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Chapter 6: Problem 12

In humans, the disease galactosemia causes mental retardation at an early age. Lactose (milk sugar) is broken down to galactose plus glucose. Normally, galactose is broken down further by the enzyme galactose- phate uridyltransferase (GALT). However, in patients with galactosemia, GALT is inactive, leading to a buildup of high levels of galactose, which, in the brain, causes mental retardation. How would you provide a secondary cure for galactosemia? Would you expect this disease phenotype to be dominant or recessive?

Short Answer

Expert verified
A secondary cure could involve a lactose-free diet or enzyme replacement; galactosemia is likely recessive.

Step by step solution

01

Understanding the Problem

Galactosemia is a genetic disorder where the enzyme GALT is inactive, preventing the breakdown of galactose properly. This accumulates in the body, particularly affecting the brain, and causes mental retardation.
02

Mechanism of Galactosemia

Lactose is usually broken down into glucose and galactose. The enzyme GALT is responsible for converting galactose into forms that can be further used or excreted. In galactosemia, this process is disrupted due to the inactivity of GALT.
03

Exploring a Secondary Cure

A secondary cure could involve dietary changes to limit or eliminate the intake of lactose and galactose, hence preventing their accumulation. An alternative would be enzymatic replacement therapy which aims to introduce functional GALT enzyme externally.
04

Genetic Dominance Analysis

In galactosemia, since the lack of enzyme function is necessary for the disease, it is most likely a recessive trait. This means two non-functional alleles are required to express the disease since having one functional allele typically suffices for normal function.
05

Concluding the Phenotype Nature

Given that one working allele is usually enough to produce sufficient enzyme activity, the disease phenotype would be recessive. Carriers of one non-functional allele do not typically show symptoms.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Galactosemia
Galactosemia is a type of genetic disorder that affects how the body processes a simple sugar called galactose. Galactose is a component of lactose, which is found in milk and dairy products. In a healthy body, galactose is converted into energy by the enzyme galactose-1-phosphate uridyltransferase (GALT). However, individuals with galactosemia have an inactive GALT enzyme, leading to an accumulation of galactose in their system.

This accumulation can be particularly harmful to the brain, potentially resulting in severe symptoms like mental retardation, liver damage, or even cataracts. Early diagnosis is crucial for managing galactosemia.
  • Newborns are often screened for this condition at birth.
  • A lactose-free diet can prevent severe symptoms by avoiding galactose buildup.
Dietary management must begin as soon as possible to avoid any long-term complications associated with this condition.
Enzyme Deficiency
Enzyme deficiency occurs when the body lacks a specific enzyme necessary for the conversion of certain substances. In the case of galactosemia, the body's inability to produce the enzyme GALT hinders the metabolism of galactose.

Enzymes are essential proteins that facilitate and speed up biochemical reactions. Without the necessary enzyme, various metabolic processes are halted. This deficiency can result in various accumulated substances, leading to manifestations like those seen in galactosemia.
  • Enzymes act as biological catalysts, making biochemical reactions faster and more efficient.
  • A deficiency can be inherited, as seen in genetic disorders like galactosemia.
  • Enzyme replacement therapy is a possible therapeutic approach, where missing enzymes are externally supplied.
Understanding enzyme deficiencies helps in developing effective treatments and managing symptoms of disorders like galactosemia.
Recessive Trait
Galactosemia is an example of a recessive trait. Recessive traits manifest in an individual only when two copies of a non-functional gene (alleles) are inherited, one from each parent.

In genetics, alleles are forms of a gene. An individual inherits alleles from both parents, which determine the specific traits or disorders one might display. For galactosemia, an individual needs to inherit two non-functional alleles, leading to the inactivity of the GALT enzyme.
  • Recessive traits require both alleles to be defective for the trait to be expressed.
  • Carriers, with only one copy of the non-functional allele, do not usually show symptoms.
  • Pedigree charts can help trace the inheritance patterns of recessive traits in families.
Recognizing the recessive nature of galactosemia is vital in genetic counseling and prenatal testing.

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Most popular questions from this chapter

In common wheat, Triticum aestivum, kernel color is determined by multiply duplicated genes, each with an \(R\) and an \(r\) allele. Any number of \(R\) alleles will give red, and a complete lack of \(R\) alleles will give the white phenotype. In one cross between a red pure line and a white pure line, the \(\mathrm{F}_{2}\) was \(\frac{63}{64}\) red and \(\frac{1}{64}\) white. a. How many R genes are segregating in this system? b. Show the genotypes of the parents, the \(\mathrm{F}_{1}\), and the \(\mathrm{F}_{2}\). c. Different \(F_{2}\) plants are backcrossed with the white parent. Give examples of genotypes that would give the following progeny ratios in such backcrosses: (1) 1 red: 1 white (2) 3 red : 1 white, (3) 7 red : 1 white. d. What is the formula that generally relates the number of segregating genes to the proportion of red individuals in the \(\mathrm{F}_{2}\) in such systems?

In the nematode \(C .\) elegans, some worms have blistered cuticles due to a recessive mutation in one of the bli genes. Someone studying a suppressor mutation that suppressed bli-3 mutations wanted to know if it would also suppress mutations in \(b l i-4 .\) They had a strain that was homozygous for this recessive suppressor mutation, and its phenotype was wild type. a. How would they determine whether this recessive suppressor mutation would suppress mutations in bli- 4 ? In other words, what is the genotype of the worms required to answer the question? b. What cross(es) would they do to make these worms? c. What results would they expect in the \(\mathrm{F}_{2}\) if (1) it did act as a suppressor of bli-4? (2) it did not act as a suppressor of bli- 4 ?

Four homozygous recessive mutant lines of Drosophila melanogaster (labeled 1 through 4) showed abnormal leg coordination, which made their walking highly erratic. These lines were intercrossed; the phenotypes of the \(\mathrm{F}_{1}\) flies are shown in the following grid, in which "+" represents wild-type walking and "-" represents abnormal walking: $$\begin{array}{rrrrr} & 1 & 2 & 3 & 4 \\ \hline 1 & \- & \+ & \+ & \+ \\ 2 & \+ & \- & \- & \+ \\ 3 & \+ & \- & \- & \+ \\ 4 & \+ & \+ & \+ & \- \\ \hline \end{array}$$ a. What type of test does this analysis represent?? b. How many different genes were mutated in creating these four lines? c. Invent wild-type and mutant symbols, and write out full genotypes for all four lines and for the \(\mathrm{F}_{1}\) flies. d. Do these data tell us which genes are linked? If not, how could linkage be tested? e. Do these data tell us the total number of genes taking part in leg coordination in this animal?

For several years, Hans Nachtsheim investigated an inherited anomaly of the white blood cells of rabbits. This anomaly, termed the Pelger anomaly, is the arrest of the segmentation of the nuclei of certain white cells. This anomaly does not appear to seriously burden the rabbits. a. When rabbits showing the Pelger anomaly were mated with rabbits from a true-breeding normal stock, Nachtsheim counted 217 offspring showing the Pelger anomaly and 237 normal progeny. What is the genetic basis of the Pelger anomaly? b. When rabbits with the Pelger anomaly were mated with each other, Nachtsheim found 223 normal progeny, 439 with the Pelger anomaly, and 39 extremely abnormal progeny. These very abnormal progeny not only had defective white blood cells, but also showed severe deformities of the skeletal system; almost all of them died soon after birth. In genetic terms, what do you suppose these extremely defective rabbits represented? Why were there only 39 of them? c. What additional experimental evidence might you collect to test your hypothesis in part \(b\) ? d. In Berlin, about 1 human in 1000 shows a Pelger anomaly of white blood cells very similar to that described for rabbits. The anomaly is inherited as a simple dominant, but the homozygous type has not been observed in humans. Based on the condition in rabbits, why do you suppose the human homozygous has not been observed? e. Again by analogy with rabbits, what phenotypes and genotypes would you expect among the children of a man and woman who both show the Pelger anomaly? (Data from A. M. Srb, R. D. Owen, and R. S. Edgar, General Genetics, 2 nd ed. W. H. Freeman and Company, \(1965 .)\)

In Drosophila, the autosomal recessive bw causes a dark brown eye, and the unlinked autosomal recessive \(s t\) causes a bright scarlet eye. A homozygote for both genes has a white eye. Thus, we have the following correspondences between genotypes and phenotypes: \(\begin{aligned} s t^{+} / s t^{+} ; b w^{+} / b w^{+} &=\text {red eye (wild type) } \\ s t^{+} / s t^{+} ; b w / b w &=\text { brown eye } \\ s t / s t ; b w^{+} / b w^{+} &=\text {scarlet eye } \\ s t / s t ; b w / b w &=\text { white eye } \end{aligned}\) Construct a hypothetical biosynthetic pathway showing how the gene products interact and why the different mutant combinations have different phenotypes.
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