Question

Instead of moving on to linkage, as originally planned, we're going to use this tutorial to revisit some concepts many of you needed more practice on. When genes are on different chromosomes, they sort independently and follow the laws of segregation and independent assortment, regardless of their inheritance pattern or any interactions (epistasis) between them. Basically, genotypes are NOT phenotypes. Inheritance patterns like dominant-recessive, co-dominant, and so on, and any gene-gene interactions affect the phenotype. Inheritance Patterns Example: Snapdragons can have seven petals or five petals, and can be red, white, or pink. The gene for petal number and the gene for petal color are on different chromosomes. Five petals are dominant to seven petals, while red and white show incomplete dominance with heterozygotes being pink. Let's designate some alleles: Five petals = F; Seven petals = f Red = R1; White = R2; Pink = R1R2 If we cross heterozygotes for petal number, we have - Ff X Ff In the offspring, we'd expect 1/4 FF, 1/2 Ff, 1/4 ff, or 1 FF: 2Ff: 1ff (genotypic ratio) F (five petals) is dominant to f (seven petals), so the phenotypic ratio is: 3F:1f (3 dominant trait: 1 recessive trait) If we cross heterozygotes for petal color, we have - R1R2 X R1R2 In the offspring, we'd expect 1/4 R1R1, 1/2 R1R2, 1/4 R2R2, or 1 R1R1: 2 R1R2: 1 R2R2 (genotypic ratio) R1 (red) and R2 (white) show incomplete dominance, with heterozygotes (R1R2) being pink. So, the phenotypic ratio is - 1 red: 2 pink: 1 white NOTE: the genotypic ratios in both crosses are the same, the difference in inheritance pattern only changes the phenotype ratios. Two Genes If multiple genes contribute to a trait, we can see gene interactions. Let's look at two genes with two alleles each - Gene A (alleles: A, a) and Gene B (alleles: B, b), where A is dominant to a, and B is dominant to b. If we assume the genes sort independently and we perform a dihybrid cross, what phenotypic ratios do we expect? (AaBb X AaBb) Phenotype A = genotype A_ (dominant so the second allele can be either A or a) Phenotype a = genotype aa Phenotype B = genotype B_ Phenotype b = genotype bb So for the two genes (sorting independently), we have the following possibilities: Phenotype A;B = genotype A_B_ Phenotype A;b = genotype A_bb Phenotype a;B = genotype aaB_ Phenotype a;b = genotype aabb This is your classic Mendelian dihybrid cross, the phenotypic ratios are 9 (A;B): 3 (A;b): 3 (a;B): 1 (a;b) In a complementation test, crossing two true-breeding mutants produce wild-type offspring, indicating that the two lines represent mutations in different genes. aaBB (mutant line 1) X AAbb (mutant line 2); offspring are AaBb What happens if we cross two F1s; what is the ratio of phenotypes in the offspring? AaBb X AaBb Just like for the incomplete dominance example we started with, the genotypic ratios will remain the same (as no gene interaction). 9 A_B_: 3 A_bb: 3 aaB_: 1 aabb But the gene interaction has to be taken into account when determining the phenotypic ratio. What are the genotypes of mutant offspring? A_bb and aaB_ What are the genotypes of the wild-type offspring? A_B_ So, the phenotypic ratio is - 9 wild type: 7 mutant (3 A_bb + 3 aaB_ + 1 aabb) Deviations from Mendelian Ratios (9:3:3:1) If we see deviations from Mendelian ratios, we can use them to infer things like patterns of inheritance or interactions between genes. From our snapdragon example, we can see if we have a trait determined by one gene and see a 1:2:1 phenotypic ratio in the offspring of a hybrid cross, the alleles likely show incomplete dominance. In crosses with two genes, offspring ratios of 9 (wild type): 7 (mutant) can indicate the genes complement one another. You have accepted a mission on Mars to study a new species of flying creatures (similar to birds, probably). Your station, Station2040, has isolated a true-breeding line that shows a four-winged recessive phenotype. On the other side of Mars, on Station1001, another researcher also found a true-breeding, recessive four-winged creature. You decide to collaborate together and determine whether the mutant phenotypes you observe are on two different genes that both affect the number of wings. You share your data with the other researcher and now have access to the mutant flying creatures from Station2040, mutant flying creatures from Station1001, along with the wild-type flying creatures. What cross would you set up to determine if you and your collaborator have found mutations on two different genes that both affect wing number or whether you found mutations in the same gene? What results would you see if the mutations are in the same gene, and what results would you see if they are on different genes?

          Instead of moving on to linkage, as originally planned, we're going to use this tutorial to revisit some concepts many of you needed more practice on. When genes are on different chromosomes, they sort independently and follow the laws of segregation and independent assortment, regardless of their inheritance pattern or any interactions (epistasis) between them. Basically, genotypes are NOT phenotypes. Inheritance patterns like dominant-recessive, co-dominant, and so on, and any gene-gene interactions affect the phenotype.

Inheritance Patterns
Example: Snapdragons can have seven petals or five petals, and can be red, white, or pink. The gene for petal number and the gene for petal color are on different chromosomes. Five petals are dominant to seven petals, while red and white show incomplete dominance with heterozygotes being pink.

Let's designate some alleles:
Five petals = F; Seven petals = f
Red = R1; White = R2; Pink = R1R2

If we cross heterozygotes for petal number, we have - Ff X Ff
In the offspring, we'd expect 1/4 FF, 1/2 Ff, 1/4 ff, or 1 FF: 2Ff: 1ff (genotypic ratio)
F (five petals) is dominant to f (seven petals), so the phenotypic ratio is: 3F:1f (3 dominant trait: 1 recessive trait)

If we cross heterozygotes for petal color, we have - R1R2 X R1R2
In the offspring, we'd expect 1/4 R1R1, 1/2 R1R2, 1/4 R2R2, or 1 R1R1: 2 R1R2: 1 R2R2 (genotypic ratio)
R1 (red) and R2 (white) show incomplete dominance, with heterozygotes (R1R2) being pink. So, the phenotypic ratio is - 1 red: 2 pink: 1 white

NOTE: the genotypic ratios in both crosses are the same, the difference in inheritance pattern only changes the phenotype ratios.

Two Genes
If multiple genes contribute to a trait, we can see gene interactions.
Let's look at two genes with two alleles each - Gene A (alleles: A, a) and Gene B (alleles: B, b), where A is dominant to a, and B is dominant to b.
If we assume the genes sort independently and we perform a dihybrid cross, what phenotypic ratios do we expect? (AaBb X AaBb)

Phenotype A = genotype A_ (dominant so the second allele can be either A or a)
Phenotype a = genotype aa
Phenotype B = genotype B_
Phenotype b = genotype bb

So for the two genes (sorting independently), we have the following possibilities:
Phenotype A;B = genotype A_B_
Phenotype A;b = genotype A_bb
Phenotype a;B = genotype aaB_
Phenotype a;b = genotype aabb

This is your classic Mendelian dihybrid cross, the phenotypic ratios are 9 (A;B): 3 (A;b): 3 (a;B): 1 (a;b)

In a complementation test, crossing two true-breeding mutants produce wild-type offspring, indicating that the two lines represent mutations in different genes.
aaBB (mutant line 1) X AAbb (mutant line 2); offspring are AaBb

What happens if we cross two F1s; what is the ratio of phenotypes in the offspring?
AaBb X AaBb

Just like for the incomplete dominance example we started with, the genotypic ratios will remain the same (as no gene interaction).
9 A_B_: 3 A_bb: 3 aaB_: 1 aabb

But the gene interaction has to be taken into account when determining the phenotypic ratio.
What are the genotypes of mutant offspring? A_bb and aaB_
What are the genotypes of the wild-type offspring? A_B_

So, the phenotypic ratio is - 9 wild type: 7 mutant (3 A_bb + 3 aaB_ + 1 aabb)

Deviations from Mendelian Ratios (9:3:3:1)
If we see deviations from Mendelian ratios, we can use them to infer things like patterns of inheritance or interactions between genes.
From our snapdragon example, we can see if we have a trait determined by one gene and see a 1:2:1 phenotypic ratio in the offspring of a hybrid cross, the alleles likely show incomplete dominance.
In crosses with two genes, offspring ratios of 9 (wild type): 7 (mutant) can indicate the genes complement one another.

You have accepted a mission on Mars to study a new species of flying creatures (similar to birds, probably). Your station, Station2040, has isolated a true-breeding line that shows a four-winged recessive phenotype. On the other side of Mars, on Station1001, another researcher also found a true-breeding, recessive four-winged creature. You decide to collaborate together and determine whether the mutant phenotypes you observe are on two different genes that both affect the number of wings.

You share your data with the other researcher and now have access to the mutant flying creatures from Station2040, mutant flying creatures from Station1001, along with the wild-type flying creatures.

What cross would you set up to determine if you and your collaborator have found mutations on two different genes that both affect wing number or whether you found mutations in the same gene? What results would you see if the mutations are in the same gene, and what results would you see if they are on different genes?

Added by Ricky S.

Question

Please give Ace some feedback