Chapter 9 Patterns of Inheritance
Assume that in cattle a spotted coat is dominant to an even coat, short horns are dominant to long horns, and the traits for coat type and horn length assort independently. In a cross between cattle that are each heterozygous for both traits, what proportion of their offspring are expected to have long horns?
1/4 Because these traits assort independently, this dihybrid cross can be considered as two separate monohybrid crosses, with the consequence that one-fourth of the offspring of cattle heterozygous for the horn length trait will express the recessive long-horn trait.
Hypercholesterolemia is a disorder in which blood cholesterol levels are elevated. The H allele is incompletely dominant to the h allele, with hh homozygotes having extremely high levels of blood cholesterol. A husband and wife are both Hh heterozygotes. What is the chance that their first child will have normal levels of blood cholesterol?
1/4 This is the probability of giving birth to an HH homozygote, the only genotype conferring normal cholesterol levels.
Two individuals decide to have children. The expected blood group genotypes are 50% of blood type A, and 25% each of blood types AB and B. What genotypes are the parents?
I^Ai x I^AI^B Since blood groups show codominance, the child of blood type A must be either IAIA or IAi. The child of blood type B must be either IBIB or IBi. The child of blood type AB can only be IAIB. Because there are more offspring expected with blood type A, we know that an A allele is being contributed from both parents. By reversing the Punnett square, the parents' genotypes are observed to be IAi and IAIB.
Which of the following is true regarding this new fetal DNA test?
A decreased number of false positive tests for both Down syndrome and Trisomy 18.
The Y chromosomes of mammals contain genes that code for _____.
"maleness" and a few other characteristics Researchers have identified a number of extra genes on the Y chromosome that are required for normal testis functioning.
If one parent is blood type AB and the other is type O, what fraction of their offspring is expected to have blood type A?
0.5 About half the offspring would be expected to inherit the IA allele and be phenotypically A.
Pea flowers may be purple (P) or white (p). Pea seeds may be round (R) or wrinkled (r). What proportion of the offspring from the cross PpRr × PpRr are expected to have white flowers and wrinkled seeds?
1/16 Pp × Pp would produce 3/4 purple and 1/4 white. Rr × Rr would produce 3/4 round and 1/4 wrinkled. (1/4)(1/4) = 1/16 would have white flowers and wrinkled seeds.
Human genetic disorders __________.
are most often recessive Although dominant genetic disorders occur, disorders caused by recessive alleles are more common.
Linked genes are inherited together. This is because linked genes _____.
are on the same chromosome Genes located on the same chromosome tend to be inherited together and are said to be linked genes. They may or may not be functionally related.
Your 28 year old friend is pregnant. Which of the following screenings is she most likely to initially have to test for Down syndrome?
blood test
DdEe pea plants can produce _____ type(s) of gametes, but a ddee plant can produce _____ type(s) of gametes.
four ... one DdEe individuals can produce the following gametes: DE, De, dE,de. A ddee plant can produce only de gametes.
Two identical twins are raised in different environments. They possess _____ genotypes and _____ phenotypes.
identical ... variable Identical twins will have identical genotypes. However, if they are raised in different environments, then their phenotypes will be variable. That is, they will show some similarities and some differences, such as in fingerprints even at the moment of birth.
A child with cystic fibrosis can be born to two parents who do not have the disease. This is because the disease _____.
is caused by a recessive allele The most common lethal genetic disease in the United States is cystic fibrosis, which is recessively inherited. The parents in this case would be heterozygotes, or carriers for the disease.
Our understanding of the role played by genes in many human characteristics is advancing rapidly in, for example, body size, performance on IQ tests, and personality traits. In this new genetic era, the role of the environment __________.
is to work with genes in complex and often unknown ways in the development of these traits Genes and environment collaborate in complicated and often unknown ways in the development of traits.
Suppose we have a pea plant with purple flowers, determined by the dominant allele P. How might you determine whether the plant is homozygous (PP) or heterozygous (Pp)?
Perform a testcross: Cross the plant with a white one, which must be homozygous recessive (pp). If the purple-flowered plant is PP, all the offspring will have purple flowers. If it is Pp, half the offspring will have white flowers.
The existence of rare XY individuals who are phenotypically normal women was instrumental in learning about human sex determination. Maleness is determined by the SRY gene found on the Y chromosome. How is it possible to be an XY woman?
The SRY locus of the Y chromosome is deleted. If SRY is deleted, the Y chromosome would still be present, but development would occur along the female path.
Which of the following is true?
The chance of having a child with a chromosomal abnormality increases with the age of the mother.
Tay-Sachs disease runs in Rebecca's family. On a family pedigree, she saw a shaded circle. This represented _____.
a female with Tay-Sachs The circle indicates a female, and the shading indicates she has the disease.
What is an allele?
an alternative version of a gene A diploid organism has two alleles for each autosomal gene. The two alleles are found at comparable locations on homologous chromosomes. The alleles may be identical or slightly different, but they affect the same genetic character.
Three characteristics assort independently in one species of insect: Red eyes (R) are dominant to black eyes (r). Blue wings (B) are dominant to white wings (b). Green bodies (G) are dominant to yellow bodies (g). A cross is made between two heterozygous insects (RrBbGg). What is the probability that the offspring will be homozygous dominant for all three characteristics?
1/64 The likelihood that the offspring will be homozygous dominant for any particular characteristic is 1/4 and can be determined using a Punnett square. The likelihood that the offspring will be homozygous dominant for all three characteristics can be determined using the rule of multiplication (1/4 × 1/4 × 1/4).
There are over 100 alleles known for the gene associated with cystic fibrosis. With current technology, it is possible to determine exactly which allele or alleles is/are carried by a person. What is the maximum number of different alleles that any person can carry?
2 Because there are two homologous chromosomes, each with one locus for this and every other gene, a person may carry up to two different alleles for this gene.
Flower color in snapdragons is an example of incomplete dominance. If a red-flowered plant is crossed with a white-flowered plant, the F1 generation has pink flowers. If a pink-flowered plant is crossed with another pink-flowered plant, the expected progeny plants will be __________.
25% red, 50% pink, and 25% white Breeding the F1 hybrids of the snapdragon cross produces F2 offspring with a phenotypic ratio of 1 red: 2 pink: 1 white.
A woman who is a carrier of hemophilia marries a man affected with hemophilia. What percentage of their sons and daughters is expected to have hemophilia?
50% of sons and 50% of daughters Because sons obtain their single X chromosome from their mother, they have a 50% chance of inheriting the homolog that carries the hemophilia allele. And because daughters inherit their father's only X chromosome and one of the two X chromosomes of their mother, in this cross they have a 50% chance of being homozygous for the hemophilia allele.
You have one tree that produces big yellow apples and another tree that produces small red apples. When the two are crossed, you find that half the offspring trees produce big red apples and half produce big yellow apples. What are the genotypes of the parents? A = big apples; a = small apples; R = red apples; r = yellow apples.
AArr and aaRr Trees that produce big yellow apples could be AArr or Aarr. Trees that produce small red apples could be aaRR or aaRr. Because all the offspring are big, the big parent must be homozygous for A. Half the apples are red and half are yellow, so the red parent must be heterozygous for red. The conclusion is that the cross must be AArr × aaRr.
We can better understand our ancestry by analyzing the __________ in males.
Y chromosome Scientists have been tracking male descendants using the Y chromosome because the Y chromosome passes essentially intact from father to son.
In lentils, the C gene has two different alleles. CSCS homozygotes have spotted seeds, CDCD homozygotes have dotted seeds, and CSCD heterozygotes have seeds with both spots and dots. This indicates that __________.
CS and CD are codominant In the case of incomplete dominance of CS and CD, the heterozygotes would have some intermediate phenotype, for example, pigmented areas bigger than dots but smaller than spots.
Imagine that long fins in zebrafish is a dominant trait. A breeder wants to set up a breeding program beginning with homozygous dominant long-finned fish. If she obtains a handful of the long-finned fish, how can she tell which, if any, of these are homozygous for the trait?
Cross the long-finned fish with short-finned fish; if the offspring are all long-finned, the long-finned parent is homozygous. This cross is a testcross (the short-finned fish must be homozygous recessive), and having all offspring show the dominant trait proves that the long-finned parent is homozygous.
An insect that has the genotype EeGGcc will have the same phenotype as an insect with the genotype __________.
EEGgcc Individuals with one or two dominant alleles of a gene are indistinguishable, so EE and Ee have the same phenotype, as do GG and Gg.
Two normal parents have three normal children: one son and two daughters. Their son and one of their daughters marry and also have normal children. Their second daughter, Mary, marries a man with a rare, recessive blood disorder. They have two children, and both children develop the blood disorder. What must be true of the genotypes of Mary's parents?
Either one of her parents or both of her parents were heterozygous for the trait. From the information provided, it is not possible to discern which of these is correct for sure. If one of Mary's siblings did develop the disorder, you could determine that both of the parents were heterozygous.
Hemophilia appears rarely in females. This is because __________.
Hemophilia appears rarely in females. This is because __________. Because hemophilia is an X-linked recessive disorder, the female must have the gene on both X chromosomes. For this to occur, one parent must have hemophilia and one must be a carrier.
In breeding pure-breeding large and small strains of mice, you cross individuals of each strain and note that their offspring are intermediate in size. Two models (explanations) to account for this result are (1) that body size in these strains is due to one gene with alleles that show incomplete dominance and (2) that body size is a polygenic trait. How could you distinguish between these models?
Intercross the F1 and see if the F2 contains three size classes (consistent with the incomplete dominance model) or if there is a range of sizes (consistent with the polygenic model).
How is the fetal DNA used in this new screening tool unique?
It is not from a cell, but is floating freely in the mother's blood.
Which of the following best summarizes current research on this new fetal DNA test?
It works well for finding errors in chromosome number.
Why are lethal dominant alleles so much more rare than lethal recessive alleles?
Lethal dominant alleles are harmful whether they are carried in homozygous or heterozygous form, so there is always strong selection against these alleles. The ability of lethal recessive alleles to "hide" from selection in heterozygotes accounts for their prevalence relative to dominant alleles, which are always visible to selection.
Akin to urban legends, there are curious genetics legends - things like eye color being determined by one gene, with a brown eye allele being completely dominant to blue. The problem comes when simple myth meets the complex reality of how eye color and many other traits are transmitted. Why is the inheritance of so many traits difficult to explain using only Mendel's view of genetics?
Mendel was correct for the traits he investigated, but his principles must be extended (not discarded) to explain many more complex patterns of inheritance. Mendel's laws are general for all sexually reproducing organisms, but they must be extended in ways Mendel never knew in order to account for many patterns of inheritance.
In an individual of genotype Aa, where are the A and a alleles physically located?
One allele is on one chromosome, and the other is in the same position (locus) on the homologous chromosome.
Imagine a particular character (such as flower color) that is determined by a single gene. If this gene is present in two forms, how can you tell which allele is dominant and which is recessive?
Perform a cross between two true-breeding individuals and observe the trait or traits expressed by the F1 individuals. The F1 plants are hybrids that contain both forms of the gene, so the trait the F1 plants display is the dominant trait.
In corn, blue kernels are produced by a dominant allele of a coloration gene, and white kernels are produced in individuals homozygous for a recessive allele of the same coloration gene. Another gene has two alleles for shape, with smooth kernels being dominant to wrinkled. A plant heterozygous for both genes is testcrossed (crossed to a homozygous recessive white, wrinkled strain). The testcross offspring consist of the following types: 1,447 blue smooth; 1,436 white wrinkled; 150 blue wrinkled; 145 white smooth. Explain the inheritance of the coloration and shape traits.
The coloration and shape genes are linked on the same chromosome. The fact that there isn't the 1:1:1:1 ratio of phenotypes expected in this cross if the alleles assorted independently and that there is a pair of large phenotypic classes (1,447 blue smooth; 1,436 white wrinkled) and a pair of small phenotypic classes (150 blue wrinkled; 145 white smooth) indicates that the genes are linked.
In a particular species of mammal, black hair (B) is dominant to green hair (b), and red eyes (R) are dominant to white eyes (r). When a BbRr individual is mated with a bbrr individual, offspring are produced in a ratio of 5 black hair and red eyes : 5 green hair and white eyes : 1 black hair and white eyes : 1 green hair and red eyes. Which of these explanations accounts for this ratio?
The genes for hair color and eye color are linked. This explains why the parental types occur more often than would be expected if the characters assorted at random.
Imagine you're counseling a couple who have undergone carrier screening for Tay-Sachs disease. The man is a carrier, and the woman does not carry the Tay-Sachs allele. How should you advise them?
They should be informed that if they have a child, the child will not have Tay-Sachs disease but will have a 50% chance of being a carrier of the Tay-Sachs allele. Because the woman will transmit a normal allele to the child, the child will not be affected; however, there is a 50% chance that the child will inherit a Tay-Sachs allele from the father and thus be a carrier.
A woman and her male partner have normal color vision. However, her father and her first son are colorblind. What is her genotype? Use C as the gene for colorblindness.
XCXc Because this is an X-linked recessive condition, she must possess at least one allele for color blindness to have an affected son. Because she has normal color vision, she must also have one normal allele.
When applied to Mendel's experiments, the term true-breeding means a self-fertilization of two plants that produces __________.
offspring identical to the parent True-breeding means that a homozygous dominant individual, when self-fertilized, would produce only homozygous dominant individuals. The same would also be true of a homozygous recessive individual only producing homozygous recessive individuals.
The inheritance of height and weight can best be described as __________.
polygenic These characteristics involve several genes.
The cross-fertilization of two different, but true-breeding, varieties of pea plants will _____.
result in hybrid plants The offspring of the crossing of different varieties are referred to as hybrids.
Cystic fibrosis is inherited in an autosomal recessive pattern. Males who have cystic fibrosis are usually sterile. Furthermore, the disease is often lethal before the age of reproduction. Even though people with the disease rarely reproduce, cases continue to arise because __________.
the harmful allele "hides" inside heterozygous individuals and one-fourth of the offspring of two heterozygotes should be afflicted Cystic fibrosis is inherited as a Mendelian recessive trait, and affected individuals usually have unaffected parents, both of whom are carriers of the recessive allele.
Two parents of mixed ethnicity have twins, one of which is born with very light skin and one of which is born with very dark skin. This is because of __________.
the polygenic nature of skin color genes Skin color is known to be affected by at least three genes, possibly more.
In a monohybrid cross, F2 refers to __________.
the second filial generation, or the "grandchildren" of the original mating pair F2 is shorthand for "second filial generation," or the "grandchildren" of the original mating pair.
Many genetic disorders can be detected before birth. Procedures include _____, which is noninvasive, or _____, which allows the chromosomes of the fetus to be examined. Alternatively, maternal blood samples can be taken and tested for _____.
ultrasound imaging ... chorionic villus sampling ... AFP Ultrasound testing is the least invasive of the tests, CVS involves sampling fetal tissue, and AFP (alpha-fetoprotein) is obtained from a maternal blood sample.
If the two characteristics that Mendel looked at in his dihybrid cross of smooth yellow peas with wrinkled green peas had been controlled by genes that were located close together on the same chromosome, then the F2 generation __________.
would have deviated from the 9:3:3:1 phenotypic ratio that is predicted by the law of independent assortment If the two characters are located on the same chromosome, they will not segregate independently.
