Genes and Chromosomes Gene 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!
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??????
XC Xc XC XCXC XCXc Y XCY XcY • Answer: Yes! One of the sons will be colorblind because he will have the genotype 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 White • Let’s do the Punnett Square
R r R RR Rr r Rr rr PUNNET SQUARE • 25% RR: Red • 50% Rr: Pink • 25% rr: White
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 genes • Genes code for a protein • If 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 superscript • HR 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:
HR HW HW HR HW HWHW HW HR HW HWHW PUNNET SQUARE • 25% ROAN • 50% WHITE
NONDISJUNCTION • Whole chromosomes or sets of chromosomes fail to separate normally during meiosis • Example: Down’s syndrome • When all chromosomes fail to separate, can cause 3N or 4N organisms.