Transcript Mendel and the Gene Idea Patterns of Inheritance

Patterns of
Inheritance
Chapter 14: Mendel and the Gene Idea
Patterns of Inheritance
Parents and offspring
often share observable
traits.
Grandparents and
grandchildren may
share traits not seen in
parents.
Why do traits
disappear in one
generation and
reappear in another?
Possible Hypotheses
The “blending” hypothesis states
that
• Example: Blue and yellow paint
blend to make green
The “particulate” hypothesis states
that
• These heritable units are
that can be passed on to the
next generation
Gregor Mendel documented a
through his
experiments with
Gregor Mendel
Austrian monk
Analyzed
• Asked why
• Tested his theories
• Worked with
Pea plants
Pisum sativum
 Advantages of pea plants for genetic
study:
• Many varieties with distinct
heritable features, or
(such as flower
color)
 Character variants (such as
purple or white flowers) are
called
•
• Each pea plant has spermproducing organs (stamens) and
egg-producing organs (carpels)
•
Traits Mendel used
True breeding plants
Began with true-breeding
varieties (pure-bred)
•
•
 Crossed with other truebreeding variety
• Offspring called
Generations
 Mendel mated two contrasting,
true-breeding varieties
• Process called
 The true-breeding parents are
the
 The hybrid offspring of the P
generation are called the
• Referred to as hybrids
 When F1 individuals selfpollinate, the
is produced
Crosses
Had lots of varieties
• 7 traits with two forms of each trait
Crossing a tall plant with a short plant is a
• A monohybrid cross is
Trait • Two variations Generations
• P - parental (true-breeding):
• F1 - first filial or son (children):
• F2 - next generation (grandkids):
What happened? How did the short plants reappear?
Mendel’s Conclusions
1.
 For example, the gene for flower color in pea plants exists
in two versions: purple flowers and white flowers
 These alternative versions of a gene are now called
 Each gene resides at a specific
 Therefore, we distinguish between an organism’s
•
•
Alleles
Genotype Terminology
If alleles are identical =
• If both alleles are recessive =
 Genotype is
• If both alleles are dominant =
 Genotype is
If both alleles are different =
•
• One dominant allele and one recessive allele ( )
An organism’s traits do not always reveal
Mendel’s Conclusions
2.
 Factors (genes) that determine traits can be
 Alleles may be
•
traits expressed in the F1 generation
•
traits not expressed in the F1 generation
 Mendel observed the same pattern of inheritance in 7
pea plant characters, each represented by two traits
 What Mendel called a “heritable factor” is what we now
call a
Mendel’s Conclusions
3.
 When Mendel crossed contrasting, true-breeding white
and purple flowered pea plants,
 When Mendel crossed the F1 hybrids,
 Mendel discovered a ratio of about
Mendel’s Law of Segregation
4.
 Thus, an egg or a sperm gets only
 This segregation of alleles corresponds to the
distribution of
Principle of Segregation
1. Two copies of each trait (gene)
• Fully expressed gene • Other gene 2. Gametes only have
3. Fertilization restores
Monohybrid & Dihybrid
Crosses
Mendel derived the law of segregation by following
Mendel identified his second law of inheritance by
following
Crosses involving two traits are called
A dihybrid cross can determine whether two
characters are transmitted to offspring
Using a dihybrid cross, Mendel developed the
Law of Independent
Assortment
 The law of independent assortment states that
 This law applies only to genes on
 Genes located near each other on the same chromosome
Probability Rules
Mendel’s laws reflect
When tossing a coin, the outcome
of one toss has
on the
outcome of the next toss
In the same way, the alleles of
one gene
The multiplication rule states that the
Example: probability of 2 coins landing heads up is
Probability Rules
 Each gamete has a chance of
carrying the
and a
chance of carrying the
• Similar to heads and tails
 Another rule is needed to figure out
the probability that an F2 plant from
a monohybrid cross will be
heterozygous rather than
homozygous
 The rule of addition states that the
probability that any
 Example: probability of one heads &
one tails is
Probability Rules
 These rules can be used to predict the outcome of crosses
involving
 A dihybrid or other multicharacter crosses are equivalent to two
or more
 In calculating the chances for various genotypes, each character
is
Punnett squares
 Probability can be depicted
through the use of a
• A diagram for predicting
the results of a genetic
cross between
 Predicts
for all possible gametes with
 Same letter used for trait
•
•
Setting up a Punnett Square
 Step 1. Designate letters which will represent the
genes/traits.
• T = tall t = short
 Step 2. Write down the genotypes (genes) of each parent.
These are often given to you or are possible to determine.
• TT (tall) X tt (short) - both homozygous or purebred
 Step 3. List the genes that each parent can contribute.
Parent 1
Parent 2
Setting up a Punnett Square
 Step 4. Draw a Punnett square and write the possible gene(s)
of one parent across the top and of the other parent along the
side.
 Step 5. Fill in each box of the Punnett square by transferring
the letter above and in front of each box into each appropriate
box. As a general rule, the capital letter goes first and a
lowercase letter follows.
Step 6. List the possible
genotypes and phenotypes of the
offspring for this cross.
Genotypic Ratio:
Phenotypic Ratio:
Practice!
1. Cross a homozygous tall plant with a short
plant. What are the genotypic and
phenotypic ratios?
2. Cross a heterozygous tall plant with a
homozygous tall plant. What are the
genotypic and phenotypic ratios?
3. Cross a heterozygous tall plant with a
short plant. What are the genotypic and
phenotypic ratios?
Testcross
 Used to determine
• Individual expressing
• Could be
 Cross with
 Make prediction with Punnett square
 If homozygous dominant:
• TT x tt
•
 If heterozygous:
• Tt x tt
•
More Practice!!!
1. In rabbits, the allele for black fur (B) is dominant over the allele for brown fur
(b). If a heterozygous male mates with a heterozygous female, what are the
chances that the offspring will have black fur?
2. In humans, dimples are dominant to no dimples. If a homozygous dominant
man reproduces with a heterozygous female, what are the chances of having a
child with no dimples?
3. In humans, freckles are dominant over no freckles. A man with freckles
reproduces with a woman with freckles, but the children have no freckles. What
chance did each child have for freckles?
4. If a man is homozygous for widow’s peak (dominant) reproduces with a woman
homozygous for straight hairline (recessive), what are the chances of their children
having a widow’s peak? A straight hairline?
5. In humans, pointed eyebrows (B) are dominant over smooth eyebrows (b).
Mary’s father has pointed eyebrows, but she and her mother have smooth. What
is the genotype of the father?