genetics 3

Dihybrid cross

A cross involving two pairs of contrasting traits (two-factor cross)

Trihybrid cross

A cross involving three pairs of contrasting traits (three-factor cross)

Mendel's 4th Postulate

Independent Assortment

What law is Mendel's 4th Postulate?

Mendel's 2nd Law

Mendel's 2nd Law

During gamete formation, segregating pairs of unit factors assort independently of each other

When does independent assortment occur?

At anaphase of meiosis I

What happens because of independent assortment?

All possible combinations of chromatids in the gametes will form with equal frequency

What is the F2 phenotypic ratio of a dihybrid cross?

9:3:3:1

What are the four F2 phenotype groups in a dihybrid cross?

Dominant for both traits; dominant for trait 1 and recessive for trait 2; recessive for trait 1 and dominant for trait 2; recessive for both traits

What fraction is dominant for both traits?

9/16

What fraction is dominant for trait 1 and recessive for trait 2?

3/16

What fraction is recessive for trait 1 and dominant for trait 2?

3/16

What fraction is recessive for both traits?

1/16

What gametes can GgWw produce?

GW, Gw, gW, gw

How many gametes can GgWw produce?

4

How many combinations are in a dihybrid Punnett square?

4 × 4 = 16

What are the F2 phenotype groups from a trihybrid cross?

8 phenotype groups

How are genotypes related to the 8 F2 phenotype groups in a trihybrid cross?

Each dominant phenotype can be produced by either a homozygous dominant or heterozygous genotype (A_); each recessive phenotype requires a homozygous recessive genotype (aa)

What is the F2 phenotypic ratio of a trihybrid cross?

27:9:9:9:3:3:3:1

Trihybrid squares

8 × 8 = 64

Forked-Line Method

A method used to solve crosses involving any number of gene pairs, providing that all gene pairs assort independently

Why is the Forked-Line Method useful?

It is easier than a Punnett square when working with larger numbers of traits

How are probabilities arranged in the Forked-Line Method?

Independent events are placed in successive columns

How do you calculate the frequency of a phenotype or genotype with the Forked-Line Method?

Multiply the probabilities along the branches

What is the probability of dominant for trait 1 AND recessive for trait 2?

3/4 × 1/4 = 3/16

What is the probability of recessive for trait 1 AND dominant for trait 2?

1/4 × 3/4 = 3/16

What is the probability of recessive for BOTH traits?

1/4 × 1/4 = 1/16

Product law of probabilities

find the probability of two or more independent events happening together by multiplying their individual probabilities

Forked Line Method is actually a visual tool that relies directly on the product law of probability rather than being a separate or competing rule

Product law shortcut

a shortcut used to find the probability of two or more independent events happening together by multiplying their individual probabilities

What genotype is used for a two-trait testcross?

Homozygous recessive for both traits

For genes G/g and W/w, what is the genotype of the homozygous recessive tester?

ggww

What is a two-character testcross used for?

To determine the genotype of an individual expressing two dominant traits whose genotype is unknown

What is the general setup of a two-trait testcross?

Individual with dominant phenotypes and unknown genotype × individual homozygous recessive for both traits

What genotype is used as the tester in a three-trait testcross?

Homozygous recessive for all three traits

What is the general setup of a three-trait testcross?

Individual with dominant phenotypes and unknown genotype × individual homozygous recessive for all three traits

How many types of gametes can a trihybrid heterozygote produce?

2 × 2 × 2 = 8

How large is a trihybrid Punnett square?

8 × 8 = 64 cells

How do you calculate the number of possible gametes?

2^n

What does n represent in the formula 2^n for possible gametes?

The number of heterozygous gene pairs being considered

What happens to different gene pairs during independent assortment?

They segregate independently of one another

What three principles are used to predict outcomes involving multiple independently assorting genes?

Segregation, independent assortment, and random fertilization

What is the general genotype setup for a two-trait testcross?

A_B_ × aabb

What is the general genotype setup for a three-trait testcross?

A_B_C_ × aabbcc

What is the purpose of a two- or three-trait testcross?

To determine the unknown genotype of an individual showing dominant phenotypes by crossing it with an individual homozygous recessive for all traits

What does a recessive phenotype among the offspring of a testcross tell you?

The unknown parent must carry the recessive allele for that gene and is heterozygous at that gene

If an individual heterozygous for two genes is testcrossed (AaBb × aabb), what offspring genotypes are expected with independent assortment?

AaBb, Aabb, aaBb, and aabb in a 1:1:1:1 ratio

If an individual heterozygous for three genes is testcrossed (AaBbCc × aabbcc), how many offspring genotype/phenotype groups are expected with independent assortment?

8 groups in equal proportions (1:1:1:1:1:1:1:1)

What are the F2 phenotype groups and corresponding genotype groups in a dihybrid cross?

Dominant/dominant = A_B_; dominant/recessive = A_bb; recessive/dominant = aaB_; recessive/recessive = aabb

What are the F2 phenotype groups and corresponding genotype groups in a trihybrid cross?

Dominant/dominant/dominant = A_B_C_; dominant/dominant/recessive = A_B_cc; dominant/recessive/dominant = A_bbC_; recessive/dominant/dominant = aaB_C_; dominant/recessive/recessive = A_bbcc; recessive/dominant/recessive = aaB_cc; recessive/recessive/dominant = aabbC_; recessive/recessive/recessive = aabbcc