1. MENDEL AND MONOHYBRIDS AP Biology Ms. Gaynor
Chapter 14 (Part 1)
MENDEL AND MONOHYBRIDS
AP Biology
Ms. Gaynor

2. Transmission (passing down) of Traits
How?
One possible explanation of heredity is a “blending” hypothesis
genetic material contributed by two parents LITERALLY mixes

3. Another Hypothesis
An alternative to the blending model is the hypothesis of inheritance (genes)
Parents pass on discrete heritable units (factors) called genes
The Novelty Gene Video

4. Gregor Johann Mendel (1843)
1st to document a mechanism of inheritance through his experiments with garden peas
Figure 14.1

5. Gregor Johann Mendel
Mendel used the scientific method to identify two laws of inheritance
Mendel discovered the basic principles of heredity
By breeding garden peas in carefully planned (CONTROLLED) experiments

6. Mendel’s Experimental Method
Why use pea plants (model organism)?
available in many varieties of traits
easy to get
could strictly control which plants mated with which
Grow quickly

7. Some genetic vocabulary
Character: an inherited feature, such as flower color
Trait: a variant of a character, such as purple or white flowers

8. Mendel observed the same pattern
In many other pea plant characters

9. Pea Plant Fertilization
Self fertilization : mate with self  produce identical offspring
TRUE or PURE breeds
Cross fertilization : mate with another  can produce different offspring
HYBRIDS
Colored Cotton Video

12. A B
SELF
CROSS

13. Procedure: Crossing pea plants

14. Mendel’s Results Mendel only looked at “either-or” characters
Ex: Purple OR white flowers
Mendel started his experiments with “true-breeding”
Made through self fertilization so plants are “TRUE” for only 1 trait
Known as HOMOZYGOUS for trait

15. What was Mendel’s Procedure?
1. He made 14 “TRUE BREEDS”
1 for EACH trait he looked at
These are the original parents
Are called the P generation

16. What was Mendel’s Procedure?
2. He used cross fertilization to mate 2 true breeds for same gene
Ex: Purple vs white flower color
3. He collected the offspring (progeny)
The hybrid (mixed) offspring of the P generation
Are called the F1 generation

17. What was Mendel’s Procedure?
4. He crossed (using cross fertilization) male and female from F1 progeny
When F1 individuals are mated together
The F2 generation is produced

18. What did Mendel Discovered?
A 3:1 ratio, purple to white flowers, in the F2 generation
P Generation
(true-breeding
parents)
Purple
flowers
White 
F1 Generation
(hybrids)
All plants had
purple flowers
F2 Generation
Where did the white color go?

20. What did Mendel’s Conclude?
Mendel reasoned that
In the F1 plants, only 1 factor (ex: purple flower) was affecting physical outcome color in hybrids
This factor was dominant and the hidden factor was recessive

21. What are Mendel’s factors?
Mendel’s “factors” are now called alleles
Alternative version or form of a gene
Figure 14.4
Allele for purple flowers
SAME locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers F f

23. single nucleotide polymorphism
SNP=
single nucleotide polymorphism

24. Recessive is…
Represented by a lowercase letter (it is NOT the letter itself, though)
an allele that does NOT produce a characteristic effect when present with a dominant allele
only expressed when present with another (identical) recessive allele
This is known as the homozygous condition
aa or hh

25. Dominant is… Represented by a uppercase letter
an allele that produces the same trait whether inherited with a another dominant allele (homozygous) or with a recessive allele (heterozygous)
Aa or AA
The allele that is expressed if present

26. Frequency of Dominant Alleles
Not necessarily better, stronger, etc. than recessive alleles
Ex: Polydactyl

27. The Law of Segregation: Mendel’s 1st Law
Each gamete ONLY gets 1 allele PER GENE

28. More about Alleles… located on homologous chromosomes
Each individual has 2 alleles for the same gene
located on homologous chromosomes
Each parent passes 1 allele for each gene to his/her offspring
Using meiosis
Alleles can be dominant or recessive
“alternative form” of a gene

29. a A

30. Law of Independent Assortment: Mendel’s 2nd Law
Says genes are inherited independently of other genes
Genes are not linked unless on the same chromosome!
Mendel assumed traits occur on different chromosomes!
Occurs during Metaphase I

31. Useful Genetic Vocabulary
Homozygous
A pair of IDENTICAL (same) alleles for that gene
Exhibits true-breeding
aa = homozygous recessive (or just recessive)
HH = homozygous dominant
Heterozygous
Pair of alleles that are different for that gene
Aa or Hh

32. More Genetic Vocabulary
An organism’s genotype
Is its genetic (DNA) makeup
A.k.a.-the allele combination (includes 2 alleles)
An organism’s phenotype
Is its physical outcome of the genotype
Ex: blue eyes or AB blood type

33. Mendelian Genetics & Punnett Squares

34. Mendelian Genetics… aka- COMPLETE DOMINANCE
If an organism is heterozygous (Hh),
The effect of the recessive allele is HIDDEN
Heterozygous and homozygous dominant have SAME phenotype
The 1st allele is “completely dominant” over the 2nd allele

35. Phenotype Genotype 3 Purple White Ratio 3:1 Ratio 1:2:1 1 Purple 2
Figure 14.6 3
Phenotype
Purple
White
Genotype PP
(homozygous) Pp
(heterozygous) pp
Ratio 3:1
Ratio 1:2:1 1
Purple 2 Pp
(heterozygous)
Purple 1 1

36. Why Did Mendel Keep Getting the SAME results?
We can answer this question using a Punnett square
a diagram (box) used to predict probabilities of possible outcomes for offspring that will result from a cross between 2 parents
SHOWS EXPECTED RESULTS (not necessarily actual)

37. Monohybrid Cross Dihybrid Cross
a cross between 2 individuals that looks at 1 trait
Ex: Just looking at the possibility of getting freckles
a cross between 2 individuals that looks at the possibilities of inheriting 2 DIFFERENT traits at one time
Ex: looking at the possibility of getting freckles AND dimples in the SAME offspring
Dihybrid Cross

38. Monohybrid Punnett Square
Mom’s genotype (Hh) x Dad’s genotype (hh)
Tall Short
Mom’s allele # Mom’s allele #2
H h
Dad’s
Allele # h
Allele # h Hh hh

39. Genotype Outcome (Ratio) vs. Phenotype Outcome (Ratio)
Genotype Possibilities = the GENOTYPE probabilities (expected results) of offspring
Ex: 50% HH and 50% Hh
Phenotype Possibilities= the PHENOTYPE probabilities (expected results) of offspring
Ex: 50% Tall 50% Short

40. Example: Heterozygous x Heterozygous
Mom’s genotype (Hh) x Dad’s genotype (Hh)
Tall Tall
H h H h
Genotype ratio = 25% HH, 50% Hh, 25% hh (1:2:1)
Phenotype ratio = 75% Tall, 25% short (3:1) HH Hh hh

41. Working Backwards…The Testcross
Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype
Genotype is not obvious…could be HH or Hh
Cross an individual with the dominant phenotype with individual that is recessive for the same trait

42. Conduct a test cross, where the unknown dominant individual is crossed with the known recessive .
H _ ?_ x hh

43. Mom’s genotype (H?) x Dad’s genotype (hh)
Test Cross
Mom’s genotype (H?) x Dad’s genotype (hh)
Tall Short
H ? H h
If all the offspring are ALWAYS tall…Mom has to be HH
If some offspring are short…Mom has to be Hh HH ?h Hh