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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 selfpollinate, 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 terminal
Seed color- yellow or green
Seed shape- round or wrinkled
Pod shape- inflated or constricted
Pod color- green or yellow
Stem 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 gene
Ex: 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 white
GENOTYPE- 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 CROSSEScrosses 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 p
½ x ½ = ¼
“What is probability of an F2 plant having genotype
YYRR?”
- probability of YR is ¼
¼ 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 Ppyyrr
Calculate 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 = 1/16
ppYyrr: ¼ x ½ x ½
= 1/16
Ppyyrr: ½ x ½ x ½
= 2/16
PPyyrr:
¼ x ½ x ½
= 1/16
ppyyrr: 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?”