Genes and Chromosomes Gene Linkage, Crossing Over, Codominance and Incomplete Dominance
Mendel and Chromosomes • Mendel never asked “Where in the cell are the factors that control heredity?” • i.e. where are the genes • He couldn’t have answered the question though • His promotion to abbot kept him too busy
Where are they? • By 1900 cell biologists had discovered most organelles and the process of mitosis and meiosis • Nucleus was logical place
Why the Nucleus? • Central Location • Large size • Lots of activity during mitosis and meiosis
Chromosome Theory of InheritanceWalter Sutton – Columbia University • Genes are located on chromosomes • Each gene occupies a specific place on a chromosome • A gene may exist in several forms, or alleles • Each chromosome contains one allele for each gene
Thomas Morgan • Embryologist at Columbia University • Worked with fruit flies
Drosophila melanogaster • Prolific reproducers: a single cross will produce 100s of offspring • 4 pairs of chromosomes • Larva hatch within two weeks
Genes on a Chromosome • Each chromosome has hundreds or thousands of genes • Genes on the same chromosome tend to be inherited together • Remember: a chromosome is passed along as a single unit during meiosis
Thomas Morgan • First to associate a specific gene with a specific chromosome • Crossed gray, normal winged homozygous parents with black, vestigial wing parents. • GGNN x ggnn = GgNn • Then test crossed GgNn x ggnn • Let’s do the Punnet Square:
Expected: • 25% Gray, Normal wing • 25% Gray, Vestigial wing • 25% Black, Normal wing • 25% Black, Vestigial wing
Actual Phenotypes: • 41.5 % Gray, Normal • 41.5% Black, Vestigial • 8.5% Gray, Vestigial • 8.5% Black, Normal
If they are linked… • Let’s let G = G and N • Let’s let g = g and n • So GG would be GGNN and … • Gg would be GgNn and … • gg would be ggnn.
If they are linked… • Don’t forget that Gg is GgNn and gg is ggnn • So we have 50% Gray bodied, Normal wing • And 50% Black bodied, Vestigial wing • This explains the variation from the original expected.
Morgan’s Hypothesis • Body color (gray or black) and wing shape (normal or vestigial) are usually inherited together in a specific combination due to GENE LINKAGE
Gene Linkage • Genes located on the same chromosome tend to be inherited together because they are part of a single chromosome that is passed along as a unit.
Morgan’s Crosses • 83% of Morgan’s flies had characteristics of their parents. (gray/normal wings or black/ vestigial wings.) (41.5% gray/normal + 41.5% black/vestigial =83%)
17% of the flies had either a)Gray, vestigial wings b)Black, normal wings (8.5% gray, vestigial + 8.5% black, normal = 17%)
Morgan and Sturtevant (his associate) hypothesized that linkages could sometimes be broken • Gray/normal and black/vestigial genes can occasionally be separated
INTERESTING QUESTION: • If body color and wing shape genes are linked, how come all of the offspring were not like their parents (gray/normal and black/vestigial)?????????????
CROSSING OVER • DURING MEIOSIS, HOMOLOGOUS CHROMOSOMES EXCHANGE PORTIONS OF THEIR CHROMATIDS
GENE MAPPING • Distance between chromosomes determines how often crossing over occurs • Further away = more crossing over • Frequency of crossing over allows mapping of position on chromosomes
Gene Mapping Diagram • Genes A & B are very close together • C & D are farther apart • A random crossover event is much more likely to separate genes C & D than A and B (which are closer together).
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.