Chromosomal Inheritance Chapter 15

Chromosomal basis of Inheritance Hereditary factors are located on chromosomes at specific loci - genes. Located in pairs –Homologous chromosomes Separated in meiosis via segregation and independent assortment.

Thomas Hunt Morgan Experimented with fruit flies – Drosophila melonagaster –Eye Color Wild type = normal –Red eyes Mutant = abnormal –White eyes Figure 15.3

Thomas Hunt Morgan Mated red eyed (w+) females with white eyed males (w) –P - generation All F 1 = red eyes Mated F 1 generation F 2 = 3:1 ratio red:white –All white eyes were male Eye color is linked on the sex chromosome –In not linked half the white eyes would be female and half male

CONCLUSION Since all F 1 offspring had red eyes, the mutant white-eye trait (w) must be recessive to the wild-type red-eye trait (w + ). Since the recessive trait—white eyes—was expressed only in males in the F 2 generation, Morgan hypothesized that the eye-color gene is located on the X chromosome and that there is no corresponding locus on the Y chromosome, as diagrammed here. P Generation F 1 Generation F 2 Generation Ova (eggs) Ova (eggs) Sperm X X X X Y W W+W+ W+W+ W W+W+ W+W+ W+W+ W+W+ W+W+ W+W+ W+W+ W+W+ W W+W+ W W W

Interactive Question 15.1

Linked Genes Each chromosome contains hundreds or thousands of genes Genes on the same chromosomes are usually inherited together – linked gene –Not inherited together if crossing over takes place

Parental Types vs. Recombinants Parental Type - The offspring's phenotype resembles the parents Recombinant - Offspring’s phenotype is different than the parents 50% each = non linked –Caused by Independent assortment – genetic recombination Less than 50% recombinants = linked genes SIO

Interactive Questions

Crossing Over Causes recombination in linked genes Occurs during prophase I of meiosis between non- sister chromatids of homologous chromosomes –Recombinant frequency

Linkage Mapping The further apart 2 genes are on a chromosome the greater the chance of crossing over – greater recombinant frequency Map unit = recombinant frequency

Interactive Question 15.4

Sex Chromosomes X-Y system – mammals –XX = female –XY = male X-O system – some insects – grasshoppers, cockroaches –XX = female –X = male Z-W system – birds, fish, some insects –ZW = female –ZZ = male Haplo-diploid system – bees and ants – no sex chromosomes –Diploid – females –Haploid - males Figure 15.9b–d 22 + XX 22 + X 76 + ZZ 76 + ZW 16 (Haploid) 32 (Diploid) (b) The X–0 system (c) The Z–W system (d) The haplo-diploid system

Sex linked genes Linked to X chromosomes Males can only pass sex linked genes to daughters Females can pass sex linked genes to sons or daughters –Males will express sex linked genes – only have one X

Sex Linked Recessive Disorders More common in males than females –Only one X Females will only express if they are homozygous recessive –Only way to get the disorder is if BOTH parent have it –EXAMPLES Color blindness Muscular Dystrophy Hemophilia

X Inactivation In mammals only one X chromosome is expressed in somatic cells Second X condenses to become a barr body Barr bodies are reactivated during gamete formation

Interactive Question 15.5

Nondisjunction Homologous chromosomes do not separate in anaphase of meiosis Result is aneuploidy –Trisomy – 3 chromosomes n + 1 –Monosomy – 1 chromosome n – 1 –Polyploidy – 3 or more chromosomes

Chromosomal Alterations A B CD E FG H Deletion A B C E G H F A B CD E FG H Duplication A B C B D E C F G H A A MN OPQR B CD EFGH B CDEFGH Inversion Reciprocal translocation A BPQ R M NOCDEF G H A D CBEFH G

Chromosomes Alterations TypeExplanationExample 1. DeletionRemoval of a chromosome segment ABCDE  ABDE 2. DuplicationRepetition of a chromosomal segment ABCDE  ABBCDE 3. InversionReversal of a chromosome segment ABCDE  ABDCE 4. TranslocationMovement of a segment on one chromosome to another - nonhomologous ABCDE  FGCDE FGHIJ  ABHIJ

Full Chromosomal Disorders Syndromes –Down Syndrome – Trisomy 21 –Kleinfelters – XXY Sterile –Trisomy X – XXX –Turner syndrome – XO Sterile

Altered Chromosomal Disorders Cri du Chat –Deletion on chromosome 5 Leukemia – CML –Reciprocal translocation between chromosome 9 and 22

Chromosomal Mutations Bubble Map Summarize – give a specific example of a disorder caused by each.

Genomic Imprinting Genes on autosomal chromosomes that are expressed depending on whether they come from the mother or father Insulin growth factor – only the paternal gene is activated –Maternal gene is imprinted - methylated

Organelle Inheritance Chloroplasts are inherited through the cytoplasm from the egg NOT the pollen Mitochondria are also passed in the cytoplasm of the egg –Not carried in sperm cells

Free Choice

Summary