6.3
Mendel and Heredity
The student is expected to:
3F research and describe
the history of biology and
contributions of scientists
and
6F predict possible outcomes of
various genetic combinations
such as monohybrid crosses,
dihybrid crosses and nonMendelian inheritance
TEKS 3F, 6F
History of Genetics
• Gregor Mendel was
an Austrian monk
and scientist who
was in charge of the
monastery garden.
Mendel studied
garden peas.
6.3
Mendel and Heredity
TEKS 3F, 6F
KEY CONCEPT
Mendel’s research showed that traits are inherited as
discrete units.
6.3
Mendel and Heredity
Mendel laid the groundwork for genetics.
• Traits are distinguishing
characteristics that are
inherited.
• Genetics is the study of
biological inheritance patterns
and variation.
• Gregor Mendel showed that
traits are inherited as discrete
units.
• Many in Mendel’s day
thought traits were blended.
TEKS 3F, 6F
6.3
Mendel and Heredity
TEKS 3F, 6F
Mendel’s data revealed patterns of inheritance.
• Mendel made three key decisions in his experiments.
– use of purebred plants
– control over breeding
– observation of seven
“either-or” traits
Pea plants happened to be a good
choice to study because:
– They are self-pollinating.
– He had different pea plants that were truebreeding.
– True-breeding - means that they are
homozygous for that trait.
– EX. if the plants self-pollinate they produce
offspring identical to each other and the
parents.
When discussing generations’
traits, we label them as following:
• The true-breeding parental generation is
called the “P generation”.
• The offspring of the two parental plants is
called the “F1 generation”.
• A cross between F1 generation would be
called “F2 generation.”
6.3
Mendel and Heredity
TEKS 3F, 6F
• Mendel used pollen to fertilize selected pea plants.
– P generation crossed to produce F1 generation
– interrupted the self-pollination process by removing male
flower parts
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
TEKS 3F, 6F
• Mendel allowed the resulting plants to self-pollinate.
– Among the F1 generation, all plants had purple flowers
– F1 plants are all heterozygous
– Among the F2 generation, some plants had purple
flowers and some had white
Original cross
(P) Parental Generation
(true breeding)
F1 Generation (offspring)
Cross pollination
F2 Generation (Cross of
F1 Generations)
Mendel’s Investigations
• Mendel saw that when he crossed plants
with different versions of the same trait (P
generation), the F1 offspring were NOT
blended versions of the parents.
• The F1 plants resembled only one of the
parents.
Tall
x
short 
all tall…
6.3
Mendel and Heredity
• Mendel observed patterns in the first and second
generations of his crosses.
TEKS 3F, 6F
6.3
Mendel and Heredity
TEKS 3F, 6F
• Mendel drew three important conclusions.
– Traits are inherited as discrete units.
– Organisms inherit two copies of each gene, one from
each parent.
– The two copies segregate
during gamete formation.
– The last two conclusions are
called the law of segregation.
purple
white
Mendel concluded:
• 1. Biological inheritance
is determined by
“factors” that are passed
from one generation to
the next.
• Factors were later
defined as “genes”– Mendel discovered all of
this without the
knowledge of DNA!
Mendel concluded:
•
In Mendel’s plants, there was one gene for
each trait.
For example, there was one gene for plant
height.
–
But, there were two versions of this gene: one for a
tall plant and one for a short plant.
Mendel concluded:
– Alleles: Different versions of the same
gene
• Remember, genes are used to make proteins.
• Each allele contains the DNA that codes for a
slightly different version of the same protein
• This gives us the different characteristics for
each trait
2. Principal of dominance:
•
Some alleles are dominant and some
alleles are recessive.
– Recessive alleles are able to be masked
– Dominant alleles mask recessive alleles
•
The trait that was represented in the F1
generation was the dominant trait.
2. Principal of dominance:
• How many alleles do you have for each
gene? Two
• Where do they come from?
One comes from mother and one comes from father.
3. Segregation:
•
•
Observation: After seeing that his F1
plants looked like only one generation of
the P generation plants, Mendel wanted
to know what happened to the recessive
alleles.
Question: Did they disappear?
3. Segregation:
• Experiment: Mendel self-pollinated the F1
plants, or crossed the F1 plants with each
other, to produce the F2 generation. From
his F1 crosses, Mendel observed:
– The versions of the traits coded for by
recessive alleles reappeared in the F2 plants.
– The recessive trait was still there!
3. Segregation:
– About 25% (or ¼) of the F2 plants exhibited
the recessive version of the trait. In this case
the recessive phenotype is short. The
dominant phenotype, tall, was found in 75%
(or ¾) of the F2 plants.
P generation
F1 generation
F2 generation
Segregation of alleles during meiosis:
• When the F1 plants produce gametes (sex cells)
and self-pollinate, the two alleles for the same
gene separate from each other so that each
gamete carries only one copy of each gene.
• Remember, gametes are haploid. In the
example, we use “T” to represent the dominant,
tall allele and “t” to represent the recessive, short
allele.
6.4
Traits, Genes, and Alleles
The student is expected to:
6A identify components of
DNA, and describe how information
for specifying the traits of an
organism is carried in the DNA
and
6F predict possible outcomes of
various genetic combinations such
as monohybrid crosses, dihybrid
crosses and non-Mendelian
inheritance
TEKS 6A, 6F
6.4
Traits, Genes, and Alleles
TEKS 6A, 6F
KEY CONCEPT
Genes encode proteins that produce a diverse range
of traits.
6.4
Traits, Genes, and Alleles
TEKS 6A, 6F
The same gene can have many versions.
• A gene is a piece of DNA that directs a cell to make a
certain protein.
• Each gene has a locus, a
specific position on a pair of
homologous chromosomes.
6.4
Traits, Genes, and Alleles
TEKS 6A, 6F
• An allele is any alternative form of a gene occurring at a
specific locus on a chromosome.
– Each parent donates
one allele for every
gene.
– Homozygous
describes two alleles
that are the same at a
specific locus.
– Heterozygous
describes two alleles
that are different at a
specific locus.
6.4
Traits, Genes, and Alleles
• Alleles can be represented using letters.
– A dominant allele is
expressed as a phenotype
when at least one allele is
dominant.
– A recessive allele is
expressed as a phenotype
only when two copies are
present.
– Dominant alleles are
represented by uppercase
letters; recessive alleles by
lowercase letters.
TEKS 6A, 6F
6.4
Traits, Genes, and Alleles
TEKS 6A, 6F
• All of an organisms genetic material is called the Genome
• A Genotype refers to the makeup of a specific set of alleles
• A Phenotype is the physical expression of a trait
Key Terms in Mendelian Genetics:
• Dominant- allele that can mask;
represented by capital letters (B, D, F,
etc.)
• Recessive- alleles that can be masked;
represented by lower case letters (b, d, f,
etc.)
Key Terms in Mendelian Genetics:
• Phenotype- observable traits (brown eyes,
yellow seed pods)
• Genotype- actual alleles; describes the
genetic characteristics (BB, dd, Ff)
Phenotype: brown eyes
Genotype: could be BB,
or Bb
Key Terms in Mendelian Genetics:
• Homozygous (True-Breeding)- having two
identical alleles for the same trait (TT, tt);
“homo” means same
• Heterozygous- having two different alleles
from the same trait (Tt); “hetero” means
different
6.4
Traits, Genes, and Alleles
TEKS 6A, 6F
• Both homozygous dominant and heterozygous genotypes
yield a dominant phenotype.
• Most traits occur in a range
and do not follow simple
dominant-recessive patterns.
6.5
Traits and Probability
The student is expected to:
3F research and describe the history
of biology and contributions of
scientists;
6F predict possible outcomes of
various genetic combinations such
as monohybrid crosses, dihybrid
crosses and non-Mendelian
inheritance;
6G recognize the significance of
meiosis
to sexual reproduction
TEKS 3F, 6F, 6G
6.5
Traits and Probability
KEY CONCEPT
The inheritance of traits follows the rules of
probability.
TEKS 3F, 6F, 6G
6.5
Traits and Probability
TEKS 3F, 6F, 6G
Punnett squares illustrate genetic crosses.
• The Punnett square is a grid system for predicting all
possible genotypes resulting from a cross.
– The axes represent
the possible gametes
of each parent.
– The boxes show the
possible genotypes
of the offspring.
• The Punnett square
yields the ratio of
possible genotypes and
phenotypes.
6.5
Traits and Probability
TEKS 3F, 6F, 6G
A monohybrid cross involves one trait.
• Monohybrid crosses examine the inheritance of only one
specific trait.
– homozygous dominant-homozygous recessive: all
heterozygous
6.5
Traits and Probability
TEKS 3F, 6F, 6G
– heterozygous-heterozygous—1:2:1 homozygous
dominant: heterozygous:homozygous recessive; 3:1
dominant:recessive
6.5
Traits and Probability
TEKS 3F, 6F, 6G
• heterozygous-homozygous recessive—1:1
heterozygous:homozygous recessive; 1:1
dominant:recessive
• A testcross is a cross between an organism with an
unknown genotype and an organism with the recessive
phenotype.
6.5
Traits and Probability
TEKS 3F, 6F, 6G
A dihybrid cross involves two traits.
• Mendel’s dihybrid crosses with heterozygous plants
yielded a 9:3:3:1 phenotypic ratio.
• Mendel’s dihybrid crosses
led to his second law,
the law of independent
assortment.
• The law of independent
assortment states that
allele pairs separate
independently of each
other during meiosis.
6.5
Traits and Probability
TEKS 3F, 6F, 6G
Heredity patterns can be calculated with probability.
• Probability is the likelihood that something will happen.
• Probability predicts an average number of occurrences, not
an exact number of occurrences.
number of ways a specific event can occur
• Probability =
number of total possible outcomes
• Probability applies to
random events such as
meiosis and fertilization.
6.6
Meiosis and Genetic Variation
TEKS 6F, 6G
The student is expected to:
6F predict possible outcomes of various
genetic combinations such as monhybrid
crosses, dihybrid crosses and nonMendelian inheritance
and
8G recognize the significance of meiosis to
sexual reproduction
6.6
Meiosis and Genetic Variation
TEKS 6F, 6G
KEY CONCEPT
Independent assortment and crossing over during
meiosis result in genetic diversity.
6.6
Meiosis and Genetic Variation
TEKS 6F, 6G
Sexual reproduction creates unique combinations of genes.
• Sexual reproduction creates unique combination of genes.
– independent assortment of chromosomes in meiosis
– random fertilization of gametes
• Unique phenotypes may give a reproductive advantage to
some organisms.
6.6
Meiosis and Genetic Variation
TEKS 6F, 6G
Crossing over during meiosis increases genetic diversity.
• Crossing over is the exchange of chromosome
segments between homologous chromosomes.
– occurs during prophase I of meiosis I
– results in new combinations of genes
6.6
Meiosis and Genetic Variation
TEKS 6F, 6G
• Chromosomes contain many genes.
– The farther apart two genes are located on a
chromosome, the more likely they are to be separated
by crossing over.
– Genes located close together on a chromosome tend to
be inherited together, which is called genetic linkage.
• Genetic linkage allows the distance between two genes to
be calculated.