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Huntington's Disease is a progressive neurological condition with a dominant inheritance pattern. What causes the defective version of this gene to cause disease?

part of the gene is missing in people with the disease

the gene is completely missing in people with the disease

the gene is overly active and its effects are damaging to the brain

the gene has accumulated a "stutter" that causes it to have abnormally long stretches of the same protein

the gene is blocked from being transcribed and no protein can be made from it

Hemophilia is a genetic flaw in the production of a protein needed for blood to clot. It affects almost exclusively boys. Why can we say with confidence that a child has no risk of inheriting this condition from a healthy father, but usually inherits it from a healthy mother?

the gene that codes for this protein is silent on the paternal chromosome (imprinting) and can only be translated from the maternal chromosome

the gene only causes disease when present in males because women have other genetic factors that produce different clotting proteins related to pregnancy and childbirth

the gene is present on the X chromosome, so females can be healthy with one affected and one normal chromosome, but males will either have a defective X chromosome or a normal one, and if he has a normal X chromosome, he will pass it to all his children

the gene is present on the X chromosome, and since males inherit their X chromosome from their fathers, a healthy father can't transmit the disease

in males, the gene accumulates more errors and is more likely to produce defective protein

Rett's syndrome is a genetic condition that causes apparently normal infants to undergo rapid neurological regression and develop unusual behaviors and physical symptoms. It normally affects exclusively girls. How does this work?

girls have two X chromosomes, so both must be affected, while males only have one X chromosome, which doesn't produce enough protein to cause disease

girls have two X chromosomes, which allows them to have one functioning copy of the gene, while boys have only a defective copy, and without one working copy, the condition is usually lethal before birth

the gene accumulates a "stutter", producing an abnormal chain of the same amino acid, and this only occurs on maternal chromosomes

the defective gene is silenced on the maternal chromosome (imprinting), so girls, who have two X chromosomes do not produce as much of the protein

the condition is so mild in boys that it is generally not recognized as a genetic defect

Angelman's syndrome causes severe intellectual disabilities and unusual behavior known as "happy puppet" syndrome. Prader-Willi syndrome causes mild to moderate intellectual delay, distinctive appearance, and an incessant need to eat, causing morbid obesity. Both genes are located on chromosome 15. How else are they related?

they are more and less severe forms of the same genetic defect depending on how severely the gene is damaged

chromosome 15 is particularly at risk of breaking or other forms of damage, causing it to accumulate an unusual number of defects

Angelman's is recessively inherited, while Prader-Willi is dominantly inherited

both conditions are caused by a genetic "stutter" producing an abnormal repeat of one amino acid, with Angelman's having a greater number of repeats than Prader-Willi

This part of the chromosome is imprinted, and defects causing Angelman's occur because the paternal gene is damaged and the maternal gene is silenced and cannot replace it, while the opposite occurs with Prader-Willi

Sonic Hedgehog is a gene critical to development of the fetus. Among other things, it dictates the width of the face, the division of the brain into hemispheres, and the development of limbs. Defects in Sonic Hedgehog can cause severe abnormalities or almost no abnormalities in individuals in the same family. How does this work?

Sonic Hedgehog is a critical gene, but some of its functions are redundant (are also carried out by other genes), so the function of the other genes can dictate how severe the expression of the Sonic Hedgehog mutation is

Sonic Hedgehog is transmitted on the X chromosome, and since males only have one, and in females one is randomly inactivated, the severity of the defect depends on which parent it is inherited from

in early fetal development, some cells recognize that the production of Sonic Hedgehog is defective and turn off the defective version

Sonic Hedgehog genes can accumulate a "stutter" that causes an abnormal chain of a repeated amino acid, and the more repeats, the more severe the condition

the distribution of Sonic Hedgehog in the body is affected by genes, so the gene may only be defective in certain body parts

Trisomy 21, otherwise known as Down Syndrome, occurs when an individual has three copies of chromosome 21. Affected individuals have developmental delays and are prone to heart defects and other health problems, although some of them function very well in structured environments. Trisomy 21 is the most common trisomy diagnosed in newborns. Why is it so common?

chromosome 21 is particularly small, which increases the likelihood that an extra copy will accidentally be transmitted

chromosome 21 carries especially important genes, so trisomy of this chromosome causes a recognizable syndrome, while trisomy of other chromosomes may not be noticed

Down Syndrome is extremely easy to recognize and diagnose, while symptoms of other trisomies can be more vague and harder to diagnose

trisomy in any chromosome other than chromosome 21 is lethal, and embryos with trisomy of other chromosomes are usually miscarried in very early pregnancy or die immediately after birth

Down Syndrome is not any more common than other trisomies, but it is particularly well-known because this condition is often represented in the media

Sickle cell anemia is a recessive condition in which individuals with two defective genes have red blood cells that "sickle" or change shape when deprived of oxygen, causing damage to blood vessels and organs. Although it is a serious condition and it can be deadly, it is an unusually common condition, especially in people whose ancestors came from particular areas of the world. Why hasn't this condition been eliminated by evolution if it is so harmful?

the condition does not affect people until after they have had children, so it doesn't affect whether they pass on the gene

while two defective copies of the gene can be lethal, one defective copy (a carrier) increases an individual's resistance to death by malaria by making red blood cells harder for the parasite to live in

the defective gene only becomes harmful when the individual is exposed to certain toxins that were not common in the environment of our ancestors

people from the regions where this gene is common are particularly prone to inbreeding, which makes the condition more common

if people are never in a situation where their blood has a shortage of oxygen, the disease will never cause them difficulties

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