Transcript Genetics: The Science of Heredity

Genetics: The Science of
Heredity
Chapter 3
Gregor Mendel
• Mendel studied pea plants in order to
understand genetics.
• He cross-pollinated pea plants in order to find
out how the traits of the parent plants related to
the traits of the offspring (For Example: Would a
tall plant crossed with a short plant produce a tall
or a short plant?) From this he found a pattern.
• He also studied other traits in pea plants such as
seed shape, seed color, pod shape, etc.
Dominant and Recessive Traits
• From the pea plant experiments, Mendel found that
some traits could mask or hide other traits. For instance,
dominant traits would overrule recessive traits.
• Example: Black fur is dominant over white fur in rabbits.
That means that if a bunny has one black fur gene
paired with one white fur gene, the bunny will most likely
have black fur.
• However if a bunny has one white fur gene paired with
another white fur gene, the bunny will most likely have
white fur. This is because the only way to have a
resulting recessive trait is to pair two recessive genes.
Discussion Questions
• What is another way that you could wind up with
a black bunny besides having a black fur gene
combined with a white fur gene?
• If a tall pea plant is dominant and a short pea
plant is recessive, than what combinations would
produce a tall pea plant? What combinations
would produce a short pea plant?
Symbols
• We can use symbols to represent dominant and
recessives genes and traits.
• A dominant trait can be represented with a
capital letter. For instance the black fur of the
rabbit could be represented as B.
• A recessive trait can be represented with the
lower case letter of the dominate trait. For
instance the white fur of the rabbit could be
represented b.
• If a rabbit inherits both a black fur gene and a
white fur gene you would symbolize it as Bb.
Discussion Questions
• How could you symbolize a rabbit that
inherits 2 recessive white fur genes?
• How could you symbolize a rabbit that
inherits 2 dominant black fur genes?
Phenotype vs. Genotype
• A phenotype is the written version of a
physical trait. For instance if you are given
the symbolic version Bb for a rabbit, you
would know that the phenotype is black.
The word black is the written version of
this symbol and it is the phenotype.
• However, the symbolic code Bb would be
the genotype.
Discussion Questions
• What are 2 possible genotypes for a tall pea
plant? (Remember tall is the dominant gene)
• What is the phenotype of tt in a pea plant?
• What is the phenotype of BB in a rabbit?
• What is the genotype of a white rabbit?
Punnett Square
• Scientists use Punnett Squares to predict
the possible combinations of inherited
genes.
• You must start with a square consisting of
four boxes.
This Punnett Square the possibility of eye
color outcome in humans offspring. Brown is
the dominant eye color gene over green.
Questions:
1. What color are the mother’s eyes?
2. What color are the father’s eyes?
3. How many possible combinations
of outcomes are there?
4. How many of those possible
outcomes show the offspring having
brown eyes?
5. How many of those possible
outcomes show the offspring having
green eyes?
6. So what percentage of the time
would the offspring be brown eyed?
Activity
• Create a Punnett Square with the
combination of a black mama rabbit whose
genotype is BB and a white papa rabbit
whose genotype is bb. Find the possible
outcomes for the offspring.
• What percentage of the time would the
offspring have black fur?
• What percentage of the time would the
offspring have white fur?
DNA
• DNA (Deoxyribonucleic Acid) is the genetic
material that carries information about an
organism that is passed from parent to offspring.
• DNA is unique to each individual and has the
appearance of a double helix.
• DNA is located within chromosomes.
Chromosomes
• The body cells of humans contain 23 pairs
or 46 chromosomes total.
• Genes are located on the chromosomes.
• Each pair of chromosomes are made of
one gene from the mother and one gene
from the father.
Mitosis Review
• Do you remember what mitosis was?
Definition of Mitosis
• Mitosis was the middle phase of the cell
cycle in which body cells are in the
process of dividing and reproducing two
new daughter cells.
• This phase is when one copy of the DNA
is distributed into each of the two daughter
cells.
Meiosis
• Meiosis has a similar name to mitosis, but it is a
very different process.
• Meiosis happens when sex cells, not body cells,
are produced. This process differs because only
half of the chromosomes (23) are given to the
new cells.
• This is because, the offspring of the female and
male sex cells is a combination of both the
female and male chromosomes. This gives the
new offspring a total of 46 chromosomes.
Mutations
• A mutation is a “change” that occurs in the DNA code.
• For instance, a white coat on a buffalo calf (which is very
unlikely).
• Sometimes, the mutation can result in a favorable
change that helps the organism in their environment.
• However, the mutation can result in a
harmful change that can hurt the
organism.
• For instance, the white buffalo calf would
not be hurt by this mutation because it is a
farm animal.
• Can you think of a way in which this same
mutation could have been hurtful to this
calf if living in the wild?
• Some mutations can neither be hurtful or
helpful.
Homework
• Read pages 86-99 and complete the
following questions: page 91- #’s 2 & 3
and page 99- #’s 2 & 3
• Read pages 102-112 and complete the
following questions: page 106- #’s 2 & 3
and page 112- #3