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