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Dominant or Recessive?
Inherited Traits
Living things inherit traits, or
characteristics/parents. Ex. – Eye
color, hair color, flower color.
Genes and Chromosomes
Genes – Hereditary information.
What else do genes do?
Determine traits of an organism - Ex. Shape
of ears, you have a backbone/legs, hair color
Genotype and Phenotype
Genotype – Genes passed to offspring.
Phenotype – Physical appearance.
Ex. Black hair
Genotype determines Phenotype.
Genes are located on Chromosomes.
Chromosomes – Hold genetic info.
1 chromosome contains many different
genes.
More on Genes and Chromosomes
Cells in different kinds of
organisms contain different
numbers of chromosomes.
Ex. – Cats – 38 / Dogs - 78
Human baby = 23 chromosomes
from mom/23 from dad.
Human body cells=46 chromosomes
in 23 pairs.
Each pair has a number from 1-23.
Chromosome 1 from dad/mom have
similar genetic information.
This is true for the other 22 pairs.
Homologous Chromosomes
Genes that determine a specific
trait are in matching locations
on paired, or homologous,
chromosomes.
Genes for ear shape are located in
same place on 2 homologous
chromosomes.
Mom
Dad
Dominant and Recessive Alleles
Many traits are controlled by a
single gene.
Ex.- 1 gene = Earlobes attached
1 gene = Earlobes free
Trait
Different forms of same gene are Hair
called Alleles.
Type
Alleles will be either:
Dominant = Stronger Trait
Recessive = Weaker Trait
Let’s 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 for a trait is written as
2 letters.
Capital letters = Dominant Alleles
Lowercase letters = Recessive Alleles
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
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. What makes males and females different at the chromosome level?
12. How many pairs of chromosomes does each human cell have?
13. Describe how the chromosome pairs are numbered and how the genetic information is
arranged.
14. What are homologous chromosomes?
15. What are alleles?
16. What is meant by the terms dominant and recessive?
17. Give some dominant and recessive traits.
18. How do you distinguish between dominant and recessive traits when written?
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.
Ex.- Ff
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
___________________________
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
Sex Chromosomes
In humans, females have 2 “X”
chromosomes. So, each egg contains one
“X” chromosome.
Males have both an “X” and a “Y”
chromosome. Each sperm cell contains
either an “X” or a “Y” chromosome.
XX = Female
XY = Male
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.
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.