Concepts Summary

There are several difficuties in applying traditional genetic techniques to the study of human traits, including the inability to conduct controlled crosses, long generation time, small family size, and the difficulty of separating genetic and environmental influences.

A pedigree is a pictorial representation of a family history that displays the inheritance of one or more traits through several generations.

Autosomal recessive traits typically appear with equal frequency in both sexes. If a trait is uncommon, the parents of a child with an autosomal recessive trait are usually heterozygous and unaffected; so the trait tends to skip generations. When both parents are heterozygous, approximately 1/4 of their offspring will have the trait. Recessive traits are more likely to appear in families with consanguinity (mating between closely related persons).

Autosomal dominant traits usually appear equally in both sexes and do not skip generations. When one parent is affected and heterozygous, approximately 1/2 of the offspring will have the trait. When both parents are affected and heterozygous, approximately 3/4 of the offspring will be affected. Unaffected people do not normally transmit an autosomal dominant trait to their offspring. X-linked recessive traits appear more frequently in males than in females. Affected males are usually born to females who are unaffected carriers. When a woman is a heterozygous carrier and a man is unaffected, approximately 1/2 of their sons will have the trait and 1/2 of their daughters will be unaffected carriers. X-linked traits are not passed from father to son.

X-linked dominant traits appear in males and females, but more frequently in females. They do not skip generations. Affected men pass an X-linked dominant trait to all of their daughters but none of their sons. Heterozygous women pass the trait to 1/2 of their sons and 1/2 of their daughters.

Y-linked traits appear only in males and are passed from father to all sons.

Analysis of twins is an important technique for the study of human genetic characteristics. Dizygotic twins arise from two separate eggs fertilized by two separate sperm; monozygotic twins arise from a single egg, fertilized by a single sperm, that splits into two separate embryos early in development. Concordance is the percentage of twin pairs in which both members of the pair express a trait. Higher concordance in monozygotic than in dizygotic twins indicates a genetic influence on the trait; less than 100% concordance in monozygotic twins indicates environmental influences on the trait.

Adoption studies are used to analyze the inheritance of human characteristics. Similarities between adopted children and their biological parents indicate the importance of genetic factors in the expression of a trait; similarities between adopted children and their genetically unrelated adoptive parents indicate the influence of environmental factors.

Genetic counseling provides information and support to people concerned about hereditary conditions in their families.

Genetic testing includes screening for disease-causing alleles in newborns, the detection of people heterozygous for recessive alleles, presymptomatic testing for the presence of a disease-causing allele in at-risk people, and prenatal diagnosis.

Common techniques used for prenatal diagnosis include ultrasound, amniocentesis, chorionic villus sampling, and maternal blood sampling. Preimplantation genetic diagnosis can be used to select for embryos that are free of a genetic disease.

(important terms pedigree (p. 134) proband (p. 135) consanguinity (p. 136) dizygotic twins (p. 141) monozygotic twins (p. 141) concordance (p. 142)

genetic counseling (p. 145) newborn screening (p. 147) heterozygote screening

(p. 147) presymptomatic genetic testing (p. 147)

ultrasonography (p. 147) amniocentesis (p. 147) chorionic villus sampling

maternal blood testing

(p. 149) fetal cell sorting (p. 150) preimplantation genetic diagnosis (p. 150)

Worked Problems

1. Joanna has short fingers (brachydactyly). She has two older brothers who are identical twins; they both have short fingers. Joanna's two younger sisters have normal fingers. Joanna's mother has normal fingers, and her father has short fingers.

Joanna's paternal grandmother (her father's mother) has short fingers; her paternal grandfather (her father's father), who is now deceased, had normal fingers. Both of Joanna's maternal grandparents (her mother's parents) have normal fingers. Joanna marries Tom, who has normal fingers; they adopt a son named Bill who has normal fingers. Bill's biological parents both have normal fingers. After adopting Bill, Joanna and Tom produce two children: an older daughter with short fingers and a younger son with normal fingers.

(a) Using correct symbols and labels, draw a pedigree illustrating the inheritance of short fingers in Joanna's family.

(b) What is the most likely mode of inheritance for short fingers in this family?

(c) If Joanna and Tom have another biological child, what is the probability (based on you answer to part b) that this child will have short fingers?

(a) In the pedigree for the family, note that persons with the trait (short fingers) are indicated by filled circles (females) and filled squares (males). Joanna's identical twin brothers are connected to the line above with diagonal lines that have a horizontal line between them. The adopted child of Joanna and Tom is enclosed in brackets and is connected to the biological parents by a dashed diagonal line.

(b) The most likely mode of inheritance for short fingers in this family is autosomal dominant. The trait appears equally in males and females and does not skip generations. When one parent has the trait, it appears in approximately half of that parent's sons and daughters, although the number of children in the families is small. We can eliminate Y-linked inheritance because the trait is found in females. If short fingers were X-linked recessive, females with the trait would be expected to pass the trait to all their sons, but Joanna (III-6), who has short fingers, produced a son with normal fingers. For X-linked dominant traits, affected men should pass the trait to all their daughters; because male II-1 has short fingers and produced two daughters without short fingers (III-7 and III-8), we know that the trait cannot be X-linked dominant. It is unlikely that the trait is autosomal recessive because it does not skip generations and approximately half of the children of affected parents have the trait.

(c) If having short fingers is autosomal dominant, Tom must be homozygous (bb) because he has normal fingers. Joanna must be heterozygous (Bb) because she and Tom have produced both short- and normal-fingered offspring. In a cross between a heterozygote and homozygote, half of the progeny are expected to be heterozygous and half homozygous (Bb X bb : 1/2 Bb, 1/2 bb); so the probability that Joanna's and Tom's next biological child will have short fingers is 1/2. 2. Concordance values for a series of traits were measured in monozygotic twins and dizygotic twins; the results are shown in the following table. For each trait, indicate whether the rates of concordance suggest genetic influences, environmental influences, or both. Explain your reasoning.

Monozygotic Dizygotic

Characteristic concordance (%) concordance (%)

Monozygotic Dizygotic

Characteristic concordance (%) concordance (%)

(a) ABO blood type

0 0

Post a comment