Crazy Chromosomes!

Ch. 15

Journal

• How many pairs of chromosomes are there per cell? Which law of Mendel’s produces this specific number?

• What occurs during nondisjunction? Name a disorder caused by nondisjuntion

Nondisjunction

• Results in aneuploidy = zygote with abnormal chromosome #

• Monosomic When zygote has only 1 copy of a gene (2n – 1)

• Trisomic When zygote has 1 additional copy of a gene (2n + 1)

VIDEO

Trisomy 21

• Trisomy 21 (AKA Down Syndrome) • A common trisomic genetic disorder caused by

extra 21st chromosome

Alterations of Chromosomes

What might cause chromosomes to alter their genetic sequence?

errors during meiosis or damage like radiation

4 way to alter sequence:

1) Deletion

2) Duplication

3) Inversion

4) Translocation

Occur during synapsis

Chromosome Alterations PIC

All can be harmful because can either be lethal or alter individual phenoype by making different/faulty protein

Disorders from Alterations

*Many are so dangerous = spontaneously abortion

1) Aneuploidy of Sex ChromosomesA. XXY (1 in 500-1000 males) - Klinefelter Syndrome

- affects male sex organs causes male to be infertile

- may develop female characteristics

B. X (1 in 2500) monosomy X (Turner Syndrome)

-sterile females sex organs not mature

C. XYY (1 in 1000 males)

- develop normally… may be taller than average

D. XXX (1 in 1000 females) - trisomy X

-develop normally… may be taller than average

Aneuploidy of Sex ChromosomesPICS

Disorders from Alterations

*Many are so dangerous = spontaneously abortion

2) Structurally Altered Chromosome DisordersA. Cri du chat (“cry of the cat”) – deletion on chromosome 5

- severely intellectually disabled- small head, unusual facial features- cry like distressed cat (video) usually die as infants early

childhood

Chromosomes outside the nucleus… Huh? What? No Way!Inheritance of organelle genes

What organelle(s) would have their own genes?

Why might this be?

Mitochondrial DNA – a small portion of your DNA that only comes from mom … how can this be?

can be used to trace lineage

you and your siblings and your mother all have identical mitochondrial DNA!

Mitochondrial DNA … beneficial or harmful for males?

ARTICLE

Linked Genes

What are linked genes?

How do they differ from sex-linked genes?

How would you relate linked genes to Mendel’s law of independent assortment? What is the relationship?

Linked Genes

DO GENES ALWAYS STAY LINKED?

in other words

ARE LINKED GENES ALWAYS INHERITED TOGETHER?

Genetic Recombination and Linkage

• In pea plants, yellow seeds are dominant to green seeds and round seeds are dominant to wrinkled. Perform a dihybrid cross between a homozygous recessive plant and a plant that is heterozygous for both traits.

• ½ share same genotype as parents = Parental Type

• ½ share different genotype than parents = recombinantsWe say that there is a 50% frequency of recombinants when

these two plants are crossedThis is a common outcome for unlinked genes that undergo

I.A.

Genetic Recombination and Linkage

• Morgan’s fly testing showed that only 17% of his believed to be “linked” genes showed recombinants

• Concluded that sometimes these linked genes can become unlinked!• This occurs during what we now call crossing over

(synapsis)

• This creates new combinations of chromosomes (by splitting linked genes)

DRIVING DIVERSITY and ultimately NATURAL SELECTION among a species

Genetic Mapping

• One of Morgan’s students, Alfred Sturtevant predicted that the farther apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency.

• Let’s look at an example

Sturtevant’s linkage map of 3 genes in Drosophila

• 3 genes: body-color (b), wing-size (vg), and cinnebar (cn) –one of many genes affecting eye color

• Observed recombination frequencies:• cn and b = 9%• cn and vg = 9.5%• b and vg = 17%

*Crossing over would occur most frequently between genes b and vg

• He decided to “map” these out on a chromosome• 1 map unit is = to 1%

recombination frequency

Sturtevant’s linkage map of 3 genes in Drosophila

• Let’s practice!

Linkage Practice

A Wild type fruit fly (heterozygous for gray body and normal wings) is mated with a black fly with vestigial wings.OFFSPRING: 778- wild type785- black-vestigial158- black- normal wings162- gray body-vestigial wings

What is the recombination frequency between these genes?

A Wild type fruit fly (heterozygous for gray body and red eyes) is mated with a black fly with purple eyes.OFFSPRING: 721- gray body/red eyes751- black body/purple eyes49- gray body/purple eyes45- black body/red-eyes

What is the recombination frequency between these genes?

Determine the sequence of genes along a chromosome based on the following recombination frequencies:A-B = 8 map unitsA-C = 28 map unitsA-D = 25 map unitsB-C = 20 map unitsB-D = 33 map units

Determine the sequence of genes along a chromosome based on the following recombination frequencies:A-C = 20 map unitsA-D = 10 map unitsB-C = 15 map unitsB-D = 5 map units