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 X b = No color blindness X B = 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 Mapping Problem Mating: AaBb x aabb A = Long antennae a = Short antennae B = Green eyebrows, and b = Blue eyebrows. Say you make this mating, and your actual results look like this: Long Green Long Blue Short Green Short Blue - 850

Calculate Linkage Map Distance One linkage map unit (LMU) is 1% recombination. Thus, the linkage map distance between two genes is the percentage recombination between those genes. In this case, we have a total of 300 recombinant offspring, out of 2000 total offspring. Map distance is calculated as (# Recombinants)/(Total offspring) X 100. What is the LMU of the two genes?

Linkage Map Summary 1.How do you tell your genes are linked? 2.How do you identify the parental linkage? 3.How do you calculate linkage map distance?

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

Organelle Genes Extranuclear genes (found in cytoplasm) Mitochondrial genes Chloroplast genes NOT distributed to the offspring in the Mendelian fashion Come from mother – why?

Mitochondrial Genes Most make up the ETC and ATP synthase Defects in these genes will affect energy production – Most severely nervous system and muscular system