Biology 11 Enriched: Genetics UnitChapter 15

Name: _______________________ ________

Biology 11 Enriched: Genetics Unit

Chapters: 15, this is required reading.

Big Ideas:Big Idea Chapters Illustrative Examples

3. Living systems retrieve, transmit and respond to information essential to life processes.3.A.4 The inheritance pattern of many traits cannot be explained by simple Mendelian genetics.

15.1, 15.2, 15.3, 15.5

• Sex-linked genes reside on sex chromosomes (X in humans) • In mammals and flies, the Y chromosome is very small and carries few genes • In mammals and flies, females are XX and males are XY; as such, X-linked recessive traits are always expressed in males • Some traits are sex limited, and expression depends on the sex of the individual, such as milk production in female mammals and pattern baldness in males

Part A: Chromosome Theory of Inheritance

-The Chromosomal Theory of Inheritance began to develop in 1902 as a result of a convergence between Mendel’s ideas of inheritance and the observed behaviour of chromosomes.

-Remember that when Mendel proposed his ideas of inheritance there was no knowledge of chromosomes, genes or alleles!

-This theory states that Mendelian genes had specific loci along chromosomes and that these chromosomes undergo segregation and independent assortment.

-This section of notes focuses on providing more supporting evidence to this theory and extending your knowledge of inheritance.

Part B: Chromosomal Behaviour Shaping Mendelian Inheritance-Thomas Hunt Morgan, an experimental embryologist, worked with fruit flies in the early 20th century. His experiments provided evidence that chromosomes are the location of Mendel’s heritable factors.

1. Morgan’s Choice for Experimental Organism-Why are fruit flies good organisms for genetic research?______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

-Wild Type: Phenotype for a character most commonly seen in the natural population.

-Mutant Phenotypes: Traits that are alternatives to the wild type. They are the products of mutations in the wild-type allele.

-What is the possible genotypes for the flies shown right?_____________________________________________

_____________________________________________

2. Correlating Behaviour of a Gene’s Alleles with Behaviour of a Chromosome Pair-Examine the Experiment and Result portion of

the diagram right. What is revealed about the

gene for eye colour?

_______________________________________

_______________________________________

_______________________________________

_______________________________________

_______________________________________

_______________________________________

_______________________________________

_______________________________________

-Imagine that the eye colour gene was located on an autosomal chromosome. Predict the phenotypes (including gender) of the F2 flies in this hypothetical cross.

-Morgan’s work showed that a specific gene is carried on a specific chromosome and that genes located on a sex-chromosome show a unique pattern of inheritance.

Part C: Sex-Linked Genes Exhibit Unique Patterns of Inheritance1. Chromosomal Basis of Sex-Humans and other mammals have X and Y sex chromosomes.-Short segments at the end of the Y chromosome are homologous with regions on the X to allow pairing during meiosis.

-The diagram right illustrates other sex chromosome systems in other animals.

-Sex-linked gene: Gene located on either sec chromosome.-X-linked genes: Located on the X chromosome. Approximately 1,100, many of which are not related to sex characteristics.-Y-linked genes: Located on the Y chromosome. 78 genes, half of which are only expressed in the testes.

2. Inheritance of X-Linked Genes-Examine the Punnett Squares below. What are some patterns you observe in the transmission of X-linked alleles (particularly colourblindness)?

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

-If a colour-blind woman married a man who had normal colour vision, what would the probable phenotypes of their children be?

3. X Inactivation in Female Mammals-If genes code for the production of proteins, do women make twice as much proteins linked to the X chromosome as men?___________________________________________________

___________________________________________________

___________________________________________________

___________________________________________________

___________________________________________________

___________________________________________________

-Barr Body: Condensed, inactivated X chromosome. Most of the genes on this chromosome are not expressed. Inactivation is done through the attachment of methyl groups.

-Examine the diagrams right. Is there a pattern to the creation of Barr Bodies?___________________________________________________

___________________________________________________

___________________________________________________

___________________________________________________

Part D: Linked Genes are Near to Each Other on the same Chromosome1. How Linked Genes Affects Inheritance-Linked Genes: Genes that are commonly inherited together as they are close to each other on the same chromosome.

-Do you think linked genes will follow Mendel’s normal laws of inheritance?

______________________________________________________________________________

-Examine one of Morgan’s experiments with flies below. Are the two genes being examined linked?

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

-Genetic Recombination: Production of offspring with combinations of traits different from those found in either P generation parent.

2. Genetic Recombination and Linkage

Recombination of Unlinked Genes: Independent Assortment of Chromosomes-As discussed already, crosses involving two characters that are not linked produce offspring of two types:

a) Parental Type: Phenotype that matches one of the parents.b) Recombinant Type: New combinations of phenotypes.

-The diagram above shows a 50% frequency for recombinants. Based on this do you think that these genes are linked? ______________________________________________________________________________

Recombination of Linked Genes: Crossing Over-Morgan re-visited the test cross that suggested some fly genes are linked.

-What process do you think created the relatively small number of recombinants in this cross?____________________________________

*A simple statistical test can be conducted to determine whether alleles are inherited together due to a linkage or whether they just happen to assort together by chance. This is called the Chi-Square (X2) Test.

-The diagram below summarizes how linkage can affect the inheritance of genes.

3. Mapping the Distance Between Genes Using Recombination Data-The discovery of linked genes and recombination motivated one of Morgan’s students, Alfred H. Sturtevant, to devise a method for making a genetic map.

-Genetic Map: Ordered list of genetic loci along a chromosome.

-Linkage Map: Genetic map based on the frequencies of recombination between markers during crossing over of homologous chromosomes.

-Map Units: Distance between genes on a chromosome. 1 map unit = 1% recombination frequency.

-The interpretation of recombinant data is more complex than depicted above

a) Some genes are physically connected, but genetically unlinked. For example, crossing over will always occur between two genes that are very far apart on the same chromosome, making them appear to be on different chromosomes that are assorting independently

b) Frequency of crossing over is not uniform over the length of the chromosome, meaning that map units do not correspond to physical units. A linkage map shows the order of genes on a chromosome, not their precise locations (new technology creates cytogenetic maps that show this).

Part E: Genetic Disorders Are Caused by Alterations to Chromosome Number and Structure1. Abnormal Chromosome Number-Nondisjunction: Where homologous pairs do not separate in meiosis I or sister chromatids do not separate in meiosis II.

-What is the difference between aneuploidy and polyploidy?_______________________________________________

_______________________________________________

_______________________________________________

_______________________________________________

_______________________________________________

_______________________________________________

-Some types of aneuploidy are called monosomic (2n-1) and trisomic (2n +1). These occur in 10 to 15% of pregnancies and are the main reason for loss of pregnancy.

-Some types of polyploidy are triploidy (3n) and tetraploidy (4n). These are common in Kingdom Plantae and, in general, polyploidy organisms are more phenotypically normal than aneuploids.

2. Alterations of Chromosome Structure-Errors in meiosis or due to environmental mutagens can lead to a breakage of a chromosome, leading to four possible changes in chromosome structure:

a) Deletion: Chromosomal fragment is lost. More likely to occur during meiosis. Large deletions are normally lethal.

b) Duplication: Fusion of a chromosomal fragment from a sister chromatid. More likely to occur during meiosis. Tend to be harmful.

c) Inversion: Fusion of a chromosomal fragment in its original position, but in the reserve orientation.

d) Translocation: Fusion of a chromosomal fragment to a non-homologous chromosome. Tend to be harmful.

-

Do you think that reciprocal translocations between non-homologous chromosomes or inversions are harmful? Why?______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

3. Human Disorders Due to Chromosomal Alterations

-Take your own notes from the text on the following human genetic disorders:

Trisomy 21______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

Klinefelter Syndrome (XXY Males)______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

XYY Males______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

Trisomy X (XXX Females)______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

Turner Syndrome (X0 Females)______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

Cri du Chat______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

Chronic Myelogenous Leukemia

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

Part F: Some Inheritance Patterns are Exception to Mendelian Inheritance1. Genomic Imprinting-Genomic Imprinting: Expression of an allele in offspring depends on whether the allele is inherited maternally or paternally. These genes are mostly on autosomes and not sex chromosomes.

-Attaching a methyl group to cytosine will either silence or activate the expression of an allele.

-Affects only a small fraction of mammalian genes.

2. Inheritance of Organelle Genes-Not all genetic material in a eukaryotic cell is found in the nucleus. Where could the other genes be? -Mitochondira, chloroplasts

-These extranuclear genes are not inherited according to Mendelian rules, but through the maternal line.