Sex-Linked Traits

Sex-Linked Traits

Nondisjunction and Autosomal Disorders Humans have 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes Remember:

Females-XX Males-XY Nondisjunction Chromosomal Disorders The most common error in meiosis occurs when homologous

chromosomes fail to separate. This mistake is known as nondisjunction, which means not coming apart. Nondisjunction may result in gametes with an abnormal number of chromosomes, which can lead to a disorder of

chromosome numbers. Chromosomal Disorders If two copies of an autosomal chromosome fail to separate during meiosis, an individual may be born with three copies of that chromosome. This condition is known as a trisomy, meaning three bodies.

Monosomy 1 copy only; a shortage Trisomy 21 The most common form of trisomy is known as Down syndrome. There is an extra copy of chromosome

21 (3 copies). This extra chromosome only occurs on the 21stautosome pair. Chromosomal Disorders Nondisjunction of the X chromosomes can lead to a disorder known as Turners syndrome.

A female with Turners syndrome usually inherits only one X chromosome. Women with Turners syndrome are sterile, which means that they are unable to reproduce. Their sex organs do not develop properly. The chromosome arrangemment is 45X or 45XO instead of 46XX as in a normal female.

Chromosomal Disorders In males, nondisjunction may cause Klinefelters syndrome, resulting from the inheritance of an extra X chromosome, which interferes with meiosis and usually prevents these individuals from reproducing. There have been no reported instances of babies being born without an X chromosome, indicating that this chromosome contains genes that are vital for the survival

and development of the embryo. With karyotype47XXY, this person will appear male, but he is sterile. A normal male is 46XY. Autosomal Disorders Before genetic testing or

complete understanding of DNA, observers noticed that genetic traits were passed from one generation to another in predictable patterns. Autosomal Disorders The genetic mutation is on an

autosome, one of the chromosomes that are not an X or Y. Autosomal dominant only need one copy of abnormal gene to inherit disease

1. Huntingtons Disease A progressive brain disorder that causes uncontrolled movements, emotional problems, and loss of thinking

ability (cognition) Most common formadultonset usually appears in the 30s or 40s Huntingtons Disease Cause- The allele for this disease contains a long string of bases in which the codon CAGcoding for the amino acid glutaminerepeats over and over again, more than

40 times. The greater the number of codon repeats, the earlier the disease appears, and the more severe are its symptoms. 2. Neurofibromatosis Tumors grow on nerve tissue

and produce skin and bone abnormalities 3. Dwarfism A person of short stature as an adult; Achondroplasia causes 70% of all dwarfism

Arms and legs are short in comparison to trunk and head Dwarfism is not a disease, but leads to a greater risk of some health problems Autosomal recessive need two copies of abnormal

gene to inherit disease 1.Tay-Sachs Classical form manifests in infancy, fatal before 5 years old; juvenile and adult forms less severe Developmental retardation, paralysis, dementia, blindness, startle reaction

to sound, retinal cherry-red spot; common among Jewish & French Canadian ancestry. 2. Albinism Little or no pigment in

eyes, skin or hair Altered genes that do not make usual amounts of pigment

If both parents have albinism, child will not necessarily inherit it although the chance is much higher; still a mystery

Vision ProblemsPeople with albinism always have problems with vision (not correctable with eyeglasses). Vision problems in albinism result from abnormal development of the retina and abnormal patterns of nerve connections between the eye and the brain. It is the presence of these eye problems that

defines the diagnosis of albinism. 3. PKU (Phenylketonuria) Inability of body to utilize amino acid phenylalanine Must limit protein in diet Left untreated, causes brain development problems,

progressive mental retardation, and seizures 4. Cystic Fibrosis Affects mucus & sweat glands Mostlylungs,

pancreas, liver, intestines, sinuses, sex organs Mucus is usually watery; keeps the linings of certain organs moist and prevents them from drying out or getting infected

In CF the abnormal gene causes mucus to become thick & sticky; Mucus builds up in the lungs & blocks the airways Cystic Fibrosis Cystic fibrosis (CF) is most common among people of European ancestry.

Cause- deletion of just three bases in the gene for a protein called cystic fibrosis transmembrane conductance regulator (CFTR). Cystic Fibrosis CFTR normally allows chloride ions (Cl) to pass across cell membranes.

The loss of these bases removes a single amino acidphenylalaninefrom CFTR, causing the protein to fold improperly. The misfolded protein is then destroyed. Cystic Fibrosis With cell membranes unable to transport chloride ions, tissues throughout the body malfunction.

Effects of Cystic Fibrosis One of the most common fatal genetic disorders Genetic Advantages In the United States, the sickle cell allele is carried by approximately 1 person in 12 of African ancestry,

and the CF allele is carried by roughly 1 person in 25 of European ancestry. Why are these alleles still around if they can be fatal for those who carry them? Genetic Advantages Most African Americans today are descended from

populations that originally lived in west central Africa, where malaria is common. Malaria is a mosquito-borne infection caused by a parasite that lives inside red blood cells.

Genetic Advantages Individuals with just one copy of the sickle cell allele are generally healthy, and are also highly resistant to the parasite, giving them a great advantage against malaria.

Genetic Advantages More than 1000 years ago, the cities of medieval Europe were ravaged by epidemics of typhoid fever. Typhoid is caused by a bacterium that enters the body through cells in the digestive system. The protein produced by the CF allele helps block the entry of this bacterium.

Individuals heterozygous for CF would have had an advantage when living in cities with poor sanitation and polluted water, andbecause they also carried a normal allelethese individuals would not have suffered from cystic fibrosis.

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