LEQ: How do the events of meiosis account for Mendel’s laws? 9.18

Chromosome Theory of Inheritance Genes occupy a specific location on a chromosome Chromosomes undergo segregation and independent assortment

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

LEQ: How are linked genes inherited? 9.19 and 9.20

Linked Genes Genes found on the same chromosome Genes found on the same chromosome Linked genes do NOT independently assort Linked genes do NOT independently assort Look and behave like a monohybrid Punnett square Look and behave like a monohybrid Punnett square

Thomas Hunt Morgan American embryologist – early 1900’s, studied fruit flies, identified the process of “crossing over” by studying linked genes American embryologist – early 1900’s, studied fruit flies, identified the process of “crossing over” by studying linked genes Why is Drosophila melanogaster a good organism to study? Why is Drosophila melanogaster a good organism to study? Few chromosomes Few chromosomes Small Small Easy to care for Easy to care for Lots of offspring Lots of offspring Quick generation time Quick generation time

How do you get genetic diversity with linked genes? Crossing over occurs. Chromatids with new gene combinations are referred to as recombinant chromatids.

Recombination Frequency The percentage of recombinants from a cross. In this example Morgan observed 17% recombination frequency.

LEQ: How is gender determined? 9.22

Sex Determination Systems X-Y: humans & fruit flies; XX = female; XY = male; sperm determines sex X-Y: humans & fruit flies; XX = female; XY = male; sperm determines sex X-O: insects (grasshoppers & roaches) O = absence of a sex chromosome; XX = female; X- = male; sperm determines sex by having an X or having no sex chromosome X-O: insects (grasshoppers & roaches) O = absence of a sex chromosome; XX = female; X- = male; sperm determines sex by having an X or having no sex chromosome Z-W: fish, butterflies, & birds; ZZ = male; ZW = female; egg determines sex Z-W: fish, butterflies, & birds; ZZ = male; ZW = female; egg determines sex Number: ants & bees; females are diploid – fertilized eggs; males are haploid – unfertilized eggs Number: ants & bees; females are diploid – fertilized eggs; males are haploid – unfertilized eggs

LEQ: How are some traits linked to our gender? 9.23

Sex Linked Genes Genes that are located on sex chromosomes Genes that are located on sex chromosomes Thomas Hunt Morgan identified sex linked traits by studying eye color in fruit flies Thomas Hunt Morgan identified sex linked traits by studying eye color in fruit flies He noticed that white eye color is more common in males than females He noticed that white eye color is more common in males than females

Fruit Fly Eye Color

Figure 15.7 Eggs Sperm (a) (b) (c) XNXNXNXN XnYXnY XNXnXNXn XNYXNY XNXnXNXn XnYXnY XnXn Y XNXN Y Y XnXn XnXn XnXn XNXN XNXN XNXN XNXN XNXnXNXn XNYXNY XNYXNY XNYXNYXNYXNY XnYXnY XnYXnY XNXnXNXn XNXnXNXn XNXnXNXn XNXNXNXN XnXnXnXn

Some disorders caused by recessive alleles on the X chromosome in humans Some disorders caused by recessive alleles on the X chromosome in humans Color blindness (mostly X-linked) Color blindness (mostly X-linked) Duchenne muscular dystrophy Duchenne muscular dystrophy Hemophilia Hemophilia © 2011 Pearson Education, Inc.

X Inactivation in Female Mammals In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development The inactive X condenses into a Barr body The inactive X condenses into a Barr body If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character © 2011 Pearson Education, Inc.

Early embryo: X chromosomes Allele for orange fur Allele for black fur Two cell populations in adult cat: Cell division and X chromosome inactivation Active X Inactive X Active X Black fur Orange fur