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Unformatted text preview: Chapter 13 Chromosomes, mapping, and the MeiosisInheritance Connection
Learning Objectives: 1. To understand the concept of genes and chromosomes 2. To understand the concept of linkage and crossingover 3. To Understand the concept of sex linkage 4. To Understand the sex determination mechanism in animals 5. To Understand the importance of human genetic disorders including those produced by nondisjunction during meiosis Genes are located on Chromosomes Chromosomes are of two types: 1. Autosomal chromosomes 2. Sex chromosomes Linkage and CrossingOver
Definition Linkage Groups Example of Linkage Example of Linked Genes Crossingover 1. Genes that are on the same chromosome are said to be linked. 2. Linked genes do not normally assort independently. 3. However, linked genes can assort, sometimes a little and sometimes independently, because of the process of crossingover. 4. Crossingover is the process where sister chromatids of different homologues break and exchange pieces of their chromosomes. 5. Crossingover occurs in prophase I of meiosis. We can see the results of crossingover = chiasmata An Example of Linkage with CrossingOver Question? Why are the Crossover Products so Rare? Answer: Because the rate of crossingover is related to the distance between the two genes. The farther two genes are apart the greater the probability that a crossingover event will occur somewhere between them. Therefore, genes that are farther apart will crossover more often than genes close together. Interestingly, the percent of crossover progeny is used as a measure of how far apart two genes are on the chromosome. By convention, 1% crossingover is equal to 1 map unit on the chromosome. This assumes that the rate of crossingover along any length of the chromosome is the same. Question: How far apart on the chromosome in map units are the genes for adipose fin size and head color? Answer: In our example, there were 10 individuals out of a 100 that resulted from crossingover. Therefore, 10% of the individuals resulted from crossingover. Therefore, adipose fin size and head color are 10 maps units apart. Number of Crossover Progeny 58 Map distance = = = 17.8 MU Total Number of Progeny 326 Let's Do Some Sample Calculations
Phenotype in the F2 Generation Number of Progeny Large adipose, Red head 45 40 30 25 Small adipose, Green head 45 40 30 25 Large adipose, Green head** 5 10 20 25 Small adipose, Red head** 5 10 20 25 100 100 100 100 Distance in Map Units = ? ? ? ? **= crossover progeny The Big Question?
Under What Conditions Can Genes Assort Independently? Answer: The Sum Probability Law: The probability of either A, B, C, or D event occurring is the sum of the individual probabilities.
Chromosome 19 in Salmon Adipose Size X1 X2 X3 X4 X5 X6 Head Color The probability of crossingover is the same for each "X" unit of distance along the chromosome. The probability of a crossover within any "X" unit of distance is 1/ 100. Therefore the probability of a crossingover event occurring somewhere between X1 and X6 = 1/100 + 1/100 + 1/100 + 1/100 + 1/100 + 1/100 = 6/100 = 0.06. Comprenez vous? Sex Linkage Definition: Hemizygous Example In Fruit Flies Example in Humans Hemophilia (XLinked)
Affected individuals and Carriers In guppies, some of the brilliant colors of the male are controlled by genes on the Y chromosome. Ylinked genes follow a patrilineal pattern being passed on down the generations from father to son. Other colors are controlled by autosomal genes. Sex Determining Mechanisms in Animals
1. Mammals, humans, Drosophila 2. Birds 3. Grasshoppers 4. Honeybees 5. Turtles and Alligators Dosage Compensation in Mammalian Xlinked Genes
1. To keep the amount of gene product in Xlinked genes the same in males and females, one of the females X chromosomes is inactivated and becomes what is known as a Barr Body Example of XChromosome Inactivation The Calico Cat Major Causes: 1. Single nucleotide Polymorphisms caused by mutation (SNPs) Human Genetic Diseases 2. Chromosomal Mutations Euploidy Cope's Tree Frog, Hyla chrysoscelis Diploid Species 2N = 24 Eastern Gray Tree Frog, Hyla versicolor Tetraploid Species 2N = 48 How Unique Am I (or YOU)? In 1620 (before you were born?) my ancestor Peter Aspinwall and his brother immigrated to the United States from England. All 400+ Aspinwall's living in the United States today, to my knowledge, are direct ancestors of one of these two brothers. The important question is: Do I share many chromosomes (genes) with my distant grandfather Peter? Discussion Time The Answer The simple answer is: NO. Each generation an individual's genetic makeup gets diluted by because of sexual reproduction. For example, my son Eric has only of my chromosomes since his mother donated the other . My grandchildren will have only of my chromosomes. Since 1620, 387 years have elapsed. Assuming that the average generation time is 20 years, there have been roughly 19 generations separating me and Peter. Therefore, on the average, I will possess (1/2)19 or 1 chromosome out of 524,288. Now, since I only have 46 chromosomes total, there is a pretty low probability of having any of Peter's Aspinwall's chromosomes. Therefore, is there any genetic basis for genealogy as presently employed? Am I depressed or What? Nope I'm Pretty Cheerful
I'm pretty cheerful because even though Peter and I don't share many chromosomes in common we do share the Y chromosome. The Y chromosome in my body is exactly the same as Peter's Y chromosome barring a few mutations. The Y chromosome is the only biological basis for the patrilineal system (ie children with the same father's name sharing a common lineage). Are all the girls in the class depressed? Nope!!! In a matrilineal system mothers and daughters would possess the same genes in common. The only biological basis for this is the presence of mitochondrial DNA which is inherited maternally from mother to daughter. ...
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- Fall '11