The Chromosomal Basis of Interhitance Chapter 15 The Chromosomal Basis of Interhitance

The Chromosome Theory Thomas Hunt Morgan worked with Drosophila (dew loving) and showed a connection between chromosomes and inheritance

Female Male

Chromosome theory The unifying theory stating that inheritance patterns may be generally explained by assuming that genes are located in specific sites on chromosomes, and found in a linear sequence.

Figure 15.2 Morgan’s first mutant

Chromosome Map of Drosophila (Fruit Flies)

Fruit Fly Inheritance

Recombination due to Crossing Over

Sex linked traits occur because… they are determined by genes located on (linked) either the X or Y chromosome, but not on both.

Sex linked traits… Hemophilia: an X-linked genetically inherited recessive disease in which one or more of the normal blood clotting factors is not produced. Hemophilia most often afflicts males.

Other X - Linked traits Hemophilia Red-green color blindness Duchene muscular dystrophy Fragile X-syndrome

Colorblind persons can see the circle but not the star

Y - linked trait… This trait always occurs when the gene is present…but we can’t know if it is dominant. Why not?

X Inactivation in Female Mammals During fetal development, one female X chromosome become inactive and lies along the inside of the nuclear membrane Which X chromosome inactivates is random and varies from cell to cell, however during mitotic division the same X chromosome remains inactive The inactive X chromosome is called a Barr body

BAR BODIES Only occurs in female mammals since they have 2 X chromosomes, unlike men Females consist of a mosaic of two types of cells; those with active and those with inactive X chromosomes

X Inactivation in Female Mammals Tortoiseshell cats show mosaicism with patches of orange and black fur. Therefore, all tortoiseshell cats are female.

Alterations of Chromosome Number Nondisjunction – members of a pair of homologous chromosomes fail to move apart properly during meiosis I or sister chromosomes do not separate during meiosis II. One gamete receives two of the same type of chromosome and the other receives no copy.

Nondisjunction

Alterations of Chromosome Number Aneuploidy – an abnormal number of chromosomes Monosomic – a chromosome is missing (2n - 1) Trisomic – a chromosome is present in triplicate (2n + 1) Polyploidy – an organism with two or more complete chromosome sets Triploidy – 3n Tetraploidy – 4n

Alterations of Chromosome Structure Caused by a breakage of a chromosome

Translocation

Human Disorders Due to Chromosomal Alterations Down’s syndrome affects 1 out of 700 births in the US Results from a trisomy of chromosome 21 (2n+1) Characteristic facial features, short stature, heart defects, susceptibility to respiratory infection, and mental retardation Individuals are also more prone to develop leukemia and Alzheimer's disease; these diseases are located on chromosome 21 Risk – 0.04% for women under 30 1.25% for women over 30

Human Disorders Due to Chromosomal Alterations Down’s syndrome

Human Disorders Due to Chromosomal Alterations Klinefelter’s Syndrome XXY sex chromosomes Individual usually sterile Have both male and female secondary sexual characteristics Occurs in offspring of mother’s over 40 years old.

Human Disorders Due to Chromosomal Alterations XYY Condition Males who have an extra Y chromosome Tend to be very tall and have subnormal intelligence 1 in 1000 births

Human Disorders Due to Chromosomal Alterations Turner’s Syndrome Monosomy X chromosome; individual is female 1 in 5000 births Usually sterile

Human Disorders Due to Chromosomal Alterations Triple X Syndrome Trisomy – XXX chromosomes 1 in 1000 births Usually a healthy female

Human Disorders Due to Chromosomal Alterations Trisomy chromosome 8 Mental retardation, skeletal deformities, internal organs malformed 1 in 3,000 births Patau Syndrome Trisomy 13 Malformations of major organs Usually die within 1 year 1 in 7,500 – 10,000 births 88% maternal; 12% paternal

Patau’s Syndrome – Trisomy 13

Human Disorders Due to Chromosomal Alterations Edward Syndrome Trisomy 18 Thin, frail children with short eyelids, low set ears, webbed fingers & toes, and skin folds. In males, the testes do not descend 20-30% die in the first month 90% die by the age of 1 1 in 5,000 births Affects girls more than boys

Human Disorders Due to Chromosomal Alterations Cri du Chat Deletion of the short arm of chromosome 5 High pitched cry, low birth weight Difficulties sucking and swallowing Mental retardation Most die in infancy – those that live are severely delayed in development 1 in 50,000 live births

Constructing a Pedigree Use the appropriate symbols: Unaffected Male Unaffected Female Affected Male Affected Female Male carrier of trait Mating of Offspring 2. Label each generation down the left hand side of your pedigree. 3. Label each individual in your pedigree with his or her name. 4.  It is easiest to construct your pedigree working from the most recent generation backwards.