Genetics 2
• Genetics 1 Review
• Segregation
• Genotype and Phenotype
• Incomplete dominance and
codominance
Genetics 1 Review
 In the previous lecture, you learned about Gregor
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Mendel and his study of pea plants.
What is a Punnet Square and what does it help us
predict?
What are alleles? What does it mean to have a
dominant and a recessive allele?
How is it possible for two tall pea plants to have a
short pea plant offspring?
What does homozygous and heterozygous mean?
Segregation
 Alleles are different forms of a gene. For example,
the gene for height in pea plants has two alleles, (T)
and (t). (T) is the allele for Tall height and (t) is the
allele for short height.
 Gregor Mendel wanted to understand why two
heterozygous tall (Tt) pea plants could produce a
short (tt) offspring. He said that these alleles must
segregate, or separate during reproduction.
Segregation and Gametes
 Segregation is the separation of alleles during sexual
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reproduction.
When plants and animals reproduce, their cells
separate so that their offspring get half of the
genetic material from each parent.
That is, half of your cells are from your mom and half
are from your dad.
Gametes are sex cells (Like sperm and egg cells)
During sexual reproduction, each parent produces a
gamete that has half of its genetic material.
Phenotype and Genotype
 Two vocabulary words that you should know are
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phenotype and genotype.
Genotype refers to the genetic make-up of an organism.
For example, a heterozygous pea plant (Tt) has a
genotype that has both the tall (T) and short (t) alleles for
height.
Phenotype refers to the physical characteristics of an
organism.
For example, even though the heterozygous (Tt) pea
plant carries the short gene, it will still be tall, because
the tall allele (T) is dominant. The phenotype is tall.
Incomplete Dominance and Codominance
 As we learned already, there are dominant and
recessive alleles for various traits.
 However, sometimes alleles are not completely
dominant over others.
 For example, the alleles for red and white flowers
show incomplete dominance.
 Incomplete dominance: when one allele is not
completely dominant over another.
Incomplete dominance and Codominance
 The alleles for red and white flowers are: Red (R)
and White (r).
 When a flower is homozygous Red, its alleles are
___ ___ .
When a flower is homozygous White, its alleles are
___ ___ .
When a flower is heterozygous, its alleles are ___ ___.
Incomplete Dominance
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RR=Red
Rr= Pink
rr=White
Incomplete Dominance
 What happens if you cross two heterozygous pink
flowers (Rr x Rr) ??
What are the
phenotypes of the
offspring of the
two pink flowers?
Codominance
 Codominance: both alleles contribute to the
phenotype.
 In other words, both colors show up evenly. They do
not mix like in incomplete dominance.
 Example of codominance: A brown bird and a black
bird are crossed and their offspring have some brown
feathers and some black feathers.
 Do you understand the difference between
Codominance and Incomplete Dominance?