Concept explainers
CASE STUDY |A genetic flip of the coin
On July 11, 2008, twin sons were born to Stephan Gerth from Germany and AddoGerth from Ghana. Stephan is very fair-skinned with blue eyes and straight hair; Addo is dark-skinned, with brown eyes and curly hair. The first born of the twins, Ryan, is fair-skinned, with blue eyes and straight hair; his brother, Leo, has light brown skin, brown eyes, and curly hair. Although the twins' hair texture and eye color were the same as those of one or the other parent, the twins had different skin colors, intermediate to that of their parents. Experts explained that the blending effect of skin color in the twins resulted from quantitative inheritance involving at least three different gene pairs, whereas hair texture and eye color are not quantitatively inherited. Using thisas an example of quantitative genetics, we can ask the following questions:
What approach is used in estimating how many gene pairs are involved in a quantitative trait? Why would this be extremely difficult in the case of skin color in humans?
Case summary:
Twin sons were born to Stephan Gerth from Germany, who has fair-skinned with blue eyes and straight hair; and Addo Gerth from Ghana, who has dark-skinned with brown eyes and curly hair. One of their twins have fair skin, blue eyes, and straight hair whereas another boy has light brown skin, brown eyes, and curly hair. Experts explained that blending effect of skin color in the babies is resulted from quantitative inheritance involving at least three different gene pairs. Hair texture and eye color are not quantitatively inherited.
Characters in the case:
Stephen Gerth and Addo Gerth, who have twins and one of the twins have blended skin color of the parents.
Adequate information:
Among the two twin sons of Stephen and Addo, one son is born with light brown colored skin. That is considered as a blended trait.
To determine:
The approach used to estimate number of genes involve in the quantitative trait and whether this is difficult in case of skin color in human beings.
Given information:
The blending effect of skin color in the babies is resulted from quantitative inheritance involving at least three different gene pairs. Hair texture and eye color are not quantitatively inherited.
Explanation of Solution
When many different factors combine to produce a distinctive trait then it is known as quantitative inheritance. Skin color can be considered as an example of polygenic inheritance. This is because many genes collectively influence the phenotypic expression of the trait.
The number of gene pairs can be estimated by determining the number of different phenotypic categories found. The number can be determined by using following equation that is 2n+1, where n is the number of polygenes.
In case of human skin color, it would be extremely difficult because of presence of so many distinct phenotypic characters. It is hard to determine the number of shades found in human beings. There are nearly eight loci are found in human beings for skin colors.
Therefore, it can be concluded that number of gene pairs can be estimated by using equation 2n+1. It is hard to determine the number of shades for skin color in human due to presence distinct phenotypic categories.
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Chapter 21 Solutions
Essentials of Genetics (9th Edition) - Standalone book
- Two autosomal mutations include albinism and dwarfism. Albinism (a) is recessive, and dwarfism (D) is dominant. Complete a dihybrid cross of two people that are heterozygous for normal pigmented skin. One person is normal height and has no genetic trace of dwarfism in the family. The other person has dwarfism but has a mother who is normal height. 21. Complete a full dihybrid Punnett square (6pts).arrow_forwardConcordance studies of twins for a neurodegenerative disorder show MZ= 46% and DZ= 15%. Further studies have shown a possible link to a gene on chromosome 9, however, there are some individuals in the study who have the allele but do not develop the disorder (group 1), and there are other individuals who do not have the allele yet develop the disorder (group 2). Amita's older sister and maternal uncle have this disorder. Currently, Amita & her 2 younger brothers do not show symptoms. Amita's paternal grandfather was rumored to have this disorder. 1. Draw the pedigree for Amita's family and determine the mode of inheritance if any. 2. Explain how the 2 groups in the study could be possible? 3. What would you tell Amita about the heritability of this disorder?arrow_forward10 cM separates two hypothetical autosomal human genes. The dominant alleles are have complete penetrance and will result to Crossed eyes (e+) and short thumbs (th+). Four children are born to a normal guy and cross-eyed, small-thumbed woman. Two of the children have short thumbs and the other two have crossed eyes. She is carrying her fifth child. What is the probability that this fifth child will be cross-eyed and have short thumbs?arrow_forward
- In human beings, the gene for red‑green colorblindness (r) is sex‑linked and recessive to its allele for normal vision (R), while the gene for freckles (F) is autosomal and dominant over its allele for nonfreckled (f). A nonfreckled, normal‑visioned woman whose father was freckled and colorblind, marries a freckled, colorblind man whose mother was nonfreckled. What is the probability that the first child born to the couple will either be a freckled, colorblind boy or a non‑freckled, normal visioned girl or a non-freckled, normal visioned boy? What is the probability that the first four children born to the couple will be freckled and normal visioned girls?arrow_forward1-Gigantism is being traced in a family through a pedigree. Its mode of inheritance is thought to be autosomal recessive. You examine the pedigree and decide that the mode of inheritance is not correct. What is the correct mode of inheritance? 2-Provide 2 pieces of evidence for the mode of inheritance selected for Gigantism. (i.e.- Individual 45 is afflicted and passed it on to all of her offspring, indicating that it is dominant. Or there is a gendered pattern of inheritance seen when Individual 99 passed it on to only his female offspring.)arrow_forwardDuchenne muscular dystrophy is an X-linked, recessive disorder in which muscles waste away early in life, resulting in death in the teens or twenties. A man and woman in their late thirties have five children—three boys (ages 1, 3, and 10 years) and two girls (ages 5 and 7 years). The oldest, boy shows symptoms of the disease. What are the probabilities that their other children will develop the disease? Give only typing answer with explanation and conclusionarrow_forward
- Hemophilia and color blindness are both recessive conditions caused by genes on the X chromosome. To calculate the recombination frequency between the two genes, you draw a large number of pedigrees that include grandfathers with both hemophilia and color blindness, their daughters (who presumably have one chromosome with two normal alleles and one chromosome with two mutant alleles), and the daughters sons. Analyzing all the pedigrees together shows that 25 grandsons have both color blindness and hemophilia, 24 have neither of the traits, 1 has color blindness only, and 1 has hemophilia only. How many centimorgans (map units) separate the hemophilia locus from the locus for color blindness?arrow_forwardA couple was referred for genetic counseling because they wanted to know the chances of having a child with dwarfism. Both the man and the woman had achondroplasia (MIM 100800), the most common form of short-limbed dwarfism. The couple knew that this condition is inherited as an autosomal dominant trait, but they were unsure what kind of physical manifestations a child would have if it inherited both mutant alleles. They were each heterozygous for the FGFR3 (MIM 134934) allele that causes achondroplasia. Normally, the protein encoded by this gene interacts with growth factors outside the cell and receives signals that control growth and development. In achrodroplasia, a mutation alters the activity of the receptor, resulting in a characteristic form of dwarfism. Because both the normal and mutant forms of the FGFR3 protein act before birth, no treatment for achrondroplasia is available. The parents each carry one normal allele and one mutant allele of FGRF3, and they wanted information on their chances of having a homozygous child. The counsellor briefly reviewed the phenotypic features of individuals with achondroplasia. These include facial features (large head with prominent forehead; small, flat nasal bridge; and prominent jaw), very short stature, and shortening of the arms and legs. Physical examination and skeletal X-ray films are used to diagnose this condition. Final adult height is approximately 4 feet. Because achondroplasia is an autosomal dominant condition, a heterozygote has a 1-in-2, or 50%, chance of passing this trait to his or her offspring. However, about 75% of those with achondroplasia have parents of average size who do not carry the mutant allele. In these cases, achondroplasia is due to a new mutation. In the couple being counseled, each individual is heterozygous, and they are at risk for having a homozygous child with two copies of the mutated gene. Infants with homozygous achondroplasia are either stillborn or die shortly after birth. The counselor recommended prenatal diagnosis via ultrasounds at various stages of development. In addition, a DNA test is available to detect the homozygous condition prenatally. What if the couple wanted prenatal testing so that a normal fetus could be aborted?arrow_forwardA couple was referred for genetic counseling because they wanted to know the chances of having a child with dwarfism. Both the man and the woman had achondroplasia (MIM 100800), the most common form of short-limbed dwarfism. The couple knew that this condition is inherited as an autosomal dominant trait, but they were unsure what kind of physical manifestations a child would have if it inherited both mutant alleles. They were each heterozygous for the FGFR3 (MIM 134934) allele that causes achondroplasia. Normally, the protein encoded by this gene interacts with growth factors outside the cell and receives signals that control growth and development. In achrodroplasia, a mutation alters the activity of the receptor, resulting in a characteristic form of dwarfism. Because both the normal and mutant forms of the FGFR3 protein act before birth, no treatment for achrondroplasia is available. The parents each carry one normal allele and one mutant allele of FGRF3, and they wanted information on their chances of having a homozygous child. The counsellor briefly reviewed the phenotypic features of individuals with achondroplasia. These include facial features (large head with prominent forehead; small, flat nasal bridge; and prominent jaw), very short stature, and shortening of the arms and legs. Physical examination and skeletal X-ray films are used to diagnose this condition. Final adult height is approximately 4 feet. Because achondroplasia is an autosomal dominant condition, a heterozygote has a 1-in-2, or 50%, chance of passing this trait to his or her offspring. However, about 75% of those with achondroplasia have parents of average size who do not carry the mutant allele. In these cases, achondroplasia is due to a new mutation. In the couple being counseled, each individual is heterozygous, and they are at risk for having a homozygous child with two copies of the mutated gene. Infants with homozygous achondroplasia are either stillborn or die shortly after birth. The counselor recommended prenatal diagnosis via ultrasounds at various stages of development. In addition, a DNA test is available to detect the homozygous condition prenatally. What is the chance that this couple will have a child with two copies of the dominant mutant gene? What is the chance that the child will have normal height?arrow_forward
- A couple was referred for genetic counseling because they wanted to know the chances of having a child with dwarfism. Both the man and the woman had achondroplasia (MIM 100800), the most common form of short-limbed dwarfism. The couple knew that this condition is inherited as an autosomal dominant trait, but they were unsure what kind of physical manifestations a child would have if it inherited both mutant alleles. They were each heterozygous for the FGFR3 (MIM 134934) allele that causes achondroplasia. Normally, the protein encoded by this gene interacts with growth factors outside the cell and receives signals that control growth and development. In achrodroplasia, a mutation alters the activity of the receptor, resulting in a characteristic form of dwarfism. Because both the normal and mutant forms of the FGFR3 protein act before birth, no treatment for achrondroplasia is available. The parents each carry one normal allele and one mutant allele of FGRF3, and they wanted information on their chances of having a homozygous child. The counsellor briefly reviewed the phenotypic features of individuals with achondroplasia. These include facial features (large head with prominent forehead; small, flat nasal bridge; and prominent jaw), very short stature, and shortening of the arms and legs. Physical examination and skeletal X-ray films are used to diagnose this condition. Final adult height is approximately 4 feet. Because achondroplasia is an autosomal dominant condition, a heterozygote has a 1-in-2, or 50%, chance of passing this trait to his or her offspring. However, about 75% of those with achondroplasia have parents of average size who do not carry the mutant allele. In these cases, achondroplasia is due to a new mutation. In the couple being counseled, each individual is heterozygous, and they are at risk for having a homozygous child with two copies of the mutated gene. Infants with homozygous achondroplasia are either stillborn or die shortly after birth. The counselor recommended prenatal diagnosis via ultrasounds at various stages of development. In addition, a DNA test is available to detect the homozygous condition prenatally. Should the parents be concerned about the heterozygous condition as well as the homozygous mutant condition?arrow_forwardProblem) A university student is studying the inheritance of 2 genes of pea plants. Color of plant: Blue is dominant (B), and yellow is recessive (b) Spikes or no spikes: Having spikes is dominant (G), and having no spikes is recessive (g). Which of the following findings would indicate to the student that these two traits are located close together on the same chromosome? a) The traits show independent assortment b) All gametes of a double heterozygous individual (BbGg) are either BG or bg c) A double heterozygous individual has four types of gametes in approximately equal proportions: BG, Bg, bG, and bg d) Student finds that there is frequent crossing over between the 2lociarrow_forward1. Duchenne’s muscular dystrophy (DMD) is a devastating X-linked recessive disease that causes muscle wasting and is eventually fatal. Meredith’s brother Craig has DMD but Meredith is healthy. You may find it beneficial to draw the family pedigree. a) What is the simple probability (as a percentage) that Meredith is a carrier of a mutation causing DMD? b) Meredith herself has two healthy children, a boy and a girl. Is this anterior (ancestral) or posterior (modifying) information? c) If Meredith IS a carrier for DMD (i.e. scenario A is correct), what is the probability (as a percentage) of her having these two healthy children?arrow_forward
- Human Heredity: Principles and Issues (MindTap Co...BiologyISBN:9781305251052Author:Michael CummingsPublisher:Cengage Learning