Exploring Mendelian Genetics Going Beyond Simple Dominance: X-linked (sex-linked) Traits Incomplete Dominance Codominance Multiple Alleles Blood Types Polygenic Traits Pleiotropy Epistasis Multifactorial

SeX-linked Traits Trait can be dominant or recessive Probability of inheritance is altered because the trait is on the X chromosome Females- XX Males- XY

Examples of X-linked traits Colorblindness Hemophilia Duchenne Muscular Dystrophy Female carriers do not express phenotype

Colorblindness Test

Incomplete Dominance In some cases, neither allele truly dominates over the other. No allele is really dom. or rec. The heterozygous genotype shows a MIX of the two traits. Example- Four O’Clocks, Snapdragons R- gene for red flowers, W- gene for white flowers: RR- red, WW- white, RW- pink

Codominance In some cases, both alleles are dominant. No allele is really recessive. The heterozygous genotype shows BOTH of the two traits. Example- Chicken feathers B- gene for black feathers, W- gene for white feathers: BB- black, WW- white, BW- “erminette” Black and White! Rhododendron and Roan Cow

Multiple Alleles Many genes have more than just two alleles for a trait Remember, you can still only have 2 alleles at a time b/c of the diploid nature of our cells It is still just ONE gene, but lots of possibilities Example: Alleles for rabbit fur C-full color, dominates over Cch, Ch, c Cch – chinchilla, dominates over Ch and c Ch- himalayan, dominates over c c- albino, recessive to all

Polygenic Traits Many traits result from the interaction of several genes. Multiple genes, perhaps on different chromosomes even, produce one phenotype Polygenic traits can produce a large range of phenotypes Examples: human skin color (at least 4 genes), human eye color, human height http://www.athro.com/evo/gen/geframe.html (eye color explained)

Capital letters = dark; small letters = light; more dark alleles = darker!!

Gradation of Human Skin Color

Height in Humans Range of phenotypes resulting from polygenic trait

Human Blood Types Multiple alleles, Polygenic and Codominance! Multiple alleles- A, B, O Polygenic- one gene controls type, another gene controls rH factor (+, -) Codominance- A and B are codominant but both dominate over O

Multiple Alleles Most genes have more than two alleles in a population—called multiple alleles. The ABO blood groups in humans are determined by three alleles, IA, IB, and i. Both the IA and IB alleles are dominant to the i allele The IA and IB alleles are codominant to each other. Because each individual carries two alleles, there are six possible genotypes and four possible blood types.

Pleiotropy Ability of 1 gene to affect more than one phenotype Discovered because Mendelian #s didn’t work Most common examples include some physical phenotype and viability “phenotype” http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/mendel/mendel5.htm http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/mendel/mendel5.htm nice example of the coat color in mice functioning also as a lethal gene

Epistasis (p380) The interaction between 2 genes to control a single phenotype; you need both genes to receive a certain outcome. A “cascade” of proteins

NATURE VERSUS NURTURE WHAT DO YOU THINK?

Nature vs. Nurture Environmental effects National Children’s Health Study (JH) Twin studies Norm of Reaction Range of phenotypic possibilities over which there may be variation due to environmental influence

Environment and Phenotype Himalayan Rabbits - Enzyme coding for black fur is active only at low temperatures. Black fur only occurs on extremities.

Other Chromosomal Actions Linkage  Genes occurring on the same chromosome can be inherited together X-Inactivation  if men only need one X chromosome, do women need both X’s? If not, which one “works”? Non-disjunction  how gametes can end up with the wrong # of chromosomes in meiosis

Linkage Really it is the chromosomes that are segregated independently, not necessarily individual genes. Some genes are LINKED if they are on the same chromosome Ex: you get all of the genes on chromosome 1 from your mom if you get her chr.1 Is that always the case though? What do you know might happen?

Crossing Over Depending on how FAR APART genes are on chromosomes, they may be switched during meiosis Prophase I Must be homologous

Gene Maps Where actual genes are located on chromosomes. Discovered by Alfred Sturtevant in Thomas Morgan’s lab at Columbia in 1931. ~ 1 million bp = 1 map unit (m.u.) or centimorgan By looking at statistical inheritance patterns, he calculated the “recombination frequency” of alleles. If genes are far apart, crossing over is more likely 1 cM = 1% chance a marker at one genetic locus on a chromosome will be separated from a marker at a second locus due to crossing over in a single generation

X-Chromosome Inactivation Mary Lyon Men are XY: One X chromosome is enough Women are XX: The “extra” copy is not needed. It condenses into a “Barr body.” Remember, you have trillions of cells. Which X chromosome that “switches off” is random in each cell

Mosaicism

Nondisjunction “Not coming apart” Homologous chromosomes fail to separate in meiosis Result in Trisomy or Monosomy Examples: Klinefelter syndrome (trisomy X) Down syndrome (trisomy 21) Turners syndrome (X_ monosomy)