Genetics Pedigree Analysis and Inheritance Patterns in Humans (CH6)

Pedigree

Pictorial representation of a family history; a family tree that outlines the inheritance of one or more characteristics.

Proband

The person from whom the pedigree is initiated.

Autosomal recessive traits

Traits that tend to skip generations and appear only when both parents carry a copy of the gene for the trait, which is more likely when the parents are related.

Familial hypercholesterolemia

An autosomal dominant condition where blood cholesterol is greatly elevated due to a defect in the LDL receptor.

Autosomal dominant trait

A trait that might skip generations when a new mutation arises or the trait has incomplete penetrance.

Cystic fibrosis (CF)

A condition that is autosomal recessive, indicated when the CF phenotype is not present in parents but appears in one of their children.

Controlled mating

Not possible in human genetics, making the study of inheritance challenging.

Long generation time

A characteristic of human biology that complicates genetic studies.

Small family size

A feature of human biology that limits the study of genetics.

Consanguine mating

Mating between relatives, which increases the likelihood of autosomal recessive traits appearing in pedigrees.

X-linked recessive traits

Traits that are passed down through the X chromosome and can affect males and females differently.

X-linked dominant traits

Traits that are expressed in individuals with only one copy of the gene on the X chromosome.

Y-linked traits

Traits that are passed down through the Y chromosome and affect only males.

Testcross

A genetic cross used to determine the genotype of an individual with a dominant phenotype.

Generation time

The average time between the birth of an individual and the birth of its offspring.

Inheritance of characteristics

The transmission of traits from parents to offspring, often studied through pedigrees.

Human genetics challenges

Include the impossibility of controlled matings, limited family sizes, and short generation times.

Phenotype

The observable characteristics or traits of an individual, which can be influenced by genetics.

Genetic testing

The analysis of DNA, RNA, chromosomes, proteins, and metabolites to detect genetic disorders.

Mutation

A change in the DNA sequence that can lead to genetic variation or disease.

Penetrance

The proportion of individuals with a particular genotype that actually express the associated phenotype.

Autosomal traits

Traits that are determined by genes located on autosomes, not sex chromosomes.

Alexander disease

An autosomal dominant trait that can skip generations.

X-linked recessive trait

A trait that is more commonly seen in males and is passed from mothers to sons.

X-linked dominant trait

A trait that affects offspring with a 1/2 probability for both sons and daughters if the father is affected.

Characteristic of X-linked recessive traits

The trait can skip a generation and is more commonly seen in males.

Higher penetrance in males

A scenario where an autosomal recessive trait is more frequently expressed in males than in females.

Proportion of offspring affected by X-linked dominant trait

1/2 sons and 1/2 daughters will be affected if the father has the trait.

Trait transmission in X-linked recessive inheritance

The trait is not passed from fathers to sons.

Affected son's mother in X-linked recessive trait

An affected son's mother either has the trait or is heterozygous (carrier).

Conclusion from X-linked dominant trait study

If none of the sons have the trait and all daughters do, the mother has the X-linked dominant trait.

Mutation origin in X-linked traits

The mutation likely arose during meiosis of either parent.

Trait skipping generations

Common in autosomal dominant traits, indicating incomplete penetrance.

Characteristic NOT expected in X-linked recessive pedigree

The trait is commonly passed from fathers to sons.

Environmental influence on Alexander disease

Only the environment can cause Alexander disease.

Reasoning for trait skipping a generation

No reasoning could explain this if Alexander disease is autosomal dominant.

X-linked dominant trait inheritance

A male affected with an X-linked dominant trait will pass it to all daughters and none of his sons.

X-linked recessive trait characteristic

Whenever a daughter receives the trait, the father is affected.

X-linked dominant trait conclusion

If the father has the trait, he will pass it to all daughters.

Autosomal recessive trait vs X-linked recessive trait

X-linked recessive traits are only passed to sons from mothers, not from fathers.

Affected offspring from X-linked dominant parent

1/2 of the offspring will be affected if the father has the trait.

X-linked dominant trait in family study

The father has the X-linked dominant trait if none of the sons have it and all daughters do.

Y-linked trait

A trait that is passed from fathers to sons only.

Genetic mosaicism

A condition where an individual has somatic cells that are genotypically different from one another.

Concordant trait

The trait shared by both members of a twin pair.

Concordance

The percentage of twin pairs that are concordant for a trait.

Dizygotic twins

Nonidentical twins.

Monozygotic twins

Identical twins.

Autosomal recessive trait

A trait that can skip generations and may appear in offspring if both parents are carriers.

In vitro fertilization

A process where several eggs are fertilized outside the body before being implanted.

Somatic cells

Cells that make up the body tissues and organs, excluding germ cells.

Embryogenesis

The process of development of an embryo from the fertilization of the egg to the formation of a fetus.

Twin studies

Research studies that compare the traits of twins to assess the influence of genetics and environment.

Mosaicism

A genetic condition resulting from abnormal separation of chromosomes during early embryogenesis.

Genetic makeup

The unique combination of genes that an individual possesses.

Fetus

An unborn offspring in the later stages of development, after the embryonic stage.

Conjoined twins

Twins that are physically connected to each other at birth.

Abnormal separation of chromosomes

A process that can lead to genetic mosaicism.

Affected heterozygous males

Males who carry one copy of a dominant allele for a trait.

Genetic inheritance

The process by which genetic information is passed from parents to offspring.

Concordance rate of cancer in monozygotic twins

12%

Concordance rate of cancer in dizygotic twins

15%

Concordance rate of epilepsy in monozygotic twins

59%

Concordance rate of epilepsy in dizygotic twins

19%

Genetic component of epilepsy

Epilepsy has a strong genetic component.

Environmental component of asthma

Asthma is linked strongly to an environmental component.

Concordance of traits

A trait exhibits 100% concordance in both monozygotic and dizygotic twins.

Importance of genes and environment

Studying twins and adoptions can help assess the importance of genes and environment.

Interpretation of low concordance values

Such low values make interpretation difficult.

Genetic factors in cancer occurrence

The occurrence of cancer is mostly linked to genetic factors.

Environmental factors in cancer occurrence

The occurrence of cancer is mostly linked to environmental factors.

Equal influence of factors in cancer

The occurrence of cancer is linked equally to both genetic and environmental factors.

Single gene causation

Epilepsy is not caused by a single gene.

Penetrance of epilepsy

Epilepsy is not 100% penetrant.

Asthma in adopted children

If adoptive parents develop asthma, then their adopted child is more likely to have asthma.

Genetic factors in asthma

Asthma is linked strongly to a genetic component.

Concordance in monozygotic twins

Monozygotic twins tend to look more similar.

Concordance in dizygotic twins

Dizygotic twins share only 50% of their genes on average.

Study of twins and adoptions

Can help assess the importance of genes and environment.

Cancer interpretation

The occurrence of cancer is linked equally to both genetic and environmental factors.

Genetic factors are extremely important.

A conclusion drawn when a trait exhibits 100% concordance in both monozygotic and dizygotic twins.

Genetic factors are somewhat important.

A conclusion drawn when a trait does not exhibit 100% concordance in both monozygotic and dizygotic twins.

Genetic factors are unimportant.

A conclusion drawn when environmental factors are determined to play a larger role than genetic factors.

Both genetic and environment factors are important.

A conclusion drawn when both types of factors contribute to the differences in a trait.

Adoption studies assumptions.

Adoptees have no contact with their biological parents after birth, the adoptive parents and biological parents are not related, and the environments of biological and adoptive parents are independent.

Genetic counseling.

Provides information related to hereditary conditions.

Reasons for seeking genetic counseling.

Genetic disease runs in the family, help is needed in understanding the results of a prenatal test, parents know they are carriers for a recessive disease, and the members of a couple are closely related to each other.

Considerations before genetic testing.

Genes at other loci could cause the disease, the disease may be influenced by environmental factors, and the disease may not be 100% penetrant.

A sample of fetal blood.

Obtained by inserting a needle into the umbilical cord.

Adermatoglyphia (ADG).

An autosomal dominant disorder characterized by the absence of fingerprints.

Hallmark of adermatoglyphia.

Individuals with the condition have a parent with the condition.

Autosomal dominant disorder.

A disorder that is seen more commonly in males than females and skips generations.

Fingerprints in adermatoglyphia.

Individuals with the condition have fingerprints that are all whirls.

Higher concordance for monozygotic twins than for dizygotic twins

Indicates that the environment is not involved in the condition.

Cohort of adopted children with OCD

Indicates a genetic component when biological parents have OCD but adoptive parents do not.

Genetic Counseling and Genetic Testing

Provide information to those concerned about genetic diseases and traits.

Preimplantation genetic diagnosis

Determines the presence of disease-causing genes in an embryo at an early stage, before it is implanted in the uterus.

Prenatal genetic testing

Determines the presence of disease-causing genes or chromosomes in a developing fetus.

Amniocentesis

Involves withdrawing amniotic fluid from the uterus, not the fallopian tubes, to perform prenatal testing on fetal tissue.

Tay-Sachs disease

An autosomal recessive condition that can be tested for using chorionic villus sampling.