CHAPTER 9 Patterns of Inheritance Part 3

Human Genetic Analysis  Since humans live under variable conditions, in different places, and have long life spans like the geneticists who study the inheritance patterns of specific traits, genetic analysis of human traits is very difficult.  In addition, humans select their own mates, reproduce if and when they want, have relatively small families, there are just not enough offspring produced by a single family in order to make studying the inheritance pattern of a specific trait in a family easy.  Therefore, inheritance patterns in humans are usually studied by tracking observable genetic disorders that appear in families.  Geneticists gather information from multiple generations and graph their results in standardized charts of genetic connections called pedigrees.

A Pedigree

Human Genetic Analysis  Studying pedigrees can tell us if genetic abnormalities and disorders are caused by a dominant or recessive allele, and whether the allele is on an autosome or a sex chromosome.  A genetic abnormality is defined as a rare or uncommon version of a trait that is not inherently life-threatening. An example would be individuals who are born with more than five digits on their hands and/or feet.  On the other hand, a genetic disorder sooner or later causes medical problems that may be severe and is characterized by a specific set of symptoms called a syndrome. An example would be cystic fibrosis.  Pedigrees also allow geneticists to determine the probability that a certain genetic disorder will show up in future generations of a family.

Human Genetic Analysis  Finally, we will see as we analyze pedigrees that alleles that cause severe genetic disorders are usually rare in populations because they negatively affect the health and reproductive ability of the affected individual.  Why don’t they completely disappear? There are several reason for this.  First, sometimes mutations reintroduce them to populations.  Second, sometimes heterozygous individuals act as carriers. Their dominant allele masks the effects of the harmful allele so that the individual is still healthy but can pass the harmful allele on to their offspring.  Finally, the harmful allele may provide some kind of survival advantage in a hazardous environment, increasing the likelihood of survival of those individuals who possess the harmful allele.

Human Genetic Disorders  Pedigrees that show a relatively large number of affected individuals with males and females being approximately equally affected usually indicate that the mode of inheritance of the trait is autosomal dominant.  However, some genetic disorders that are autosomal dominant tend not to run in families because affected individuals do not live long enough to reproduce. (Ex. Progeria)  On the other hand, other autosomal dominant genetic disorders can persist in families if their expression does not interfere with reproduction (Ex. Achondroplasia) or if the symptoms do not appear until after the age of reproduction (Ex. Huntington’s disease).

Autosomal Dominant Genetic Disorder

Human Genetic Disorders  Pedigrees that show a very small number of affected individuals with males and females being approximately equally affected usually indicate that the mode of inheritance of the trait is autosomal recessive.  Other indicators that a trait may be autosomal recessive would be if an affected child has two unaffected biological parents and/or if the trait seems to skip generations.  Since autosomal recessive traits require an individual to have two copies of the recessive allele (one from each parent) in order to be affected, they occur infrequently in a population.  In addition, two affected parents will always have affected children and two heterozygotes have a ¼ chance of having an affected child.

Autosomal Recessive Genetic Disorder

Human Genetic Disorders  The X chromosome carries more than 6% of all human genes.  Mutations of genes on this chromosome are known to cause more than 300 genetic disorders.  Certain clues that a disorder may be sex-linked recessive include the fact that more males are affected than females.  Since males must only get one copy of the mutation to be affected, while women must get two, males are affected more than females by these traits.  Also, an affected father will never pass this trait on to his son since the father passes a Y chromosome to his son, not an X. However, all daughters of an affected male will be carriers. In turn, any carrier female has a ½ chance of passing the mutation on to her son, thereby having an affected son.

Sex-Linked Recessive Genetic Disorder

Human Genetic Disorders  Sex-linked dominant disorders tend to be more rare than sex-linked recessive disorders because they tend to be lethal in male embryos.  However, since females have two X chromosomes, their one functional allele will dampen the effects of the mutated one, resulting in more females being affected than males.

Sex-Linked Dominant Genetic Disorder Give an example of a sex-linked dominant disorder and explain the symptoms of this disorder.

Homework 9: Modes of Inheritance What is the most likely mode of inheritance for each of the traits indicated in the pedigrees a-e above?