C15 DNA
C15 Genetics Continued
Chromosome theory of inheritance took form ~1902. Thomas
Morgan Hunt, at Columbia University,1 st to associate a specific gene with a specific chromosome, early 1900’s. He studied Drosophila melanogaster and found the eye color of fruit flies is located on the X chromosome.
Wild type – most common phenotype in nature, ex. red eyes in fruit fly.
Mutant phenotype – alternatives to the wild type, ex. white eyes in fruit fly.
Linked genes – genes located on the same chromosome that tend to be inherited together.
Genetic recombination – the production of offspring with new combinations of traits inherited from two parents.
2 types of offspring can be produced:
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Parental types – offspring matching a parental phenotype.
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Recombinants – offspring that do not match either parent’s phenotype.
The further apart linked genes are, the greater the chance for cross-overs.
Maximum value =50% which is equal to the percentage of recombinants from genes on different chromosomes
(non-linked). Morgan found 17% for linked genes. (Crossing over accounts for the recombination of linked genes).
Mendel’s work:
Seed color and flower color are both on chromosome 1, but so far apart, that linkage is not observed. Plant height and pod shape are linked, but Mendel did not report these in dihybrids. Crossover data helps biologists map genes.
Linkage map – a genetic map based on recombination frequencies, portrays sequence but not exact locations.
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C15 DNA
Cytological maps – locate genes with respect to chromosomal features, such as stained bands.
Sex Chromosomes
50/50 chance of being male or female. XX is female, XY is male. “If you have a Y, you’re a guy.”
In humans, the anatomical signs of sex begin to emerge when the embryo is about two months old. 1990, British team idenitified SRY, sex-determining region of Y. In the absence of SRY, the gonads develop into ovaries. It is likely that SRY codes for a protein that regulates many other genes. Recently, researchers have identified more genes on the Y chromosome that are required for normal testicular function.
Sex-linked genes – genes located on a sex chromosome (usu. X).
Males cannot pass a X-linked gene to their sons, only their daughters.
Hemizygous – describes male X-linked traits expressed because they stand alone.
X-linked disorders (these are also recessive):
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Colorblindness – difficulty discriminating between red and green colors.
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Duchenne muscular dystrophy – progressive weakening of muscles and loss of coordination due to lacking a key muscle protein called dystrophin.
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Hemophilia – a clotting factor, Factor VIII, deficiency which leads to excessive bleeding when injured. Treated with Factor
VIII replacement therapy. (Plagued the royal families of
Europe).
The X chromosome becomes almost completely inactivated in female mammals during embryonic development. The inactive chromosome condenses into a Barr body . (Barr-body chromosomes are reactivated in the ovary cells that give rise to ova). As a result, females can be a mosaic for certain traits. The female calico cat exemplifies this.
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Nondisjunction – when chromosomes or chromatids fail to separate properly during meiosis.
Aneuploidy – offspring with an abnormal chromosome number.
Trisomic – having three chromosomes of one type, ex. Trisomy 21.
Monosomic – having a missing chromosome, ex. Turner’s syndrome.
Polyploidy – having more than two complete chromosome sets.
Triploidy means having 3 n ; tetraploidy is 4 n .
Alterations of chromosome structure:
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Deletion – when a chromosomal fragment is lost during cell division.
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Duplication – when a fragment joins the homologous chromosome and you have extra nucleotides.
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Inversion – when a fragment breaks off and reattaches to the proper chromosome, but is now upside down.
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Translocation – when a fragment attaches to a nonhomologous chromosome.
Down syndrome – Trisomy 21, characterized by shorter lifespan, characteristic facial features, short stature, heart defects, susceptibility to respiratory infection, and mental retardation. Only autosomal trisomy humans can survive beyond infancy. Increased risk in older mothers. One out of every 700 children born in the US.
A small portion of Down sydrome persons have the usual 46 chromosomes, but the third chromosome 21 is attached to another chromosome by translocation.
XXY – Klinefelters syndrome, 1 in every 2000 live births, have male sex organs, but the testes are abnormally small and the man is sterile.
XYY – taller than average, some studies link to anti-social behaviors.
XXX – Trisomy X, one in every 1000 live births, cannot be distinguished from XX except by karyotype.
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C15 DNA
XO – Turner syndrome, females of shorter stature and sterile. May take growth hormones and other female hormones to help normalize.
One in every 5000 live births.
Cri du chat – “Cry of the cat,” caused by deletion in chromosome 5.
Symptoms include mental retardation, small head, unusual facial features, and a cry that sounds like a distressed cat’s mewing. Usu. die in infancy or early childhood.
Chronic myelogenous leukemia (CML) – caused by a portion of chromosome 22 switching places with a tip of chromosome 9.
Genomic imprinting – the parental effect on gene expression.
Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.
Thought to happen via methylation (adding of methyl groups to nucleotides at specific loci which inactivates that allele). Ex, Prader-
Willi syndrome patients have mental retardation, obesity, short stature, and small hands and feet. Angelman syndrome patients have uncontrollable laughter and jerky movements…Both caused by deletion of a segment of chromosome 15. If the child inherits it from his father, the result is Prader-Willi, from the mother, Angelman syndrome. So far, at least 20 mammalian genes have been found that are subject to imprinting, and there may be a couple of hundred more.
Fragile X syndrome – named for the physical appearance of the X chromosome which hangs by a thread of DNA. Most common form of mental retardation. More severe when inherited from mother than father. More males have this disorder.
Extranuclear genes exhibit a non-Mendelian pattern of inheritance . Maternal inheritance is the rule for plastids in plants, and mitochondrial genes in plants and mammals. Ex, person with mitochondrial myopathy suffers weakness and muscle deterioration due to impaired mitochondria from mother’s ova.
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