Heredity – Unit 2 lesson 4
What is Heredity?
Heredity is when traits are passed from
parents to offspring
Gregor Mendel
Born in Austria, 1822
Entered a monastery at 21.
Monks taught him science.
Working in the monastery garden, he
discovered the
principles of
heredity
The Mystery of Pea Plants
• Mendel was curious why pea plants
had different traits (stem length, seed
color, etc.)
• Pea plants were a good choice to
experiment with because they grow
quickly and there are many different
characteristics
• Pea plants can also self pollinate (they
have male and female sex cells) and
parent traits can be controlled.
Characteristics and Traits
*
A characteristic is a feature that has different forms in a population
(hair color, height)
*
A trait is the different form of a characteristic
(blond, black or brown hair, or tall or short height)
Mendel Used the Scientific Method
*
Ask a question – How are traits inherited?
*
Form a hypothesis – It is passed from parent to offspring
*
Test the hypothesis – Breed plants and offspring
*
Analyze the results – There are patterns
*
Conclusion – Traits are inherited in predictable patterns
*
Communicate results – Publish results
First Generation Experiment
* He used pea plants to study
7 different characteristics
* He crossed (bred) a plant
with purple flowers with a
Plant with white flowers. All
the offspring had purple
flowers.
* What happened to the trait for white flowers?
Dominant and Recessive Traits
* Mendel found that
one trait was always
present and one disappeared.
* He called the trait that
showed the dominant trait.
* The trait that disappeared
or was covered was called
the recessive trait.
Second Generation
* The
purple offspring self pollinated
and the recessive trait reappeared.
* For every 3 plants with purple flowers,
there was 1 with white flowers.
* This pattern appeared for
all 7 traits tested.
Discovering Mendel
*
It was only 30 years after Mendel died that his work got the
attention and value it deserved.
He is known at the “Father of Genetics.”
Where do traits come from?
*
Genes – carry instructions for an inherited trait
*
Different forms of a gene for a trait (dominant or
recessive) are called alleles
*
The dominant allele is shown with a capital letter.
*
The recessive allele is shown with a lower case letter.
Check your understanding!
Purple flower color in peas is dominant over white flowers.
Which of the following pairs of alleles would give purple flowers?
A.
PP
B.
Pp
C.
pp
D.
Both PP and Pp
Check your understanding!
Which of the following pairs of alleles would give white flowers?
A.
PP
B.
Pp
C.
pp
D.
Both PP and Pp
Genes
The genotype is the genetic makeup
of an organism. It is the combination
of alleles for a specific trait
(the combination of letters).
The phenotype is the appearance
of the trait or what you see.
There can be any number of alleles
for a gene, but an individual can only
have two alleles (one on each
homologous chromosome)
Check your understanding
A, B and O are the ________________ of the human blood type ______________
A.
genes, gene
B.
genes, allele
C.
alleles, gene
D.
alleles, allele
Genotypes
If the two alleles are the same
( two recessive bb or two dominant BB),
we same the genotype is homozygous.
If the alleles are different, (Bb)
we call the genotype heterozygous.
Check your understanding
Which of the following is a homozygous genotype?
A.
AA
B.
Aa
C.
aa
D.
Both AA and aa
Punnett Squares
A Punnett square is uses to organize all the
possible combinations of offspring from a
particular parent.
Each square has one of each parents’
alleles
Each box shows the probability of a
particular phenotype and genotype in the
offspring.
Try this!
Tongue rolling is a dominant trait. You only need one allele. See if you can roll your tongue.
See if your parents and siblings can roll their tongues.
Now figure out your genotype (homozygous dominant, homozygous recessive or heterozygous.)
Not so simple dominance
Co-dominance – Each allele has equal dominance. For example, in blood type, A and B types are
equally dominant. So if you have the alleles AB, then you are AB type. O is recessive to both, so you
can have the alleles AA and AO and be type A for blood type.
Incomplete dominance – Neither allele is dominant so both alleles create a new phenotype. For
example, in some flowers the red color allele (R) and the white color allele (W )produce pink flowers
Genes and Traits
Sometimes, one gene influences more than one trait.
Some traits, like eye color, are the result of several genes working together.
Remember, the environment also plays a role. You may have genes to be tall, but if you do not
eat a healthy diet, you may not reach your full potential height