HONORS
Background
• With the exception of gametes (aka egg
and sperm), all cells in your body are
diploid and contain two copies of each
chromosome.
– One copy from mom (in her egg)
– One copy from dad (in his sperm)
How many total chromosomes make up who
you are?
Background
• Gene: section of DNA that provides the instructions for
making a protein.
• Since a gene is a piece of a chromosome and we have two of every
chromosome, we have two copies of instructions to make every
protein in our bodies.
• Alleles: different versions of the same gene.
• We inherited a full set of chromosomes (containing genes) from each of
our parents, but we may not have inherited the same version of every
gene.
- Ex. Mom gave you a gene for blue eyes and dad gave you a gene for brown eyes.
• Homologous Chromosomes: the matching
chromosomes from our mom and dad.
• They contain the same genes in the same locations, but may contain
different alleles.
Gregor Mendel
• Gregor Mendel – Austrian monk who used pea
plants to learn about genetics. He is known as the
“Father of Genetics”.
3 Laws of Inheritance:
– Law of Dominance
– Law of Segregation
– Law of Independent
Assortment
Mendel’s Experiments
In his experiments crossing pea plants he made sure that:
1. He had control over breeding.
– Mendel used pea plants that
typically self-pollinate.
2. He used only purebred
plants.
– Purebred = type of organism
whose ancestors are
genetically uniform.
3. He observed only “either-or”
traits.
– Chose pea shape, pea
color, pod shape, pod color,
plant height, flower color.
What is a Cross?
• Cross – mating of 2 organisms
P = parental generation
F1 = first generation
F2 = second generation
Significance of Alleles
•
•
Alleles have assigned letters:
- Uppercase for dominant (Ex. A)
- Lowercase for recessive (Ex. a)
Remember, each individual has 2 alleles (one from
each parent) for every gene.
• Homozygous – 2 of
the SAME alleles.
– Ex. AA or aa
• Heterozygous – 2
DIFFERENT alleles.
- Ex. Aa
Law of Dominance
A dominant (strong) allele will express itself over
a recessive (weak) allele.
– Dominant = if present, allele will always have
that trait expressed (seen).
• Ex. If brown hair is dominant over blonde hair, b represents the
allele for blonde hair and B represents brown hair.
• If you inherited the B allele from one or both of your parents, you will
have brown hair. (whether you are BB or Bb = brown)
– Recessive = allele will only have that trait
expressed (seen) when the dominant allele is
NOT present.
• The only way to get blonde hair is to inherit the b allele from both
parents. (bb = blonde hair)
Genotype vs. Phenotype
• Genotype – the actual
alleles inherited.
– Ex: Genes that code for
flower color such as FF,
Ff, or ff
• Phenotype – the
physical
traits/characteristics
seen in an organism.
– Ex: Purple flowers
Law of Segregation
When chromosomes separate in meiosis, each
gamete (egg or sperm) will receive only one
chromosome from each pair.
Example:
• If a man has alleles for
brown hair and blonde
hair, he is heterozygous
(Bb) for hair color.
• His sperm cells can
contain the allele for
brown hair (B) OR the
allele for blonde hair (b).
• Each sperm only gets
ONE of the alleles.
Law of Independent
Assortment
The assortment of chromosomes for one trait
doesn‘t affect the assortment of chromosomes
for another trait.
This allows for any combination of
maternal and paternal
chromosomes to be passed on
because homologous chromosomes
line up RANDOMLY during
metaphase.
Punnett Squares
• Punnett Square – a diagram that shows the probability
of inheriting traits from parents with certain genes.
– Monohybrid cross: a cross between two organisms looking at one trait.
Possible
genotypes
of offspring
PRACTICE
with Monohybrid Crosses
1. In pea plants, round seeds are dominant to wrinkled
seeds. Cross a homozygous recessive plant with a
homozygous dominant plant.
2. Cross two heterozygous plants.
3. Cross a heterozygous plant with a plant that has wrinkled
seeds.
Dihybrid Crosses
Used when finding the possible genotypes for
offspring when considering two traits at the
same time.
Example #1: Cross two tomato plants that are both
heterozygous tall (Tt) / red (Rr). Note: the recessive traits in
tomatoes are short and yellow.
1. Write genotypes of parents: Mom =
Dad =
2. Sort the alleles. (Ask yourself, what combinations could
go into the gametes during meiosis?)
Dihybrid Crosses
Example #1: Cross two tomato plants that are both
heterozygous tall (Tt) / red (Rr). Note: the recessive traits in
tomatoes are short and yellow.
3. Write these
combinations on
the sides of the
Punnett square. (In
this case, we will use the
same combinations on
the top and the side
because the mother and
father had the same
genotype.)
4. Combine the alleles
from the top and left
to fill in the square.
5. Determine the
phenotypic ratio.
Dihybrid Crosses
Example #2: In peas, purple flowers are dominant to white, and
tall plants are dominant to short. A white and short plant is
crossed with a plant that is heterozygous for both traits.
1. Write genotypes:
Mother =
Father =
2. Sort the alleles.
3. Write these
combinations on
the sides of the
Punnett square.
4. Combine the alleles
to fill in the square.
5. Determine the
phenotypic ratio.
Dihybrid Crosses
Example #3: In humans, brown eyes are dominant to blue eyes,
and brown hair is dominant to blonde. Cross a homozygous
dominant brown-eyed blonde man, with a blue-eyed and
heterozygous brunette woman.
Using Probability
• Probability can be used to determine the
possibility of getting a specific genotype.
– This is especially helpful when looking at more
than 2 genes at a time.
• How to:
1. Perform a monohybrid cross for each
individual gene and determine the odds of
getting each genotype in fraction form.
2. Multiply the odds to determine the likelihood of
getting this and this and this and etc.
Using Probability
• Example:
– Erica has the genotype AABbcc for three different genes.
– Carlos has the genotype AaBbCc for those same three genes.
– Determine the probability of Erica and Carlos having a child who
is heterozygous for all three genes (AaBbCc) .
Using Probability
• Example:
– Erica has the genotype AABbcc for three different genes.
– Carlos has the genotype AaBbCc for those same three genes.
– Determine the probability of Erica and Carlos having a child who
is heterozygous for all three genes (AaBbCc) .
½ Cc
½ cc
Therefore, the probability of being AaBbCc = ½ x ½ x ½ = 1/8