Heredity -- Exam 4

Recessive genes are usually expressed in humans only when both alleles are exactly the same, or homozygous. This means that an individual must inherit two copies of the recessive allele in order for it to be expressed. If an individual has one dominant allele and one recessive allele, the dominant allele will mask the expression of the recessive allele. Therefore, it is only when both alleles are the same, either two recessive alleles or two dominant alleles, that the recessive gene will be expressed.

The sex chromosomes of a normal male are XY. In humans, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The presence of the Y chromosome determines the development of male characteristics. Therefore, the correct answer is XY.

Homozygous refers to a genetic condition where an individual has two identical alleles for a particular trait. In this case, the two alleles express exactly the same information for the trait, meaning that they are identical. This is why the correct answer is homozygous. Heterozygous refers to a condition where an individual has two different alleles for the trait, monogamous refers to a type of relationship, and hemizygous refers to a condition where only one allele is present for a particular trait.

A couple whose blood types are A and B may have a child with any of the blood types A, B, AB, or O. This is because the child inherits one blood type allele from each parent, resulting in a possible combination of A and B alleles, which can give rise to any of the mentioned blood types.

Multiple-allele inheritance refers to a pattern of inheritance where a trait is determined by more than two alleles. In the case of the ABO blood group, there are three alleles (A, B, and O) that determine the blood type. Each person inherits two of these alleles, resulting in four possible blood types: A, B, AB, and O. This example demonstrates how multiple alleles can interact to determine a specific trait, in this case, the blood type.

Phenotypic characteristics are those that can be observed or determined through superficial inspection of an individual. These traits are influenced by a combination of genetic and environmental factors, and can include physical attributes such as hair color, eye color, and height. The term "phenotypic" refers to the observable traits that are expressed as a result of an individual's genotype and interactions with the environment.

The statement "A woman has blond hair and brown eyes" is best described as indicating phenotype. Phenotype refers to the observable physical characteristics of an individual, such as hair color and eye color. In this case, the woman's phenotype is being described based on her blond hair and brown eyes. The other options (recessive traits, genotype, and allelic pairs) do not specifically address the physical appearance of the woman, but rather refer to different aspects of genetic inheritance.

Recessive genetic disorders are more frequent than disorders inherited as dominant because carriers of recessive genetic disorders can pass them on to their offspring without being aware that they are carriers. Unlike dominant genetic disorders, where the presence of the disorder is usually expressed in the individual, carriers of recessive disorders may not show any symptoms themselves but can still pass on the disorder to their children. This increases the likelihood of recessive disorders being passed down through generations and becoming more prevalent in a population.

Genetic screening can be done before conception by carrier recognition or during fetal testing. This means that individuals who are planning to have children can undergo genetic screening to determine if they are carriers of certain genetic disorders. Additionally, pregnant individuals can also opt for genetic screening during fetal testing to identify any potential genetic abnormalities in the fetus. This allows for early detection and informed decision-making regarding the pregnancy.

Based on the given information in Figure 29.1, the genotype ratio for the offspring would be 1:2:1. This means that for every one individual with a certain genotype, there would be two individuals with another genotype, and one individual with a different genotype.

Genetic diversity is not solely due to inherited genes on the sex chromosomes. It is primarily a result of processes such as crossing-over of chromosomes, independent assortment of chromosomes, and segregation of chromosomes. These mechanisms occur during meiosis, the process of cell division that produces gametes. During crossing-over, genetic material is exchanged between homologous chromosomes, leading to new combinations of genes. Independent assortment and segregation further contribute to genetic diversity by randomly distributing and separating chromosomes during cell division. Therefore, genetic diversity is a result of these processes rather than solely influenced by the sex chromosomes.

Polygene inheritance refers to the inheritance of a trait that is controlled by multiple genes. In the case of stature (height) in humans, it is likely that multiple genes contribute to determining an individual's height. Each gene may have a small effect on height, and the combination of these genes determines the overall height of an individual. This is supported by the observation that height tends to vary continuously in a population, rather than being limited to a few distinct categories. Therefore, polygene inheritance is a plausible explanation for the inheritance of stature in humans.

The mutant gene in Amy's skin cell will replicate itself when the cell divides, but it will not be passed on to her offspring. This is because mutations in somatic cells, such as skin cells, are not heritable and do not affect the genetic material that is passed on to future generations. Only mutations in germ cells, such as eggs or sperm, can be inherited by offspring. Therefore, while the mutant gene may continue to exist in Amy's body, it will not be transmitted to her children.

An individual who is heterozygous for a particular trait, yet expresses both alleles of that trait, is an example of incomplete dominance. In incomplete dominance, neither allele is completely dominant over the other, resulting in a phenotype that is a blend or combination of the two alleles. This is different from dominance, where one allele is completely dominant over the other, and from recessive inheritance, where the recessive allele is not expressed in the heterozygous state. Sex-linked inheritance refers to the inheritance of genes located on the sex chromosomes, which is not applicable to the given scenario.

Newborn infants are not routinely screened for color blindness at birth. Color blindness is a condition where individuals have difficulty distinguishing between certain colors. While color blindness can be diagnosed through various tests, it is not considered a critical condition that requires immediate intervention or treatment at birth. On the other hand, conditions like imperforate anus, PKU (phenylketonuria), and congenital hip dysplasia are routinely screened for at birth as they require early detection and intervention for proper management and treatment.

Dominant alleles are called so because under most circumstances they suppress the expression of other alleles. This means that when a dominant allele is present, it will mask the effects of any recessive alleles that may also be present for the same trait. As a result, the dominant allele's characteristics will be expressed in the phenotype, while the recessive allele's characteristics will remain hidden. This is why dominant alleles are often more visible or prevalent in a population compared to recessive alleles.

A chromosomal aberration in which part of a chromosome is lost is known as deletion. This occurs when a segment of the chromosome breaks off and is lost during cell division. This can result in the loss of important genetic material, leading to various genetic disorders or abnormalities. Translocation refers to the movement of a segment of one chromosome to another non-homologous chromosome. Crossing-over is the exchange of genetic material between homologous chromosomes. Inversion is the reversal of a segment within a chromosome.

Gene mutations in the sex chromosomes of humans tend to become visibly expressed more frequently in males. This is because males have one X and one Y chromosome, while females have two X chromosomes. If a gene mutation occurs on the X chromosome, it is more likely to be expressed in males because they only have one copy of the X chromosome, whereas females have two copies. Therefore, any mutation on the X chromosome in males is not masked by a second copy, making it more likely to be visible.

The given information in Figure 29.1 shows that all the alleles present are lowercase "g", indicating a homozygous recessive genotype (gg). Therefore, the genotype of the offspring designated by the blank square would also be gg.

Drugs and nutrition can alter normal gene expression because certain substances in drugs and nutrients can interact with genes and affect their activity. This can lead to changes in the way genes are expressed, resulting in altered protein production and cellular function. These changes can have various effects on an individual's health and development.

A female infant is born with several hundred oocytes, each one genetically unique because of independent assortment and random crossover. During meiosis, the process by which oocytes are formed, chromosomes pair up and exchange genetic material through a process called crossover. This results in the shuffling and recombination of genetic material, leading to genetic diversity and uniqueness among the oocytes. Independent assortment also contributes to this uniqueness, as each pair of chromosomes aligns independently during meiosis, further increasing genetic variation.

Huntington's disease is a genetic disorder characterized by the degeneration of the basal nuclei of the brain. This degeneration leads to the progressive loss of control over movement, as well as cognitive and psychiatric symptoms. The disease is caused by a mutation in the huntingtin gene, which leads to the production of a toxic protein that damages the brain cells. Therefore, the correct answer is degeneration of the basal nuclei of the brain.

Based on the given information, the phenotype ratio is 1:3. This implies that for every one individual with a certain phenotype, there are three individuals with a different phenotype.

Amniocentesis is the most common type of fetal testing. This procedure involves the collection of a small sample of amniotic fluid, which surrounds the fetus in the womb. The fluid is then tested for various genetic abnormalities, chromosomal disorders, and other conditions. Amniocentesis is typically performed between 15 and 20 weeks of pregnancy and is considered a safe and reliable method for detecting potential health issues in the fetus. It provides valuable information to expectant parents and allows for informed decision-making regarding the pregnancy.

If C were an incomplete dominant trait, the phenotype ratio for the offspring would be 1:2:1. This means that for every one individual with the dominant phenotype (CC), there would be two individuals with the heterozygous phenotype (Cc), and one individual with the recessive phenotype (cc). This ratio is characteristic of incomplete dominance, where the heterozygous genotype results in an intermediate phenotype between the two homozygous genotypes.

In sex-linked inheritance, the expression of a recessive allele can occur even when only one copy is present. This is because sex-linked genes are located on the sex chromosomes, specifically the X chromosome in humans. In males, who have one X and one Y chromosome, any recessive allele on the X chromosome will be expressed because there is no second copy of the X chromosome to mask its expression. Therefore, sex-linked inheritance is the main way a recessive allele can be expressed with only one copy present.

In meiosis, crossing-over refers to the exchange of genetic material between homologous chromosomes, leading to the creation of genetically unique spermatozoa. This process increases genetic variation among humans. Independent assortment refers to the random alignment and separation of homologous chromosomes during meiosis, resulting in different combinations of genetic material in the produced spermatozoa. This also contributes to the variation among humans. Chromosome segregation alone does not account for the genetic diversity, as it only ensures the separation of chromosomes into different cells. Therefore, the correct answer is crossing-over and independent assortment only.

Incomplete dominance occurs when neither allele is completely dominant over the other, resulting in a phenotype that is a blend of both alleles. In the case of sickle-cell anemia, individuals who inherit one copy of the sickle-cell allele have a milder form of the disease called sickle-cell trait, while those who inherit two copies of the sickle-cell allele have the more severe form of the disease. This demonstrates incomplete dominance because neither the normal allele nor the sickle-cell allele is completely dominant over the other.