alleles in gene pair are identical

Results and Conclusions of
Mendel’s Experiments
Mendel began his experiments by
cross pollinating pure breeding plants
• His studies focused on only one characteristic
at a time for example, flower color. This type
of cross is called a Monohybrid cross
Mendel developed a mathematical vocabulary
to identify different generations of pea plants
• P1: Parent generation
• F1: First (“filial”meaning offspring) generation
Offspring of P1 cross P1 x P1
• F2: Second Generation - Offspring of F1 cross
All F1 plants are
purple (Ww )…
Where did the WHITE
form go?
3 out of 4 offspring with
purple flowers
What is the resulting ratio of purple to
white flowers in the F2 generation?
• Purple 3:1 white
Express this answer as a percent
• 75% purple: 25 % white
Now let’s look at another…
Mendel Concluded….
1) Parents transmit information
about traits to their offspring.
• Mendel called this information a “factor.”
Today we know that “factors” are really genes.
2) Each parent must be contributing a
form of a gene to the offspring.
• From our studies in the last
unit, we know this is true
because chromosomes
came in pairs called
homologous chromosomes.
Each homolog contains the
same genes, so a gene pair
exists for each gene on the
Gene Pair
Gene locus
2) Each parent must be contributing a
form of a gene to the offspring.
• The two alleles in a gene pair can • Gene pairs are described in
two ways:
1) identical or
1) homozygous: alleles in
gene pair are identical Pure
2) different
breeding plants are
• For example, a gene pair which
codes for seed color in pea
2) heterozygous: alleles in
plants could contain:
gene pair are different
1) yellow, yellow (purebred or
Hybrid plants are
2) green, green homozygous)
3) yellow, green (hybrid) heterozygous
3. Symbols can be used to represent
• If you use an uppercase “W” to represent
purple as the dominant form of pea color and
use a lowercase “w” to represent white as the
recessive form of pea color, you can illustrate
the allele combinations for the following pea
plants. (WW, Ww, ww)
4. Genetic makeup and observed
characteristics are not always the same!
• the exact form of the alleles
in a gene pair
• Example: WW, Ww, ww
• the observable or expressed
form of a trait resulting
from a gene pair
• Example: yellow or green
seed color
In pea plants, Green pod color is
dominant over yellow.
Homozygous dominant
Homozygous recessive
Heterozygous recessive?? No such thing!
As a result of his many years of
careful study, Mendel’s work was
summarized into three main laws.
Mendel’s Laws
1. The law of Dominance
• States that if two alleles in a gene pair are
different, then one allele can control the trait
and the other one can be hidden.
• Example: Green pod color is seen in plants
with the genotype Gg
Mendel’s Laws
2. The law of Independent Assortment
• States that gene pairs segregate into gametes
randomly and independently of each other.
• At what stage of meiosis does this occur?
Metaphase I and Anaphase I of meiosishomologs randomly separate
Mendel’s Laws
3. The law of Segregation
• States that each pair of genes segregates, or
separates, during meiosis and each gamete
contains one gene from each gene pair.
• What is the chance that a gamete will end up
with a maternal homolog at any one
chromosome pair?
• ½ or 50%
• The chance that all 23 homologs from one parent
will end up in a single gamete is 1 in 8 million
What is a monohybrid cross?
• A monohybrid cross is when a scientist studies
the inheritance of one trait at a time.
How is the following statement “Each parent
must be contributing a form of a gene to the
offspring” supported by what you know about
meiosis and homologous chromosomes?
The chromosomes from each homologous pair
come from the parents (one from each). Each
homologous pair carries a pair of genes (one
gene on each homolog). Therefore one copy
of each gene comes from each parent.
What is the difference between
homozygous and heterozygous?
• Homozygous means each gene has the same
• Heterozygous means that each gene in the
pair has different alleles.
What is the relationship between
phenotype and genotype?
• Genotype is what alleles are found in a pair of
genes. Phenotype is the outward appearance
of those alleles.
What does dominant and recessive mean?
• A dominant allele can control a trait and hide
a recessive allele. A recessive allele will only
show if the dominant allele is not present.

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