patterns of inheritance

Rules governing expression of single-gene or monogenic traits, These rules help explain how traits can be inherited from remote ancestors as well as more directly from our parents

mendelian genetics

One with expression controlled by the quality of its two alleles, one on each chromosome of a pair, (more than one gene input would be considered polygenic)

single gene trait

The initial parental generation of a family or group being observed for a specific trait or traits, is the start of the focus for the traits

P1 generation

Succeeding generations of offspring (progeny) produced from the parental generation, are about 2-3 lines

F generations

Traits or disorders in which heterozygous alleles are equally expressed, also referred to as incomplete dominance, red sock in white laundry load 

codominant expression

is expressed whether the person is homozygous or heterozygous for the gene alleles

single gene trait

Two dominant alleles are both expressed —-

equally

Trait about equal in distribution between males and females, No carrier status—heterozygous person expresses the trait, Trait appears in every generation— transmitted from parent to child

autosomal dominant

autosomal dominant: Risk for affected heterozygous individuals to transmit affected allele is —- with each pregnancy

50%

autosomal dominant: Risk for affected homozygous individuals to transmit affected allele is —- with each pregnancy

100%

Traits: Blood type A, Blood type B, Free earlobes, Long eyelashes, Oval face shape, Straight thumbs, Tongue rolling, Widow’s peak

AD

Disorders: Achondroplasia, DMT2, Ehlers-Danlos syndrome, Huntington disease, Marfan syndrome, Polycystic Kidney Disease**, Polydactyly, Retinitis pigmentosa**, Von Willebrand disease

AD

Degree to which the proportion of people with a given genotype actually express the expected phenotype, Gene is present and either is or is not expressed (all or none) - regardless of degree of expressivity, ex: polydactyl brothers having different levels of expression will have the same level of ——

penetrance

—penetrant traits/disorders: Achondroplasia, Blood type, Huntington disease

highly

—penetrant disorders: Hereditary breast/ovarian cancer (~60% to 80%), Polydactyly (~80%)

variable

Variance in expression of any given single-gene trait, Gene is always expressed, but degree of expression can range from slight to excessive, not predictably heritable - ex: A slightly affected parent may have a profoundly affected child who then produces a slightly affected grandchild

expressivity

A —— can be expressed whether a person is homozygous or heterozygous for the gene alleles

single gene trait

Traits/disorder expressed only when both alleles are present (person is homozygous), allele may be present in a family for many generations without overt expression, Trait expresses nearly equal distribution between male and female family members, Trait often appears first in siblings rather than in the parents of affected children

autosomal recessive

About 25% of the members of a family with a —- trait express it, Carrier status possible (one affected allele), Two carrier parents can transmit the trait and have an affected child (25% risk/pregnancy)

AR

Traits: Attached earlobes, Cleft chin, Hitch-hiker thumb, Short eyelashes, Straight hairline, Type O blood

AR

Disorders: Cystic Fibrosis, Hereditary hemochromatosis, Phenylketonuria (PKU), Sickle-Cell Disease/trait, Tay-Sachs disease

AR

autosomal recessive: what is the risk per pregnancy of an AR trait/disorder if there are two carrier parents

25%

Most of the genes on the —- chromosome code for somatic cell functions important for both males and females, and relatively few genes code for female sexual differentiation

X

Incidence of trait, or disorder, much higher among males in a family than among females (possibly exclusive to males), Trait cannot be transmitted father to son, Transmission from affected father to all daughters (obligate carriers) and from carrier mothers to sons and daughters

X linked recessive

X linked: Female carriers have —- risk (each pregnancy) of transmitting the allele to their offspring of either gender

50%

X linked: If no sons are born to carrier mothers, the trait/disorder may not be —— for many generations

overtly expressed

X linked: If no daughters or affected sons are born to affected fathers who have children with carrier mothers, the trait/disorder is —-

not transmitted further

X linked: —-homozygotes may not survive pregnancy or may have very severe disease

female

Traits: Color blindness (red/green), Male pattern baldness

X linked recessive

Disorders: Classic Hemophilia, Duchenne Muscular Dystrophy, Fragile X syndrome

X linked recessive

In somatic cells of females, one X chromosome is randomly inactivated and exists as a —-, and its alleles are not expressed, Inactivation of one X prevents a “double dose” of X-chromosome alleles, Random inactivation occurs early in embryonic development and does not change within a tissue as it grows

Barr body

Neither dominant nor recessive, involves several genes working together, each contributing a small amount to trait/disorder expression, Each gene variant adds to or takes away from phenotypic expression, combines the influence of one or more genes with environmental influences, aka multifactorial or polygenic

complex traits/disorders

for polygenic traits must reach a threshold level for overt expression

risk alleles

Most adult-onset health problems are —- such as diabetes mellitus, obesity, cancer, Show “regression to the mean” expression (tall fathers having shorter sons), Phenotypic expression can be altered by modifier genes

complex

When more risk alleles are present in one family than in the general population, the liability model shifts the threshold to the —-, increasing the likelihood of expression

left

Product of one zygote splitting into one or more equal parts during embryogenesis, Initially, noncoding DNA regions and gene alleles are identical at all loci - if one twin is affected, the other twin will have it too but may have different expressivity (if the other twin doesn’t have it may have more of an environmental influence than genetic)

monozygotic twins

Product of two or more separately fertilized eggs, Degree of genetic similarity for these siblings is the same as for any other children conceived by the same parents - one twin affected does not always mean the other will be

dizygotic twins

is the frequency with which a specific trait/disorder is found in both members of a set of twins

twin concordance

When concordance for a disorder is about the same among monozygotic and dizygotic twins, the environmental influence for expression is probably —- than the genetic influence

stronger

Risk of another child in a family being affected when one child is already affected, Easier to calculate for monogenic disorders following known inheritance patterns, additional factors to consider: More than one child with the disorder, Other close family members are affected, Disorder is present in a child of the gender less frequently affected

recurrence risk