CROSSES (ONE TRAIT)
1.
CALCULATE MATHEMATICALLY OR USE A PUNNETT SQUARE
PARENTS (BOTH HYBRIDS): Rr x Rr, WHERE R IS DOMINANT RED FLOWER
2.
3.
a.
THE PROBABILITY OF A RED FLOWER IS ¾ (OR 75%, or 0.75)
b.
THE PROBABILITY OF A WHITE FLOWER IS ¼ (Or…?)
c.
CAN DETERMINE PHENOTYPIC OR GENOTYPIC RATIOS
TEST CROSSES ARE USED WHEN GENOTYPE OF DOMINANT PHENOTYPE IS
UNKNOWN (I.E., RR OR Rr)
a.
EITHER GENOTYPE LOOKS RED, SO SECOND ALLELE IS UNKNOWN
b.
CROSS WITH HOMOZYGOUS RECESSIVE
INCOMPLETE DOMINANCE
a.
RESULTS IN OFFSPRING OF THE P GENERATION THAT HAVE A
PHENOTYPE SOMEWHERE BETWEEN THE TWO PARENTS
i)
PHENOTYPE IS SEEN IN THE HETEROZYGOTE
ii)
RESULTS IN DIFFERENT PHENOTYPIC RATIOS THAN MENDEL
PREDICTED
b.
EXAMPLE: FOUR O’CLOCK FLOWERS
i)
RR = RED FLOWERS
ii)
rr = WHITE FLOWERS
iii)
Rr = PINK FLOWERS
iv)
PHENOTYPIC RATIOS ARE 1:2:1 RED/PINK/WHITE INSTEAD OF 3:1
RED/WHITE (SOMETIMES THE SYMBOLS FOR INCOMPLETE
D.
DOMINANCE USED ARE R AND W INSTEAD OF R AND r, WHERE
RW IS THE HETEROZYGOTE)
4.
CODOMINANCE
a.
OCCURS WHEN BOTH ALLELES ARE EXPRESSED IN THE HETEROZYGOTE
b.
NEITHER GENE IS DOMINANT OVER THE OTHER
c.
EXAMPLE: ROAN HORSES
i)
RR IS A RED HORSE
ii)
RR’ IS A ROAN HORSE (BOTH RED AND WHITE HAIRS VISIBLE)
iii)
R’R’ IS A WHITE HORSE
PREDICTING RESULTS FOR DIHYBRID CROSSES (TWO TRAITS)
1.
IS A CROSS BETWEEN INDIVIDUALS WITH 2 PAIRS OF CONTRASTING TRAITS
WHAT YOU SHOULD KNOW AFTER STUDYING THIS SECTION:
1.
EXPLAIN HOW YOU MIGHT GO ABOUT DETERMINING THE GENOTYPE AND PHENOTYPE
OF A PURPLE-FLOWERING PEA PLANT.
2.
WHAT IS THE EQUATION USED TO DETERMINE PROBABILITY? IN WHAT WAYS CAN
PROBABILITY BE EXPRESSED?
3.
IF YOU WERE TO CROSS PINK-FLOWERING FOUR O’CLOCKS WITH WHITE-FLOWERING
FOUR O’CLOCKS, WHAT RESULTS WOULD YOU EXPECT? PROVIDE A PUNNET SQUARE
TO SUPPORT YOUR ANSWER.
4.
EXPLAIN HOW YOU WOULD USE A PUNNETT SQUARE TO PREDICT THE PROBABLY
OUTCOME OF A MONOHYBRID CROSS.
5.
6.
EXPLAIN THE DIFFERENCE BETWEEN A MONOHYBRID CROSS AND A DIHYBRID CROSS.
GIVE AN EXAMPLE OF EACH.
(DEMONSTRATE UNDERSTANDING) THE OFFSPRING OF TWO SHORT-TAILED CATS HAVE
A 25 PERCENT CHANCE OF HAVING NO TAIL, A 25 PERCENT CHANCE OF HAVING A LONG
TAIL, AND A 50 PERCENT CHANCE OF HAVING SHORT TAIL. BASED ON THIS
INFORMATION, WHAT CAN YOU HYPOTHESIZE ABOUT THE GENOTYPES OF THE
PARENTS?
DIHYBRID CROSS-TREE METHOD