Download - The Chromosomal Basis of Inheritance
The Chromosomal Basis of Inheritance
Chapter 15
Review
• Mitosis• Meiosis• Chromosome• Genotype and Phenotype• Mendelian Genetics
Thomas Hunt Morgan
• Studied Drosophila melanogaster– Large number of
offspring– Small, easy to care for– 4 chromosomes with
easily observable phenotypes
Drosophila melanogaster Phenotypes
• Wild type: red eyes• Variations: eye color, body color, wing shape• Males and females are easy to tell apart
Morgan’s Cross
• Red eyed female x White eyed male (w+) (w)
• F1 100% red eyed (wild type)
• F2 3:1 ratio BUT only males had white eyes
Morgan’s Conclusion
• The gene for eye color must be carried on the X chromosome and NOT an autosome
• Higher probability of a male having the recessive phenotype
Sex Linked Traits
Color Blindness• Xb = No color blindness• XB = color blindness
• Determine the crosses (both phenotype and genotype) for the following crosses
1. Color blind father x normal mother yields one color blind son.
2. Normal father, carrier mother.3. Normal father, color blind mother.4. What cross will yield a color blind daughter?
Sex Linked Traits
• Called hemizygous • Do occur in females but males have a higher
probability of inheriting the trait• Duchenne muscular dystrophy, hemophilia,
color blindness
X Inactivation in Females
• Having two X chromosomes is a lot of genes!• One X chromosome will be turned off – Barr body: X chromosome condenses and will be
near the nuclear envelope– Ovaries – Barr body will be duplicated for viable egg
cells– In development, different X chromosomes could be
turned off • About ½ of the cells display the mother’s traits and half
display the father’s
Linked Genes
• Genes on the same chromosome that tend to be inherited together
• Morgan believed body color and wing shape were inherited together
Genetic Recombination
• Offspring show combinations of traits not found in the parents
• How does this happen?
Recombinants
• Mendel’s peas• YyRr x yyrr
• Which genotype and phenotypes are recombinants? Which are parental types?
Crossing Over• During Meiosis I (Prophase I)• Homologous chromosome pairs come
together forming a tetrad– Crossing over• Each chromosome will cross with the other in the pair• Parts of the chromosome will be exchanged
Recombination Frequency
• How likely is it that the two genes will be linked?
• Based on how close they are on the chromosome– Closer they are, more likely they will be linked
• Linkage map: genetic map based on recombination frequency
Linkage Map for Drosophila melanogaster
15.4 Alterations of chromosome number or structure cause some
genetic disorders
Abnormal Chromosome Number
• Nondisjunction: chromosomes do not separate correctly– Meiosis I or II
Aneuploidy
• Gamete with abnormal number of chromosomes unites with a normal gamete
• Offspring will have abnormal number of chromosomes
• Monosomic (2n – 1)• Trisomic (2n + 1)• Polyploidy: triploidy (3n), tetraploidy (4n)
Chromosomal Mutations
• Involve changes in the number or structure of chromosomes
• 4 types:– Deletion– Duplication– Inversion– Translocation
Deletion and Duplication
• Deletion:– Loss of all or
part of a chromosome
• Duplication:– Produce extra
copies of the chromosome
Inversion and Translocation
• Inversion:– Reverse direction of
parts of the chromosome
• Translocation:– Part of one chromosome
breaks off and attaches to another chromosome
Human Disorders caused by Chromosomal Alterations
• Down Syndrome (Trisomy 21)• Kleinfelter Syndrome (XXY)• Turner Syndrome (monosomy of the X
chromosome)
Inheritance Patterns
• Not all fall into set patterns of equal inheritance
• Genomic Imprinting– Effect of the allele for a certain trait depends on
which parent passed on the trait– Could be expressed in different strengths
Genetic Imprinting in Humans
Prader-Willi Syndrome• Deletion of part of
chromosome 15
• Learning difficulties, short stature, and compulsive eating
• Defective chromosome came from dad
Angleman Syndrome• Deletion of part of
chromosome 15
• Learning difficulties, speech problems, seizures, jerky movements, and an unusually happy disposition
• Defective chromosome came from mom