1. LECTURE 6 : GENETICS Introduction to Genetics and heredity
Gregor Mendel – a brief bio
Genetic terminology (glossary)
Monohybrid crosses
Patterns of inheritance
Dihybrid crosses
Test cross
Beyond Mendelian Genetics – incomplete dominance

2. Introduction to Genetics
Genetics is the study of genes.
Inheritance is how traits, or characteristics, are passed on from generation to generation.
Character: a heritable feature, such as flower color
Trait: a variant of a character, such as purple or white flowers
Each trait carries two copies of a unit of inheritance, one inherited from the mother and the other from the father

3. Alternative forms of traits are called alleles
Chromosomes are made up of genes, which are made up of DNA.
Genetic material (genes,chromosomes, DNA) is found inside the nucleus of a cell.
Gregor Mendel is considered “The Father of Genetics"

4. Chromosomes carry the hereditary information (genes)
GENETICS – branch of biology that deals with heredity and variation of organisms.
Chromosomes carry the hereditary information (genes)
Arrangement of nucleotides in DNA
DNA  RNA  Proteins

5. Chromosomes (and genes) occur in pairs Homologous Chromosomes
New combinations of genes occur in sexual reproduction
Fertilization from two parents

6. Themes of Mendel’s work
Variation is widespread in nature
Observable variation is essential for following genes
Variation is inherited according to genetic laws and not solely by chance
Mendel’s laws apply to all sexually reproducing organisms

7. Mendel’s Experimental, Quantitative Approach
Mendel used the scientific approach to identify two laws of inheritance
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments
Mendel chose to work with the garden pea (Pisum sativum)
Because they are available in many varieties, easy to grow, easy to get large numbers
Because he could strictly control which plants mated with which

8. Mendel’s peas
Mendel looked at seven traits or characteristics of pea plants:

9. Mendel was the first biologist to use Mathematics – to explain his results quantitatively.
Mendel predicted
The concept of genes
That genes occur in pairs
That one gene of each pair is present in the gametes

10. Genotype – the genetic makeup of an organisms
Phenotype – the physical appearance
of an organism (Genotype + environment)
Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait.
P = Parental generation
F1 = First filial generation; offspring from a genetic cross.
F2 = Second filial generation of a genetic cross

11. Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers

12. Phenotype vs Genotype

13. Mechanism of gene transmission
Law of Segregation
Mechanism of gene transmission
Gametogenesis:
alleles segregate
Fertilization:
alleles unite

14. Monohybrid cross Parents differ by a single trait.
Crossing two pea plants that differ in stem size, one tall one short
T = allele for Tall
t = allele for dwarf
TT = homozygous tall plant
t t = homozygous dwarf plant
T T  t t

15. Monohybrid cross for stem length:
P = parentals
true breeding,
homozygous plants:
T T  t t
(tall) (dwarf)
T t
(all tall plants)
F1 generation
is heterozygous:

16. Punnett square A useful tool to do genetic crosses
For a monohybrid cross, you need a square divided by four….
Looks like
a window
pane…
We use the
Punnett square
to predict the
genotypes and phenotypes of
the offspring.

17. Using a Punnett Square TT and t t T T  t t
STEPS: 1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 3. draw a p-square Parent genotypes:
TT and t t
Cross
T T  t t

18. Punnett square T T t T T  t t T t
4. "split" the letters of the genotype for each parent & put them "outside" the p-square
5. determine the possible genotypes of the offspring by filling in the p-square
6. summarize results (genotypes & phenotypes of offspring)
T T
T T  t t
T t t
Genotypes:
100% T t
Phenotypes:
100% Tall plants

19. Monohybrid cross: F2 generation
If you let the F1 generation self-fertilize, the next monohybrid cross would be:
T t  T t
(tall) (tall)
Genotypes:
1 TT= Tall
2 Tt = Tall
1 tt = dwarf
Genotypic ratio= 1:2:1
T t
T T
T t
t t T t
Phenotype:
3 Tall
1 dwarf
Phenotypic ratio= 3:1

20. Secret of the Punnett Square
Key to the Punnett Square:
Determine the gametes of each parent…
How? By “splitting” the genotypes of each parent:
If this is your cross
T T  t t
The gametes are: T T t t

21. Once you have the gametes… t t t t
T t T T

22. Shortcut for Punnett Square…
If either parent is HOMOZYGOUS T T  t t t
Genotypes:
100% T t T
T t
Phenotypes:
100% Tall plants
You only need one box!

23. Understanding the shortcut…
T t T =
T t T T
Genotypes:
100% T t
Phenotypes:
100% Tall plants

24. If you have another cross…
A heterozygous with a homozygous T t  t t
You can
still use the
shortcut! t
Genotypes:
50% T t
50 % t t
T t
t t T
Phenotypes:
50% Tall plants
50% Dwarf plants t

25. Another example: Flower color
For example, flower color:
P = purple (dominant)
p = white (recessive)
If you cross a homozygous Purple (PP) with a homozygous white (pp): 
P P p p
ALL PURPLE (Pp)
P p

26. Cross the F1 generation:
P p P p 
Genotypes: PP Pp
1 pp P p
P P
P p
p p P
Phenotypes:
3 Purple
1 White p

27. Mendel’s Principles 1. Principle of Dominance:
One allele masked another, one allele was dominant over the other in the F1 generation.
2. Principle of Segregation:
When gametes are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell (egg/sperm) receives only one kind of gene.

28. Dihybrid crosses
Matings that involve parents that differ in two genes (two independent traits)
For example, flower color:
P = purple (dominant)
p = white (recessive)
and stem length:
T = tall t = short

29. Dihybrid cross: flower color and stem length
TT PP  tt pp
(tall, purple) (short, white)
Possible Gametes for parents
T P and t p
F1 Generation: All tall, purple flowers (Tt Pp)
tp tp tp tp TP
TtPp

30. Dihybrid cross: flower color and stem length (shortcut)
TT PP  tt pp
(tall, purple) (short, white)
Possible Gametes for parents
F1 Generation: All tall, purple flowers (Tt Pp)
T P
T P
t p
t p
T t P p

31. (tall, purple) (tall, purple)
Dihybrid cross F2
If F1 generation is allowed to self pollinate, Mendel observed 4 phenotypes:
Tt Pp  Tt Pp
(tall, purple) (tall, purple)
TP Tp tP tp
Possible gametes:
TP Tp tP tp
Four phenotypes observed
Tall, purple (9); Tall, white (3); Short, purple (3); Short white (1) TP Tp tP tp
TTPP
TTPp
TtPP
TtPp
TTpp
Ttpp
ttPP
ttPp
ttpp

32. Dihybrid cross 9 Tall purple 3 Tall white 3 Short purple 1 Short white
TP Tp tP tp TP Tp tP tp
TTPP
TTPp
TtPP
TtPp
TTpp
Ttpp
ttPP
ttPp
ttpp
Phenotype Ratio = 9:3:3:1

33. Dihybrid cross: 9 genotypes
Genotype ratios (9): Four Phenotypes:
1 TTPP
2 TTPp
2 TtPP
4 TtPp
1 TTpp
2 Ttpp
1 ttPP
2 ttPp
1 ttpp
Tall, purple (9)
Tall, white (3)
Short, purple (3)
Short, white (1)

34. Principle of Independent Assortment
Based on these results, Mendel postulated the 3. Principle of Independent Assortment:
“Members of one gene pair segregate independently from other gene pairs during gamete formation”
Genes get shuffled – these many combinations are one of the advantages of sexual reproduction

35. Relation of gene segregation to meiosis…
There’s a correlation between the movement of chromosomes in meiosis and the segregation of alleles that occurs in meiosis

36. Test cross
When you have an individual with an unknown genotype, you do a test cross.
Test cross: Cross with a homozygous recessive individual.
For example, a plant with purple flowers can either be PP or Pp… therefore, you cross the plant with a pp (white flowers, homozygous recessive)
P ?  pp

37. Test cross
If you get all 100% purple flowers, then the unknown parent was PP…
P P
P p p
If you get 50% white,
50% purple flowers,
then the unknown
parent was Pp…
P p
P p
p p p

38. Dihybrid test cross??
If you had a tall, purple plant, how would you know what genotype it is?
tt pp
?? ?? 
1. TTPP
2. TTPp
3. TtPP
4. TtPp

39. Incomplete Dominance Snapdragon flowers come in many colors.
If you cross a red snapdragon (RR) with a white snapdragon (rr)
You get PINK flowers (Rr)!
R R
r r 
Genes show incomplete dominance when the heterozygous phenotype is intermediate.
R r

40. Incomplete dominance R r R r
When F1 generation (all pink flowers) is self
pollinated, the F2 generation is 1:2:1
red, pink, white
Incomplete Dominance
R r
R R
R r
r r R r

41. Incomplete dominance What happens if you cross a pink with a white? 
A pink with a red? 

42. Exceptions To Mendel’s Original Principles
Pleiotropy
Environmental effects on gene expression
Linkage
Sex linkage
Incomplete dominance
Codominance
Multiple alleles
Polygenic traits
Epistasis

43. Summary of Genetics Chromosomes carry hereditary info (genes)
Chromosomes (and genes) occur in pairs
New combinations of genes occur in sexual reproduction
Monohybrid vs. Dihybrid crosses
Mendel’s Principles:
Dominance: one allele masks another
Segregation: genes become separated in gamete formation
Independent Assortment: Members of one gene pair segregate independently from other gene pairs during gamete formation