Download File

Basics of Genetics
DNA, Genes, and Chromosomes
Figure 14.1 A genetic cross
Cross Fertilization and Peas as Ideal Model Plant
Monohybrid Cross
• P = parental (true
breeding)
• F1 = first generation
offspring
• F2= second generation
(made from F1)
Law of Dominance
• In two true breeding plants for opposing traits
in P generation, all offspring in F1 generation
will have one of those traits (dominant trait)
• Recessive trait hidden in F1 generation
Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
What is the approximate ratio of dominant : recessive
traits in the F1 generation when 2 homozygotes (one
dominant, one recessive) are crossed?
Figure 14.x2 Round and wrinkled peas
Law of Segregation
• Mendel suggested that alleles separate during
formation of the sex cells, or gametes
• Each somatic cell has 2 copies of each gene…
• Each sex cell has only one copy…
• The two copies must separate during
formation of sex cells (meiosis!)
• If the sex cell only has one copy of each gene,
how is chromosome number restored in
somatic cells?
Figure 14.4 Mendel’s law of segregation (Layer 2)
Law of Segregation
Why isn’t there this
type of segregation
(with products with
different
chromosomes) in
mitosis– the
chromosomes still
separate at
metaphase?
P = purple p = white
PP
P p
Pp
Gametes
G
g
What
pp
types of
gametes
must both
organisms
in the P
generation
give?
Figure 14.5 Genotype versus phenotype
Genotype vs. Phenotype
**Genotype = genetic makeup
**Phenotype = outward appearance /trait expression
**Just because you have a gene for a trait, doesn’t mean
you always express it (carriers)
Gamete Assortment
• Does the segregation of a pair of alleles affect the
segregation of another pair of alleles?
• EXAMPLE: A pregnant female already has 1 child.
Imagine that there are only 2 potential eye colors
(blue and brown) and hair colors (brown and
blonde). Her first child has blonde hair and blue
eyes. Is it possible that she can have a child with
blonde hair and brown eyes? Why or why not?
What if they always stayed
together?
P
(gametes)
F1
(gametes)
F2
RY and ry
stayed together
Law of Independent Assortment
• We don’t see this…The offspring have a
combination of traits from both parents!
• How can we explain this?
Independent Assortment
P
(gametes)
F1
(gametes)
(FOIL)
Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross
Summary
• Genes control heredity and are passed
down from each parent
• When 2 or more alleles exist for a single
trait, some may be dominant and some
may be recessive
• The 2 alleles are segregated during
formation of gametes
• Genes assort independently of each other
Punnett Squares– Monohybrid Crosses
• Do pg. 10 #1 in
your packets
independently
!
Testcrosses
Can you tell the
genotype of this
flower (assuming
purple color is a
dominant trait in
flowers) just by
looking at it?
Figure 14.6 A testcross
Testcross
•Dominant
phenotype with
unknown
genotype
crossed with
homozygous
recessive
Dihybrid Cross
• Same as
monohybrid
cross– just
more boxes
• Be sure to
combine all
possible
gametes
(independent
assortment)