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

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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


Genes and chromosomes

  • 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!


Red green color blind

Red-Green Color Blind?


Red green color blind1

Red-Green Color Blind?


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??????


Genes and chromosomes

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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