chapter 14
DESCRIPTION
CHAPTER 14. INTRODUCTION TO GENETICS. MODELS OF HEREDITY. 1. BLENDING MODEL - genetic material contributed by the two parents mixes - over many generations, a freely mating population will give rise to a uniform population of individuals - everyday observation contradicts this model - PowerPoint PPT PresentationTRANSCRIPT
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CHAPTER 14
INTRODUCTION TO GENETICS
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MODELS OF HEREDITY
1. BLENDING MODEL- genetic material contributed by the two parents
mixes - over many generations, a freely mating population
will give rise to a uniform population of individuals- everyday observation contradicts this model- does not explain why traits sometimes skip
generations
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2. Particulate model (the gene idea)- parents pass on discrete heritable units (genes)
that retain their separate identities in offspring
Modern genetics began with the work of Gregor Mendel, who documented this particulate model of inheritance
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Figure 14.0x Mendel
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Figure 14.0 Painting of Mendel
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GREGOR MENDEL
Mendel begin breeding garden peas around 1857 to study inheritance
- there was a long tradition of breeding plants at the monastery where he lived
- he probably chose to work with peas because there are many varieties
CHARACTER- a heritable feature that varies among individuals, such as flower color
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TRAIT- a variant for a character, such as purple or white flowers
By using peas, Mendel was also able to control which plants mated
- each pea flower has both male (stamens) and female (carpel) parts
- these plants usually self-fertilize
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Mendel cross-pollinated the plants:- he removed immature stamens from a plant
before they produced pollen- he then dusted the carpel of with pollen from
another flower
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Figure 14.1 A genetic cross
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Mendel began all of his experiments with TRUE-BREEDING
- when the plants self-pollinate, all offspring are of the same variety
Mendel crossed 2 true-breeding varieties (example: white vs. purple flowers)
- the parents are called the P1 generation
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- their hybrid offspring are called the F1 generation
- allowing the F1 generation to self-pollinate produced the F2 generation
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RESULTS OF THE EXPERIMENTS
When Mendel crossed pure purple and pure white flowered plants, he got all purple flowers in the F1 generation
- if the blending model had been correct, these flowers should have been pale purple
What happened to the white?- when Mendel allowed F1 plants to self-pollinate,
white flowers reappeared
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Figure 14.2 Mendel tracked heritable characters for three generations
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- his results worked out to about 3 purple to 1 white flower
Mendel reasoned that the factor for white flowers did not disappear in the F1 plants, but only the purple-flower factor was affecting flower color in this generation
- Mendel called the purple color a DOMINANT TRAIT and the white color a RECESSIVE TRAIT
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Mendel observed other characters with the same results:
Flower position- axial or terminalSeed color- yellow or greenSeed shape- round or wrinkledPod shape- inflated or constrictedPod color- green or yellowStem length- tall or dwarf
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Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
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LAW OF SEGREGATION
Mendel developed a hypothesis to explain his results that can be broken down into 4 parts:
1. Alternative versions of genes account for variations in inherited characters
- ALLELES- alternative versions of a geneEx: Gene is flower color, alleles are purple
and white
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- the purple allele and the white allele are 2 DNA variations possible at the flower color locus on one of a pea plant’s chromosomes
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Figure 14.3 Alleles, alternative versions of a gene
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2. For each character, an organism inherits two alleles, one from each parent
- RECALL: a diploid organism has homologous pairs of chromosomes, one from each parent
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3. If the 2 alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance
4. The two alleles for each character segregate (separate) during gamete production
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- an ovum and a sperm each get only one of the two alleles that are present in the somatic cells of the organism
- this is where the name of the law, the LAW OF SEGREGATION, comes from
PUNNETT SQUARE- a diagram used to predict the results of a genetic cross
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Figure 14.4 Mendel’s law of segregation (Layer 2)
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SOME IMPORTANT VOCABULARY
HOMOZYGOUS- an organism having a pair of identical alleles for a character
Ex: a pea plant that is true-breeding for purple flowers (PP)
- can also be for white flowers (pp)HETEROZYGOUS- an organism having 2 different
alleles for a gene- for the flowers, would be Pp- produces a purple flower
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PHENOTYPE- an organism’s traits- this is the PHYSICAL APPEARANCE or
ABILITY- for the flowers, the phenotypes are either purple
or whiteGENOTYPE- an organism’s genetic makeup- for the flowers, the phenotypes could be PP, pp,
or Pp
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Figure 14.5 Genotype versus phenotype
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TESTCROSS
A TESTCROSS is a genetic cross performed when the genotype of one of the parents is unknown
- the genotype of the parent can be determined by looking at the offspring
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LAW OF INDEPENDENT ASSORTMENT
The Law of Segregation was derived by performing breeding experiments using only a single character
- the F1 hybrids produced in these crosses are called MONOHYBRIDS (Monohybrid crosses)
Mendel also performed DIHYBRID CROSSES- crosses involving 2 separate characters
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Mendel studied seed color and seed shape- he knew that the allele for yellow seeds is
dominant over green seeds, and the allele for round seeds is dominant over wrinkled seeds
- he crossed 2 true-breeding plants that differed in BOTH characters:
YYRR x yyrr
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Mendel wondered if the 2 characters, seed color and seed shape, were transmitted from parents to offspring as a package
- in other words, will Y and R alleles always stay together?
Or- are they inherited independently of each
other?
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For both possibilities, the F1 plants were heterozygous for both traits:
YyRr- Mendel needed to see what would happen
when these plants self-pollinated (in the F2 generation)
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THE RESULTS
Experimental results supported the hypothesis that each character is independently inherited
- the 2 alleles for seed color segregate independently of the 2 alleles for seed shape
- Mendel always ended up with the 9:3:3:1 phenotypic ratio
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The independent segregation of each pair of alleles during gamete formation is now called the LAW OF INDEPENDENT ASSORTMENT
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PROBABILITY
RECALL:The probability scale ranges from 0 to 1- an event certain to occur has a probability of 1- an event certain NOT to occur has a probability of
0- with a coin, the chance of tossing heads is ½, tails
is ½
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This can be applied in fertilization:- the ovum has ½ chance of carrying a
dominant allele, and ½ chance of carrying a recessive allele
- the same odds apply to the sperm- like 2 separate coin tosses, allele segregation
occurs 2 independent events
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Rule of Multiplication
“What is the chance that 2 coins tossed simultaneously will land heads up?”
- we will find the probability for each independent event and then multiply these events together
½ x ½ = ¼
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“An F1 plant is Pp for purple flower color. What is the probability that an F2 plant will have white flowers?”
- both ovum and sperm must carry a p½ x ½ = ¼
“What is probability of an F2 plant having genotype YYRR?”
- probability of YR is ¼¼ x ¼ = 1/16
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Rule of Addition
“What is the probability that an F2 plant from a monohybrid cross will be heterozygous?”
- there are 2 ways that this can occur:* the dominant allele can come from the ovum and the recessive from the sperm, or vice versa
- to find the probability that an event can occur in 2 or more different ways, we add the probabilities
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¼ + ¼ = ½
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We can combine these rules for more complicated crosses:
PpYyRr x PpyyrrCalculate the fraction of offspring predicted to
show the recessive phenotypes for at least 2 of the 3 traits :
Ppyyrr, ppYyrr, ppyyRr, PPyyrr, ppyyrr- we will combine the rules
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PpYyRr x Ppyyrr
ppyyRr: ¼ (prob. of pp) x ½ x ¼ = 1/16ppYyrr: ¼ x ½ x ½ = 1/16Ppyyrr: ½ x ½ x ½ = 2/16PPyyrr: ¼ x ½ x ½ = 1/16ppyyrr: ¼ x ½ x ½ = 1/16___________________________________Chance of at least 2 = 6/16 recessive traits or 3/8
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Try it:PPYyrr x PpyyRr
“What is the probability of getting dominant phenotypes for at least 2 of the 3 traits?”
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