Genetics 3401 Chapter 4

Simple Mendelian

Refers to inheritance patterns that follow Mendel's laws, including the concepts of dominant and recessive alleles.

• Ex. Tt = tall VS. tt = short

Incomplete Penetrance

Refers to a situation where not all individuals with a specific genotype express the expected phenotype.

• dominant phenotype isn’t expressed with one dominant allele

Incomplete Dominance

Heterozygote has a phenotype that is the intermediate between either homozygous.

• Ex. Red and white flowers make pink flowers

Overdominance

When the heterozygote has a trait that is more beneficial than either homozygote

• Ex. Sickle‑cell trait (AS) protects against malaria more than AA or SS

Codominance

Both alleles expressed equally; no blending

• ABO blood type - I^A I^B = AB blood, both antigens expressed

• Heterozygote expresses both alleles simultaneously

X-Linked

Gene located on X chromosome; males express whichever allele they carry.

• Ex. Red‑green color blindness — more common in males (XbY)

Sex-Influenced

Trait expressed differently in males vs females (dominant in one, recessive in the other)

• Ex. Pattern baldness — dominant in males, recessive in females

Sex‑Limited

Trait appears in only one sex

• Ex. Milk production in female mammals; male peacock feather display

Lethal Alleles

Alleles that can cause organismal death

• Yellow coat color allele in mice (YY = lethal)

Ressesive Human Diseases

3 Diseases

Hemizygous

Being male and having X-Linked alleles in genotype which code for a ressesive phenotype.

What makes alleles dominant?

• toxic gain-or-function - Ex. ALS or gerthigs disease

• dominant-negative

• Haploinsufficiency - mutant is loss of function causes wild type phenotype

Polydactyl

people who carry the dominant allele for extra toe,

example of incomplete penetrance

if 60% of the heterozygotes carrying a dominant allele exibit the trait allele, the trait is 60% penetrant

Environmental Effects

• fur changing color during differing temperatures

• Humans affected by PKU disease - causes mental retardation, inborn error of metabolism and require a restricted diet

heterozygote advantage

when a heterozygote has a greater reproductive success (AKA overdominance)

• ex sickle cell anemia

• A and S hemoglobin

Homodimers

Has 2 different subunits that code for the same gene

• Ex. A1A2 heterogygotes - make A1A1 A2A2 homodimers and A1A2 heterodimers

• E1 = higher temperatures, E2 = lower temperatures

Blood Types (Codominance)

• O - (ii) - surface antigens: neither A or B - Against A and B (universal Donor)

• A - (IAIA or IAi) - surface antigens: A - Agaisnt B

• B - (IBIB or IBi) - surface antigens: B - Agaisnt A

• AB - (IAIB) - surface antigens: A and B - none

Y-Linked genes

• AKA holandric genes

• transmitted from father to son

Huntingtons Disease

• produces a toxic mutant huntingtin protein that slowly damages neurons.

• Develops later in life because neuronal injury accumulates over decades, and symptoms appear only after the brain can no longer compensate. Aging also reduces the brain’s ability to clear toxic proteins, accelerating onset.

pleiotropy

• One gene → many traits

• Effects arise because the gene’s protein is used in multiple tissues or pathways

• cystic fibrosis - single CFTR mutation disrupts chloride transport in many tissues, producing multiple distinct symptoms across the lungs, pancreas, sweat glands, intestines, and reproductive system.

  • can be expressed in different cell types

  • different stages of life

  • can affect cell function in more than one way

Porphyria Variegata

• autosomal dominant disorder

• defect in the PPOX enzyme

• causes a buildup of porphyrins

• leads to skin sensitivity to sunlight and episodes of abdominal pain and nerve problems.

Epistasis

one gene masks the other

Coat color in mice — one gene can hide the pigment gene’s effect.

Ex. even if you have 1C or 1P it would be masked by the cc and pp (9:7 ratio)

Complementation

Two parents with similar recessive phenotypes produce wild‑type offspring because their mutations are in different genes.

Two albino plants → normal‑colored offspring

Ex. CCpp x ccPP make all CcPp

Gene modification

An allele of one gene changes how another gene’s alleles affect the phenotype

Modifier genes altering eye color intensity

Ex. (9:4:3 ratio) - Agouti coat color where german sheppard color can be based on A and C toghether

Gene Redundancy

Several copies (paralogs) of genes perform the same function; losing one doesn’t change phenotype, but losing both does.

Duplicate genes for leaf development in plants

its a 15:1 ratio and only recessive for both lowercase alleles