Chromosomal Theory of Inheritance Chapter 15

Learning target 1 I can explain the chromosome theory of inheritance. I can explain why linked genes do not assort independently. I can explain how sex linked inheritance works in mammals and in birds. I can distinguish between parental and recombinant phenotypes. I can explain how crossing over can unlink genes. I can create a linkage map. I can describe the inheritance patterns and symptoms of color blindness, Duchenne muscular dystrophy and hemophilia. Learning target 1

The Theory Genes are located on chromosomes Chromosomes segregate and independently assort during meiosis

Evidence Cytologists (study cells): Geneticists (study genes): 1879 – Mitosis worked out 1890 – Meiosis worked out Geneticists (study genes): 1860 – Mendel proposed laws of segregation & independent assortment 1900 – Mendel’s work rediscovered

Linkage & Chromosome maps Linked genes are genes that are located on the same chromosome and DO NOT assort independently Why some traits tend to be inherited together, such as red hair and freckles We also know that chromosomes will also ‘cross-over’ during gamete formation Based on the number of crossovers observed, we are able to determine the order of genes, and relative distance of genes from one another

Genetic Recombination Due to crossing over, offspring may or may not look like their parents Parental types do Recombinant types don’t Crossing over can unlink genes on the same chromosome, depending on distance from each other

Making a Linkage Map In fruit flies, gray body is dominant to black body and normal wings are dominant to vestigial wings. Flies heterozygous for both gray bodies and normal wings are crossed with flies that had black bodies and vestigial wings. The following results were obtained: The results indicate that the genes for wings and body color are on the same chromosome. The recombinant offspring are a result of crossing over. How many map units (expressed as a percent) apart are the two genes? Phenotype Number of flies Gray body/normal wings 482 Black body/vestigial wings 472 Gray body/vestigial wings 103 Black body/normal wings 92

The formula for calculating recombination frequency is: Which phenotypes are recombinants? Gray body/vestigial wings (103) Black body/normal wings (92) Use the formula: (103+92)/1149 = 17% This means that the genes for body color and wing shape are located 17 map units from each other on the chromosome

Sex Determination Determined by presence of Y chromosome in humans XX: female XY: male Other systems in birds, insects: XO: insects ZW: birds Haplo-diploid: bees

SRY gene Sex determination region On Y chromosome Triggers events that lead to testicular formation

Sex-linkage (or, X-linkage) Some traits are only inherited because they are on the X chromosome Typically recessive traits Males inherit these more often Why?

Sex linked disorders Colorblindness Hemophilia Duchenne Muscular Dystrophy

Sample cross Colorblindness XCXc x XCY In any sex-linked cross: XC = normal vision Xc = colorblind XCXc x XCY In any sex-linked cross: Affected males Xc get from Mom Affected females get one Xc from Mom and one from Dad More males affected than females

I can explain how X inactivation results in Barr Bodies. I can describe the process of X inactivation in female mammals. Explain how this phenomenon produces the tortoiseshell coloration in cats. Learning target 2

X inactivation Females are XX but only need one X one X condenses and genes become silenced Inactivation happens during embryonic development Inactivation is random Females are mosaics of 2 cell types paternal X inactive maternal X inactive

I can describe how alterations of chromosomes occur and how they can cause genetic disorders I can explain how nondisjunction can lead to aneuploidy I can Identify human disorders caused by chromosomal alterations; trisomy 21, Klinefelter syndromes, turner syndrome. I can identify alterations in chromosome structure as the result of: deletions, duplications, inversions, and translocations. Learning target 3

Whole Chromosome Mutations Nondisjunction Failure of chromosomes to separate during meiosis

Disorders due to Nondisjunction Down Syndrome (trisomy 21) Klinefelter Syndrome – male who is XXY Turner Syndrome – female who is XO

Chromosomal Alterations Mutations of parts of chromosomes Replication error or crossing over error

I can identify and explain inheritance patterns that are exceptions to the standard chromosome theory. I can explain how genomic imprinting works. Learning target 4

Back to Mendel for a minute… Looking at Mendel’s peas, it didn’t matter if the traits came from the maternal or paternal parent Each trait would have the same bearing on the offspring Scientists, within the past decade or so, have found some genes that are inherited differently depending on which parent passed it along

Genomic Imprinting Variations in phenotype depending on whether the allele is passed on from the male or female parent This differs from sex linkage due to the fact that most imprinted genes are found on autosomes

Imprinting Occurs during gamete formation These genes are expressed differently in eggs or sperm – the trait is ‘silenced’ in one gender The developing embryo will only express the trait from one parent in all of its body cells Effect is determined by which chromosome’s gene is expressed

Example – Igf2 Insulin-like growth factor 2 (Igf2) is needed in mice for normal growth and development In crosses between wild type and homozygous recessive dwarf mice, individuals produced heterozygous offspring Offspring differed in phenotype depending on whether the gene was maternal or paternal

I can explain how inheritance works with extra-nuclear genes found in organelles such as mitochondria. Learning target 5

One More Genetic Exception… Not all genes are found on nuclear chromosomes Organelles such as mitochondria and chloroplasts have circular DNA molecules that carry genes These are almost always passed on from the mother because the egg houses these organelles, while the sperm only carries chromosomes