Exploring Mendelian Genetics

Going Beyond Simple Dominance:1. X-linked (sex-linked) Traits2. Incomplete Dominance3. Codominance4. Multiple Alleles Blood Types5. Polygenic Traits6. Pleiotropy7. Epistasis8. 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!

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

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

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 NATURE VERSUS NURTURENURTURE

WHAT DO YOU THINK?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)