3.4: Pedigree Analysis
Background Information
Recall that autosomal chromosomes are all the chromosomes in a genome that are not the sex chromosomes. For disorders that are caused by a dominant allele there are some trends that you will see in a population of a family tree that carries that allele. Since you only need 1 copy of a dominant allele to express a dominant trait, autosomal dominant disorders often will occur in every generation of a family. If two individuals who do not have the disorder reproduce, it is impossible for any of their children to be affected by the disorder. Huntington’s disease is an example of a disease caused by an autosomal dominant allele.
If the gene (and its resulting disorder) reduces the chance of surviving or reproducing, its frequency should decrease over time. This is because if the disorder results in death before a person reaches reproductive age, they will never get the chance to pass the deleterious allele to the next generation. This is not always the case however, because some disorders do not become evident until after the individual has reached reproductive age, and already unknowingly passed the disorder to their children. This is the case with Huntington’s disease, a debilitating and deadly disease that leads to a serious degeneration of the nervous system, and does not appear until the age of 30 or later. In other instances, such as achondroplasia, this disorder does not affect the person’s life expectancy significantly or impact normal reproductive events, and will therefore persist generation after generation.
Autosomal recessive inheritance means that a person can be a carrier of the deleterious allele, but not be affected with the disorder. Cystic fibrosis, phenylketonuria (PKU), and albinism are all examples of autosomal recessive traits. This inheritance pattern has characteristic trends in a population. Heterozygotes are typically symptom-free carriers and only homozygous recessive individuals are affected. Although there are exceptions to this rule, such as sickle cell anemia, where heterozygotes display some symptoms, but they are so mild that it generally does not affect their overall life expectancy. Sickle cell anemia is an example of a disorder that displays codominance. Because inheritance of an autosomal recessive disorder requires 2 copies of the allele to be expressed, these types of disorders tend to skip generations. Two heterozygous parents have a 50 percent chance of producing heterozygous children and a 25 percent chance of producing a homozygous recessive child. When both parents are homozygous recessive, then all children will be affected.
Sex-linked traits : Disorders that are carried on the X chromosome have very different patterns in a population. Male offspring will always inherit the only X chromosome they carry from their mother. Males cannot inherit the X chromosome from their father, since the paternal X chromosome would result in a female offspring. Clues that a trait is a recessive X-linked:
- More males than females are affected because males only get one shot at an X chromosome. They cannot be carriers for a disorder, they either have it, or they don’t. Females need two copies, therefore their chances of inheriting both deleterious alleles is greatly diminished compared to males.
- An affected father cannot pass trait to sons, and in order for a female to be affected by a sex-linked disorder, her father MUST express the disorder as well.
- X-linked dominant disorders are often fatal, and do not result in viable offspring that survive outside of the womb and are exceedingly rare. For this reason, we will not address X-linked dominant disorders.
- A note on X-linked annotation. With autosomal alleles, the correct annotation is to use capital letters for dominant and lower case for recessive. Genes found on the X chromosome however, are annotated a bit differently. Capital and lowercase are still used, but most often these are added as a sub- or superscript next to the X. In this example Punnett square for example, the mother who is a carrier for, let’s say color blindness, would be annotated XBXb.
Exercise 3: Pedigree Analysis
Inheritance patterns of traits can often be determined by analyzing a pedigree , a chart that depicts several generations of parents and their children and illustrates the individuals that are affected by a particular trait. In a pedigree males are depicted with squares and females with circles. When an individual is affected by the trait being studied then their square or circle is black. When the trait is absent from an individual then their square or circle is white. Parents are connected by a horizontal line and children are shown beneath the parents, connected to the parents with a vertical line. Each generation is shown on the same horizontal line.
Figure X. How to read pedigrees. “BI-102 General Biology 2” by Linn Benton Virtual College. 7/2020, CC BY. https://www.oercommons.org/courses/b...biology-2/view
Pedigree 1
Things to note visually in this pedigree:
- Both male and females are affected at about the same percentage
- A large percentage of offspring at each generation are affected
- There are no offspring with the disease that don’t have at least one affected parent
- Is this disorder autosomal or sex-linked?
- Is this disorder most likely caused by a dominant or recessive gene?
- Circle the individual(s) that proves your choices.
-
What are the genotypes of the individuals labeled in the pedigree?
- Grandfather:
- Son:
- Grandson 1:
- Daughter 1:
- Granddaughter 1:
Pedigree 2
Things to note visually in this pedigree:
- A much higher percentage of males are affected by this disorder.
- Females are only affected if their father is also affected.
- An offspring is affected by the disorder even though both parents are not.
- Is this disorder autosomal or sex-linked?
- Is this disorder most likely caused by a dominant or recessive gene?
- Circle the individual(s) that proves your choices.
-
What are the genotypes of the individuals labeled in the pedigree?
- Grandfather:
- Grandmother:
- Granddaughter 1:
- Grandson 1:
- Daughter 1:
Pedigree 3
- Is this disorder autosomal or sex-linked?
- Is this disorder most likely caused by a dominant or recessive gene?
- Circle the individual(s) that proves your choices.
-
What are the genotypes of the individuals labeled in the pedigree?
- Grandfather:
- Grandson 1:
- Daughter 1:
- Granddaughter 1:
- Daughter-in-Law:
- Son 1:
Pedigree 4
- Is this disorder autosomal or sex-linked?
- Is this disorder most likely caused by a dominant or recessive gene?
- Circle the individual(s) that proves your choices.
-
What are the genotypes of the individuals labeled in the pedigree?
- Grandfather:
- Son 1:
- Daughter-in-Law:
- Granddaughter 1
- Grandson 1
Pedigree 5
- Is this disorder autosomal or sex-linked?
- Is this disorder most likely caused by a dominant or recessive gene?
- Circle the individual(s) that proves your choices.
-
What are the genotypes of the individuals labeled in the pedigree?
- Grandfather:
- Grandmother:
- Daughter 1:
- Granddaughter 1:
- Grandson 1:
Questions for Review
- In typical situations, how many chromosomes are inherited from our mother? How many from our father?
- What does it mean if someone is a carrier for a trait?
- Why is it that even though some dominant disorders are fatal, they continue to persist in human populations?
- Explain why disorders caused by a dominant allele cannot have carriers of the disorder.
- Why is it possible for two individuals who express a dominant disorder to have children who are unaffected?
- A son receives his X-chromosome from:
- Explain why men cannot be carriers for an X-linked trait.
Testing Understanding Post-Lab
-
Imagine you are a genetic counselor, and a couple who is planning on starting a family comes to you for information. Charles was married once before, and he and his first wife had a child who suffered from cystic fibrosis (an autosomal recessive disorder). His current wife Elaine’s brother died of cystic fibrosis, but both parents did not have the disorder.
- What is Charles’ genotype?
- What is the genotype of Elaine’s parents?
- What is the genotype of Elaine’s brother?
- What is the chance that Elaine is a carrier for the disorder?
- A couple is both phenotypically normal, but their son suffers from hemophilia, an X-linked recessive disorder. What percent of their children are likely to suffer from hemophilia? What percent are likely to be carriers for the disorder?
- Roger and Gayle are both of normal phenotype but have a son with Muscular Dystrophy, a sex-linked recessive disorder. Gayle is pregnant and amniocentesis shows the fetus is male. What is the probability that the fetus will suffer from muscular dystrophy?