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Chapter 10
Mendel and Heredity
(CH 10.1)
The Origins of Genetics
Mendel and Others Studied Pea Plants.
• The passing of traits from parents to
offspring is called heredity.
• Can be used to alter crop plants and
domestic animals to give them traits more
useful to humans.
Mendel and Math
• Austrian monk, Gregor Johann Mendel, carried
out experiments where he bred different
varieties of the garden pea Pisum sativum.
• Mendel was the first to develop rules that
accurately predict patterns of heredity which
formed the basis of genetics (the branch of
biology that focuses on heredity).
• Mendel crossed (mated) a variety of pea plants
that had purple flowers with a variety that had
white flowers. All the offspring had purple
flowers. However, when the offspring were
crossed, the second group of offspring had both
white and purple flowers!
Useful Features in Pea Plants
• Garden pea is small, grows easy and
quickly, and produces many offspring.
Results can be obtained quickly with many
offspring to count.
• Only has two colors, purple and white (no
intermediates or blended colors).
• Can self-pollinate or cross-pollinate to
control matings (in cross-pollinate, remove
the stamen or male parts from flowers to
better control mating).
Mendel Observed that Traits are
Expressed in 3 Simple Ratios
•
•
Initial experiments were monohybrid
crosses (cross that involves one pair of
contrasting traits, i.e. tall vs short).
Allowed each pea plant to self-pollinate
for several generations to ensure truebreeding (only produce one particular
trait) called the parental generation or P
generation which are the first two
individuals that are crossed in a breeding
experiment.
•
•
Then cross-pollinated
two P generation plants
that had contrasting traits
(purple vs white) and
called the offspring the
first filial generation, or
the F1 generation.
Recorded the number of
F1 plants expressing
each trait.
Finally, allowed the F1
generation to selfpollinate. Called the F1
offspring the second filial
generation, or the F2
generation. Again, each
was counted.
Mendel’s Results
• F1 plants showed only one form of the trait, the
white disappeared! But when the F1 selfpollinated, the missing trait reappeared in the F2
generation.
• Mendel formed a ratio of the purple vs white.
Total # of plants: 929. Total number of purple:
705 and total number of white: 224. So, Mendel
wrote a ratio, 705:224, or after being reduced,
3:1.
705/224= 3.15 (round to 3), 224/224= 1, so 3:1
• For each of the 7 traits Mendel studied, he found
the same 3:1 ratio of plants expressing
contrasting traits in the F2 generation (pg 262)
Mendel’s Theory
Mendel’s Work Became Theory of
Heredity
• Before Mendel, people thought that
offspring was just a blend of the parents
(tall mixed with short gave you medium).
Mendel’s results didn’t support this. He
correctly concluded that each pea has two
separate “heritable factors” for each trait –
one from each parent.
(Remember, haploid or half plus haploid or
half gives diploid or whole?)
Mendel’s Hypotheses
These 4 theories make up the Mendelian
theory of heredity (foundation of
genetics)
• For each inherited trait, an individual has
two copies of the gene – one from each
parent.
• There are alternative versions of genes.
(ex. Purple vs white) Different versions of
a gene are called alleles.
•
When two different alleles occur together,
one of them may be completely
expressed, thus the other may have no
observable effect on the organism’s
appearance. Mendel described the
expressed form as dominant. The trait
that did not show when the dominant was
present, was called recessive. One
form of the trait is always dominant and
the allele for the other form was always
recessive.
•
When gametes are formed, the alleles for
each gene in an individual separate
independently of one another (remember
independent assortment?) Thus,
gametes carry only one allele for each
inherited trait. When gametes unite
during fertilization, each gamete
contributes one allele. Each parent can
contribute only one of the alleles bc of
the way gametes are produced during
meiosis.
Mendel’s Findings in Modern Terms
• Dominant alleles are indicated by writing the
first letter of the trait as a capital letter (P for
purple flowers). Recessive alleles are indicated
by writing the first letter of the dominant trait in
lowercase (p for white flowers).
• If the two alleles present in an individual are the
same, the individual is said to be homozygous
for that trait (PP for purple, pp for white).
• If the alleles present in an individual are
different, the individual is heterozygous (Pp for
purple).
• The set of alleles a person has is called the
genotype. The physical appearance of a trait is
called a phenotype.
Laws of Heredity
• Law of Segregation: two alleles for a trait
segregate (separate) when gametes are formed
(remember, first chromosomes are separated in
anaphase I, then the actual chromatids are split
at the centromere in anaphase II?)
• Law of Independent Assortment: the alleles of
different genes separate independently of one
another during gamete formation. This means
that the inheritance of one trait did not influence
the inheritance of any other trait.
• Units of heredity are portions of DNA called
genes…
•
•
•
Studying Heredity
Punnet Squares Can Predict the
Expected Results in Crosses
A Punnett Square (named after
Reginald Punnett) is a diagram
that predicts the expected
outcome of a genetic cross by
considering all possible
combinations of gametes in the
cross (predicts the probability of
genotypes and phenotypes).
List one parent's gametes across
the top of the square, and the
other parent's gametes down the
side of the square.
Combine all of the possible
gametes from each parent to
produce all of the possible
combinations of alleles in the
Dark eyes (D) are dominant
to light eyes (d)
•How many offspring will exhibit a
phenotype of dark eyes? Light eyes?
•How many heterozygous genotypes
are there? Homozygous?
• The smallest Punnet square has four boxes and only
considers one trait. This type of cross is called a
monohybrid cross.
• A dihybrid cross considers two traits and has 16
boxes in its Punnet square. In order to do this, you
must consider how the four alleles from either parent
can combine to form gametes.
• For two parents that are heterozygous for Round and
Yellow (RrYy), the possible gametes could be RY, Ry,
rY, or ry for each parent. These are written on the top
and left side of the cross.
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
• How many offspring would be round and yellow?
Round and green? Wrinkled and yellow? Wrinkled
Determining Unknown Genotypes
If you know the phenotype but not the genotype:
• A test cross can be performed on a dominant
phenotype by crossing it with a homozygous
recessive in order to determine if the genotype
of that specimen is homozygous dominant or
heterozygous dominant.
• Ex: We cross a plant that is yellow with a plant
that is green. If all of the offspring are yellow, we
know that the parent is homozygous dominant.
If there is a green offspring, the parent is
heterozygous.
Probabilities Can Also Predict the
Expected Results of Crosses
• Probability :likelihood that a specific event will occur
and can be presented as words (1 out of 1), decimals
(1), percentages (100%), or fractions (1/1)
Probability = number of one kind of possible outcome
total number of all possible outcome
• This formula can be used to predict specific alleles
being present in the gamete. A parent has two alleles
(let’s say one each for green and yellow) that can
possibly passed down to the gamete. The probability
that the gamete will get the allele for yellow is ½. For
green it is ½ also because the gamete will only receive
one allele (green or yellow) from that parent, and there
are two total possible colors (1/2).
• Probability = number of one kind of possible outcome
total number of all possible outcome
• Both parents have to be considered when calculating probability.
Alleles from one parent does not depend on alleles from the other
parent, they are considered to be independent. In order to find the
probability that a combination of this event will occur, you multiply
the separate probabilities together. (1/2 * 1/2 = 1/4)
• How many total possible
outcomes are there?
• What is the probability that an
offspring will have dark eyes?
• What is the probability that an
offspring will be heterozygous
for dark eyes?
• What is the probability that an
offspring will have light eyes?
Family Pedigrees Can be Used
to Study How Traits are Inherited
(CH12)
• Pedigree - a family history that shows how a trait is inherited over
several generations.
• People who are affected by
a specific trait are usually
marked with a colored circle
(female) or colored square
(male) and the trait is usually
recessive.
• People who are not affected
and are not carriers are
usually marked with a white
circle or square.
• People who are carriers of
the trait are marked with half
colored/ half white circles or
squares.
Horizontal lines indicate mating, and vertical lines
• Are there any carriers in this
indicate offspring.
pedigree example?
Autosomal or Sex-linked trait
• What is an autosome?
• A chromosome that is not linked with sex
determination.
• What do you think the difference is between traits that
are autosomal and traits that are sex-linked?
• An autosomal trait will show up equally in both sexes. A
sex-linked trait is usually seen in only one gender
(mostly males).
• Most sex-linked traits are recessive and located on the X
chromosome. Because males have only one X
chromosome, if the allele is recessive on the X
chromosome, they will have the trait. (The Y isn’t big
enough to carry anything.)
• Females that have two X chromosomes both with the
recessive alleles will have the trait.
Patterns of Heredity Can Be
Complex
Most Traits are not Controlled
by Simple Dominant-Recessive
Alleles
• Ex: A horse with red hair mates with a
horse with white hair, what color hair does
the colt have?
• The colt exhibits both red and white hair,
but why?
• This trait has a more complex pattern of
heredity than just simple dominantrecessive patterns.
Traits Influenced by Several
Genes
• A polygenic trait is a trait that is
influenced by several genes that can be
on the same chromosome or a different
chromosome.
• Ex. Of polygenic traits:
•
•
•
•
Hair color
Eye color
Height
Skin color
Intermediate Traits
• Incomplete dominance is when an
individual displays a trait that is
intermediate between two parents.
• Ex: Mix a red plant with a white plant and
get a pink plant.
• Ex: Mix a straight hair parent with a curly
hair parent and get a wavy hair child.
Traits with Two Forms
Displayed at the Same Time
• When two dominant alleles are expressed
at the same time and both forms of the
traits are seen, the trait is codominant.
• Ex: Previous horse hair trait (Roan horse
has red and white hairs)
Traits Controlled by Genes with
three or more alleles
• These genes have multiple alleles
• When traits are controlled by genes with
multiple alleles, an individual can have
only two alleles for that gene
• Ex: ABO blood types
Traits Influenced by the
Environment
• Phenotypes can be effected by
environmental factors such as:
– Soil acidity
– Temperature
– Nutrition
– Sun Exposure
Indentical twins are used to study
environmental influences
Some Traits are Caused by
Mutations
• Changes in genetic material are called
mutations.
• The effects of mutations are genetic
diseases.
• Most mutations are carried by recessive
alleles.
Recessive Genetic Disorders
• Sickle cell anemia: mutated allele causes
hemoglobin to sickle
– Red blood cells rupture easily and carries less oxygen
to the blood.
– Protects against Malaria
• Hemophilia: sex-linked trait on X chromosome
that impairs the blood’s ability to clot
• Cystic Fibrosis: mucus clogs many organs
caused by a defected chloride-ion transport
protein
Genetic Disorders Caused by
Dominant Alleles
• Huntington’s Disease (HD): causes loss
of muscle control, physical spasms,
severe mental illness, and death
– Most people do not realize that they have the
allele for this until they have children
Detecting and Treating
Genetic Disorders
• Most cannot be cured
• Sometimes you can treat it if it is caught at
an early stage
• May want to undergo genetic counseling
before becoming a parent
– Genetic Counseling is a form of medical
guidance that informs people about genetic
problems that could affect their babies
• Future gene technology might allow us to
replace defected genetic material with copies
of healthy genes in a process called gene
therapy