CHAPTER 14
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Transcript of CHAPTER 14
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- does not explain why traits sometimes skip
generations
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
Figure 14.0x Mendel
Figure 14.0 Painting of Mendel
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
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
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
Figure 14.1 A genetic cross
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
- their hybrid offspring are called the F1 generation
- allowing the F1 generation to self-pollinate produced the F2 generation
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
Figure 14.2 Mendel tracked heritable characters for three generations
- 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
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
Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
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
- 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
Figure 14.3 Alleles, alternative versions of a gene
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
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
- 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
Figure 14.4 Mendel’s law of segregation (Layer 2)
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
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
Figure 14.5 Genotype versus phenotype
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
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
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
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?
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)
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
The independent segregation of each pair of alleles during gamete formation is now called the LAW OF INDEPENDENT ASSORTMENT
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
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
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
“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
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
¼ + ¼ = ½
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
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
Try it:PPYyrr x PpyyRr
“What is the probability of getting dominant phenotypes for at least 2 of the 3 traits?”