LEQ: How do the LEQ: How do the events of meiosis events of meiosis account for Mendel’s account for Mendel’s laws?laws?9.189.18

Chromosome Theory of Chromosome Theory of InheritanceInheritance

Genes occupy a specific location on a Genes occupy a specific location on a chromosomechromosome

Chromosomes undergo segregation and Chromosomes undergo segregation and independent assortmentindependent assortment

* Segregation occurs when homologous * Segregation occurs when homologous chromosomes are separated during chromosomes are separated during anaphase I.anaphase I.* Independent Assortment occurs when * Independent Assortment occurs when homologous chromosomes line up homologous chromosomes line up together during metaphase I.together during metaphase I.

LEQ: How are linked LEQ: How are linked genes inherited?genes inherited?9.19 and 9.209.19 and 9.20

Linked GenesLinked Genes

Genes found on the Genes found on the same chromosomesame chromosome

Linked genes do NOT Linked genes do NOT independently assortindependently assort

Look and behave like a Look and behave like a monohybrid Punnett monohybrid Punnett squaresquare

Thomas Hunt MorganThomas Hunt Morgan

American embryologist – American embryologist – early 1900’s, studied early 1900’s, studied fruit flies, identified the fruit flies, identified the process of “crossing process of “crossing over” by studying linked over” by studying linked genesgenes

Why is Why is Drosophila Drosophila melanogastermelanogaster a good a good organism to study?organism to study? Few chromosomesFew chromosomes SmallSmall Easy to care forEasy to care for Lots of offspringLots of offspring Quick generation timeQuick generation time

How do you get genetic How do you get genetic diversity with linked diversity with linked

genes?genes?Crossing over occurs. Chromatids with new gene combinations Crossing over occurs. Chromatids with new gene combinations

are referred to as recombinant chromatids.are referred to as recombinant chromatids.

Recombination FrequencyRecombination FrequencyThe percentage of recombinants from a cross. In this example The percentage of recombinants from a cross. In this example

Morgan observed 17% recombination frequency. Morgan observed 17% recombination frequency.

LEQ: How is gender LEQ: How is gender determined?determined?9.22 9.22

Sex Determination Sex Determination SystemsSystems

X-Y: humans & fruit flies; X-Y: humans & fruit flies; XX = female; XY = male; XX = female; XY = male; sperm determines sexsperm determines sex

X-O: insects (grasshoppers X-O: insects (grasshoppers & roaches) O = absence of & roaches) O = absence of a sex chromosome; XX = a sex chromosome; XX = female; X- = male; sperm female; X- = male; sperm determines sex by having determines sex by having an X or having no sex an X or having no sex chromosomechromosome

Z-W: fish, butterflies, & Z-W: fish, butterflies, & birds; ZZ = male; ZW = birds; ZZ = male; ZW = female; egg determines female; egg determines sexsex

Number: ants & bees; Number: ants & bees; females are diploid – females are diploid – fertilized eggs; males are fertilized eggs; males are haploid – unfertilized eggshaploid – unfertilized eggs

LEQ: How are some LEQ: How are some traits linked to our traits linked to our gender?gender?9.239.23

Sex Linked GenesSex Linked Genes

Genes that are located Genes that are located on sex chromosomeson sex chromosomes

Thomas Hunt Morgan Thomas Hunt Morgan identified sex linked identified sex linked traits by studying eye traits by studying eye color in fruit fliescolor in fruit flies

He noticed that white He noticed that white eye color is more eye color is more common in males than common in males than femalesfemales

Fruit Fly

Eye C

olo

rFru

it Fly E

ye C

olo

r

Figure 15.7Figure 15.7

Eggs Eggs Eggs

Sperm Sperm Sperm

(a) (b) (c)

XNXN XnY XNXn XNY XNXn XnY

Xn Y XN Y YXn

Xn Xn

XN

XN

XN XNXNXn XNY

XNY

XNY XNY

XnY XnYXNXn XNXn

XNXnXNXN

XnXn

Some disorders caused by recessive alleles on the Some disorders caused by recessive alleles on the X chromosome in humansX chromosome in humans

Color blindness (mostly X-linked)Color blindness (mostly X-linked)Duchenne muscular dystrophyDuchenne muscular dystrophyHemophiliaHemophilia

© 2011 Pearson Education, Inc.

X Inactivation in Female X Inactivation in Female MammalsMammals

In mammalian females, one of the two X In mammalian females, one of the two X chromosomes in each cell is randomly chromosomes in each cell is randomly inactivated during embryonic development inactivated during embryonic development

The inactive X condenses into a The inactive X condenses into a Barr bodyBarr body

If a female is heterozygous for a particular gene If a female is heterozygous for a particular gene located on the X chromosome, she will be a located on the X chromosome, she will be a mosaic for that charactermosaic for that character

© 2011 Pearson Education, Inc.

Early embryo:

X chromosomesAllele fororange fur

Allele forblack fur

Two cellpopulationsin adult cat:

Cell division andX chromosomeinactivation

Active XInactive X

Active X

Black fur Orange fur