genes and chromosomes
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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

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genes and chromosomes

Genes and Chromosomes

Gene Linkage, Crossing Over, Codominance and Incomplete Dominance

sex linkage
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
slide3
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
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
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??????
slide8
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
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
INCOMPLETE DOMINANCE
  • Josef Kolreuter – crossed white carnations (rr) with Red (RR) carnations.
  • Got all Pink!!! (Rr)
incomplete dominance1
INCOMPLETE DOMINANCE
  • Did red and pink blend together????
did the colors blend
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
punnet square
R

r

R

RR

Rr

r

Rr

rr

PUNNET SQUARE
  • 25% RR: Red
  • 50% Rr: Pink
  • 25% rr: White
incomplete dominance2
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
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
CODOMINANCE
  • What does dominant mean?
  • In codominance, both alleles are expressed
codominance example
CODOMINANCE EXAMPLE
  • On sports teams, often have two captains. Do both captains have equal representation?
  • Sure they do!!!!
codominance1
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:
  • 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
Try This!
  • What would you get if you crossed a Roan with a White haired cow?
  • Do the punnett square:
punnet square1
HR

HW

HW

HR HW

HWHW

HW

HR HW

HWHW

PUNNET SQUARE
  • 25% ROAN
  • 50% WHITE
nondisjunction
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.
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