1. Lecture #24 **change PPT Guide # to 27
Introduction to Mendel
Honors 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 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

4. Gregor Johann Mendel 1843
Documented a mechanism of inheritance through his experiments with garden peas
Figure 14.1

5. Gregor Johann Mendel
Mendel discovered the basic principles of heredity
By breeding garden peas in carefully planned experiments

6. Mendel’s Experimental Method
Why use pea plants?
available in many varieties
easy to get
he could strictly control which plants mated with which
Grow quickly

7. 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

10. SELF
Cross B A

11. Procedure: Crossing pea plants

12. Mendel’s Procedures Mendel chose to track
Only those characteristics (traits) that varied in an “either-or” manner
Mendel also made sure that
He started his experiments with varieties that were “true-breeding”

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. Mendel’s Generations Original Parents Are called the P generation
The hybrid (mixed) offspring of the P generation
Are called the F1 generation
When F1 individuals self-pollinate
The F2 generation is produced

17. 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

18. 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

19. Generations (in general)
P = parental generation of a cross
F1 = the first generation after the parental (the results of the 1st cross)
F2 = a cross between F1 individuals yields F2 (2nd cross)

20. 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?

22. 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

23. 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

24. Alleles Alleles = alternative forms of genes (Mendel’s “factors”)
Each individual has 2 alleles for the same gene (because there are 2 homologous chromosomes)
Each parent passes one allele for each gene to his/her offspring

25. h or H

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

27. Recessive is… Recessive
an allele that does NOT produce a characteristic effect when present with a dominant allele
only expressed when the determining allele is present in the homozygous condition
aa or hh

28. Dominant is…
Dominant
an allele that produces the same phenotypic effect whether inherited with a homozygous or heterozygous allele
Aa or AA, Hh or HH
The allele that is expressed
Not necessarily better, stronger, etc.

29. Mendel’s Law of Segregation
The two alleles for a characteristic separate (segregate) during gamete formation (Anaphase I of meiosis) end up in different gametes
Each gamete gets 1 allele ONLY

30. Mendel’s Model Mendel developed a model to predict inheritance
Mendelian Genetics states
That heterozygotes will have the SAME phenotype as homozygous dominant individuals
Aka-Complete Dominance

31. Mendelian Genetics COMPLETE DOMINANCE
If the 2 alleles at a locus differ (heterozygous)
Then the dominant allele determines organism’s appearance
The recessive allele has no noticeable effect on organism’s appearance (it is hidden)
The 1st allele is “completely dominant” over the 2nd allele

32. More Genetic Vocabulary
An organism’s phenotype
Is its physical appearance
Ex: Blue eyes, brown hair, etc
An organism’s genotype
Is its genetic makeup
A.k.a.-the allele combination
Ex: aa, Aa, AA