1. Chapter 8 Introduction to Genetics

2. 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.

3. 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?

4. 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

5. P = parental generation = original pair of plants
F1 = first filial generation= first generation
Hybrids: offspring from parents with different traits

6. Tracking Generations Parental generation P mates to produce
First-generation offspring F1
mate to produce
Second-generation offspring F2

7. P GENERATION: purple x white flowers F1 GENERATION: all purple flowers
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

8. Inheritance is determined by factors that are passed down
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:
Inheritance is determined by factors that are passed down
GENES: the factors that determine traits
Contrasting characters are different forms of a gene called ALLELES

9. Mendel’s Principle of Dominance
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

10. 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

11. 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

12. 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)

13. SEGREGATION

14. 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.

15. Punnett Squares What 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

16. Punnett Square for TT x Tt
Punnett Square for YY x yy

17. 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?

18. 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!

19. 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

20. 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

21. 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.

22. INDEPENDENT ASSORTMENT
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

23. Independent Assortment
Metaphase I: A A a a A A a a B B b b b b B B
Metaphase II: A A a a A A a a B B b b b b B B
Gametes: B B b b b b B B A A a a A A a a
1/4 AB
1/4 ab
1/4 Ab
1/4 aB

24. Summary of Mendel’s Principles
Inheritance of characteristics is determined by genes which are passed to offspring
If 2+ alleles of a trait exist, some alleles may be dominant, others may be recessive
Sexually reproducing organisms have 2 copies of each gene which segregate during gamete formation
Alleles for different genes segregate independently

25. Beyond Dominant and Recessive Alleles
Genetics is more complicated
Some alleles are neither dominant nor recessive
Many traits are controlled by multiple alleles or multiple genes

26. Other Inheritance Patterns…
Incomplete Dominance
Codominance
Multiple Alleles
Polygenic Traits

27. Incomplete
Dominance
Homozygous
parent (RR) X
Homozygous
Parent (rr)
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)
All F1 are
heterozygous X
F2 shows three phenotypes in 1:2:1 ratio

28. Incomplete Dominance X All F1 offspring heterozygous for flower color:
homozygous parent X
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:

29. 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

30. 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

31. 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

32. 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

33. 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.