Sex Chromosomes, Linkage, and Organelle Inheritance PDF

Chapter 15 Inheritance of Sex Chromosomes, Linked Genes, and Organelles  Non-Mendelian Inheritance Patterns Some traits do not show the inheritance patterns predicted by Mendel. Instead, the ratios and pedigree patterns are the result of principles that Mendel knew nothing about. For...

Chapter 15 Inheritance of Sex Chromosomes, Linked Genes, and Organelles  Non-Mendelian Inheritance Patterns Some traits do not show the inheritance patterns predicted by Mendel. Instead, the ratios and pedigree patterns are the result of principles that Mendel knew nothing about. For example: genes carried in the sex chromosomes are transmitted differently in males and in females. genes that are close together on the same chromosome do not undergo independent assortment. mitochondria and chloroplast DNA follow their own inheritance pattern.  Human Sex Chromosomes The small region of homology between the sex chromosomes allows them to line up and segregate from Loading… one another during anaphase of meiosis I, contributing to the pattern of sex-linked traits seen in pedigrees.  Segregation of the Sex Chromosomes Random fertilization should result in a 1:1 ratio of XX and XY progeny. At birth, sex ratios show a slight excess of males.  X-Linked Genes T. H. Morgan discovered a white-eyed male in a jar of red-eyed fruit flies. After crossing the white- eyed male with a wild- type red-eye female, all F1 progeny had red Loading… eyes. The F1s showed a typical recessive inheritance pattern. When two F1s were crossed, a pattern of inheritance was revealed that differed from Mendelian predictions. Inheritance Patterns of X-linked Genes This pattern of inheritance suggests that the allele for white eyes was inherited in a pattern other than the Mendelian patterns that were understood at the time. X-Linkage The X-linked recessive alleles are expressed in males because males have only one X chromosome. Heterozygous XX Cross An X chromosome carrying the recessive allele can “crisscross,” or alternate, between the sexes in successive generations.  Morgan’s White-Eye Fruit Fly Crosses The hypothesis of X-linkage explained the original data and allowed predictions for future crosses that further supported the hypothesis.  Nondisjunction: Rare Meiotic Events  Nondisjunction The genotypes XXX and OY were never observed. Bridges concluded that embryos with these chromosomal constitutions were unable to survive. From the phenotypes of the exceptional fruit Loading… flies and their chromosome constitutions, Bridges concluded that the white-eye gene (or any X-linked gene) is physically present on the X chromosome. Bridges’ demonstration that genes are present on chromosomes provided the first experimental evidence of nondisjunction. X-Linked Recessive Mutation in Humans: Color Blindness  Crossing Over Without Recombination When crossing over occurs outside the interval between genes, there is no recombination between the alleles of the genes. Nonrecombinant chromosomes have the same configuration of alleles as one of the parental chromosomes.  Crossing Over With Recombination Crossing over occurs in prophase I of meiosis. Crossing over results in two recombinant and two nonrecombinant chromosomes at the end of meiosis II. Recombinant chromosomes show a nonparental combination of alleles.  Frequency of Recombination The physical distance between two genes on the chromosome affects their recombination frequency. The closer the two genes, the less likely crossing over will occur. The frequency of recombination between any two genes ranges from 0% to 50%.  Y-Linked Genes Y-linked genes are present in a unique area of the Y chromosome. They do not cross over with the X chromosome. Y-linked genes are only passed from fathers to sons because daughters inherit the X chromosome from their fathers. Y Chromosome Haplotypes and Ancestry There is only one copy of the Y chromosome, so the genotype of the Y chromosome is called a haplotype. We can use the accumulation of mutations over time to follow a lineage of Y chromosomes. Worldwide Y Chromosome Lineages  Inheritance of Mitochondrial and Chloroplast DNA Mitochondria and chloroplasts are ancient organelles of eukaryotic cells originally acquired when engulfed by prokaryotic cells. They have their own genomes that contain genes for many of the enzymes that carry out the organelles’ functions. Depending on the organism, the organelles may have a different pattern of inheritance.  Patterns of Inheritance in Cytoplasmic Organelles Three important patterns of inheritance in cytoplasmic organelles are: Maternal inheritance, in which the organelles in the offspring cells derive from those in the mother. Paternal inheritance, in which the organelles in the offspring cells derive from those in the father. Biparental inheritance, in which the organelles in the offspring cells derive from those in both parents. Inheritance of Mitochondrial DNA Inheritance of Mitochondrial DNA and Ancestry Mutations in mitochondrial DNA accumulate over time. Mitochondrial DNA can be used to trace ancestry because it does not undergo recombination and is maternally inherited.