Bio 203: Midterm 2

What are pedigrees?

Tool used by geneticists to evaluate traits in family trees, trach sources of disease and infer genetic mechanisms

What are the different ways people can be represented in a pedigree?

• Sex (square for male, circle female, diamond for unspecified)

• Phenotypic expression (filled in)

• Carrier (dot in unfilled)

• Proband (little arrow with p)

• Twins (line between = identical)

• Consanguinity (double lines)

How to find “coordinates” of person in pedigree?

By generation that is identified by roman numeral and family members denoted by Arabic numerals

How do autosomal recessive traits appear in a pedigree?

• Appears equally in both sexes

• Tends to skip generations

• Affected kids can be born to unaffected parents

• If both parents heterozygous, 9331

• Appears more frequently in consanguineous marriages

How do autosomal dominant traits appear in a pedigree?

• Appears in both sexes with equal frequency

• Does not skip generation

• Affected offspring must have an affected parent, unless there’s a new mutation

• Unaffected parents do not transmit the trait

What is Huntington’s Disease?

• Autosomal dominant, nucleotide repeats

• Always fatal

• Late onset in 30s or 40s

• Leads to neuron degeneration

• Dominant due to presence of mutant proteins that make neurons unstable

Who is Nancy Wexler?

Discovered location of the huttington’s gene and its autosomal dominant inheritance

• Her family was affected by HD

• Based research partially on pedigree analysis and gel electrophoresis

How do x-linked recessive traits appear in a pedigree?

• Usually more males affected than females

• Affected sons often born to unaffected mothers

• Can skip generations

• Never passed from father to son

• All daughters of affected fathers are carriers

What is hemophilia classified as?

X-linked recessive

How do x-linked dominant traits appear in a pedigree?

• Often, more females are affected (just because there are 2 X-chromosomes)

• Does not skip generations

• Affected sons must have affected mother (will never pass father to son)

How do y-linked traits appear in a pedigree?

• Only males are affected

• Passed from father to son

• Does not skip generations

Why are y-linked traits rare?

Because Y chromosome only has ~80 genes compared to ~1500 on X chromosome

How can incomplete penetrance be represented in a pedigree?

• Based on darkness of color, lighter color could mean that person has the genotype, but is not expressed (does not express the phenotype at 100%)

• If pedigree based on phenotype, can still be helpful when figuring out if something is incompletely dominant

Who were the Hapsburgs?

Royal family that had a lot of inbreeding, last of line could not reproduce + had lots of issues

What is inbreeding?

Probability that a pair of alleles are identical by descent

What is the difference between being identical by descent and identical by state?

• IBD: 2 copies of same allele descended from same copy in a common ancestor

• By state: two copies of allele are the same in structure and function, but descended from two different copies in ancestors

What is the inbreeding coefficient F?

Probability that two alleles are identical by descent in potential offspring

• 0 is 0% chance of IBD

• 1 is 100% (theoretically, never actually get this high)

How to calculate inbreeding constant?

(1/2)ⁿ * number of loops, with n = number of ppl in one loop, excluding individual

Genetically speaking, does having IBD alleles matter?

IBD could lead to two copies of a LOF mutation —> disease; mainly bad if the pedigree has a rare recessive disease

• Only becomes very risky when repeated

What are genetic screens?

Panel of hundreds of genes of parents to determine your own alleles (can see if you’re a carrier) and chance of your child having it

When is Mendel’s Law of Independent Assortment true?

Is only strictly true with different genes are on completely separate chromosomes (1 in 22)

What is genetic linkage?

Tendency of DNA sequences close to one another on a chromosome to be inherited together

• Locks certain genotypes together

What is recombination?

An exchange event between homologous chromosomes

• Increases genetic variation

• Daughter chromosomes have new combinations of alleles not found in parent chromosomes

What are the two reasons why we are unique (from our parents)?

• De novo mutations

• Recombination

What is ancestry?

Determined by your ancestors because they directly contribute their genetics… to an extent…

What is genealogical ancestry?

Determined by the ancestors that exist in your family tree

• As early as seven generations back, there are some ancestors from whom we don’t share any genetic material with (not considered genetic ancestors)

What is genetic ancestry?

Determined by which ancestors share DNA with you, and how much

• There are people who are genealogically considered ancestors who are not genetic ancestors

What is nonindependence?

Linked genes (no independent assortment, no 9331)

What are recombinant and nonrecombinant chromosomes?

Nonrecombinant refers to a chromosome with the original combination of alleles, recombinant means that crossing over has occurred on this chromosome

What is repulsion (trans configuration)?

Wild type allele and mutant allele are found on the same chromosome

What is coupling (cis configuration)?

One chromosome contains both wild-type alleles, one chromosome contains both mutant alleles