Genetics chap 7 pedigree analysis

Human genetics research is constrained by several factors, which is what?

Controlled mating is not possible

Humans have a long generation time

human family sizes tend to be small

Pedigrees are _______

Powerful tools for understanding how traits are inherited. A pictorial representation of a family history, essentially a family tree that outlines the inheritance of one or more characteristics

Obligate carrier for a trait on a pedigree

Symbol with a dot in the center (carries the gene but does not have the trait)

Asymptomatic carrier on a pedigree

Unaffected at this time but may later exhibit the trait (symbol depends on specific convention, often an open symbol with a line or notation)

Deceased person on a pedigree

Symbol with a diagonal line through it

Proband of a pedigree

The first affected family member coming to the attention of a geneticist

Adoption on a pedigree

Brackets enclose adopted persons; dashed line denotes adoptive parents; solid line denotes biological parents.

Consanguinity

Mating between related persons (indicated by a double line connecting the parents).

What is the frequency of Autosomal recessive traits

Usually appear with equal frequency in both sexes

Autosomal recessive traits usually skip

Generations

Parents of Autosomal recessive traits

Affected offspring are typically born to unaffected parents (who are carriers)

Consanguinity is more likely to appear in which traits?

Autosomal recessive traits, which appear among the progeny of related parents

Frequency of Autosomal Dominant traits

Usually appear with equal frequency in both sexes

Do Autosomal dominant traits skip generations?

No

Transmission of Autosomal dominant traits

Affected individuals must have at least one affected parent (unless it is a new mutation). Unaffected individuals do not transmit the trait

Frequency of X-linked recessive traits

Appear more frequently in males than in females

Transmission of X-linked recessive traits

• Affected males do not pass the trait to their sons but pass the allele to all their daughters, making them carriers.

• Affected sons are usually born to unaffected mothers (carriers).

• The trait often skips generations.

• Never passed from father to son.

In X-linked recessive traits, males are ________

Hemizygous for X-linked traits

Frequency of X-linked dominant traits

Both males and females are affected; often more females than males.

Does not skip generations

Transmission of X-linked dominant traits

• Affected males pass the trait to all their daughters and none of their sons.

• Affected females (if heterozygous) pass the trait to about half of their sons and half of their daughters.

Frequency of Y-linked traits

Only males are affected

Transmission of Y-linked traits

Passed from a father to all his sons, and does not skip generations

Dizygotic (DZ) twins

Nonidentical twins, share about 50% of their genes (like regular siblings). Develop from two eggs fertilized by two sperm.

Monozygotic (MZ) twins

Identical twins, share 100% of their genes. Develop from a single egg fertilized by a single sperm, which then splits.

Concordant trait

A trait shared by both members of a twin pair.

Concordance

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

Higher concordance in Monozygotic twins compared to Dizygotic twins suggests a ________

Significant genetic component

Similar concordance in both Monozygotic and Dizygotic twins suggests _______

Environmental factors play a larger role

Adoption studies

• Compare the traits of adopted individuals with both their biological and adoptive parents

A strong correlation between the adoptee and biological parents suggests _______ while a correlation with adoptive parents suggests _________

Genetic influence ; Environmental influence

Genetic counseling

• Provides information to individuals concerned about hereditary conditions.

• It's an educational process covering diagnosis, symptoms, treatment, mode of inheritance, genetic risks, and reproductive options.

• Genetic counselors are professionals who assist with this process.

Genetic testing

• Aims to detect genetic conditions in individuals, families, or during pregnancy.

• Can inform reproductive choices and allow for early detection and potential prevention or management of conditions.

• The Online Mendelian Inheritance in Man (OMIM) database lists thousands of conditions with a simple genetic basis.

Reasons for seeking genetic counseling

• Family history of genetic disease.

• Birth of a child with a genetic disease, birth defect, or chromosome abnormality.

• Child with intellectual disability or a family history of it.

• Advanced maternal age (typically ≥≥ 35 years).

• Consanguineous parental relationships.

• Recurrent pregnancy loss or infertility.

• Exposure to teratogens (substances causing birth defects).

• Interpretation of genetic test results.

• Both parents are known carriers of a recessive disease or belong to a high-risk ethnic group.

Ultrasonography

Uses sound waves to create images of the fetus, can detect physical abnormalities like cleft lip and palate.

Amniocentesis

Performed around 15-18 weeks of pregnancy. A needle is inserted through the abdomen into the amniotic sac to withdraw amniotic fluid containing fetal cells. These cells can be cultured for chromosomal, DNA, or biochemical analysis.

Chorionic Villus Sampling (CVS)

Performed earlier, around 10-12 weeks of pregnancy. A catheter is inserted through the vagina and cervix to obtain a sample of chorionic villi (part of the placenta). These cells can be used directly for testing, often without culturing. CVS carries a higher risk than amniocentesis.