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Mendel’s Second Law
• The Law of Independent Assortment


Mendel also crossed plants of two traits.
• because two traits are involved in these crosses
they are called a dihybrid cross.
Mendel crossed true breeding tall plants that had
green pods (TTGG) with true breeding short
plants that had yellow pods (ttgg) to produce the
F1 generation

in this case the true breeding plants will produce only one
type of gametes
TTGG → will produce gametes with the TG genes
ttgg → will produce gametes with the tg genes
♀
tg
tg
TG
TtGg
TtGg
TG
TtGg
TtGg
♂


the phenotypic ratio of the F1
generation:
100% tall and green pods
the genotypic ratio of the F1
generation
100% heterozygous


Mendel then crossed the F1 generation to produce
an F2 generation
in this case the plants of the F1 generation
produce four different types of gametes
TtGg → will produce gametes with the:
TG genes (tall, green)
Tg genes (tall, yellow)
tG genes (short, green)
tg genes (short, yellow)
♀
♂
TtGg → will produce
gametes with the:
TG genes
Tg genes
tG genes
tg genes
TG
Tg
tG
tg
TG
TTGG
TTGg
TtGG
TtGg
Tg
TTGg
TTgg
TtGg
Ttgg
tG
TtGG
TtGg
ttGG
ttGg
tg
TtGg
Ttgg
ttGg
ttgg
TT = tall
GG = green
Tt = tall
Gg = green
tt = short
gg = yellow
Phenotypes
Tally
Tall & Green
Pods
9
Tall & Yellow
Pods
3
Short & Green
Pods
3
Short & Yellow
Pods
1

for every dihybrid cross that Mendel carried he
got the 9:3:3:1 ratio (when he crossed the F1
generation).
• this ratio is what is expected if the segregation
of alleles for one gene had no influence on the
segregation of alleles of another gene.
Law of Independent Assortment
• The two alleles of one gene segregate (assort)
independently of the alleles for other genes
during gamete formation
Pleiotropic Genes
• a gene that affects more than one
characteristic
• example: Sickle-cell anemia
• the normal hemoglobin is produced by the
allele HbA
• in sicke-cell anemia the individual has two
copies of the mutated allele Hbs
Pleiotropic Genes
• the mutation cause abnormally shaped
hemoglobin that cannot deliver oxygen to the
cells.
• causes fatigue, enlarged spleen, pneumonia
and major organ damage.
• a heterozygous individual has resistance to
malaria but an increased chance of having
homozygous recessive offspring.
Homework



“5.Dihybrid Cross” from your workbook
(questions 1-5)
Journal questions
Read pages 613-617