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Genetics- lecture 6- Mendel

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winniesmith1's version from 2017-05-14 18:09

Section 1

Question Answer
GeneSequence of DNA capable of producing a product (Protein, trait, characteristics etc.). Each parent contributes one set of chromosomes (genes)
Allele alternative form of a gene
LocusSite/position of a gene on chromosome.
Homozygote Trait with similar (like/identical) types of alleles
HeterozygoteTrait with unlike types of alleles
Dominant allele Allele which are phenotypically expressed in both homozygotes and heterozygotes.
Recessive allele Not expressed when paired with a dominant allele
Codominant allele Both alleles can express. (Most common classification of genes)
Cellsfundamental units of living things
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Section 2

Question Answer
What did Mendel study?Pea plants by artificial fertilisation. Peas have relatively short life cycle, many varieties available. He chose seven different traits to compare. (mainly round and wrinkled)
What is Mendels 1st law or segregation principleMembers of a single gene pair (alleles) are always SEGRAGATED (separated) during gamete formation (germ cells, sperm, ovum). Traits or characters are usually determined by a pair of genes (one from each parent). No Blending of genes. Genotypic ratio: 1:2:1 (25%: 50%: 25%). Phenotypic ratio: 3:1 (75%: 25%).
When doing a monohybrid cross; Round vs Wrinkled what ratios are producedGenotypic ratio: 1:2:1 and a phenotypic ratio 3:1
What phenotypic ratio does a dihybrid cross give you9:3:3:1
What is Mendels 2nd law or independent AssortmentDihybrid or Trihybrid crosses . Behaviour of one set of genes is NOT dependent on the other set of genes. Independently assort themselves. Phenotypic Ratios 9:3:3:1
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Section 3

Question Answer
What is a chi squared testTest of significance. Compares Observed numbers with Expected numbers (according to some rule, for example Mendel’s 1st or 2nd law phenotypic or genotypic expectations)
What does chi squared confirmConfirms whether your experiment is according to expectation or not
How do you calculate degrees of freedom Nm of categories -1 (for 2x2 table). Others= (no. of cat -1) X (no. of classes -1)
How do you know if a chi- value is significant If observed Chi-value is more than Table value then the differences are Significant. If Less, then the differences are NOT SIGNIFICANT, i.e., Results are as expected.
How is mendel's law used in humans-Pedigree Analysis -Most commonly used tool in genetics
What does pedigree analysis doIllustrates the relationship among family members, shows which family members are affected with disease and which are unaffected.
In pedigree analysis what does an arrow denoteThe proband, the first individual diagnosed in the pedigree.
How is a pedigree constructedUse standard set of symbols. One generation per row (oldest at the top). Generations are given Roman numerals (I, II, III, IV, etc.). Individuals within a generation (row) are given Arabic numerals (1, 2, 3 ). Siblings are shown in order of birth (from left to right).
How do we analyse a pedigreeTrial and error. Consider one pattern of inheritance at a time for each mating and try to find evidence against it. Use features of inheritance! Patterns of inheritance follow Mendelian rules; but Mendelian ratios are rarely observed! Assumption: for rare traits unaffected people entering into a family pedigree (for example, by marriage) are considered homozygous normal
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Section 4

Question Answer
What is mendelian inheritanceMendelian traits, or single gene disorder, fall into 5 categories or modes of inheritance based on where the gene for the trait is located and how many copies of the mutant allele are required to express the phenotype.
What is autosomal dominant inheritance the locus is on an autosomal chromosome and only one mutant allele is required for expression of the phenotype.
What is autosomal recessive inheritancethe locus is on an autosomal chromosome and both alleles must be mutant alleles to express the phenotype
What is X-linked recessive inheritance the locus is on the X chromosome and both alleles must be mutant alleles to express the phenotype in females.
What is X-linked dominant inheritancethe locus is on the X chromosome and only one mutant allele is required for expression of the phenotype in females.
What is Y-linked inheritance the locus is on the Y chromosome.
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Section 5

Question Answer
Describe autosomal dominant inheritance -More than 3700 AD traits (mostly diseases) known. -Relatively Rare! 1 in 10,000 individuals affected. -Females and males exhibit the trait in equal proportions. -Males and females are equally likely to transmit trait to their offspring. -No skipping of generations: if an individual has disease/trait one parent must also have it. -Vertical transmission pattern - disease phenotype is usually seen in one generation after another. -Father to son transmission may be observed. -Each event has a probability of 0.5 regardless of how many affected or unaffected children are born
Describe a non-medical dominant trait Allele for widow's peak
Name some other autosomal dominant diseases-Achondroplasia (Dwarfism). -Marfan syndrome. -Familial Hypercholesterolemia. -Huntington’s disease. -Myotonic dystrophy. -Nail-patella syndrome.
What is achondroplasia -Common cause of dwarfism. -Characterised by short limbs relative to trunk length, prominent forehead, low nasal root and redundant skin folds in the arms and legs. -Occurs in all populations and affects 1 in 25,000 to 1 in 40,000 children -Mutation in a gene for fibroblast growth factor receptor 3 (FGFR3), which causes an abnormality of cartilage formation.
Describe autosomal recessive inheritance1. Affected individuals have parents with normal phenotypes, unaffected (heterozygotes, carriers). 2. Two heterozygous carriers can have ¼ of affected off-springs. 3. Horizontal pattern with several generations of unaffected individuals, but then several siblings in one generation are affected!  Skips generations! 4. Equally prevalent in both males and females. 5. Parents of affected children may be related. The rarer the trait in the general population, the more likely a consanguineous mating is involved.
Name some autosomal recessive traits-Cystic Fibrosis. -Gaucher disease. -Glycogen storage disease. -Phenylketonuria. -Sickle cell Anemia. -Homocystinuria. -Tay-Sachs disease. -Thalassemia.
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Section 6

Question Answer
Describe X-linked dominant diseases-The locus for a gene for a particular trait or disease lies on the X chromosome. -Trait observed in both males and females. -Affected males (hemizygote) ALWAYS transmit the trait to their daughters, but to NONE of their sons. -Affected females (heterozygotes and homozygotes) and will transmit the trait to both sons and daughters. -Trait does not skip generation. -Males are usually more severely affected than females. May be lethal in males. -Two times more common in females.
Name some examples of X-linked dominant diseases-Rett Syndrome. -Chondrodysplasia Punctata
Describe X-linked recessive diseases-More males than females are affected. -All the sons of an affected mother will be affected. -Half the sons of a carrier mother will be affected. -All daughters of carrier mothers will be normal, but half will be carriers. -Affected males do not transmit the trait to their sons. -Trait often skips a generation.
Name some examples of X-linked recessive diseases-Duchenne and Becker Muscular Dystrophies, -Colour blindness, -Haemophilia, -Retinitis Pigmentosa.
Describe Y-linked diseases-Holandric, genes encoded by Y chromosomes (100), mainly responsible for testis and sperm formation . -Passed from male to male. -Very rare. -11 “disease” genes have been identified, mainly leading to infertility. -Non-medical trait example: Hairy pinnae.
Name (8) complicating factors for inheritance patterns 1. New mutation. 2. Delayed age of onset. 3. Reduced penetrance (a dominant mutant allele may not express the clinical phenotype). 4. Variable expression (different family members show different features of a syndrome). 5. Heterogeneity (different allelic mutations leading to the same phenotype). 6. Pleiotropy: Genes that exert effects on multiple aspects of physiology or anatomy are pleiotropic (e.g. Cystic Fibrosis, Sickle Cell anaemia). 7. Genomic Imprinting (differential expression of genes depending on parental origin) . 8. Anticipation (increased severity of dominant condition in successive generations).
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