Concept 14.4: Many human traits follow Mendelian patterns of inheritance Flashcards
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
pedigree
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.
widow’s peak
Because the widow’s-peak allele is dominant, all individuals who lack a widow’s peak must be
homozygous recessive (ww).
The two grandparents with widow’s peaks must have the Ww genotype, since some of their offspring are
homozygous recessive.
The offspring in the second generation who do have widow’s peaks must also be heterozygous, because they are the products of
Ww x ww matings.
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).
homozygous (WW) or heterozygous (Ww)
figure 14.15 pedigree analysis
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
¾ (¼ WW + ½ Ww).
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
¼.
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:
¾ (chance of widow’speak) × ¼ (chance of inability to taste PTC) = 3⁄16 (chance of widow’s peak and inability to taste PTC).
Thousands of genetic disorders are known to be inherited as simple
recessive traits
An allele that causes a genetic disorder (let’s call it allele a) codes for either a malfunctioning protein or
no protein at all.
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
sufficient amount of the specific protein.
Thus, a recessively inherited disorder shows up only in the homozygous individuals (aa) who inherit a recessive allele from each
parent
Although phenotypically normal with regard to the disorder, heterozygotes may transmit the recessive allele to their offspring and thus are called
carriers
figure 14.16 albinism: a recessive trait
A mating between two carriers corresponds to a Mendelian F1 monohybrid cross, so the predicted genotypic ratio for the offspring is
1 AA : 2 Aa : 1 aa.
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 .
albinism
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
2/3 chance.
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
reproduce
Tay-Sachs disease, which we described earlier in this chapter, is disproportionately high among
Ashkenazic Jews, Jewish people whose ancestors lived in central Europe.
The probability of passing on recessive traits increases greatly, however, if the man and woman are close
relatives
This is because people with recent common ancestors are more likely to carry the same recessive alleles than are .
unrelated people
Thus, these consanguineous (“same blood”) matings, indicated in pedigrees by double lines, are more likely to produce offspring
homozygous for recessive traits—including harmful ones
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.
cystic fibrosis
The normal allele for this gene codes for a membrane protein that functions in the transport of chloride ions between certain cells and the
extracellular fluid.
These chloride transport channels are defective or absent in the plasma membranes of children who inherit two
recessive alleles for cystic fibrosis.
The result is an abnormally high concentration of intracellular chloride, which causes an uptake of
water due to osmosis.
This in turn causes the mucus that coats certain cells to become
thicker and stickier than normal.
The mucus builds up in the pancreas, lungs, digestive tract, and other organs, leading to multiple (pleiotropic) effects, including
poor absorption of nutrients from the intestines, chronic bronchitis, and recurrent bacterial infections.
Untreated, cystic fibrosis can cause death by the age of
5
The most common inherited disorder among people of African descent is __________________________, which affects one out of 400 African-Americans.
sickle-cell disease
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
hemoglobin protein of red blood cells
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
long fibers that deform the red cells into a sickle shape
Sickled cells may clump and clog small blood vessels, often leading to other symptoms throughout the body, including
physical weakness, pain, organ damage, and even stroke and paralysis.
figure 14.17 sickle cell disease, biointeractive animation sickle cell
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
homozygotes.
biointeractive video the making of the fittest
Although many harmful alleles are recessive, a number of human disorders are due to dominant alleles. One example is achondroplasia, a form of
dwarfism that occurs in one of every 25,000 people
figure 14.18 achondroplasia a dominant trait
A lethal recessive allele is only lethal when homozygous; it can be passed from one generation to the next by
heterozygous carriers because the carriers themselves have normal phenotypes
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
children
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
Huntington’s disease
Once the deterioration of the nervous system begins, it is irreversible and inevitably
fatal.
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
chromosome 4, and the gene was sequenced in 1993.
This information led to the development of a test that could detect the presence of the Huntington’s allele in an
individual’s genome
Heart disease, diabetes, cancer, alcoholism, certain mental illnesses such as schizophrenia and bipolar disorder, and many other diseases are
multifactorial.
In these cases, the hereditary component is
polygenic.
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
recessive allele.
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
amniocentesis
figure 14.19 testing a fetus for genetic disorders
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
chorionic villus sampling (CVS)
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
chorionic villus sampling (CVS)
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 .
complications in a small percentage of cases
In 2008, the Chinese government initiated a program of fetal testing to detect a harmful genetic blood disorder called
β-thalassemia.
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.
phenylketonuria (PKU)
Children with this disease cannot properly metabolize the
amino acid phenylalanine.
