Do We Need Some Review??
Draw a punnett square for a di-hybrid
cross between RrYy and RRyy
Draw a punnett square for a di-hybrid
cross between TtGg and ttgg
Draw a punnett square for a di-hybrid
cross between EeJj and EeJj
The Generations
• The original pair (the parents) in the cross is known
as the parent generation.
• The offspring from those parents are known as the
F1 generation.
• We know that the
dominant alleles over
power the recessive
alleles, but the
recessive alleles are still
present.
• Recessive alleles are
expressed only when
the genes are
homozygous recessive.
Parent Generation
RR X rr
• Let’s draw a punnett square for this cross.
F1 Fertilization
Rr X Rr
• Let’s draw a punnett square for this cross.
The Recessive Allele
• Throughout the
generations, the
recessive allele was still
present, just not
expressed until the
alleles were
homozygous recessive
• In order for this to
happen, the alleles
need to separate.
Law of Segregation
• The separation of alleles occurs during the
formation of gametic cells.
• What are gametic cells, again?
Let’s Draw It!
Parent Generation Gametes RR x rr
F1 Generation Gametes Rr x Rr
Independent Assortment
• During segregation, when alleles separate, different
alleles for different genes don’t need to separate to
the same gamete.
Independent Assortment
• The alleles for hair color
and eye color separate
independently.
• Just because an
offspring gets blonde
hair, it doesn’t mean it
will have blue eyes.
Heredity is a little more
complicated than just Dominant
and Recessive…
Co-Dominance
• What does “co” mean?
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Co-Author
Co-Chair
Co-Director
Co-teach
Cooperate
Codominance
• Neither allele is more
dominant than the
other.
• Both phenotypes are
expressed.
“Erminette”
Let’s Make a Punnett Square
Roan
Let’s Make a Punnett Square
Incomplete Dominance
• Neither allele is
dominant or recessive.
• Heterozygous
phenotype is a
combination of the two
homozygous
phenotypes.
4 o’clock Flowers
Codominance or Incomplete
Dominance?
Codominance or Incomplete
Dominance?
Codominance or Incomplete
Dominance?
Codominance or Incomplete
Dominance?
Worksheet
Multiple Alleles
• So far we have talked
about genes with only
two alleles….
• Some genes can have
more than two alleles
that express a certain
trait
Multiple Alleles
• While there are
multiple alleles
available in a
population; each
individual still only has
two alleles in their DNA
• Still follows Dominant
and Recessive rule
– Spectrum
Let’s Draw a Spectrum
• Most Dominant
• Least Dominant
Polygenic Traits
• Look at the term
“polygenic”
– Poly=??
– Genic=??
Polygenic Traits
• Traits that are
controlled by 2 or more
genes
• Results in a wide range
of phenotypes
– Skin color (4 + genes)
• Comes more into play in
evolution
Sex Linked Genes
• Remember gametes??
• They are determined by
“sex chromosomes”
(23rd pair); all other
chromosomes are
called “autosomes” (122)
• Sex chromosomes are
labeled X or Y
– XX- female
– XY- male
Fertilization
• Every Egg (female
gamete) gives 1 X
• Sperm (male gamete)
gives either an X or Y
Sex Linked Genes
• Genes that are on a sex
chromosome (X or Y).
– Only males have a Y
chromosome, which is
inherited from their dad
– A gene only carried on
the Y chromosome will
show up only in males
– Males only have one X
chromosome
Sex Linked Genes
• Females receive two X
chromosomes, one
from their mother and
one from their father.
– Have a “back up” X
chromosome if a gene
on the x chromosome is
defective
– Most sex linked
disorders are found on
the X chromosome
Sex Linked Genes
• Color Blindness (the
genes reside on the X
chromosome)
• Hemophilia
• Fragile X Syndrome
Let’s Draw a Punnett Square