1. “Mendel & Basic Heredity” Write everything that is underlined
Section 6-3:
“Mendel & Basic Heredity”
Write everything that is underlined

2. KEY CONCEPT
Mendel showed that traits are inherited as definite, identifiable, units (genes), some of which are dominant over others.

3. Mendel was the “Father” of Genetics.
A. Genetics
The study of inheritance patterns = predictable variations between offspring of the same parents.
Traits are inherited characteristics.
Traits are inherited as definite units – later called “genes”
Some genes dominate others.
Mendel studied ordinary garden peas.

4. Mendel’s data revealed patterns of inheritance.
B. Mendel’s Experiments: He made some key decisions
1. He used “True-breeding” plants
Plants whose offspring always
look like the parents
2. He “controlled” the breeding
(fertilization) of plants
3. He observed seven“either-or” traits
- like purple OR white flower color
- TALL or short height
- green or yellow pea color, etc.

5. 3. Pea plant characteristics
- Mendel Studied 7 “Either-or” Traits of Pea Plants
The traits included …
Pea Shape (round or wrinkled)
Pea Color (yellow or green)
Flower Color (purple or white)
Pod Shape (smooth or constricted)
Pod Color (yellow or green)
Flower Position (axial or terminal)
Plant Height (tall or short)

6. Mendel’s data revealed patterns of inheritance.
True-breeding Plants and Fertilization
Pea plants have both male and female reproductive organs
Male gametes/sex cells (sperm) are called “pollen”
Female gametes/sex cells (eggs)
True-breeding plants “self-pollinate”, meaning fertilize themselves
Offspring are always identical to the parent (because the parent has just one type of gene for the trait)

7. Mendel’s data revealed patterns of inheritance.
Cross: Breeding 2 different plants together… (using
pollen & eggs from separate plants)
Mendel cut away the male part of 1 parent and
used pollen from a 2nd parent to fertilize it.
This is also Called “Cross-pollination” or “a cross”
Mendel controlled the
fertilization of his pea plants
by removing the male parts,
or stamens.
He then fertilized the female
part, or pistil, with pollen from
a different pea plant.

8. Cross: Produced 3 generations: P, F1, and F2 (P = parent; F = child)
P: Parent generation. 2 true-breeding plants were cross-pollinated, to produce the F1 generation
The F1 offspring (of the P’s) were self-pollinated and produced the F2 generation
Mendel controlled the
fertilization of his pea plants
by removing the male parts,
or stamens.
He then fertilized the female
part, or pistil, with pollen from
a different pea plant.

9. F1 - ALL F1 offspring had purple flowers.
- Mendel let the (all purple flower) F1 plants self-
pollinate to produce the F2 generation.
- What do you
expect the F2’s
to look like?

10. F2
¾’s of the F2 plants had purple flowers and ¼ had white flowers (see below)

11. - Mendel observed patterns in the F1 and F2 generations.
Results: Patterns.
- Mendel observed patterns in the F1 and F2 generations.
What are these patterns?
Remember how the F1’s compared to the P’s… then, look below to see the F2 results.

12. Pattern 1: Pattern 2: Results: Patterns
100% of the F1 offspring looked like one of the parents
For example, all F1’s had purple flowers – none were white
Pattern 2:
The F2 offspring were mixed in each trait, 75% of one variation and 25% of the other
For example, 3 plants with purple flowers and 1 with white flowers
That’s a ratio of 75 to 25 (75:25), or 3:1 (3 to 1).

13. Conclusions: Mendel made 4
1. Traits are inherited as discrete units (we call genes)
“Law of Segregation” (has 2 conclusions):
2. Organisms inherit two copies
of each gene, one from each
parent.
3. The two copies segregate during gamete formation
Each copy goes to a different
egg or sperm
(Think of Meiosis and that
gametes are haploid, having
one copy of each chromosome)
purple
white

14. Additional Conclusion?
First, he had two questions:
Why were 100% of the F1’s purple (and none white)?
Why were just 75% of the F2’s purple (and 25% white)?
Purple seemed to “overpower” white….
Conclusion 4?

15. Conclusion 4: Dominance.
- Some inherited versions of a gene for a trait
dominate the other(s)!
Ex: For flower color trait, the purple flower gene
dominates the white flower gene, so plants
with a mix (“hybrids”) of one purple and one white
gene, will have purple flowers, only.

16. …test. This is only a test. Had this been a real ….
Pea pod shapes are either Wrinkled or Round. Round is dominant (so wrinkled is recessive).
(1) What would the pods of the offspring of a true-breeding Wrinkled-pod parent and a true-breeding Round-pod parent look like?
(2) If two of these offspring were cross-bred, how many of their offspring would be Round? How many would be wrinkled?

17. ALL of the offspring (F1’s) would be Round.
Answer….
ALL of the offspring (F1’s) would be Round.
(Each would receive 1 dominant (round) and 1 recessive (wrinkled)
allele from their true-breeding parents. This is called “heterozygous”
– a mix: “1 Round (R) & 1 wrinkled (r) allele”, or “Rr”)
(2) Seventy-five percent (75%) of the F2 offspring would
be Round and twenty-five percent would be wrinkled.
(Each of the heterozygous F1 “rounds” (above) could give either a
dominant or a recessive allele to an F2 offspring. - Does this blow
your mind right in front of your face?)
F2 Parents: Rr Rr
Give either R R or r = RR or Rr
or r R or r = RR or rr

18. WHAT genes do the F2 offspring receive?!
F1 (parents): Rr Rr (both heterozygous) Give either R + R or r = ? pairs or r + R or r = ? pairs F2 (offspring): = RR or Rr (Four possible = Rr or rr outcomes) RR (1 way) = homozygous dominant = Round peas Rr (2 ways) = heterozygous (dominant) = Round peas rr (1 way) = homozygous recessive = wrinkled peas Outcomes: ¾ Round and ¼ wrinkled

19. Punnett Squares: We show the possible offspring combinations with a diagram called a Punnett Square

20. Using Punnett Squares
- Genes/alleles are represented by a capital letter for a
dominant trait and a lower case for a recessive trait:
- ie, round pea pods = R; wrinkled pea pods = r
- Put one parent’s gene combinations across the top
and one across the side of the square
- Fill in the 4 boxes of the Punnett Square with the
letters above and to the side

21. Dominant and one recessive Allele for pod shape.
Using Punnett Squares
Both parents have one
Dominant and one recessive
Allele for pod shape.
- that’s called “Heterozygous”
Offspring - one of four (1/4) has two dominant alleles
That’s “Homozygous Dominant”
One of four (1/4) has two recessive alleles
That’s “Homozygous Recessive”
Two of four (2/4 = ½) have a mix of one dominant and one recessive
That’s “H “