Understanding Patterns of Inheritance through Pedigree Analysis Pamela Engel1, 3 and Julie Hoover-Fong, M.D., Ph.D.2, 3 1. Glen Burnie Senior High School, Glen Burnie, MD 2. Greenberg Center for Skeletal Dysplasias McKusick-Nathans Institute of Genetic Medicine Johns Hopkins University 3. Geneticist-Educator Network of Alliances (GENA). Overview: This lesson was designed to help high school biology students understand and recognize patterns of inheritance as illustrated by pedigrees. The usual sequence for determining patterns of inheritance in high school text books is to teach recognizable Mendelian patterns through Punnett square analysis, and introduce other, nonmendelian patterns later. After students have mastered Punnett squares, then they are introduced to pedigree analysis. All too often students associate pedigrees with the last Punnett square they were taught, usually sex-linked traits. This lesson was designed to break up this sequence and link the student understanding of inheritance to pedigrees first, and then move toward specific analyses of inheritance with Punnett squares. As students have just finished studying asexual and sexual reproduction/mitosis and meiosis, it is hoped that students will be able to relate the passing of traits to their understanding of chromosomal segregation and assortment, and that this will help them better recognize the relationship between traits, chromosomes, and genes. State Standards: 3.3.2 – The student will illustrate and explain how expressed traits are passed from parent to offspring 1.4.2 – The student will analyze data to make predictions, decisions, or draw conclusions. 1.4.6 – The student will describe trends revealed by data. Science Concepts: 1. A gene is a sequence of DNA on chromosome which codes for a protein or an RNA 2. Each individual receives two copies of a gene. 3. One allele for a gene may always be expressed when present (dominant). 4. One allele may only be expressed when two copies are present (recessive). 5. The genes for X-linked traits are located on the X-chromosome.

6. Sons receive only one X-chromosome from their mothers. They only have one copy of each of the genes on the X-chromosome. 7. Patterns of inheritance over several generations within a family may be analyzed using pedigrees Student Misconceptions: 1. Genes and chromosomes are the same thing OR genes and chromosomes are not related to each other in any way. 2. Inheritance of traits is not related to chromosomes or chromosomal movements during meiosis. 3. All pedigrees are for determining X-linked traits. 4. Alleles are traits 5. Genotype and phenotype are the same Learning Outcomes: 1. The learner will be able to analyze pedigrees to determine phenotype and genotypes of individuals by writing a justification (claim, evidence, reasoning, rebuttal) 2. The learner will be able to analyze pedigrees to determine patterns of inheritance (dominant, recessive, X-linked) in writing (claim, evidence, reasoning, rebuttal) Curriculum Materials: 1. Biology – Holt Winston Rhinehart 2004 (AACPS Single-text adoption program) Prior Knowledge: 1. Students know and understand the terms, homozygous, heterozygous, dominant, recessive, phenotype, genotype, gene, allele, and chromosome. 2. Students understand that they inherit one-half of their chromosomes from each parent. 3. Students understand that genes are carried on chromosomes, and they code for traits.

Engagement: Student scenario – They are geneticists and have been asked to construct a pedigree for their family to understand how family members are related and how physical traits and genes can be passed through the family. The pedigree construction and analysis they will perform in this exercise is introduced as key part of patient care in a genetics clinic. a. Students are given a pedigree worksheet used by genetic counselors and geneticists at JHMI in daily clinical practice. As the different symbols and notations are explained, students develop their own pedigree of their family, identifying themselves as the proband. b. After the family pedigree is constructed for three generations, students are asked to pick a trait in their family and color in all the individuals that carry or express that trait. Exploration: 1. Students are asked to examine their pedigree to determine how their trait is inherited. The focus is on identifying how the trait was passed from parent to offspring, not necessarily dominance or recessive characteristics, but who do you think gave you this trait, your mom or your dad? Who did they get it from? 2. This lends to some discussion of new traits that occur in every family due to spontaneous mutations of previously unaffected (normal) genes. Thus neither parent may have a trait, but a child (offspring) does. 3. Additional discussion about the impossibility of sharing traits and genes between step siblings, a common part of pedigrees today. The drawn 3 generation pedigree is an excellent visual aid to understand the lack of relatedness in this instance. 4. Students try to determine how the trait was passed from generation to generation. Explanation: 1. Students are directed to presentation on pedigrees. 2. Clinical descriptions of genetic disorders that represent different types of inheritance patterns are presented to students along with typical pedigrees that demonstrate this pattern. Images of the classic features of these disorders, frequency of the disorders in the population as well as a typical representative pedigree are presented to students.

3. Students are shown basic identifying characteristics for each type of inheritance pattern. 4. Disorders are chosen for their appeal to high school students – a. Marfan Syndrome b. Cystic Fibrosis c. Achondroplasia d. Fragile X syndrome 5. Students are introduced to Punnett squares as a way to determine probability of inheritance from parents. Exploration: 1. Students are directed back to their family pedigrees. 2. Using the identifying characteristics presented to them, students analyze the pattern of inheritance in their family, and determine if it is an autosomal dominant, autosomal recessive, or X-linked trait. In some cases, students picked a “trait” such as wearing glasses. Since the purpose of this was to focus on identifying the pattern, they were still able to use the characteristics to find a pattern that best fit their family. Extension: 1. After students felt somewhat familiar with pedigrees and identifying patterns, they were given actual pedigrees to analyze. 2. Working in pairs, the students were given four different pedigrees to analyze. In each case they were to justify their answer by writing a paragraph using the Claim Evidence Reasoning rubric. They have used this rubric for data analysis throughout the school year. Evaluation: Students were assessed informally by teacher observation, and formative quizzes on the subject. Summative evaluation was the Anne Arundel County Benchmark Exam for the third marking period.

Predicting inheritance pattern from pedigree analysis A man who had purple ears came to the attention of a human geneticist. The human geneticist did a pedigree analysis and made the following observations: In this family, purple ears proved to be an inherited trait due to a single genetic locus. The man's mother and one sister also had purple ears, but his father, his brother, and two other sisters had normal ears. The man and his normal-eared wife had seven children, including four boys and three girls. Two girls and two boys had purple ears. 1.

Draw a pedigree for this family.

2.

Identify the index case/proband.

3. Is this trait autosomal dominant, autosomal recessive, or X-linked recessive? Use the CERR Rubric to justify your answer to number. a. Claim – what pattern of inheritance is demonstrated in this pedigree? b. Evidence – Identify the individuals in the pedigree on which you base your claim. c. Reasoning – Explain how you arrived at your claim, which particular parent/offspring inheritance pattern helped you arrive at your conclusion and why. d. Rebuttal – how do you know it is not another pattern of inheritance?

Individuals who lack an enzyme needed to form the skin pigment melanin are called albinos. Normal skin pigmentation is the normal phenotype, and is dominant. N represents a normal allele or version of the pigmentation gene. Albinism is the abnormal phenotype and is recessive. If the pigmentation gene is abnormal, it is represented by n. The genotype for albinism is represented as nn. If you cannot determine if an individual with normal pigmentation is heterozygous or homozygous, use N_. ALTERNATIVE SENTENCES for this question: N represents a normal allele or version of the pigmentation gene. If the pigmentation gene is abnormal, it is represented by n. nn represents the genotype for albinism. If you cannot determine if an individual with normal pigmentation is heterozygous or homozygous, use N_. Refer to FIGURE II and identify the genotype of each individual. Draw a chart listing the individuals and their genotypes. FIGURE II - ALBINISM PEDIGREE

1. 2. 3.

How many individuals had the genotype Nn ? How many were N_? Write a response using the CERR Rubric. a. Claim – Which individuals do you know were Nn? b. Evidence – what evidence in the pedigree helped you determine this? c. Reasoning – Explain your reasoning at arriving at this conclusion. d. Rebuttal – why can’t you determine the second allele for the individuals you labeled as N_?

The Blue People Of Troublesome Creek Six generations after a French orphan named Martin Fugate settled on the banks of eastern Kentucky's Troublesome Creek with his redheaded American bride, his great-great-great great grandson was born in a modern hospital not far from where the creek still runs. The boy inherited his father's lankiness and his mother's slightly nasal way of speaking. What he got from Martin Fugate was dark blue skin. "It was almost purple," his father recalls. Doctors were so astonished by the color of Benjy Stacy's skin that they raced him by ambulance from the maternity ward in the hospital near Hazard to a medical clinic in Lexington. Two days of tests produced no explanation for skin the color of a bruised plum. A transfusion was being prepared when Benjy's grandmother spoke up. "Have you ever heard of the blue Fugates of Troublesome Creek?" she asked the doctors. "My grandmother Luna on my dad's side was a blue Fugate. It was real bad in her," Alva Stacy, the boy's father, explained. "The doctors finally came to the conclusion that Benjy's color was due to blood inherited from generations back." Benjy lost his blue tint within a few weeks, and now he is about as normal looking a seven-year-old boy as you could hope to find. His lips and fingernails still turn a shade of purpleblue when he gets cold or angry a quirk that so intrigued medical students after Benjy's birth that they would crowd around the baby and try to make him cry. "Benjy was a pretty big item in the hospital," his mother says with a grin. Dark blue lips and fingernails are the only traces of Martin Fugate's legacy left in the boy; that, and the recessive gene that has shaded many of the Fugates and their kin blue for the past 162 years. Given below is a pedigree of some of the blue people of Troublesome Creek I II III IV 1.

Is this trait autosomal dominant, autosomal recessive, or X-linked?

2. Identify the carriers in generation three for the “Blue” trait by completing the pedigree. Can’t confirm all the carriers in the youngest generation b/c may be Bb or BB. 3. If B = normal skin, and b = blue skin, what is the genotype of the affected individuals?

Examine the pedigree below.

1. Identify the index case in this pedigree 2. Is this trait autosomal dominant, autosomal recessive, or X-linked recessive? Justify your response using the CERR Rubric. a. Claim – which pattern of inheritance is illustrated in this pedigree? b. Evidence – what evidence for this is seen in the pedigree. c. Reasoning – Explain why this evidence helped you arrive at your conclusion. d. Rebuttal – why isn’t this a pedigree for another pattern of inheritance?

Characteristics of Pedigree Analysis Autosomal Dominant 1. Every affected individual has an affected biological parent. There is no skipping of generations. 2. Males and females have an equally likely chance of being affected. The recurrence risk of each child of an affected parent is 1/2. 3 Normal siblings of affected individuals do not transmit the trait to their offspring. Autosomal Recessive. 1. Males and females are equally likely to be affected. 2. On average, the recurrence risk to the unborn sibling of an affected individual is 1/4. 3. The trait is characteristically found in siblings, not parents of affected or the offspring of affected. 4. Parents of affected children may be related. The rarer the trait in the general population, the more likely a consanguineous mating is involved. X-Linked Recessive 1. As with any X-linked trait, the disease is never passed from father to son. 2. Males are much more likely to be affected than females. If affected males cannot reproduce, only males will be affected. 3. All affected males in a family are related through their mothers. 4. Trait or disease is typically passed from an affected grandfather, through his carrier daughters, to half of his grandsons.

Sources for Pedigrees http://www.biology.arizona.edu/Human_bio/problem_sets/human_genetics/10Q. html //www.biology.arizona.edu/Human_bio/problem_sets/human_genetics/05Q.html http://www.horton.ednet.ns.ca/staff/selig/handouts/bio12/mengenetics/pedigree s.pdf http://www.uic.edu/classes/bms/bms655/lesson7.html http://www.uic.edu/classes/bms/bms655/lesson5.html http://www.uic.edu/classes/bms/bms655/lesson4.html

NB: While the questions and pedigrees were originally obtained from the sites above, modifications were made to help students focus on key ideas, and to clarify the information for students understanding.

Vocabulary Introduced during Explanation. Pedigree A family tree describing the occurrence of a heritable character in parents and offspring across many generations Autosome A chromosome that is not involved in determining sex. Humans have 22 pairs of autosomes and 1 pair of sex chromosomes (X and Y) Carrier An individual who is heterozygous for a given gene, with one normal allele and one potentially harmful recessive allele. They are normal but can pass on the harmful allele to their offspring. Affected individual

Person who has the heritable trait in a pedigree

Index case/proband investigation

Initial member of a family being studied in a genetics

X-linked trait A trait that is determined by a gene on the X chromosome. • Females receive two copies of the X chromosome, one from the mother and one from the father, so they have two alleles for the trait. • Males only receive one X chromosome from their mothers, and only have one allele for the trait. They receive the Y chromosome from their father. Summary

GENA project Images from Google and some text from GeneTests.org

3 Types of Inheritance • Autosomal dominant • Autosomal recessive • X-linked inheritance

•Characteristics •Pitfalls •Diseases examples

3 Types of Inheritance • Autosomal dominant • Autosomal recessive • X-linked inheritance

Autosomal Dominant Marfan Syndrome •Abnormal connective tissue •Eyes: myopia (near sighted, lens Dislocation •Skeletal: bone overgrowth (pectus), laxity, limbs too long for trunk •Heart: dilation, abnormal valves

Autosomal Dominant Marfan Syndrome •25% de novo; 75% inherited •1 out of 5,000 people

Autosomal Dominant Marfan Syndrome

Autosomal Dominant: Pedigree Characteristics •

Each child of an affected person has a 50% risk of inheriting the gene mutation.

•

Males and females are equally likely to be affected.

•

The condition is seen in sequential generations, affecting 50% of individuals in each generation on average.

Marfan Syndrome Marfan Syndrome

Autosomal Dominant: Pitfalls in recognizing this inheritance •

Incomplete penetrance. Some people who have the gene mutation do not show the clinical effects.

•

Penetrance limited to one gender. If prostate cancer risk inherited in autosomal dominant manner, women who inherit the mutation are not affected, but can pass the mutation on to their sons.

Marfan Syndrome Marfan Syndrome

•

Variable expressivity. The gene mutation has variable clinical manifestations: the disorder may range from mild to severe or a range of problems may occur in people with the mutation.

•

New mutation. An affected person may be the first person in the family with the condition, from mutation arising for 1st time in sperm, egg, or embryo

•

Germline mosaicism. A new mutation may arise in testis or ovary, resulting in an unaffected parent transmitting the condition to two or more children

Another Autosomal Dominant Condition: Achondroplasia

Average

•80% born to average stature parents •One mutation in the FGFR3 gene is responsible for 99% of all achondroplasia

What if both parents have achondroplasia? Are their children affected too?

What if both parents have achondroplasia? Are their children affected too?

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What if both parents have achondroplasia? Are their children affected too?

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What if both parents have achondroplasia? Are their children affected too?

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AVERAGE STATURE

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3 Types of Inheritance • Autosomal dominant • Autosomal recessive • X-linked inheritance

What is cystic fibrosis? •Lung disease •Lots of lung secretions & infections •Deficient digestive enzymes •Excess salt in sweat •Failure to thrive •Male infertility •Sinus disease

Autosomal Recessive: Pedigree Characteristics Cystic Fibrosis

•

Have features of the disease when mutations are present in both copies of a gene pair.

•

When parents are both carriers - that is, each carries a gene mutation in one gene and has a normal copy of the other gene their children each have a 25% chance of being affected. Of the children who are unaffected, 2/3 will be carriers.

Cystic fibrosis carrier Cystic fibrosis affected

•

If only one parent is a carrier, children will not be affected.

•

Siblings of a person with the condition have a 25% chance of being affected.

Autosomal Recessive: Pedigree Characteristics R

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When parents are both carriers - that is, each carries a gene mutation in one gene and has a normal copy of the other gene their children each have a 25% chance of being affected.

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Autosomal Recessive: Pitfalls to detect this type of inheritance Cystic Fibrosis

Lack of family history. Usually no family history of other affected family members Cystic fibrosis carrier since AR conditions are most likely to occur in siblings rather than in earlier generations. Cystic fibrosis affected In small families, multiple affected siblings are uncommon.

Autosomal recessive disease in sequential generations. Occasionally autosomal recessive conditions occur in sequential generations: if the autosomal recessive condition allows a person to reach adulthood and to have children, and if Uniparental disomy. If a couple in which only the carrier rate for the condition is high, one partner is a carrier has an affected child, a person with the condition may marry a it may rarely be due to uniparental disomy: in carrier; their children would have a 50% this case both gene mutations are inherited from chance of inheriting the condition. the parent who is a carrier, due to an error in the Misassigned paternity. If the biologic father of formation of sperm or ovum. an affected individual is someone other than the person assumed to be the father, De novo mutations. Although rare, de novo incorrect carrier test results might occur mutations account for ~1% of gene mutations (the apparent father would usually not be a carrier) and risk of additional affected and provide another explanation for the birth of an affected child when only one parent is a carrier. children could be misstated.

Typical Recessive inheritance

What are the chances her child will have CF? ●

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•This brother and sister have CF •She can have children; less likely for him

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Typical Recessive inheritance

What are the chances her child will have CF? ●

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•This brother and sister have CF •She can have children; less likely for him

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Typical Recessive inheritance

What are the chances her child will have CF? ●

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•This brother and sister have CF •She can have children; less likely for him

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What happens if her partner is a carrier? ●

What are the chances her child will have CF?

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1/24 carrier rate of CF in Caucasians

What happens if her partner is a carrier? ●

What are the chances her child will have CF?

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2 carrier children 2 with CF!! 50% chance of having a carrier 50% chance of having a child with CF

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1/24 carrier rate of CF in Caucasians

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If the child is affected, the pedigree may look dominant even though it is a recessive condition.

3 Types of Inheritance • Autosomal dominant • Autosomal recessive • X-linked inheritance

X-linked inheritance: Pedigree Characteristics X-linked recessive disease usually occurs in males who have inherited a recessive Xlinked mutation from their mother. Rarely, the disease may be seen in females who have inherited mutations in the same gene X-linked from both parents. More typically, the mother is a carrier and is unaffected, although it is not uncommon for female carriers of X-linked disorders to have mild clinical manifestations related to the disorder — for example, carriers of hemophilia may have mild bleeding problems.

X-linked inheritance: Pedigree Characteristics •

A male child of a woman who is a carrier has a 50% risk of inheriting the disorder.

•

A female child of a woman who is a carrier has a 50% risk of inheriting the gene mutation and thus being a carrier herself.

•

An affected male - if able to reproduce - will pass on the gene mutation to all daughters, who are therefore obligate carriers. The affected male never passes the disease on to a son.

•

The typical family history for an X-linked recessive condition is of disease in maternal uncles. A woman who has both a brother and a son affected with an X-linked disease is also an obligate carrier.

X-linked inheritance: Pitfalls • Small families. Small family size and few male children may make the pattern of an X-linked recessive disorder difficult to discern. • New mutation. An affected male may be the first person in the family with the condition, due to a mutation arising for the first time in sperm, egg or embryo. • Germline mosaicism. A new mutation may arise in testis or ovary, resulting in a parent who can pass on the condition or the carrier state to children, without being either affected (in the case of a male parent) or a carrier (in the case of a female parent).

Fragile X • Fragile X syndrome is the most common single-gene cause of inherited mental retardation. It is caused by a CGG trinucleotide repeat expansion in the FMR1 gene on the X chromosome. • Any child with developmental delay of unknown etiology should be considered for fragile X syndrome testing. Family history of mental retardation and/or suggestive physical features can make the diagnosis more likely. • A diagnosis of fragile X syndrome can inform the family about a child's prognosis and enable the child to receive needed services; this information also informs the family about their future reproductive risk as well as risk to other family members.

Fragile X • Fragile X syndrome is inherited as an X-linked disorder. • Family history is often unremarkable. • In some families, however, males on the maternal side of the family, such as maternal uncles or male cousins, have mental retardation. WHY WE DO PEDIGREES! • Females can show milder features of fragile X syndrome, including developmental disability.

Why is it called Fragile X?

The repeats makes the structure fragile

Fragile X •

• •

•

Suspected in males and females with unexplained developmental delay or mental retardation Hyperactivity, social anxiety, perseverative speech common large head circumference long face prominent forehead large ears prominent jaw macroorchidism **not always present in young Females can have similar characteristics, but typically milder 1 in 5000 males have fragile X

Fragile X Fragile X syndrome is associated with increased CGG repeats in the FMR1 gene on the X chromosome

Excess repeats methylate the FMR1 gene That turns off the FMR1 gene That causes features of Fragile X

Description

CGG repeat size

Normal range

6 to 40

Intermediate range

41 to 58

Premutation range

59 to 200

Affected range

Greater than 200

CGG repeats Description

CGG repeat size

Normal range

6 to 40

Intermediate range

41 to 58

Premutation range

59 to 200

Affected range

Greater than 200

But those with >35 repeats have ↑ risk of premature ovarian failure (POF); 20% with >35 repeats have POF All males here have fragile X associated tremor/ataxia Syndrome (FXTAS); pass to all daughters and she will become carrier to pass on to male children. No mental retardation. Risk to increase size when pass on ANTICIPATION, but can pass premutation on for generations.

Affected male with mental retardation and facial features. A female with full mutation is typically more mild, but 50% chance to pass on.

Other genetic conditions

Turner Syndrome • Short stature • Sometimes spatial and language difficulties • Usually infertile • Abnormal pubertal development • Normal IQ • 98% die before birth • 1 in 5000

Klinefelter Syndrome • Male sex organs • Small testes, sterile • Breast enlargement and other feminine body characteristics • Normal intelligence • 1 in 500 to 1,000

22q11 Deletion Facial anomalies Congenital heart defect Speech, learning difficulties Hypocalcemia T-cell deficiency Palate anomalies (cleft) Asymmetric crying face

FISH for deletion Deletion by karyotype

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