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What is a dihybrid cross?

Agenda

• Quiz• Sex Linked Traits• Mutations

Sex Determination

• Thomas Hunt Morgan – studied fruit flies in the early 1900’s

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

XY XX

X Y X X

X X

X

Y

XX

FemaleXX

Female

XY

Male

XY

Male

50% Female; 50% Male

Sex Linkage

• Sex Linkage: the presence of a gene on a sex chromosome (X or Y)

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?

Worksheet

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

Chromosome Mapping

• The percentage of crossing-over between the genes for 2 traits is equal to the distance between them on a chromosome

• This distance is measured in map units

MUTATIONS

Mutations

• Mutation: change in DNA

VIDEO

Mutations

• Germ-cell mutation: occurs in sex cells– Affect the offspring– Example – Down Syndrome

• Somatic mutation: occurs in body cells– Affect the individual– Example - Cancer

Possible Effects of Mutations

• Lethal mutations: cause death, often before birth

• Beneficial mutations: provide variation needed for evolution

• No effect

Chromosome Mutations

• Chromosome Mutations: a change in the chromosome structure or loss/addition of entire chromosome

Chromosome Mutations

• Deletion: loss of piece of chromosome due to breakage

Chromosome Mutations

• Deletion: loss of piece of chromosome due to breakage

– Example – Cri du chat; deletion of part of chromosome 5

Chromosome Mutations

• Duplication: part of chromosome is duplicated/copied

Chromosome Mutations

• Duplication: part of chromosome is duplicated/copied

– Example – Charcot-Marie Tooth Disease

Chromosome Mutations• Inversion: piece breaks off and reattaches to

SAME chromosome in wrong order

Chromosome Mutations

• Inversion: piece breaks off and reattaches to SAME chromosome in wrong order

– Example – Hemophilia (this disorder is also X linked)

Chromosome Mutations

• Translocation: piece breaks off and reattaches to DIFFERENT chromosome

Chromosome Mutations

• Translocation: piece breaks off and reattaches to DIFFERENT chromosome

– Example – Cancer

Chromosome Mutations

• Nondisjunction: chromosomes do not separate during meiosis

Chromosome Mutations

• Nondisjunction: chromosomes do not separate during meiosis

– Example – trisomy, monosomy

Gene Mutations

• Gene Mutations: involve large segments of DNA (genes) or a single nucleotide

– Causes possible changes in the amino acid sequence

Gene Mutations

• Point Mutation: substitution, addition, or deletion of a single nucleotide

Gene Mutations

• Point Mutation: substitution, addition, or deletion of a single nucleotide

– Example – sickle cell anemia

Gene Mutations

• Frame Shift Mutation: occurs when the number of nucleotides inserted or deleted is not a multiple of 3