chapter 8 introduction to genetics the work of gregor mendel what is genetics? the study of heredity...
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Chapter 8 Introduction to Genetics
The Work of Gregor Mendel What is Genetics? the
study of heredity
Gregor Mendel’s Peas Pollen: plant’s sperm Egg Cells: plants
reproductive cells Fertilization: joining of
pollen + egg cells develops intos embryo in a seed
Born in 1822.His work with pea
plants laid the foundation for
Genetics.
Working with pea plants… Self-pollinating: pollen fertilizes egg cells in the
SAME flower (single parent reproduction) True-breeding: offspring genetically identical to
parents due to self-pollination Cross-pollination: combining reproductive cells
from 2 DIFFERENT parent plants
Mendel could cross- breed a purple flower with a white flower…
What do you think is the color of the offspring?
Genes and Dominance TRAIT: specific characteristic (seed
color, plant height, etc)
What did Mendel do in his pea plant experiments?
Studied 7 different traits each with contrasting characters (ex) Height, short or tall
He crossed the plants (with contrasting characters) and looked at their offspring
P = parental generation = original pair of plantsF1 = first filial generation= first generationHybrids: offspring from parents with different traits
Tracking Generations
Parental generation P
mates to produce
First-generation offspring F1
mate to produce
Second-generation offspring F2
CROSS-POLLINATION: Mendel cut the male parts of one flower (ouch!) and dusted the female parts with pollen from another flower.
P GENERATION: purple x white flowers
F1 GENERATION: all purple flowers
HYBRID PLANTS
What happened in Mendel’s crosses?
All the offspring only had one of the parent’s characters…the other parent’s character disappeared!!
Mendel’s Conclusions:
1. Inheritance is determined by factors that are passed down
2. GENES: the factors that determine traits
3. Contrasting characters are different forms of a gene called ALLELES
some alleles are dominant, some are recessive DOMINANT ALLELE: form of trait that will
always be exhibited; usually expressed in capitals
RECESSIVE ALLELE: form of trait is only exhibited when the dominant allele is NOT present
(ex) Allele for tall is dominant for and the allele for short is recessive
Mendel’s Principle of Dominance
What happened to the recessive allele?
Mendel wanted to know if the recessive allele disappeared from the F1 plants.
F1 CROSS: He self-crossed the F1 generations to make F2 offspring
THE F2 GENERATION…
The recessive traits reappeared!!
~¼ plants had white flowers, the recessive trait
Summary of Crosses:
tall plants X short plants tall plants
P P F1
tall plants self-pollinating ¼ short, ¾ tall
F1 Cross F2
Explaining the F1 Cross
Why did the recessive allele reappear? At some point, the recessive allele had to separate from the dominante allele. This is called…
SEGREGATION: separation of alleles occurs during formation of gametes (eggs & sperm) in anaphase II of meiosis
F1 plants inherited 1 tall allele & 1 short allele from parents When gametes are formed, the two alleles segregate from
each other each gamete has 1 copy of each gene So, 2 different types of gametes are formed (one w/ tall allele,
one w/ short allele)
SEGREGATION
Probability & Punnett Squares
Mendel realized that the principles of probability can explain the results of genetic crosses.
PROBABILITY: likelihood an event will occur (ex) Flip coin 3x in a row, 1/8 chance it will be heads all 3 times ( ½ x ½ x ½ )
The pattern in which alleles segregate is random…just like a coin flip! So which ever allele gametes receive is also random.
Punnett SquaresWhat is a Punnett Square? A diagram showing the possible genetic
combinations from a particular cross Can be used to predict and compare the genetic
variations that will result from a cross
What do the letters represent in a punnett square? Letters represent alleles: capital = dominant
lowercase = recessive
Homozygous: has two identical alleles for a trait (ex) TT or tt
Heterozygous: has two different alleles for the same trait (ex) Tt
Punnett Squarefor TT x Tt
Punnett Squarefor YY x yy
Genotype vs Phenotype
GENOTYPE: the genetic makeup of an organism (ex) TT
PHENOTYPE: the physical characteristics exhibitied (ex) tall plant
In the Punnett Square shown
What is the genotype of the offspring?
What is the phenotype?
Probability and Segregation
F2 generation from Tall F1 plants ¾ tall, ¼ short
3:1 ratio of tall to short plants
Punnet squares work to predict outcomes, so Mendel’s ideas about segregation are accurate!
Exploring Mendelian Genetics
Does the gene that determines flower color have anything to do with the gene for height?
(ex) Do all tall plants have purple flowers?
Mendel performed TWO-FACTOR CROSSES: crossing 2 different genes and following traits as they pass from one generation to the next
Two-Factor Cross: F1
Two Genes: shape of pea & color of pea The Cross: Round yellow peas x wrinkled green peas
RRYY x rryy
What are the possible alleles parent 1 can pass? RY What are the possible alleles parent 2 can pass? ry Draw a Punnett Square for this cross. All F1 were RrYy (round and yellow) or HYBRIDS
This cross does not answer question, but provides hybrids for next cross
Two-Factor Cross: F2
F1 Generation = RrYy
How would these alleles segregate when F1 self-pollinated? RrYy x RrYy
Do the two dominant alleles stay together?
What are the possible alleles each parent can pass on?
There are 4 possible combinations: Ry, RY, rY, ry
Draw a Punnett Square for this cross.
The F1 Hybrid cross produces a 9:3:3:1 phenotype ratio
Mendel found that the 2 alleles (seed shape & seed color) don’t influence each other’s inheritance
This is called the principle of Independent Assortment: genes for different traits can segregate independently during the formation of gametes
INDEPENDENT ASSORTMENT
Independent Assortment
Metaphase I:
Metaphase II:
Gametes:
1/4 AB 1/4 ab 1/4 Ab 1/4 aB
A A A A
A A A A
AAAA
B B
B B
BB
B B
BBBB
a a a a
aa aa
aaaa
bb b b
bb b b
b b b b
OR
Summary of Mendel’s Principles
A. Inheritance of characteristics is determined by genes which are passed to offspring
B. If 2+ alleles of a trait exist, some alleles may be dominant, others may be recessive
C. Sexually reproducing organisms have 2 copies of each gene which segregate during gamete formation
D. Alleles for different genes segregate independently
Beyond Dominant and Recessive AllelesGenetics is more complicated
Some alleles are neither dominant nor recessive
Many traits are controlled by multiple alleles or multiple genes
Other Inheritance Patterns…
1. Incomplete Dominance
2. Codominance
3. Multiple Alleles
4. Polygenic Traits
XHomozygous parent (RR)
Homozygous Parent (rr)
All F1 are heterozygous
X
F2 shows three phenotypes in 1:2:1 ratio
Incomplete Dominance When one allele is not
completely dominant; recessive allele is not totally masked
Heterozygous phenotype is in between the two homozygous phenotypes
(ex) Red snapdragon flowers (RR) X snapdragon white (rr) flowers pink hybrid flowers (Rr)
Xhomozygous parent homozygous parent
All F1 offspring heterozygous for flower color:
Cross two of the F1 plants and the F2
offspring will show three phenotypes in a 1:2:1 ratio:
Incomplete Incomplete DominanceDominance
Codominance Both alleles contribute to the phenotype Heterozygous genotype expresses both
phenotypes (ex) Feather colors in chickens: white feathers X
black feathers speckled chicken (ex) Horse coats: red X white roan coat
Codominance: ABO Blood Types
Alleles that controls blood type are codominant
Two alleles A & B are both exhibited when paired, a third allele (i) is recessive to others AA or Ai = Type A Blood BB or Bi = Type B Blood AB = Type AB Blood ii = Type O Blood
Multiple Alleles
> 2 possible alleles for a gene Individuals can still only have 2 alleles each
but more than 2 alleles exist in a population
(ex) coat color in rabbits lots of options due to 4 different alleles
(ex) blood type is determined by multiple alleles
Polygenic Traits
Traits controlled by the interaction of 2+ genes
(ex) Fruit fly eye color (3+ different genes)
(ex) Skin color in humans (4+ different genes), eye color, height, weight
Applying Mendel’s Principles Early 1900’s Morgan used Mendel’s
principles to study fruit flies advanced study of genetics
Mendel’s Principles also apply to study the inheritance of human traits and to calculate the probability of traits appearing in the next generation.