As shown in the picture that follows, flowers of the plant...

As shown in the picture that follows, flowers of the plant Arabidopsis thaliana (mustard weed) normally contain four different types of organs: sepals (leaves), petals, anthers (male sex organs), and carpels (female sex organs). The mutant strain shown in the picture at the right has abnormal flower morphology-the flower is made up entirely of sepals! Three genes (called $S E P I, S E P 2,$ and $S E P 3$ ) function redundantly in the pathway for generating petals, anthers, and carpels. For normal flower morphology, the plant requires only one dominant, normally functioning allele of any one of these genes: $\operatorname{SEPI}(A)$ or $\operatorname{SEP} 2(B)$ or $\operatorname{SEP} 3(C) .$ Recessive mutant alleles of these genes $(a, b, \text { or } c)$ specify no protein. a. What is the genotype of the mutant plant below? b. In a trihybrid cross of the type $A A b b c c \times a a B B$ $C C,$ where all of the $F_{1}$ are $A a B b C c,$ what is the expected fraction of normal plants among the $\mathrm{F}_{2}$ progeny? c. Suggest a model to explain how the Arabidopsis thaliana genome came to acquire three redundant genes.

As shown in the picture that follows, flowers of the plant Arabidopsis thaliana (mustard weed) normally contain four different types of organs: sepals (leaves), petals, anthers (male sex organs), and carpels (female sex organs). The mutant strain shown in the picture at the right has abnormal flower morphology-the flower is made up entirely of sepals! Three genes (called $S E P I, S E P 2,$ and $S E P 3$ ) function redundantly in the pathway for generating petals, anthers, and carpels. For normal flower morphology, the plant requires only one dominant, normally functioning allele of any one of these genes: $\operatorname{SEPI}(A)$ or $\operatorname{SEP} 2(B)$ or $\operatorname{SEP} 3(C) .$ Recessive mutant alleles of these genes $(a, b, \text { or } c)$ specify no protein.
a. What is the genotype of the mutant plant below?
b. In a trihybrid cross of the type $A A b b c c \times a a B B$ $C C,$ where all of the $F_{1}$ are $A a B b C c,$ what is the expected fraction of normal plants among the $\mathrm{F}_{2}$ progeny?
c. Suggest a model to explain how the Arabidopsis thaliana genome came to acquire three redundant
genes.

Key Concepts

Genetic Redundancy

Genetic redundancy is the presence of multiple genes in an organism’s genome that perform similar or overlapping functions. This means that if one gene is nonfunctional due to a mutation, the other genes can often compensate for the loss, buffering the organism against deleterious effects. This redundancy plays an important role in the robustness of biological systems and can be a driving force for evolutionary innovation over time.

Dominance and Recessiveness

Dominance and recessiveness describe the relationship between alleles of a gene and their effects on an organism's phenotype. A dominant allele is one that masks the phenotypic effect of a recessive allele when they are present together in a heterozygote. In many cases, the presence of a single dominant allele is sufficient to produce a normal phenotype, whereas the recessive phenotype is observed only when an organism has two copies of a mutant allele that result in loss of function.

Mendelian Inheritance and Hybrid Crosses

Mendelian inheritance describes how traits are passed from parents to offspring through discrete units, or alleles, following predictable patterns as described by Gregor Mendel. In a trihybrid cross, where three independent traits are considered, the principles of independent assortment and segregation come into play to determine the expected ratios of phenotypes among progeny. This approach is a fundamental tool for predicting genetic outcomes in crosses involving multiple gene pairs.

Gene Duplication and Functional Divergence

Gene duplication is an evolutionary process where an organism's genome gains an extra copy of a gene. Over time, these duplicated genes can acquire mutations that may lead either to redundant functions or to functional divergence, where the duplicated gene takes on a new or specialized role. This mechanism is a key source of genetic novelty and complexity in evolution and can lead to the development of redundant genetic pathways that contribute to the resiliency of biological systems.

Transcript

00:01 So, the flower of plant arabidopsis normally contains four different types of organs, sepals, petals, anthers, and coppels.

00:10 The mutant strain shown in the picture at the right has abnormal flower morphology.

00:16 The flower is made up entirely of sepals.

00:19 Three genes called cep1, cep2, and cep3 function redundantly in the pathway for generating petals, anthers, and coppels.

00:30 For normal flower morphology, the plant requires only one dominant, normally functioning allele of any of the three genes, which means cep1 produced protein a or cep2 produced protein b or cep3 produced protein c.

00:47 The recessive mutant allele of these genes, ab or c, lowercase, specifies no protein.

00:52 What is the genotype of mutant plant which produces entirely of sepals? so since only one dominant allele is required to produce normal flower morphology, and this mutant has all sepals abnormal, and it has abnormal flower morphology, we can tell that it must be homozygous lower a, b, and c.

01:28 So none of the dominant allele, capital a, b, c, or c is present, or otherwise any of the a, b, or c is able to produce a functional phenotype or normal phenotype.

01:46 So we know that when you have homozygous lower a, b, and c, that means no protein a, b, or c is present, and therefore you'll have a abnormal morphology.

02:02 Second, let's take a look at this cross.

02:08 So you have a trihybrid cross, capital a homozygous, lower b homozygous, lower c homozygous, and it crossed with homozygous lower a, capital b, and capital c.

02:27 Where all of the f1 are heterozygous a, b, c, what is the expected fraction of normal plant among the f2? so we know that f1 has heterozygous a, b, and c, and you have a cross with another heterozygous a, b, and c.

02:54 This is trihybrid cross.

02:56 Now the punnett square is going to be too big, so we're going to use the forkline method.

03:00 So basically we are going to do the three genes separately.

03:05 So let's start from gene a.

03:07 So you can see the two alleles separate into two gametes.

03:23 So each gamete only has half of the genome.

03:26 So each gamete has only one allele.

03:28 And then the other pair produce, again, two different alleles, capital a and lower a.

03:40 So half of the chance, capital a, sorry, 1 fourth of the chance, homozygous capital a, half, heterozygous, 1 fourth, homozygous lower a.

04:08 Now then let's move on to b.

04:16 So it's going to be the same punnett square...