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Heredity
Why do you look like you look?
What made you the way you
are?
All of your genetic information is contained on your
CHROMOSOMES!!
GENES are on your chromosomes!!
DNA is found on your genes!!
Different organisms contain different numbers of
chromosomes!!
Dogs - 78
Cats – 38
Human body cells
46 chromosomes
23 from mom 23 from dad
23 pair of chromosomes
Each pair has a number
from 1-23.
Chromosome 1 from dad/mom
have similar
genetic information.
This is also true for the other
22 pairs.
These chromosome pairs are called
HOMOLOGOUS CHROMOSOMES
Genes that
determine a specific
trait are found in the
same location on
that pair of
chromosomes
Example: the gene
that determines ear
shape is located in
same place on
2 homologous
chromosomes.
Those homologous
chromosomes carry the
genes that make you—YOU!!
Those two make up your
GENOTYPE
Genes passed to offspring.
Genotype determines
Phenotype.
PHENOTYPE
Actual (expression)
Physical appearance.
Dominant and Recessive Alleles
Many traits are
controlled by a single
gene.
Trait
Ex.- 1 gene = Earlobes attached
1 gene = Earlobes free
Hair
Different forms of same gene Type
are called
ALLELES
Alleles will be either:
Dominant = Stronger Trait
Recessive = Weaker Trait
Look at the table to the right
for examples of human traits.
Trait Made Trait Made
By
By
Dominant Recessive
Allele
Allele
Curly or Straight
Wavy
Hair
Color
Dark
Light
Earlobe
Free
Attached
Dimples
Present
Not
Present
Identifying Genotypes and Phenotypes
Genotype trait is written as
2 letters.
Dominant Allele-Capital Letter
Recessive Allele-Lowercase Letter
The allele that causes free earlobes
is dominant over the allele for
attached earlobes.
So F is used for free earlobe gene.
What do you think is used for
attached earlobe gene?
Lowercase f
More on Genotypes and Phenotypes
Offspring receive 2 copies of
each gene-1 from each parent.
3 possible genotypes : FF, ff, or Ff.
FF and ff = Homozygous
Ff = Heterozygous
“Homo” means “the same.”
Homozygous genotype – Made up
of 2 dominant or 2 recessive alleles.
Ex.- FF or ff
“Hetero” means “different.”
Heterozygous genotype – Made up
of 1 dominant and 1 recessive allele.
Do your ears hang low?
If an organism inherits a
dominant allele, the
dominant trait is expressed.
Allele for free earlobes = dominant.
So, how would we write the genotype?
FF and Ff
___________________________
How then would we write the
genotype for attached earlobes?
ff
___________________________
Let’s Stop and Answer Some Questions
1. What do living things inherit and from where?
2. What are some things you can inherit from mom/dad?
3. What are genes?
4. What else do genes do? Give some examples.
5. What do the terms genotype and phenotype mean?
6. Genotype determines ___________________.
7. Where are genes located?
8. What is the job of chromosomes?
9. One chromosome contains many different _____________.
10. How many chromosomes do we receive from mom/dad? For a total of ?
11. How many pairs of chromosomes does each human cell have?
12. Describe how the chromosome pairs are numbered and how the genetic
information is arranged.
13. What are homologous chromosomes?
14. What are alleles?
15. What is meant by the terms dominant and recessive?
16. Give some dominant and recessive traits.
17. How do you distinguish between dominant and recessive traits when written?
Using Punnett Squares
Used to predict results of genetic
crosses.
Cross – Mating of 2 parent organisms
Punnett Squares are used to calculate
probability of genotypes/phenotypes
in offspring.
Let’s make a Punnett Square with the information below.
Task – Predict outcome of crossing
tall pea plant with short pea plant.
First, I must know the genotype of each parent plant.
There are 3 possible genotypes : TT, Tt, and tt
Genotype of pure tall plant : TT <---- It has 2 dominant alleles
Genotype of pure short plant : tt <--- It has 2 recessive alleles.
To make a Punnett Square, 1st separate the parent alleles.
Place 1 pair across top of square, and other pair down the left side.
t
t
T
Tt
Tt
T
tall
tall
T
Tt
Tt
T
tall
tall
Genotype - Genes
Dominant Gene always 1st in genotype
t
t
Phenotypes - physical
Let’s consider a cross between 2 plants with
heterozygous genotypes : Tt x Tt
t
T
T
t
TT
Tall
Tt
Tall
Tt
Tall
tt
Short
How many possible genotypes here? 3: TT, Tt, tt
How many phenotypes? Tall , Short
Probability of
tall plants?
3 out of 4,
or 75%.
Probability of
short plants?
1 out of 4,
or 25%.
We’re done! Let’s answer some questions.
Discussion Question
Suppose you crossed a pea plant that has round seeds (R) with a pea plant that has wrinkled
seeds (r). If 50 percent of the offspring have round seeds and 50 percent of the offspring have
wrinkled seeds, what are the genotypes of the parent plants? Make a Punnett Square to help
you answer the question.
1. 2 different forms of the same gene are called
A. phenotypes. C. dominant traits
B. genotypes. D. alleles
2. An organism’s expressed traits make up its
A. recessive traits C. phenotype
B. genotype
D. alleles
3. A pea plant that is homozygous dominant for seed color is crossed with a pea plant that is
homozygous recessive for seed color. Since yellow is the dominant color, the letters Y and y
represent the genes for seed color. What cross represents these 2 plants?
A. Yy x Yy
C. YY x yy
B. YY x Yy
D. Yy x yy
4. You meet a person and notice that she has dimples. What can you say for sure about the
phenotypes of her parents?
A. At least 1 of her parents has dimples. C. Neither parent has dimples.
B. Both of her parents have dimples.
D. 1 of her parents does not have dimples.