Unit 5 (Non-Mendelian Genetics)

Non-Mendelian Genetics

Many traits do not follow the ratios predicted by Mendel’s laws because of…

• Varying degrees of dominance

• Multiple genes acting together

• Some traits are determined by genes of the sex chromosomes

• Some genes are adjacent or close to one another on the same chromosome and will segregate as a unit

• Some traits are the result of non-nuclear inheritance (ex. chloroplast and mitochondrial DNA)

Complete dominance

Homozygous dominant and heterozygous individuals are phenotypically the same

Incomplete dominance

Neither allele is fully dominant

• F₁ generation has a phenotype that is a mix of the parental generation

• Ex. red x white = pink

Codominance

Two alleles that affect phenotype are both expressed

• Ex. AB blood: both A and B are expressed

Multiple alleles

Genes that exist in forms with more than two alleles

• Ex. human blood group: i^A,i^B,i

Epistasis

The phenotypic expression of a gene at one locus affects a gene at another locus

• Locus: point on a gene

• Ex. coat color in labs and some mice → one gene codes for pigment and a second gene determines whether or not that pigment will be deposited in the hair

Polygenic Inheritence

The effect of 2 or more genes acting on a single phenotype

• Ex. height, skin color

Sex-Linked genes

A gene located on either the X or Y chromosome

• Y-linked genes: genes found on the Y chromosome (very few)

• X-linked genes: genes found on the X chromosome

X-linked inheritance patterns

• Dominant: Mostly affects males, where mothers pass it to sons and fathers pass the gene to carrier daughters but not sons.

• Recessive: Mostly affects males, where mothers pass it to sons and fathers pass the gene to carrier daughters but not sons.

  • called hemizygous

  • males are most likely to have an X-linked disorder

Autosomal Inheritance patterns

• Recessive: Affects males and females equally and can skip generations

• Dominant: Affects males and females equally and typically appears in every generation

X-linked disorders

• Duchenne muscular dystrophy: progressive weakening of muscles

• Hemophilia: inability to properly clot blood

• Color blindness

X inactivation

• Females inherit 2 X chromosomes

• During development, most of the X chromosomes in each cell becomes inactive

• The inactive X in each cell of a female condenses into a Barr body (helps to regulate gene dosage in females)

Genetic Recombination

Production of offspring with a new combination of genes from parents

• parental types: offspring with the parental phenotype

• recombiants: offspring with phenotypes that are different from the parents

Linked genes

Genes located near each other on the same chromosome tend to be inherited together

• Meiosis and random fertilization generate genetic variation due to:

  • independent assortment

  • crossing over in meiosis 1

  • any sperm can fertilize any egg

Linked genes: crossing over

Linked genes show parental phenotypes in offspring at a rate higher than 50%

• During crossing over chromosomes, from one paternal chromatid and one maternal chromatid, exchange corresponding segments

• The further apart two genes are on the same chromosome, the higher the probability that a crossing-over event will occur between them, and the higher the recombination frequency

Linkage map

Genetic map that is based on recombination frequencies

• One map unit = 1% recombination frequency

• 50% recombination means that the genes are far apart on the same chromosome or on two different chromosomes

Non-Nuclear DNA

• Some traits are located on the DNA found in the mitochondria or chloroplasts

• Both chloroplasts and mitochondria are randomly assorted to gametes and daughter cells

• In animals, mitochondria are transmitted by the egg, not the sperm → therefore, all mitochondrial DNA is maternally inherited

• In plants, mitochondrial and chloroplasts are transmitted in the ovule, not the pollen → therefore, both mitochondrial and chloroplast determined traits are maternally inherited

Chi Square

A form of stastical analysis used to compare the actual results (observed) with the expected results

Interpreting results

X² > critical value: reject null hypothesis (statistically significant difference)

X² < critical value: accept null hypothesis (not statistically significant difference)

Environmental Factors

Environmental factors can influence gene expression and lead to phenotypical plasticity

• Individuals with the same genotype exhibit different phenotypes in different environments

• Examples

  • Temperatures can change coat color in rabbits and Siamese cats

  • Soil pH can affect flower color

  • UV exposure can increase melanin production in the skin

Genetic disorders

Some genetic disorders can be linked to affected or mutated alleles or chromosomal changes

• Ex. Tay-Sachs disease and Sickle cell anemia (autosomal recessive diseases)

Nondisjunction

Chromosomes fail to separate properly in meiosis I or II

• Karyotyping can detect nondisjunction

• Ex. Down Syndrome