Concept 14.4: Many human traits follow Mendelian patterns of inheritance

Question 1

Unable to manipulate the mating patterns of people, geneticists instead analyze the results of matings that have already occurred. They do so by collecting information about a family’s history for a particular trait and assembling this information into a family tree describing the traits of parents and children across the generations—a family

Answer 1

pedigree

Question 2

a three-generation pedigree that traces the occurrence of a pointed contour of the hairline on the forehead. This trait, called a ______________, is due to a dominant allele, W.

Answer 2

widow’s peak

Question 3

Because the widow’s-peak allele is dominant, all individuals who lack a widow’s peak must be

Answer 3

homozygous recessive (ww).

Question 4

The two grandparents with widow’s peaks must have the Ww genotype, since some of their offspring are

Answer 4

homozygous recessive.

Question 5

The offspring in the second generation who do have widow’s peaks must also be heterozygous, because they are the products of

Answer 5

Ww x ww matings.

Question 6

The third generation in this pedigree consists of two sisters. The one who has a widow’s peak could be either ________________________________, given what we know about the genotypes of her parents (both Ww).

Answer 6

homozygous (WW) or heterozygous (Ww)

Question 7

figure 14.15 pedigree analysis

Question 8

What is the probability that the child will have a widow’s peak? This is equivalent to a Mendelian F1 monohybrid cross (Ww × Ww), and therefore the probability that a child will inherit a dominant allele and have a widow’s peak is

Answer 8

¾ (¼ WW + ½ Ww).

Question 9

What is the probability that the child will be unable to taste PTC? We can also treat this as a monohybrid cross (Tt × Tt), but this time we want to know the chance that the offspring will be homozygous recessive (tt). That probability is

Answer 9

¼.

Question 10

Finally, what is the chance that the child will have a widow’s peak and be unable to taste PTC? Assuming that the genes for these two characters are on different chromosomes, the two pairs of alleles will assort independently in this dihybrid cross (WwTt × WwTt). Therefore, we can use the multiplication rule:

Answer 10

¾ (chance of widow’speak) × ¼ (chance of inability to taste PTC) = 3⁄16 (chance of widow’s peak and inability to taste PTC).

Question 11

Thousands of genetic disorders are known to be inherited as simple

Answer 11

recessive traits

Question 12

An allele that causes a genetic disorder (let’s call it allele a) codes for either a malfunctioning protein or

Answer 12

no protein at all.

Question 13

In the case of disorders classified as recessive, heterozygotes (Aa) typically have the normal phenotype because one copy of the normal allele (A) produces a

Answer 13

sufficient amount of the specific protein.

Question 14

Thus, a recessively inherited disorder shows up only in the homozygous individuals (aa) who inherit a recessive allele from each

Answer 14

parent

Question 15

Although phenotypically normal with regard to the disorder, heterozygotes may transmit the recessive allele to their offspring and thus are called

Answer 15

carriers

Question 16

figure 14.16 albinism: a recessive trait

Question 17

A mating between two carriers corresponds to a Mendelian F1 monohybrid cross, so the predicted genotypic ratio for the offspring is

Answer 17

1 AA : 2 Aa : 1 aa.

Question 18

Thus, each child has a 1/4 chance of inheriting a double dose of the recessive allele; in the case of albinism, such a child will have .

Answer 18

albinism

Question 19

From the genotypic ratio, we also can see that out of three offspring with the normal phenotype (one AA plus two Aa), two are predicted to be heterozygous carriers, a

Answer 19

2/3 chance.

Question 20

Recessive homozygotes could also result from Aa x aa and aa matings, but if the disorder is lethal before reproductive age or results in sterility (neither of which is true for albinism), no aa individuals will

Answer 20

reproduce

Question 21

Tay-Sachs disease, which we described earlier in this chapter, is disproportionately high among

Answer 21

Ashkenazic Jews, Jewish people whose ancestors lived in central Europe.

Question 22

The probability of passing on recessive traits increases greatly, however, if the man and woman are close

Answer 22

relatives

Question 23

This is because people with recent common ancestors are more likely to carry the same recessive alleles than are .

Answer 23

unrelated people

Question 24

Thus, these consanguineous (“same blood”) matings, indicated in pedigrees by double lines, are more likely to produce offspring

Answer 24

homozygous for recessive traits—including harmful ones

Question 25

The most common lethal genetic disease in the United States is ______________, which strikes one out of every 2,500 people of European descent but is much rarer in other groups.

Answer 25

cystic fibrosis

Question 26

The normal allele for this gene codes for a membrane protein that functions in the transport of chloride ions between certain cells and the

Answer 26

extracellular fluid.

Question 27

These chloride transport channels are defective or absent in the plasma membranes of children who inherit two

Answer 27

recessive alleles for cystic fibrosis.

Question 28

The result is an abnormally high concentration of intracellular chloride, which causes an uptake of

Answer 28

water due to osmosis.

Question 29

This in turn causes the mucus that coats certain cells to become

Answer 29

thicker and stickier than normal.

Question 30

The mucus builds up in the pancreas, lungs, digestive tract, and other organs, leading to multiple (pleiotropic) effects, including

Answer 30

poor absorption of nutrients from the intestines, chronic bronchitis, and recurrent bacterial infections.

Question 31

Untreated, cystic fibrosis can cause death by the age of

Answer 31

5

Question 32

The most common inherited disorder among people of African descent is __________________________, which affects one out of 400 African-Americans.

Answer 32

sickle-cell disease

Question 33

Sickle-cell disease is caused by the substitution of a single amino acid in the ____________________________________; in homozygous individuals, all hemoglobin is of the sickle-cell (abnormal) variety

Answer 33

hemoglobin protein of red blood cells

Question 34

When the oxygen content of an affected individual’s blood is low (at high altitudes or under physical stress, for instance), the sickle-cell hemoglobin proteins aggregate into

Answer 34

long fibers that deform the red cells into a sickle shape

Question 35

Sickled cells may clump and clog small blood vessels, often leading to other symptoms throughout the body, including

Answer 35

physical weakness, pain, organ damage, and even stroke and paralysis.

Question 36

figure 14.17 sickle cell disease, biointeractive animation sickle cell

Question 37

About one out of ten African-Americans have sickle-cell trait, an unusually high frequency of heterozygotes for an allele with severe detrimental effects in

Answer 37

homozygotes.

Question 38

biointeractive video the making of the fittest

Question 39

Although many harmful alleles are recessive, a number of human disorders are due to dominant alleles. One example is achondroplasia, a form of

Answer 39

dwarfism that occurs in one of every 25,000 people

Question 40

figure 14.18 achondroplasia a dominant trait

Question 41

A lethal recessive allele is only lethal when homozygous; it can be passed from one generation to the next by

Answer 41

heterozygous carriers because the carriers themselves have normal phenotypes

Question 42

A lethal dominant allele may be passed on, though, if the lethal disease symptoms first appear after reproductive age. In these cases, the individual may already have transmitted the allele to his or her

Answer 42

children

Question 43

a degenerative disease of the nervous system, called ___________________________, is caused by a lethal dominant allele that has no obvious phenotypic effect until the individual is about 35 to 45 years old

Answer 43

Huntington’s disease

Question 44

Once the deterioration of the nervous system begins, it is irreversible and inevitably

Answer 44

fatal.

Question 45

By analyzing DNA samples from a large family with a high incidence of the disorder, geneticists tracked the Huntington’s allele to a locus near the tip of

Answer 45

chromosome 4, and the gene was sequenced in 1993.

Question 46

This information led to the development of a test that could detect the presence of the Huntington’s allele in an

Answer 46

individual’s genome

Question 47

Heart disease, diabetes, cancer, alcoholism, certain mental illnesses such as schizophrenia and bipolar disorder, and many other diseases are

Answer 47

multifactorial.

Question 48

In these cases, the hereditary component is

Answer 48

polygenic.

Question 49

Most children with recessive disorders are born to parents with normal phenotypes. The key to accurately assessing the genetic risk for a particular disease is therefore to find out whether the prospective parents are heterozygous carriers of the

Answer 49

recessive allele.

Question 50

One of the tests that can be done to determine whether the developing fetus has Tay-Sachs disease is ___________________, which can be performed starting at the 15th week of pregnancy

Answer 50

amniocentesis

Question 51

figure 14.19 testing a fetus for genetic disorders

Question 52

In an alternative technique called _______________________________, a physician inserts a narrow tube through the cervix into the uterus and suctions out a tiny sample of tissue from the placenta, the organ that transmits nutrients and fetal wastes between the fetus and the mother

Answer 52

chorionic villus sampling (CVS)

Question 52

In an alternative technique called _______________________________, a physician inserts a narrow tube through the cervix into the uterus and suctions out a tiny sample of tissue from the placenta, the organ that transmits nutrients and fetal wastes between the fetus and the mother

Answer 52

chorionic villus sampling (CVS)

Question 53

Ultrasound and isolation of fetal cells or DNA from maternal blood pose no known risk to either mother or fetus, while the other procedures can cause .

Answer 53

complications in a small percentage of cases

Question 54

In 2008, the Chinese government initiated a program of fetal testing to detect a harmful genetic blood disorder called

Answer 54

β-thalassemia.

Question 55

One common screening program is for ______________________, a recessively inherited disorder that occurs in about one out of every 10,000–15,000 births in the United States.

Answer 55

phenylketonuria (PKU)

Question 56

Children with this disease cannot properly metabolize the

Answer 56

amino acid phenylalanine.