Chromosomal Theory of Inheritance• Chromosomes contain the units of heredity (genes)• Pair chromosomes segregate during meiosis, each sex cell has half of the number of chromosomes found in a somatic cell. (Mendel’s law of segregation)• Chromosomes assort independently during meiosis (Mendel’s law of independent assortment)• Each chromosome contain many different genes
Sex Chromosomes• Sex chromosomes (X and Y) vs. autosomes (chromosomes 1-22), Sex cells and somatic cells.• Homogametic sex -- that sex containing two like sex chromosomes. In most animals species these are females (XX). Each egg only contain one X chromosome.• Heterogametic sex --- that sex containing two different sex chromosomes . In most animal species these are XY males. Each sperm will contain either an X or Y. Therefore the father determines whether the offspring is a boy or a girl (50/50 chance)
Sex Linkage• XA = Locus on X chromosome• XX females – XA XA, XaXa - homozygotes – XA Xa – heterozygote (carrier)• XY male – XA Y, XaY – no carriers in males, therefore they are more susceptible to x-linked traits.
Red/white eye color in Drosophila• In females: – XR XR , XR Xr = red-eye female – Xr X r = white-eyed females• In males: – XR Y = red-eye male – Xr Y = white-eyed male
Examples of Sex Linked Traits• Hemophilia - Recessive• Red-Green Color Blindness - Recessive• Muscular Dystrophy - Recessive• Fragile X syndrome - Dominant
Nondisjunction• abnormal number of autosomal chromosomes when chromosomes fail to separate during replication.• 2n – 1 = monosomic• 2n + 1 = trisomic
Nondisjunction - Examples• Downs -- trisomy 21 mean life expectancy 17 years. Short in stature, round face and mental retardation• Pataus -- trisomy 13 mean life expectancy 130 days• Edwards --- trisomy 18 mean life expectancy a few weeks
Sexual Determination - MalesSingle Y = male, so XXY, XYY, XXXY all male• Klinefelter Syndrome – XXY or XXXY. Male due to Y chromosome, Testes and prostrate underdeveloped, some breast formation, no pubic or facial hair, subnormal intelligence.• Jacob’s Syndrome – XYY. Males are usually taller than average, and tend to have speech and reading problems
Sexual Determination - Females• Turner’s Syndrome – X0. Female with bull neck, short stature, nonfunctional ovaries, no puberty• Metafemale – 3 or more X chromosomes. No apparent physical abnormality except menstrual irregularities.
Chromosomal Mutation• Permanent change in chromosome structure.• Caused by exposure to radiation, organic chemicals, viruses, replication mistakes.• Only mutations in sex cells are passed onto the next generation.
Structural Changes in Chromosomes• Inversion – occurs when a chromosome segment turns around 180 degrees.• Translocation – is movement of chromosomal segments to another non- homologous chromosome• Deletion – occurs when a portion of the chromosome breaks off.• Duplication – when a portion of a chromosome repeats itself.
Linkage• When genes are on the same chromosome, they are called linked. They can show departures from independent assortment• If genes on the same chromosome are sufficiently far apart, they can segregate independently through crossing over.
Gene Mapping• By studying cross-over (recombination) frequencies of linked genes, a chromosomal map can be constructed – Distant genes are more likely to be separated by crossing-over than genes that are closer together. – Each 1% of recombination frequency is equivalent to 1 map unit
Constructing Gene Maps• Crossing over frequencies can be used to construct gene maps. For example, – Crossing over frequency of genes A and B is 3%, genes B and C is 9% and genes A and C is 12%. 3 mu 9 mu A B C
Human Genome Project• Map of the all of the genes on the human chromosomes.
Modes of Inheritance• Autosomal dominant allele [e.g., Huntingtons Disease, brown eyes] – A phenotype associated with an autosomal dominant allele will, ideally, be present in every individual carrying that allele. It will be present in close to 50% of the individuals. – Affected children usually have affected parents – Two affected parents can produce an unaffected child – Both males and females are affected equally.
Modes of Inheritance• Autosomal recessive alleles [silent carriers] – albinism, cystic fibrosis, certain types of hemophilia, Tay-Sachs disease, PKU, blue eyes. – A pedigree following a trait associated with an autosomal recessive allele is often marked by a skipping of generations. That is, children may express a trait which their parents do not. – In such a situation, both parents are heterozygote, also known as silent carriers. – Close relatives who reproduce are more likely to have affected children. – Both males and females will be affected with equal frequency – A low number of individuals normal affected
Modes of Inheritance• Sex-linked dominant alleles [sex linkage] – A sex linked dominant allele has a variation on the pattern displayed by autosomal dominant alleles. That is: • one-half of the offspring of an afflicted heterozygote female will be similarly afflicted (gender independent). • only the female progeny of males will be afflicted (because the male donates an X chromosome to his female progeny). – As with any sex-linked allele, males can pass the allele only on to their daughters, not their sons.
Mode of Inheritance• Sex-linked recessive alleles – red-green color blindness, certain types of hemophilia. – More males affected than females – An affected son can have parents who have the normal phenotype – For a female to have the characteristic, her father must also have it and the mother must be a carrier. – If a woman has the characteristic all her sons will have it – The characteristic often skips a generation from grandfather to grandson