Chromosomal Inheritance Biology 30

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) Chromosomes assort independently during meiosis (Mendel) Each chromosome contain many different genes

Sex Chromosomes Sex chromosomes (X and Y) vs. autosomes (chromosomes 1-22) 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 XY male 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

Sexual Determination - Males Single 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.

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

Nondisjunction - Examples Down's -- trisomy 21 mean life expectancy 17 years. Short in stature, round face and mental retardation Patau's -- trisomy 13 mean life expectancy 130 days Edward's --- trisomy 18 mean life expectancy a few weeks

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.

Deletions and Duplications

Inversions and Translocations

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. Crossing over% = # of recombinations total # of offspring

Crossing Over Produces Recombinations

Gene Mapping - Example Crossing over% = # of recombinations total # of offspring Crossing over % = 200 = 53% = 53mu 381 Therefore genes p and l are 53 map units apart.

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.

Pedigree Analysis

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

Modes of Inheritance Autosomal dominant allele [e.g., Huntington's 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 is 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 heterozygotes, 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