Genes and ChromosomesGene Linkage, Crossing Over, Codominance and Incomplete Dominance

Sex Linkage

1 exception to rule that every chromosome has a corresponding homologous chromosome … 1905 American – Nettie Stevens – mealworms–Female contains 20 large chromosomes– male – 19 and one small

These are sex chromosomes– Female: XX– Male: XY

Because X and Y determine sex, genes located on one of these chromosomes are called “Sex-Linked genes”

HERE’S AN EXAMPLE:

Genes associated with vision in humans are located on the X chromosome.– Males only have ONE X chromosome!!!!

This can be a problem if the X chromosome given to him by Mom has a genetic disorder!

Red-Green Color Blind?

Red-Green Color Blind?

Colorblindness:

Your mother unknowingly has the genotype XCXc. Dad is not colorblind and therefore has the genotype XCY.Will any of the children be colorblind??????

Answer: Yes! One of the sons will be colorblind because he will have the genotype XcY.

 

XC Xc

XC

Y

XCXC XCXc

XCY XcY

Revenge against Men!

Males have just one X chromosome. Therefore, if the mom gives them a recessive allele on her X chromosome, it will be expressed in the son EVEN though the mom’s phenotype did not express the allele!

INCOMPLETE DOMINANCE

Josef Kolreuter – crossed white carnations (rr) with Red (RR) carnations.Got all Pink!!! (Rr)

INCOMPLETE DOMINANCE

Did red and pink blend together????

DID THE COLORS BLEND????

NO!!!!!!! How do we know????–When crossed Rr x Rr (F1 x F1)…….–¼ Red–½ Pink–¼ WhiteLet’s do the Punnett Square

PUNNET SQUARE

25% RR: Red50% Rr: Pink25% rr: White

  R r

R

r

RR Rr

Rr rr

INCOMPLETE DOMINANCE

When mom makes a birthday cake and wants to frost it, how does she do it?

White frosting Wants to make red frosting

no red food color makes white little red food color makes pink lots of red food color makes red

INCOMPETE DOMINANCE

Same with genesGenes code for a proteinIf the protein is a pigment, like carnations:– no dose = white– single dose = pink– double dose = red

CODOMINANCE

What does dominant mean?In codominance, both alleles are expressed

CODOMINANCE EXAMPLE

On sports teams, often have two captains. Do both captains have equal representation?Sure they do!!!!

CODOMINANCE

The same is true for codominant alleles…both are equally expressed within a phenotype.Written as a capital letter with a superscriptHR or HW means the trait is hair color and red and white are both dominant.

CATTLE:

Cattle – Red hair (HR) and White hair (HW) are codominant –HRHR= Red hair–HW HW= White hair–HR HW = roan or combination – rust

colored (Red and white)

Try This!

What would you get if you crossed a Roan with a White haired cow?Do the punnett square:

PUNNET SQUARE

25% ROAN50% WHITE

  HR HW

HW HR HW HWHW

HW HR HW HWHW

NONDISJUNCTION

Whole chromosomes or sets of chromosomes fail to separate normally during meiosis Example: Down’s syndromeWhen all chromosomes fail to separate, can cause 3N or 4N organisms.