CHAPTER 15

You Must Know How the chromosome theory of inheritance connects the physical movement of chromosomes in meiosis to Mendel’s laws of inheritance The unique pattern of inheritance in sex-linked genes How alteration of chromosome number or structurally altered chromosomes (deletions, duplications, etc.) can cause genetic disorders

Linking Mendel and Chromosomes Today we know that Mendel’s “hereditary factors” are located on chromosomes So we can link Mendelian genetics to modern genetics through the genes that lie on the chromosome Chromosome Theory of Inheritance Mendelian genes have specific loci (positions) on chromosomes Chromosomes undergo Segregation & Independent Assortment

T.H. Morgan studied flies He studied Drosophila Melanogaster – Fruit fly They _____ like flies Take 2 weeks to breed Hundreds of offspring per brood Only 4 pairs of chromosomes 3 autosomes and 1pair of sex chromosomes

Converting to Morgan speak Phenotype Dominant = wild type Recessive = mutant Wild-type – normal or typical W+ Mutant type – not normal W For example, vg+ = wild type for body size (full body) vg = mutant (vestigial body, smaller size)

Morgan gets unMendelian results He crossed wild type (red eyes) with a mutant (white eyes), but did not get Mendelian results, or did he? Basis for his questioning Mendelian Genetics P = Red-eyed female + White-Eyed male F1 = ALL Red eyes F2 = 3:1 ratio of red to white eyes, BUT only MALES = white eyes

Gene Linkage Linked genes – genes located on the same chromosome that tend to be inherited together Linked genes do not adhere to the laws of Mendelian genetics

Sex-linked gene inheritance pattern

Notes on Sex-linked Traits Female = XX Male = XY Fathers pass sex-linked traits to daughters NOT sons Fathers pass the y-chromosome to sons Female expression is like any other trait Since Males have only 1 chromosome, they express the allele on their 1 X-chromosome Sex-linked traits have NOTHING to do with sex CANNOT use heterozygous and homozygous

Known Sex-linked Disorders Duchenne muscular dystrophy Progressively weakening of muscles and loss of coordination Hemophilia Blood that is unable to clot normally Due to absence of proteins required for proper clotting

X Inactivation in Females Although females receive 2 copies of alleles, one chromosome becomes inactivated during embryonic development Due to XX Chromosome inactivation is Random Inactivation is due to methylation So females and males are operating on only 1 copy Barr Body – inactivated chromosome Lies on the inside of the nuclear envelope

Mosaic-ness

Back to Morgan & Flies

Linkage Inheritance Pattern 2 Characters: body color & wing size Body Color b+ = Grey (wild type) b = black (mutant) Wing Size vg+ = normal wings vg = vestigial wings (Reduced wing size)

Some more vocab… Linked Genes – located on same chromosome Tend to be inherited together Genetic Recombination – Offspring with new combination of genes inherited from parents Parental Phenotype – offspring phenotype = at least one of the parental phenotypes Recombinants – offspring phenotype NOT = either parental phenotype

Notes on the Results 2,300 offspring Far higher proportion of parental phenotypes than expected from independent assortment Genes are inherited together There were also recombinants or non-parental phenotypes as well Conclusion = Partial linkage & Genetic recombination (recombinants or recombinant types)

Recombinant Frequency If the genes are located on different chromosomes, then the recombination frequency should be 50% In the flies, the recombinant frequency was less than 50%; it was about 17% Evidence of that the 2 genes lied on the same chromosome So some linkage but incomplete

Crossing Over Crossing Over explains why some linked genes get separated during meiosis Crossing Over occurs in Meiosis I Farther apart 2 genes = Higher P(Crossing Over) Linkage Map – genetic map based on the percentage of cross-over events Map unit – 1% recombination frequency Used ONLY for relative distances on the chromosome

Linkage Maps

Explain We know that Mendel’s seed color and flower color were on the SAME chromosome, but they did not behave as linked genes. Explain.

Nondisjunction Nondisjunction – mishap where pairs of homologs do not move apart properly during meiosis Could happen in Meiosis I or when Sister chromatids fail to separate correctly in Meiosis II One gamete receives 2 of the same type of chromosome, while another receives no copy

-somy Aneuploidy – abnormal number of chromosomes Nondisjunction could result in a cell with 2n+1 Here this cell would be considered aneuploid, and considered trisomic for that individual chromosome -somy = different number of an INDIVIDUAL chromosomes Trisomy – 3 copies of a chromosome Monosomy – only 1 copy of a chromosome 2n – 1

-ploidy Alteration of an ENTIRE CHROMOSOMAL SET Called polyploidy Triploidy = 3n Tetraploidy = 4n Polyploidy plants are fairly common, animals are less common Polyploids are more normal than aneuploids Hence, 1 chromosome extra or fewer is more disruptive, than an entire set of chromosomes extra or fewer

Chromosomal Structural Alteration

Human Disorders caused by Chromosomal Alteration Down Syndrome 1 of 700 Trisomy 21 (each cell has 47 chromosomes, not 46) Risk increases with maternal age Klinefelter Syndrome Male have extra X chromosome Possess male sex organs, but are sterile IF Female, 3 chromosomes (XXX) = healthy & normal Turner Syndrome Female with only 1 X chromosome Sterility, but only known monosomy in humans