Download 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 genetic material from the two
parents blends together
• Example: Blue and yellow paint
blend to make green
The “particulate” hypothesis states
that parents pass on discrete
heritable units
• These heritable units are genes
that can be passed on to the
next generation
Gregor Mendel documented a
particulate mechanism through
his experiments with garden peas
Gregor Mendel
Austrian monk
Analyzed patterns of
inheritance
• Asked why
• Tested his theories
making predictions
based on math
• Worked with garden
peas
Pea plants
Pisum sativum
 Advantages of pea plants for genetic
study:
• Many varieties with distinct
heritable features, or
characters (such as flower
color)
 Character variants (such as
purple or white flowers) are
called traits
• Mating of plants can be
controlled
• Each pea plant has spermproducing organs (stamens) and
egg-producing organs (carpels)
• Cross-pollination (fertilization
between different plants) easily
done
Traits Mendel used
True breeding plants
Began with true-breeding
varieties (pure-bred)
• Self-pollinate
• Always produce same
offspring
 Crossed with other truebreeding variety
• Offspring called hybrid
Generations
 Mendel mated two contrasting,
true-breeding varieties
• Process called
hybridization
 The true-breeding parents are
the P generation
 The hybrid offspring of the P
generation are called the F1
(first filial) generation
• Referred to as hybrids
 When F1 individuals selfpollinate, the F2 (second
filial) generation 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 monohybrid
cross
• A monohybrid cross is involving only one trait.
Trait - plant height
• Two variations - tall and short
Generations
• P - parental (true-breeding): tall & short
• F1 - first filial or son (children): all tall
• F2 - next generation (grandkids): tall & short
What happened? How did the short plants reappear?
Mendel’s Conclusions
1. Alternative versions of genes account for
variations in inherited characters
 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 alleles
 Each gene resides at a specific locus (location on a specific
chromosome)
 Therefore, we distinguish between an organism’s
• phenotype or physical appearance
• genotype or genetic makeup
Alleles
Genotype Terminology
If alleles are identical = homozygous
• If both alleles are recessive = homozygous
recessive
 Genotype is aa
• If both alleles are dominant = homozygous
dominant
 Genotype is AA
If both alleles are different = heterozygous
• Dominant phenotype expressed
• One dominant allele and one recessive allele (Aa)
An organism’s traits do not always reveal its genetic
composition
Mendel’s Conclusions
2. For each character, an organism inherits two
alleles, one from each parent
 Factors (genes) that determine traits can be hidden or
unexpressed.
 Alleles may be identical (true-breeding plants) or
different (F1 hybrids)
• Dominant traits expressed in the F1 generation
• Recessive 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 gene
Mendel’s Conclusions
3. If two alleles at a locus differ, then one
(dominant allele) determines the organism’s
appearance, and the other (recessive allele) has
no noticeable effect on appearance
 When Mendel crossed contrasting, true-breeding white
and purple flowered pea plants, all of the F1 hybrids
were purple
 When Mendel crossed the F1 hybrids, many of the F2
plants had purple flowers, but some had white
 Mendel discovered a ratio of about three to one, purple
to white flowers, in the F2 generation
Mendel’s Law of Segregation
4. The law of segregation states that the two
alleles for a heritable character segregate
(separate) during gamete formation and end
up in different gametes
 Thus, an egg or a sperm gets only one of the two
alleles that are present in the somatic cells of an
organism
 This segregation of alleles corresponds to the
distribution of homologous chromosomes to different
gametes in meiosis
Principle of Segregation
1. Two copies of each trait (gene)
• Fully expressed gene dominant
• Other gene - recessive
2. Gametes only have one copy
(haploid)
3. Fertilization restores pairs or two
copies (diploid)
Monohybrid & Dihybrid
Crosses
Mendel derived the law of segregation by following a
single character
Mendel identified his second law of inheritance by
following two characters at the same time
Crosses involving two traits are called dihybrid
crosses
A dihybrid cross can determine whether two
characters are transmitted to offspring as a package
or independently
Using a dihybrid cross, Mendel developed the law
of independent assortment
Law of Independent
Assortment
 The law of independent assortment states that each pair of
alleles segregates independently of each other pair of alleles
during gamete formation
 This law applies only to genes on different, nonhomologous
chromosomes
 Genes located near each other on the same chromosome tend to
be inherited together
Probability Rules
Mendel’s laws reflect the rules of
probability
When tossing a coin, the outcome
of one toss has no impact on the
outcome of the next toss
In the same way, the alleles of
one gene separate into gametes independently of another
gene’s alleles
The multiplication rule states that the probability that two
or more independent events will occur together is the
product of their individual probabilities
Example: probability of 2 coins landing heads up is 1/4
(1/2 x 1/2 = 1/4)
Probability Rules
 Each gamete has a chance of
carrying the dominant allele and a
chance of carrying the recessive
allele
• 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 one of two or
more exclusive events will occur is
calculated by adding together their
individual probabilities
 Example: probability of one heads &
one tails is 1/2 (1/4 + 1/4 = 1/2)
Probability Rules
 These rules can be used to predict the outcome of crosses
involving multiple characters
 A dihybrid or other multicharacter crosses are equivalent to two
or more independent monohybrid crosses occurring
simultaneously
 In calculating the chances for various genotypes, each character
is considered separately, and then the individual probabilities
are multiplied together
Punnett squares
 Probability can be depicted
through the use of a Punnett
square
• A diagram for predicting
the results of a genetic
cross between individuals
of known genetic makeup
 Predicts all possible offspring
for all possible gametes with
random fertilization
 Same letter used for trait
• Capital letter = dominant
allele (A)
• Lower case letter =
recessive allele (a)
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: 4/4 or
100% Tt genotype
Phenotypic Ratio: 4/4 or
100% tall
Practice!
1. Cross a homozygous tall plant with a short plant.
What are the genotypic and phenotypic ratios?
100% Tt; 100% Tall
2. Cross a heterozygous tall plant with a
homozygous tall plant. What are the genotypic and
phenotypic ratios?
50% Tt & 50% TT; 100% Tall
3. Cross a heterozygous tall plant with a short
plant. What are the genotypic and phenotypic
ratios?
50% Tt & 50% tt; 50% Tall & 50 % Short
Testcross
 Used to determine exact genotype
• Individual expressing dominant
phenotype
• Could be heterozygous or
homozygous dominant
 Cross with homozygous recessive
 Make prediction with Punnett square
 If homozygous dominant:
• TT x tt
• All offspring Tt or tall plants
 If heterozygous:
• Tt x tt
• Offspring - half Tt (tall), half tt
(short)
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?
75% 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?
0% 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?
75% chance 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?
100% Widow’s peak; 0% 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?
Bb (heterozygous)
Review Questions
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10.
Differentiate between the blending and particulate hypotheses of
inheritance.
Explain the importance of Gregor Mendel’s work with garden peas.
Also, explain why he used garden peas.
Define the following vocabulary associated with basic genetics:
character, trait, hybrid, gene, allele, locus, genotype, phenotype,
dominant, recessive, homozygous, & heterozygous.
Differentiate between the P, F1, and F2 generations.
Differentiate between monohybrid and dihybrid crosses.
Explain Mendel’s four basic conclusions regarding inheritance
patterns.
Explain the three parts to the law of segregation.
Explain the law of independent assortment.
Properly construct a Punnett square for use in solving a genetics
problem involving probability.
Explain the idea of a testcross.