Correlation between maternal age and the incidence of Down syndrome. As women age, the chances they will bear a child with Down syndrome increase. After a woman reaches 35, the frequency of Down syndrome rises rapidly.
Over a five-year period between ages 20 and 25, the incidence of Down syndrome increases 0.1 per thousand; over a five-year period between ages 35 and 40, the incidence increases to 8.0 per thousand, 80 times as great. The period of time is the same in both instances. What has changed? Answer: The proportion of egg cells with a chromosomal change producing Down syndrome. Imagine a jar containing 10,000 white marbles. Allow one marble to become red every minute. Now draw out one marble after one minute. The chance of drawing a red one is 1/10,000, or 0.1 per thousand. Wait 80 minutes and pull out another marble. The chance of drawing a red marble is now 80/10,000 or 8 per thousand.
1). In order to ensure he had pure-breeding plants for his experiments, Mendel
a). cross-fertilized each variety with each other.
b). let each variety self-fertilize for several generations.
c). removed the female parts of the plants.
d). removed the male parts of the plants.
2). When two parents are crossed, the offspring are referred to as the
c). F1 generation.
d). F2 generation.
3). A cross between two individuals results in a ratio of 9:3:3:1 for four possible phenotypes. This is an example of a
a). dihybrid cross.
b). monohybrid cross.
d). none of these.
4). Human height shows a continuous variation from the very short to the very tall. Height is most likely controlled by
a). epistatic genes.
b). environmental factors.
c). sex-linked genes.
d). multiple genes.
5). In the human ABO blood grouping, the four basic blood types are type A, type B, type AB, and type O. The blood proteins A and B are
a). simple dominant and recessive traits.
b). incomplete dominant traits.
c). codominant traits.
d). sex-linked traits.
6). Which of the following describes symptoms of sickle cell anemia?
a). poor blood circulation due to abnormal hemoglobin molecules
b). sterility in females
c). failure of blood to clot
d). failure of chloride ion transport mechanism
7). What finding finally determined that genes were carried on chromosomes?
a). heat sensitivity of certain enzymes that determined coat color
b). sex-linked eye color in fruit flies
c). the finding of complete dominance
d). establishing pedigrees
8). A genetic map can be used to determine
a). the relative position of alleles on chromosomes.
b). restriction sites on chromosomes.
c). the frequency of recombination between two genes.
d). all of these.
9). A Barr body is a(n)
a). result of primary nondisjunction.
b). inactivated Y chromosome.
c). gene that plays a key role in male development.
d). inactivated X chromosome.
10). Down syndrome in humans is due to
a). three copies of chromosome 21.
c). two Y chromosomes.
d). three X chromosomes.
Test Your Visual Understanding
1). The figure shows the results of a dihybrid cross in which Mendel was examining the inheritance of two traits: seed shape (R and r) and seed color (Y and y). Consider the results if he had examined three traits. Using these two traits and plant height (T and t), predict:
a). the genotypes and phenotypes of the parents.
b). the genotypic and phenotypic ratios of the F1 generation.
c). the genotypes of the eggs and sperm.
d). the phenotypic ratios of the F2 generation.
a). The genotype of the parents would be RRYYTT x rryytt and the phenotypes would be tall plant with round, yellow seeds x dwarf plants with wrinkled, green seeds.
b). All individuals of the F1 generation would be the same with the genotype RrYyTt and phenotype of tall plants with round, yellow seeds.
c). There would be no difference in genotype or phenotype of the eggs and sperm, which would be
RYT RYt RyT Ryt rYT rYt ryT ryt
d). The phenotypic ratio of the F2 generation would be
26/64 are round, yellow, tall
10/64 are round, yellow, dwarf
9/64 are round, green, tall
9/64 are wrinkled, yellow, tall
3/64 are round, green, dwarf
3/64 are wrinkled, yellow, dwarf
3/64 are wrinkled, green, tall
1/64 are wrinkled, green, dwarf
Apply Your Knowledge
1). Phenylketonuria (PKU) is a genetic disorder caused by the mutation of an enzyme, phenylalanine hydroxylase, that breaks down phenylalanine in the cell. If identified at birth, dietary restrictions can control the disease. If not identified, buildup of phenylalanine interferes with brain development. The frequency of PKU in the population is 1 in 12,000 births. How many people with PKU would you expect to find in a population of 250,000?
Answer: To figure out the number of PKU individuals in a population of 250,000 use proportions:
1/12,000 :: ?/250,000
250,000 x 1 = 250,000
250,000/12,000 = 20.83
Therefore, we would expect to find 20.83 individuals with PKU in a population of 250,000.
2). How might Mendel's results and the model he formulated have been different if the traits he chose to study were governed by alleles exhibiting incomplete dominance or codominance?
Answer: If Mendel had selected traits that exhibited incomplete dominance or codominance, he probably wouldn't have identified the patterns of inheritance of dominant and recessive traits. The F1 generation would have expressed a blending of the traits only to have the parental phenotypes reappearing in the F2 generation. He may still have been able to establish the laws of segregation and independent assortment, especially because the heterozygotes would be more easily identified.