How are adoption studies used to separate the effects of genes and environment in the study of human characteristics?

What are the two types of twins and how do they arise?

Huntington's disease (HD) is a genetic disorder that usually first appears in middle age and, over time, leads to involuntary movements, impaired speech, difficulty swallowing and speaking, and cognitive decline. There is no cure for $\mathrm{HD}$, and affected individuals eventually die, usually after 10 to 15 years of progressively worsening symptoms. Huntington's disease is inherited as an autosomal dominant disorder.
Presymptomatic genetic testing for $\mathrm{HD}$ is available for people with a family history of the disease. Individuals who have a parent with HD have a $50 \%$ chance of inheriting the HD gene and eventually having HD. Some of these individuals want to know if they will eventually get the disease, and they undergo presymptomatic genetic testing. Others do not want to know, given that there is no cure or effective treatment for the disease. If you were at risk for HD, would you want to undergo genetic testing? What are some reasons for and against having a genetic test for HD?

Trait $\mathrm{X}$ displays $98 \%$ concordance in both monozygotic and dizygotic twins. Do these concordances suggest that trait $\mathrm{X}$ is influenced by (a) genetic factors; (b) environmental factors; or (c) both genetic and environmental factors? Explain your reasoning.

Define the Lyon hypothesis.

The pseudoautosomal regions (PARs) of the X and Y chromosomes enable the sex chromosomes to pair and synapse during meiosis in males. Given the location of the SRY gene near PAR1, can you propose a mechanism for how the mutant X and Y chromosomes in Fig. 4.7 (in which part of the X is on the Y, and part of the Y is on the X) may have arisen during meiosis?

Other than the fact that a Y-linked trait appears only in males, how does the pedigree of a Y-linked trait differ from the pedigree of an autosomal dominant trait?

Give two similarities and two differences between gymnosperms and angiosperms.

What is the constricted region of the chromosome where the kinetochore forms?

The frequency of dizygotic twinning is higher in older women. Can you think of some possible evolutionary reasons for the occurrence of more dizygotic twins in older women?

How does the pedigree of an autosomal recessive trait differ from the pedigree of an $\mathrm{X}$-linked recessive trait?

Dizygotic twinning often runs in families, and its frequency varies among ethnic groups, whereas monozygotic twinning rarely runs in families, and its frequency is quite constant among ethnic groups. These observations have been interpreted as evidence of a genetic basis for variation in dizygotic twinning but a little genetic basis for variation in monozygotic twinning. Can you suggest a possible reason for these differences in the genetic tendencies toward dizygotic and monozygotic twinning?

A. C. Stevenson and E. A. Cheeseman studied deafness in a family in Northern Ireland and recorded the following pedigree (A. C. Stevenson and E. A. Cheeseman. 1956. Annals of Human Genetics 20:177-231).
a. If you consider only generations I through III, what is the most likely mode of inheritance for this type of deafness?
b. Provide a possible explanation for the cross between III-7 and III-9 and the results for generations IV through $\mathrm{V}$.

Draw a pedigree that represents an autosomal dominant trait, sex-limited to males, and that excludes the possibility that the trait is $\mathrm{Y}$ linked.