Dihybrid Crosses

Genetic Continuity, Mendel and Multiple Traits

Lesson Objectives

• Minds ON• Recap – Lab, Chi Square, Null Hypothesis• Monohybrid Cross• Dihybrid Cross• Practice• Take up homework• New homework

Minds ON

Looking at this image, what can we assume the genotype of the parents is? What would we call this?

Rest of pea video:http://www.youtube.com/watch?v=Mehz7tCxjSE

Yesterday’s Recap

Mom and Dad

Children

I want to make sure we are clear on the generational names. This is a MONOHYBRID.

Grandma and Grandpa

Yesterday’s Recap

• Null hypothesis: means that there is no relationship between the observed and expected results; the result of chance

• Chi Square Table:

Think of this as a percentage; the closer it is to 1.0, the closer it is to the expected result

For Lab• So, if you hypothesized that there would be a

significant relationship between the observed results and expected results (close to a 3:1 ratio), then you would REJECT the null hypothesis.

• However, if you hypothesized that there would be no significant relationship between the observed results and expected results (not 3:1 ratio), then you would ACCEPT the null hypotheses.

NOTE: The words “significant” and “not significant” can be ignored on the bottom of the Chi Square sheet (should be “Good Fit / Poor Fit”)

Monohybrid Cross• The crossing of two individuals who are

heterozygous for ONE trait (i.e. creates F2 generation)

• We are only looking at one trait (colour, etc.) hence “MONO” (=1)

• Hybrid = combo of two varieties

Dihybrid Cross• Refers to the crossing of two individuals who

are both heterozygous for TWO traits (e.g. PpTt x PPTT)

• Assumes that these traits are on different genes and can occur at the same time.

• E.g. Pod colourand seed colour

Possible Gametes

• Parent 1 genotype is: AaBb

• Parent 2 genotype is: AaBb

x2

Dihybrid Crosses

• Passing on more than one characteristic at a time

• Phenotypic Ratio: 9:3:3:1

Back to Mendel and Meiosis…

The possible combinations produced are determined by how the tetrads line up in meiosis I. This is called Independent Assortment and Mendel’s 4th Postulate.

Back to Mendel…F1 Generation (Mom and Dad)

P1 Generation (Grandma and Grandpa)

F2 Generation (kids)

Dyhybrid Cross

Dihybrid

Let’s Try Something…

Fill in the chart with the appropriate possibilities of offspring genotypes for this dihybrid cross.

Answer:

SY Sy sY sy

SY SSYY SSYy SsYY SsYy

Sy SSYy SSyy SsYy Ssyy

sY SsYY SsYy ssYY ssYy

sy SsYy Ssyy ssYy ssyy

Try this…

What are the phenotypic ratios for:

Yellow, round:Yellow, wrinkled:Green, round:Green, wrinkled:

Answer:SY¼

Sy¼

sY¼

Sy¼

SY¼

SSYY1/16

SSYy1/16

SsYY1/16

SsYy1/16

Sy¼

SSYy1/16

Ssyy1/16

SsYy1/16

Ssyy1/16

sY¼

SsYY1/16

SsYy1/16

ssYY1/16

ssYy1/16

Sy¼

SsYy1/16

Ssyy1/16

ssYy1/16

Ssyy1/16

What are the phenotypic ratios for:

• Yellow, round: 9/16• Yellow, wrinkled: 3/16• Green, round: 3/16• Green, wrinkled: 1/16

Therefore: 9:3:3:1Genotypic ratio: 1:

Take Up Homework

• Monohybrid Sheet• Genetic Questions• Check time!!

Homework

• Dihybrid genetics sheet• The rest of the Hybrid sheet• The Punnett Squares sheet