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 genesTg 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

Phenotypes Tally

Tall & Green Pods

9

Tall & Yellow Pods

3

Short & Green Pods

3

Short & Yellow Pods

1

TT = tall GG = green

Tt = tall Gg = green

tt = short gg = yellow

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