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Chromosomal Inheritance
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
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
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.
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 Produces Recombinations
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• 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 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