comprehension question about pedigree analysis

COMPREHENSION QUESTIONS

*1. What three factors complicate the task of studying the inheritance of human characteristics?
Mating cannot be controlled, so it is not possible to set up controlled mating experiments.
Humans have a long generation time, so it takes a long time to track inheritance of traits over more than one generation.
The number of progeny per mating is limited, so phenotypic ratios are uncertain.

*2. Describe the features that will be exhibited in a pedigree in which a trait is segregating with each of the following modes of inheritance: autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant, and Y-linked inheritance.
Pedigrees with autosomal recessive traits will show both affected males and females arising with equal frequency from unaffected parents. The trait often appears to skip generations. Unaffected people with an affected parent will be carriers.
Pedigrees with autosomal dominant traits will show both affected males and females arising with equal frequency from a single affected parent. The trait does not usually skip generations.
X-linked recessive traits will affect males predominantly and will be passed from an affected male through his unaffected daughter to his grandson. X-linked recessive traits are not passed from father to son.
X-linked dominant traits will affect both males and females and will be passed from an affected male to all his daughters, but not to his sons. An affected woman (usually heterozygous for a rare dominant trait) will pass on the trait equally to half her daughters and half her sons.
Y-linked traits will show up exclusively in males, passed from father to son.

*3. What are the two types of twins and how do they arise?
The two types of twins are monozygotic and dizygotic. Monozygotic twins arise when a single fertilized egg splits into two embryos in early embryonic cleavage divisions. They are genetically identical. Dizygotic twins arise from two different eggs fertilized at the same time by two different sperm. They share, on the average, 50% of the same genes.

4. Explain how a comparison of concordance in monozygotic and dizygotic twins can be used to determine the extent to which the expression of a trait is influenced by genes or environmental factors.
Monozygotic twins have 100% genetic identity, whereas dizygotic twins have 50% genetic identity. Any trait that is completely genetically determined will therefore be 100% concordant in monozygotic twins and 50% concordant in dizygotic twins. Conversely, any trait that is completely environmentally determined will have the same degree of concordance in monozygotic and dizygotic twins. To the extent that a trait has greater concordance in monozygotic twins than in dizygotic twins, the trait is genetically influenced. Environmental influences will reduce the concordance in monozygotic twins below 100%.

5. How are adoption studies used to separate the effects of genes and environment in the study of human characteristics?
Studies of adoptees, their biological parents, and their adoptive parents separate environmental and genetic influences on traits. Adoptees share similar environments with their adoptive parents (because they live in the same house and eat similar foods), but they share 50% of their genes with each of their biological parents. If adoptees have greater similarity for a trait with their adoptive parents, then the trait is environmentally influenced. If the adoptees have greater similarity for the trait with their biological parents, then the trait is genetically influenced.

*6. What is genetic counseling?
Genetic counseling provides assistance to clients by interpreting results of genetic testing and diagnosis; providing information about relevant disease symptoms, treatment, and progression; assessing and calculating the various genetic risks that the person or couple faces; and helping clients and family members cope with the stress of decision-making and facing up to the drastic changes in their lives that may be precipitated by a genetic condition.

7. Briefly define newborn screening, heterozygote screening, presymptomatic testing, and prenatal diagnosis.
Newborn screening: Newborn infants are tested for various treatable genetic disorders by sampling a few drops of their blood soon after birth.
Heterozygote screening: Normal or asymptomatic individuals in a population or community are tested for recessive disease alleles to determine the frequency of the disease allele in the population and to identify carriers, particularly if there is a relatively high incidence of the disease in the population or community.
Presymptomatic testing: People known to be at higher risk for a disease that occurs later in life are tested while they are still asymptomatic.
Prenatal diagnosis: Results from prenatal testing for any of a number of genetic conditions. Techniques such as amniocentesis or chorionic villus sampling are used to obtain tissue samples of the still developing fetus, or fetal protein or cells in the maternal circulation are characterized.

*8. What are the differences between amniocentesis and chorionic villus sampling? What is the purpose of these two techniques?
Amniocentesis samples the amniotic fluid by inserting a needle into the amniotic sac, usually performed at about 16 weeks of pregnancy. Chorionic villus sampling can be performed several weeks earlier (10th or 11th week of pregnancy) and samples a small piece of the chorion by inserting a catheter through the vagina. The purpose of these techniques is to obtain fetal cells for prenatal genetic testing.

9. What is preimplantation genetic diagnosis?
Preimplantation genetic diagnosis may be performed on embryos created through in vitro fertilization. The embryos are cultured until they reach the 8-16 cell stage, and one cell is removed from each embryo for genetic testing.