welcome what is a dihybrid cross?. agenda quiz sex linked traits mutations
TRANSCRIPT
Sex Determination
• Observed that one pair of chromosomes was different between males and females
– Large one named “X” chromosome
– Smaller one named “Y” chromosome
– XX = female; XY = male
Sex Linkage
• X-linked genes: genes found on the X chromosome– X chromosome carries more genes
• Y-linked genes: genes found on the Y chromosome
Fruit Fly Eye Color
• Fruit flies normally have red eyes
• A few males have white eyes
• Red is dominant;
white is recessive
Morgan’s Fruit Fly Experiments
• Red-eyed female (XRXR) x White-eyed male (XrY)
XR XR
Xr
Y
XRXr XRXr
XRY XRY
RESULTS:
F1 generation – all red-eyed
Morgan’s Fruit Fly Experiments
• Red-eyed female (XRXr) x Red-eyed male (XRY)
XR Xr
XR
Y
XRXR XRXr
XRY XrY
RESULTS:
F2 generation – 3 red-eyed and 1 white-eyed
** all white-eyed where males…why?
Morgan’s Conclusions
• Gene for eye color is carried on the X chromosome = eye color is an X-linked trait
• Y chromosome does not carry a gene for eye color
• Red-eyed = XRXR, XRXr , XRY• White-eyed = XrXr, XrY
In humans colorblindness (b) is an example of a sex-linked recessive trait. A male with colorblindness marries a female who is not colorblind but carries the (b) allele.
Using a Punnett square, determine the genotypic and phenotypic probabilities for their potential offspring.
In fruit flies red eye color (R) is dominant to white eyes (r). In a cross between two flies, 50% of the male and 50% of the female offspring had red eyes. The other half of the males and females had white eyes.
What are the phenotype, and all possible genotypes, of the offspring?
Linkage Groups
• 2 or more genes that are on the same chromosome are “linked”
• Linked genes tend to be inherited together
More Fruit Fly Experiment
• Gray, long-winged (GGLL) x black, short-winged (ggll)
• F1 generation = all heterozygous gray,
long-winged (GgLl)
Morgan’s Fruit Fly Experiment
• Cross F1 flies: GgLl x GgLl
• F2 generation – If alleles on different chromosomes, they assort
independently and get a 9:3:3:1 ratio– If alleles on same chromosome, get 3 gray, long-
winged: 1 black, short-winged ratio– Morgan saw roughly the 3:1 ratio
Morgan’s Fruit Fly Experiment
• Unexpected results– Some gray, short-winged (Ggll)– Some black, long-winged (ggLl)
Morgan’s Fruit Fly Experiment
• How were these alleles separated?– Alleles were rearranged through
crossing-over during meiosis
• Genes that are farther apart are
more likely to be separated by crossovers
Chromosome Mapping
• Chromosome Map: diagram that shows the possible genes on a chromosome
• Made using crossing-over data