6.3 Mendel and Heredity
Section 6-3:
“Mendel & Basic Heredity”
Write everything that is
underlined
6.3 Mendel and Heredity
KEY CONCEPT
Mendel showed that traits are inherited as definite,
identifiable, units (genes), some of which are dominant
over others.
6.3 Mendel and Heredity
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.
6.3 Mendel and Heredity
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.
6.3 Mendel and Heredity
3. Pea plant characteristics
- Mendel Studied 7 “Either-or” Traits of Pea Plants
– The traits included …
-
Pea Shape
Pea Color
Flower Color
Pod Shape
Pod Color
Flower Position
Plant Height
(round or wrinkled)
(yellow or green)
(purple or white)
(smooth or constricted)
(yellow or green)
(axial or terminal)
(tall or short)
6.3 Mendel and Heredity
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)
6.3 Mendel and Heredity
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.
6.3 Mendel and Heredity
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.
6.3 Mendel and Heredity
• F1
- ALL F1 offspring had purple flowers.
- Mendel let the (all purple flower) F1 plants selfpollinate to produce the F2 generation.
- What do you
expect the F2’s
to look like?
6.3 Mendel and Heredity
• F2
- ¾’s of the F2 plants had purple flowers and ¼ had
white flowers (see below)
6.3 Mendel and Heredity
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.
6.3 Mendel and Heredity
Results: Patterns
• Pattern 1:
– 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).
6.3 Mendel and Heredity
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
6.3 Mendel and Heredity
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?
6.3 Mendel and Heredity
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.
6.3 Mendel and Heredity
…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?
6.3 Mendel and Heredity
Answer….
(1) 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:
Give either
or
Rr
Rr
R +
r +
R or r = RR or Rr
R or r = RR or rr
6.3 Mendel and Heredity
WHAT genes do the F2 offspring receive?!
F1 (parents):
Rr
Give either
or
R
r
F2 (offspring):
=
=
Rr
+
+
(both heterozygous)
R or r
R or r
RR or Rr
Rr or rr
=
=
? pairs
? pairs
(Four possible
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
6.3 Mendel and Heredity
Punnett Squares: We show the possible offspring
combinations with a diagram called a Punnett Square
PUNNETT SQUARES
- We show the possible offspring combinations with a
diagram called a “Punnett Square”
6.3 Mendel and Heredity
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
6.3 Mendel and Heredity
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
“