Chromosomal Inheritance Chapter 12 Mader 2007-08

Chromosomal Inheritance Humans are diploid (2 chromosomes of each type) Humans have 23 different kinds of chromosomes Arranged in 23 pairs of homologous chromosomes Total of 46 chromosomes (23 pairs) per cell Mader 2007-08

The other 22 pairs of chromosomes are autosomes One of the chromosome pairs determines the sex of an individual (The sex chromosomes) The other 22 pairs of chromosomes are autosomes Autosomal chromosomes are numbered from smallest (#1) to largest (#22) The sex chromosomes are numbered as the 23rd pair Mader 2007-08

Sex Determination in Humans Sex is determined in humans by allocation of chromosomes at fertilization Both sperm and egg carry one of each of the 22 autosomes The egg always carries the X chromosome as number 23 The sperm may carry either and X or Y If the sperm donates an X in fertilization, the zygote will be female If the sperm donates a Y in fertilization, the zygote will be male Therefore, the sex of all humans is determined by the sperm donated by their father Mader 2007-08

X-Linked Alleles Genes carried on autosomes are said to be autosomally linked Genes carried on the female sex chromosome (X) are said to be X-linked (or sex-linked) X-linked genes have a different pattern of inheritance than autosomal genes have The Y chromosome is blank for these genes Recessive alleles on X chromosome: Follow familiar dominant/recessive rules in females (XX) Are always expressed in males (XY), whether dominant or recessive Males said to be monozygous for X-linked genes Mader 2007-08

Chromosomal Basis of Sex XX female Xy male Short segments at either end of they Y chromosome are homologous to regions of the X. These regions allow the X & Y to pair up during meiosis (in testes) Mader 2007-08

Chromosomal Basis of Sex In Humans Anatomical signs of sex begin when an embryo is 2 months old Hormone conditions determine the sex Presence of Y chromosomes and the SRY gene triggers the making of testes (testosterone) Absence of the SRY gene triggers the making of ovaries Mader 2007-08

Sex Linked Genes Genes carried on the sex chromosomes Females must be homozygous to be affected by recessive alleles Heterozygotes and Homozygotes are do not exist for males…due to the y chromosome Mader 2007-08

X Inactivation of Females During embryonic development one X chromosome becomes inactive (Barr Body). All mitotic divisions create cells with the same inactive X. Males and females have equal dose of gene representation Mader 2007-08

Genes in the Barr body are not expressed at this time Barr Bodies reactivate in the ovaries Ova are made so that all gametes receive the same number of active chromosomes Barr Body development is random and independent Mader 2007-08

Ex: multicolored tortoise shell in cats Sex linked traits for a heterozygote female will express half of one allele and half of the other allele…depending on which is inactive. Ex: multicolored tortoise shell in cats Mader 2007-08

Chromosomal Basis of Inheritance Thomas Hunt Morgan Studied fruit flies (Drosophila melanogaster) Studied specific genes related to specific chromosomes He determined that the gene for eye color in fruit flies were carried on the X chromosome Mader 2007-08

Eye Color in Fruit Flies Fruit flies (Drosophila melanogaster) are common subjects for genetics research They normally (wild-type) have red eyes A mutant recessive allele of a gene on the X chromosome can cause white eyes Possible combinations of genotype and phenotype: Genotype Phenotype XRXR Homozygous Dominant Female Red-eyed XRXr Heterozygous XrXr Homozygous Recessive White-eyed XRY Monozygous Dominant Male XrY Monozygous Recessive Mader 2007-08

X-Linked Inheritance Mader 2007-08

Gene Linkage When several genes of interest exist on the same chromosome Such genes form a linkage group Tend to be inherited as a block If all genes on same chromosome: Gametes of parent likely to have exact allele combination as gamete of either grandparent Independent assortment does not apply If all genes on separate chromosomes: Allele combinations of grandparent gametes will be shuffled in parental gametes Independent assortment working Mader 2007-08

Gene Linkage Mader 2007-08

Linked Genes Morgan studied 2 traits on his fruit flies Body color and wing shape Wild type (gray body b+) mutant (black body b) Wild type (normal wings vg+) mutant (short wings vg) b+b vg+vg v. bb vg vg test cross Mader 2007-08

Linkage Groups Mader 2007-08

All Parental offspring Actual results: Expected results: All Parental offspring Actual results: The proportion of parental offspring vs. nonparental offspring was very high Mader 2007-08

Genetic Recombination Explains the nonparental type offspring. Unlinked genes 50% frequency of recombination of unlinked genes Due to law of segregation and independent assortment Fig 15.2 and Fig 13.10 in text. Mader 2007-08

Linked Genes Linked genes Crossing over of linked genes when tetrads form in meiosis I. Mader 2007-08

Constructing a Chromosome Map Crossing-over can disrupt a blocked allele pattern on a chromosome Affected by distance between genetic loci Consider three genes on one chromosome: If one at one end, a second at the other and the third in the middle Crossing over very likely to occur between loci Allelic patterns of grandparents will likely to be disrupted in parental gametes with all allelic combinations possible If the three genetic loci occur in close sequence on the chromosome Crossing over very UNlikely to occur between loci Allelic patterns of grandparents will likely to be preserved in parental gametes Mader 2007-08

Rate at which allelic patterns are disrupted by crossing over: Indicates distance between loci Can be used to develop linkage map or genetic map of chromosome Mader 2007-08

Crossing Over Mader 2007-08

Complete vs. Incomplete Linkage Mader 2007-08

Linkage Mapping Genetic map based upon recombination frequencies of genes 1 map unit = 1% recombination frequency The farther apart they are the higher the chance that crossing over occurs Mader 2007-08

Recombination frequency can only have a value of up to 50%. Anything at this point is indistinguishable from genes on separate chromosomes. If the distance between genes is great enough on one chromosome, then linkage is not observed. Mader 2007-08

Chromosome Number: Polyploidy Occurs when eukaryotes have more than 2n chromosomes Named according to number of complete sets of chromosomes Major method of speciation in plants Diploid egg of one species joins with diploid pollen of another species Result is new tetraploid species that is self-fertile but isolated from both “parent” species Some estimate 47% of flowering plants are polyploids Often lethal in higher animals Mader 2007-08

Chromosome Number: Aneuploidy Monosomy (2n - 1) Diploid individual has only one of a particular chromosome Caused by failure of synapsed chromosomes to separate at Anaphase I (nondisjunction) Mader 2007-08

Diploid individual has three of a particular chromosome Trisomy (2n + 1) occurs when an individual has three of a particular type of chromosome Diploid individual has three of a particular chromosome Also caused by nondisjunction This usually produces one monosomic daughter cell and one trisomic daughter cell in meiosis I Down syndrome is trisomy 21 Mader 2007-08

Nondisjunction Mader 2007-08

Trisomy 21 Mader 2007-08

Chromosome Number: Abnormal Sex Chromosome Number Result of inheriting too many or too few X or Y chromosomes Caused by nondisjunction during oogenesis or spermatogenesis Turner Syndrome (XO) Female with single X chromosome Short, with broad chest and widely spaced nipples Can be of normal intelligence and function with hormone therapy Mader 2007-08

Chromosome Number: Abnormal Sex Chromosome Number Klinefelter Syndrome (XXY) Male with underdeveloped testes and prostate; some breast overdevelopment Long arms and legs; large hands Near normal intelligence unless XXXY, XXXXY, etc. No matter how many X chromosomes, presence of Y renders individual male Mader 2007-08

Turner and Klinefelter Syndromes Mader 2007-08

Chromosome Number: Abnormal Sex Chromosome Number Ploy-X females XXX simply taller & thinner than usual Some learning difficulties Many menstruate regularly and are fertile More than 3 Xs renders severe mental retardation Jacob’s syndrome (XYY) Tall, persistent acne, speech & reading problems Mader 2007-08

Abnormal Chromosome Structure Deletion Missing segment of chromosome Lost during breakage Translocation A segment from one chromosome moves to a non-homologous chromosome Follows breakage of two nonhomologous chromosomes and improper re-assembly Mader 2007-08

Deletion, Translocation, Duplication, and Inversion Mader 2007-08

Abnormal Chromosome Structure Duplication A segment of a chromosome is repeated in the same chromosome Inversion Occurs as a result of two breaks in a chromosome The internal segment is reversed before re-insertion Genes occur in reverse order in inverted segment Mader 2007-08

Inversion Leading to Duplication and Deletion Mader 2007-08

Abnormal Chromosome Structure Deletion Syndromes Williams syndrome - Loss of segment of chromosome 7 Cri du chat syndrome (cat’s cry) - Loss of segment of chromosome 5 Translocations Alagille syndrome Some cancers Mader 2007-08

Williams Syndrome Mader 2007-08

Alagille Syndrome Mader 2007-08